Go-to Guide for Source Options - AVS Forum
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Background Guide to Setting Source Options

Why am I writing this guide?
Simple! There has been a great deal of confusion about the settings and ‘calibrations’ of source devices. Well-intentioned users commonly misunderstand the functions of some adjustments, and end up unintentionally degrading their image quality. This FAQ /guide is an attempt to filter through some of the mess and assemble a decent background in one place, so that you can do your best to make sure that your playback chain is not degrading your video unnecessarily.

The layout:
I’ve organized this guide into two parts with background sections first, in the hope that everyone will want to read the background first before going on to adjust settings and calibrate. For you throw-away-the-manual adventurers, you can skip ahead to the adjustment section that applies to your system. The numbers match between the two parts, so if you’ve skipped ahead to the goodies of finding the right setting and it’s confusing, circling back to the background explanation may help!

Say, who’s behind this fiasco of a FAQ? What’s the conspiracy! :)
I’ve written and compiled this material with the help of Guy Kuo, Keith Jack, Stacey Spears, Joe Murphy Jr, Bob Pariseau, and the indirect help of many other knowledgeable folks, posts, books, alternate personalities etc. Thanks guys! Much of this background has already been discussed on the forums, but is scattered and obscured by popular misinformation. My hope is to unite a solid background that is sorely lacking.

Wait a minute, you’re wrong about _____!
I’ve taken great care to make sure the information is trustworthy by having some of the biggest video super-heroes check to make sure the content is solid, so you might actually find that if you read through the background you’ll discover that what you read in the past may not have been as correct as you thought. But if I have left something out or made a mistake, let me know and I’ll fix it! I have used some simplified terms, for instance I label non-linear R’G’B’ simply RGB, etc, for simplicity and clarity: this is not likely to cause any confusion since I can’t fathom any HT environment dealing in any non-corrected linear RGB values!

What tools do I need?
If you are a Home Theater enthusiast, I should certainly hope you already own a copy of Avia or Digital Video Essentials. If you’re a nut like me you own both, and perhaps also Avia PRO! For testing Studio versus PC levels Avia PRO or DVE are necessary.


1)Analog input/outputs from digital sources
2) Digital outputs and digital levels
3) Upconverting players and possible colorspace issues
4) HTPCs


BACKGROUND

1)Analog input/outputs from digital sources

Say, what the heck is an IRE?
As often as the term IRE is thrown around when discussing video, relatively few actually know what it means!
An IRE simply a representation of volts: there are with 140 IRE units in one volt. This simplified expressing the ~700mV excursion of an analog video waveform, with white represented using 100 IRE instead of an odd 714mV. As I will describe, IRE units are ambiguous at describing the intended image content unless you know whether or not the 7.5 IRE setup pedestal is present. Many do not grasp that IRE is merely another way to represent volts, and as such confuse IRE into some mysterious value that magically describes the actual image information, which cannot do without caveats.

North American NTSC standards historically used a level for the analog output of black that was 53.6mV (aka 7.5 IRE) above the blanking voltage of 0mV (0 IRE), and we still do. Other standards like HD, Japanese NTSC, etc use 0mV to output black, and lack the 53.6mV (7.5 IRE) ‘setup pedestal’ that North America uses. This IRE setup option allows you to switch between these two voltage output levels for black. Neither will necessarily provide any universal increase in picture quality: it merely alters the voltage output that is used to represent black. With a correctly designed player and a good display, upon recalibrating your display to the new source outputs, your picture would be identical. As we all know the ideal player is rare and displays vary, so in some systems one setting may be preferred over the other to prevent a degraded picture. But neither setting is universally preferred. I’ve explained how to examine the behavior of your specific system to determine which setting(if any) is preferred in the second half of this FAQ.

Test patterns from test DVDs are (unfortunately, in my opinion) often labeled or described in IRE units. These are merely labels, and they usually assume the presence of setup, thus the source outputs black at 7.5 IRE. If there is no setup then the source is outputting black at 0IRE and the labels on the test patterns are now wrong for that system. The absolute IRE levels will change depending on the behavior of the source device, and the DVD has no control over this. These charts accurately show that the encoded digital values that are the stimulus do not change, while the absolute mV outputs (hence IRE values) will change depending on the voltage for black output:

**
For Avia and Avia PRO--
7.5 IRE output:
Code:
Labeled Percent Output  Output  
IRE     Stim    IRE     mV
7.5     0.0     7.5     53.6
10      2.7     10.0    71.4
20      13.5    20.0    142.9
30      24.3    30.0    214.3
40      35.1    40.0    285.7
50      45.9    50.0    357.1
60      56.8    60.0    428.6
70      67.6    70.0    500.0
80      78.4    80.0    571.4
90      89.2    90.0    642.9
100     100.0   100.0   714.3
0 IRE output:

“Players with a 0 IRE representation for black and 100 IRE for 100% white will output IRE levels that don't match the on disc labeling would do the following. Notice that the relationship between labeled IRE and percent stimulus remains the same as for players with 7.5 IRE setup. However, the output IRE and mV change.†-GK

Code:
Labeled Percent Output  Output  
IRE     Stim    IRE     mV
7.5     0.0     0.0     0.0
10      2.7     2.7     19.3
20      13.5    13.5    96.5
30      24.3    24.3    173.7
40      35.1    35.1    251.0
50      45.9    45.9    328.2
60      56.8    56.8    405.4
70      67.6    67.6    482.6
80      78.4    78.4    559.8
90      89.2    89.2    637.1
100     100.0   100.0   714.3
**
(From Guy Kuo)

You can see that in a system that outputs black at 7.5IRE, the labels are accurate. In a system that outputs black at 0IRE, the labels must be recalculated: actual IRE levels when setup is not present = (labeled IRE-7.5) * 100/92.5. You can see that an Avia pattern labeled 10 IRE will end up about being output at a voltage about 2.5IRE (more accurately 2.7 IRE or 19.29mV) in a system without setup. As long as the display is correctly calibrated to the source, the pattern should be imaged correctly.

If that last portion seemed like too many numbers, don’t worry! The most important point you need to take away is that IRE units are merely another way of expressing mV.

It should be obvious already that DVDs are not analog. They contain digitally encoded video. As such, there are no voltages read from a DVD(it’s an optical media!). By extension, there are of course no absolute IRE units represented on a DVD. A DVD uses 8-bit digital levels from 1-254 to represent the image data. Only when that data must be output in an analog form do voltages, hence IRE, even come into play. That’s right: if you are using a digital output, there is no analog video signal, thus IRE units do not come into play in any form (cautious note: the adjustment may unintentionally affect digital output, but if designed correctly it should not.).

Because many have confused IRE units into some mysterious creature of a unit, they’ve confused digital levels and IRE as somehow being the same: i.e. that IRE are encoded on DVDs, or that DVDs only go down to 7.5 IRE while HD has better black capabilities because HD goes down to 0 IRE.

This is rubbish.

Both HD material and DVDs use the same digital levels. It is the ANALOG output standards which can vary. The encoded levels represent black just as black. The dynamic range is the same. Video encodes black at digital 16. Period. That’s black, and whether it is output at 0 IRE or 7.5 IRE the resultant image should be equally black. If there is any difference in the visible levels of black, your video chain is miscalibrated. If you connect sources that output at various voltages, you have to calibrate to EACH of these sources. Users will often connect a DVD player that outputs black at 7.5 IRE, then connect an HD source that outputs black at 0 IRE, discover that the HD source is darker, and mistakenly conclude that HD has improved black capabilities. This discovery of changed blacks is merely that you have different sources sending different signals. You must calibrate your display to each individually.

If you’ve followed this far, then you’ll see that the following types of statements(some repeated often) are complete nonsense:
Quote:
‘Movie studio A encodes their DVDs at 7.5 IRE, while studio B uses 0 IRE.’
‘Test disc X is at 7.5 IRE while test disc Y goes down to 0 IRE.’
‘You must always switch any DVD player to 0 IRE for: darker blacks/better -contrast/blacker-than-black.’
‘HD can provide better blacks because it goes down to 0 IRE.’
To reiterate, all the previous statements were nonsensical and meaningless. They represent confusion on the facts surrounding analog voltage outputs(expressed in mV or IRE units) as opposed to what is digitally encoded on the DVD.

The first two are essentially the same statement. Again, if you understand that IRE is just an expression of volts, you realize that DVDs don’t have volts on them at all. DVDs are an optical format, read digitally by a laser, not by anything magnetic/electric. Whether a picture from a DVD is output in analog form with black represented at 7.5 IRE or 0 IRE is purely a function of the player. A DVD is encoded in digital levels. Black is encoded at digital 16 and this is fixed. Whether that is then output at 0 IRE or 7.5 IRE is beyond the control of the DVD itself.

The third statement is also incorrect. In an ideal system, as long as you recalibrate between 0 IRE and 7.5 IRE, there will be no change at all in the final image. The final image will have the exact same visible black, white, and contrast in both cases. Using the 0 IRE option does not in any way output “more†data that is magically “below 7.5 IRE.†Again, data is not encoded in IRE at all. Stating that there is somehow “extra data below 7.5 IRE on a DVD—you need to set your player to 0 IRE to retrieve this data†is like saying that your car has better fuel mileage because your speedometer is labeled in MPH instead of Kilometers. The two things are completely unrelated and the statement is, well, total baloney.

The last statement is also wrong. Black is black. If switching between sources you discover that your black levels in the image are different, this means that at least one of your sources is miscalibrated. If individually calibrated to one source at 0IRE and another source at 7.5 IRE, black will appear *identical* in the final image, along with identically bright whites, and contrast.

On to the whole blacker-than-black fiasco!

Once again, this issue has been confused and entangled with IRE. If you’ve read up to now, you hopefully know that IRE units are just a representation of analog volts. The DVD is written using digital levels, not IRE at all. A common wrong statement that I see goes along these lines:

Quote:
‘Blacker-than-black is data below 7.5 IRE. You have to use the 0 IRE setting to get this ‘superblack’ data.’
WRONG! Blacker-than-black data has nothing to do with whether a source outputs at 0 IRE or 7.5 IRE. You know already that IRE is just mV, and absolute mV are not encoded in any way on the DVD. The DVD is encoded in digital values from 1-254. Reference black is encoded at digital 16, with nominal reference white at 235. Codes 0 and 255 are illegal for image data. Codes 1-15 represent data that is below reference black, hence it is called blacker-than-black (BTB). Codes 236-254 are peak whites. I will refrain from calling them ‘whiter than white’ because this implies that they shouldn’t normally be present or visible in the final picture. They should be, unlike BTB(BTB should not regularly be visible in the final calibrated image)! It is important to maintain the full range of encoded data through your video chain for the best image. When the digital image data is converted into an analog waveform at an analog output, only then does IRE enter the picture. BTB data will simply fall below whatever the IRE output level for black is. In a system that outputs black at 7.5 IRE, BTB data will be output at voltages slightly below 7.5 IRE. In a system that outputs black at 0 IRE, BTB data will be output at voltages slightly below 0mV (simply negative volts). If you’ve digested that correctly, you realize that BTB data can be maintained in BOTH situations.

Unfortunately, in reality DVD players are often designed poorly and may clip BTB data. Some will clip BTB in both settings, others will clip it in one or the other. A correctly designed DVD player will maintain BTB data at analog output regardless of whether it is set to output black at 0 IRE or 7.5 IRE.

“Why does data below black even exist? That makes no sense! What can be blacker than black!?â€
In video, headroom and footroom is important for a number of reasons. The most basic is that mastering can be less than perfect, so some “fudge-room†has always existed. However, even with ‘perfect’ mastering, data regularly extends outside reference black/reference white. Peak white data allows for highlight details to be maintained, which is common in clouds and other bright objects. BTB data helps prevent image anomalies from hard clipping of the analog waveform at black when converted to analog. BTB can also sometimes become visible as the actual black level on a CRT display floats up and down with image content because black level retention on CRTs is not perfect. The mastering engineer is viewing on a CRT display and actively changing the encoded levels so that they are imaged correctly on the display. This reverse-float compensation in black is allowed with BTB footroom. This reverse float compensation should be the only times data encoded below black is visible in the final image. If you are using a PLUGE pattern with BTB bars to calibrate, you should calibrate so that the BTB data is not visible. BTB data also helps define dithering duty patterns on DLP projectors. Lastly, BTB and peak white data is quite useful for any image processing/scaling etc applied to an image. For all these potential reasons, video engineers advocate preserving full BTB and peak white data whenever possible. Lastly, this data is helpful for any image processing that is applied on an image, such as sharpening, scaling, etc.

Now you should understand better what an IRE is, what the 7.5 IRE/0 IRE difference does NOT entail, and what blacker-than-black data really is.

For more reading, see posts in these threads:
http://www.avsforum.com/avs-vb/showt...61#post4030461
http://archive2.avsforum.com/avs-vb/...09#post3307309

This thread has some useful attached pictures that label patterns with their actual encoded values in digital levels, and not in ambiguous IRE units. The digital levels written to the disc itself are fixed and absolute even though the analog voltages vary. Knowing the digital values tells you exactly what a portion of the pattern should look like. Parts encoded 16 are unambiguously black, always!:
http://www.avsforum.com/avs-vb/showt...hreadid=418084


2) Digital outputs and digital levels

I explained IRE in background section 1, so you should already recognize that IRE only describes analog waveforms, and is ambiguous as to the actual intended image content. When using digital transmission formats like SDI, DVI, HDMI, analog measurements like IRE units or mV are useless for describing the image data. As briefly explained before, digital video uses 8-bit encoding, which creates 256 unique steps from 0-255.

Video from DVDs or other digital sources, follows Studio RGB standards which encodes reference black at level 16, and nominal reference white at level 235. Outside these bounds is the footroom and headroom that contain blacker-than-black(BTB) and peak white data. Footroom and headroom is necessary for the best possible video reproduction for the reasons discussed briefly in background section 1:

Mastering fudge room
Highlight details and undershoot
Minimizing analog waveform anomalies at D/A converters
Compensation for the inability of CRTs to clamp black levels perfectly
Determining dithering patterns on DLPs
Any image processing applied

Enter the PC.
If you’re a computer graphics person, you may recognize the 0-255 scale of digital levels. But you may be confused because with computer graphics black is at 0, white is at 255, and that’s it! There is no footroom or headroom for BTB or peak white data. Since computers and video often have to be dealt with at the same time, this creates a problem between PC levels and Studio levels.

Projectors with digital inputs have to be able to handle both video applications and computer graphics applications. They should be able to calibrate or switch their white and black levels to accommodate both Studio levels and PC levels, but some don’t. Because video sources and graphics sources may be used simultaneously in some applications, video source manufacturers tend to include an option to leave the digital outputs at Studio levels, or to re-map them to PC levels.

Mapping Studio levels to PC levels can be done a few ways. Sometimes all the levels are just shifted down 16 steps, thus clipping off BTB data, but not introducing banding/contouring or clipping highlight details. If there are other PC level sources fed to the display, their whites will be substantially brighter. The clipping of BTB data is undesirable and the brightness mis-match is also noticeable, however note that there will be no banding problems. Usually the levels are expanded: digital 16 (black) is shifted down to 0, and 235 (white) is shifted up to 255 (or sometimes a value slightly lower than 255) thus expanding the numerical range between black and white to match PC levels. Note that this doesn’t improve contrast in the final image. In this case, re-mapping Studio levels to PC levels will destroy BTB and peak white image data, and introduce banding/contouring artifacts because of the expansion. This is also undesirable.

Always try to maintain Studio levels whenever possible in your system. You should use Avia PRO or DVE to test whether or not levels are being clipped. Both discs contain patterns with both BTB and peak white data. Note that the current consumer Avia does not, it only contains data in the range or 16-235. Thus if BTB or peak white data is being clipped, you won’t see it with Avia! If you are clipping the black or white bars in Avia, then your system is doing even more severe damage to the video by clipping above black shadow details and below-white detail: very bad! Note that Avia PRO also has some very useful ramp patterns with levels encoded at equal width so as to be completely banding-free. These are very useful for observing banding/contouring caused by the playback system.

For more reading, see this thread, especially the expert posts by Don Munsil and Stacey Spears:
http://www.avsforum.com/avs-vb/showt...hreadid=416292

3) Upconverting players and possible colorspace issues

This issue is still a fairly obscure one, and at this point I am including it only for the sake of being comprehensive. My concern is that people will read this portion about colorspace problems, and blame all color problems on this issue when it may have nothing to do with what they are seeing.

Note that this specific discussion ONLY applies to component video outputs: analog YPbPr or digital HDMI YCbCr.
This issue may arise in other narrow situations, but is quite unlikely.

(I’ve labeled ITU-R Recommendations BT.601 and BT.709 simply SD 601 and HD 709 so readers can follow which is which as they read)

If you are using YPbPr/YCbCr output, you are outputting component video that is actually encoded on the DVD. Your display is then transforming this component video into a form that it can use to drive the display (usually always RGB). The problem that can arise is that Standard Definition material uses one set of equations (SD 601) to move between RGB and YPbPr/YCbCr, while High Definition material uses a slightly different equation (HD709). A DVD is a digital SD source, as such the YCbCr on it was created using the SD 601 equations and should be decoded into RGB using the same SD 601 matrix or color errors will result.
Your display will likely apply either the SD 601 or the HD 709 decoding depending on the resolution it is seeing at input. An upconverting DVD player is sending an HD signal that your TV will respond to by decoding using the HD 709 decoding(usually). Unfortunately, your upconverting DVD player may or may not have properly re-coded the component video into HD 709. If it did not, then you have a mismatch: SD 601 encoded material is being incorrectly decoded with HD 709 equations. This color encode/decode mismatch will lead to color errors.
Note again that this section is included for the sake of being comprehensive. The actual source material is unpredictable, and may or may not be encoded using the proper color matrix. In my opinion, it is worthwhile for enthusiasts and video purists to consider and fix this colorspace problem if present, but because the problem is fairly limited to mis-designed upscaling DVD players at this point, I don’t see it as a concern for the majority of users. Hopefully, manufacturers will design upconverting players correctly to twist the color space into HD 709 before output.
See especially Stacey’s images attached in post 2 of this thread, along with the discussions nearing the end of the thread:
http://www.avsforum.com/avs-vb/showt...0&pagenumber=1

4) HTPCs
Because HTPCs are a huge discussion area, I can’t hope to cover all the possible problems that can arise with all the software, drivers, hardware, settings, etc that can come into play. In any case, I just reiterate that it is important to design your HTPC and settings purposefully so that you can maintain Studio RGB for video playback, and calibrate your display to this and not to the graphics’ PC RGB levels.

See the HTPC Forum ;)




******** ON TO THE SETTINGS! ********

1) For DVD players when using Analog outputs (Composite, S-video, Component video):

When using analog outputs, many DVD players have an option to change the IRE ouput level for black between 7.5 IRE and 0 IRE. There have been many misleading posts suggesting that 0 IRE output universally provides “deeper†blacks, is necessary to maintain blacker-than-black data, increases the picture’s contrast, or otherwise will improve your picture. These recommendations are not correct. To understand why, read the explanations in background #1 that differentiate between analog output voltage levels represented by IRE, and the actual digitally encoded values on the DVD.

Q: “So which IRE output setting should I use, and since it varies between players and systems how do I know what’s right for my system?â€
A: There are two main performance considerations to take into account:

a) First is the adjustment range of your display. The most important thing to do is to make sure that your display has enough black level adjustment range(brightness) to accommodate either 0 IRE or 7.5 IRE output for black. Most displays should have plenty of adjustment range to handle both, but if you find that no matter how far you raise or lower your black level setting that your black levels remain either submerged (too dark) or elevated (too bright: grayish) with one of the output settings, make sure you use the other setting that allows the combination of your DVD player’s output and your display’s adjustment range to show black properly. If you find yourself in this situation, keep this setting, and do not continue on to the other considerations, as proper black level in the image is the most important. In most cases, though, you will find that your display will have enough adjustment range to handle both output voltages properly. If both are handled properly, then the next thing to consider in choosing between 0 IRE and 7.5 IRE is blacker-than-black (BTB) data:

b) Next to consider is the preservation of blacker-than-black data. This issue has been confused a lot, so if my recommendations don’t agree with what you’ve been told in the past, please read the explanations behind them (background #1).

Ideally, a DVD player will preserve the full range of digital data contained on a DVD when outputting an analog signal in both the 0IRE setting, and the 7.5 IRE setting. In practice, many times this is not the case. To find out whether a specific player will pass blacker-than-black data, and in which setting, it must be tested. Reviewers sometimes describe whether or not a player passes BTB data, but unless the settings and connection method for this test are revealed, the information is not very useful. Some players will correctly pass BTB data in both settings, while many others will only pass BTB data in one of the settings, and some will clip BTB data in BOTH settings. (Others will perform still worse and clip more than just BTB data, but also clip above-black shadow details; hopefully you do not have a player like this!) To test for BTB data, you must use a test disc that contains such data, namely Avia PRO, or DVE. Note that consumer Avia does not contain BTB data. If you are clipping the black bars in Avia, your system is clipping even more than just BTB data: it is clipping above-black shadow details as well. Using the PLUGE patterns on DVE is the most likely scenario for the average enthusiast, but if you do have access to Avia Pro I recommend using it.

Using DVE, I particularly like Title 12, Chps 13-15. Chp 13 has a 20% center bar that is not blindingly bright when you raise your black levels to see the three bars beside it. If you’re not familiar with DVE, the two pairs of inner bars are above black, the background is black (digital 16) and the outermost pair of bars are below black. If your system is preserving BTB data, you will see all three bars when you raise your black level at your display to observe. If BTB data is being clipped, you will only see the inner two pairs of bars: the outermost below-black bars will appear black and match the background, which is black. Chp 14 is a useful full-range ramp marked by dots at the points for reference white and black. Chp 15 has a ramp in the lower portion of the pattern that runs from slightly above black to below black. Black crosses at the center.

After you’ve tested your DVD player to see whether it passes BTB, and in which setting(s), you should choose one that maintains BTB data. Note again that this is secondary to being able to calibrate black in your system properly in the first step. I hope users will share their player-specific findings on the preservation of BTB data in both of these settings to aid others who have the same player!

If you have a well-designed player that passes BTB data in BOTH the 0IRE and 7.5IRE output modes, then you can use either one. Always note that you have to calibrate to your chosen setting. If you do not recalibrate, your picture will be wrong: the black level output on your DVD player will not match the display’s calibration which leads to elevated or submerged blacks in the final image.

Extra: The last consideration is only if you have a player that correctly passes BTB data in both settings AND your display can be calibrated properly to either output: convenience. If you have enough inputs on your display, and per-input calibration adjustments, then you can go ahead and calibrate each input to a particular source, and it won’t matter which setting you choose. However, many times multiple sources are run through switchers, receivers, etc, or there are no per-input calibrations on your display. If you have sources conforming to different IRE standards, you would need to recalibrate your display each time you switched between them. This is annoying! You may want to set your DVD player to match the other sources more closely, which should eliminate the need to recalibrate between sources. While sources always will vary slightly in analog outputs and it is ideal to have individual calibrations for each source, when using a switcher, etc, this is often not possible. In my opinion, these slight differences are a worthy sacrifice for convenience, as few will want to recalibrate between different sources at 7.5 and 0IRE each time, and not recalibrating in this situation will result in very incorrect black levels and a significantly degraded picture for the odd source that is using a different output level. Where possible, always calibrate for each individual source, as voltage output precision isn’t always perfect! Where not possible, using the same output standards can be an acceptable compromise instead of needing to recalibrate the display each time you switch sources. How acceptable a compromise this is depends on how accurately the sources hit the standard voltages. If they vary widely enough as to be objectionable with a single calibration, you may just need to live with adjusting your display calibrations frequently.


2) For DVD players when using a digital output (DVI, HDMI).

When using digital outputs, your primary concern is to get the DVD’s image data output as undisturbed as possible. As simple as this might seem, design/price constraints, sloppy engineering, and ‘features’ can get in the way of transferring the digital data from the DVD intact. Always try to avoid using any image-altering features such as picture controls. These adjustments will usually have negative impacts on picture quality.

When using digital outputs, the major adjustment option you have is the one for digital levels. As is common in consumer labeling, the labels for this can be confusing. The most common labeling will at least hopefully show that you are making an adjustment to the DVI/HDMI digital outputs, and usually the options will read ‘Normal/Enhanced’ or ‘Normal/Expanded’ or ‘Video/PC,’ or some such label. The latter is clearer, as this adjustment is choosing whether or not the digital image data is correctly output using Studio (also called Video) levels, or is incorrectly re-mapped to PC levels. You should choose to maintain Studio levels by checking to make sure this option is properly set. Usually the default setting will correctly choose the option for Studio levels. Check to make sure.

Note that the Studio/PC level option will only work for DVI and HDMI RGB. If you are using HDMI YCbCr, Studio levels should be preserved correctly(and as far as I’m aware sources aren’t screwing this up, yet…) and the option change will either have no effect or be unavailable.

Because you are using digital outputs, adjustments for analog outputs shouldn’t be a consideration. Unfortunately again, due to cost-saving designs sometimes analog output adjustments, such as those for IRE setup, are implemented digitally even though they have nothing to do with digital outputs. Here your concern is still to ensure that the digital image data is being left as undisturbed as possible. In a well-designed player, the IRE option will have no effect whatsoever when using digital outputs. If this setting causes any change in the image when using the digital outputs, you should use test patterns to see which option leaves data un-clipped. In this instance, the degree of clipping or image alterations may be severe, so Avia, DVE, or any good test disc will come in handy. In Avia, look for the moving near-black and near-white white bars; in DVE use the ramps (Title12:Chp14) and check for clipping (if you have access to Avia PRO, the Deep Ramps are excellent tests!). Use the IRE setup setting that maintains as much data as possible through to your display. The default setting may more likely be the preferred setting. Default is usually the 7.5IRE setting. This was reported to be the case on the Denon 3910; users should test this on their players and share model-specific observations to aid others!

With the hope that digital levels and IRE are conceptually disentangled, you should understand that the IRE setup option is really for the analog output, and shouldn’t have to be discussed when using digital outputs. Unfortunately, instead of implementing these analog adjustments in the analog domain, cost-conscious designs seem to be altering the digital values, which necessitates double-checking this option to make sure your digital levels are not being tampered with.

3) Colorspace issues with upconverting DVD players

This is written only for those who are using component video, either analog YPbPr or digital HDMI YCbCr output. If you are using analog RGB, DVI(which is digital RGB)or HDMI RGB output this section does not apply to you! I omitted composite and s-video because of course you cannot output HD video via either.

A DVD is a digital SD source, as such the YCbCr on it was created using the SD 601 equations and should be decoded using the same matrix or color errors will result. Upconverting players should be modifying the colorspace to match the HD 709 coding, but some do not. Most displays will simply apply SD 601 decoding to any SD inputs, and HD 709 decoding to any HD inputs. If an upconverting player left the SD 601 colorspace encode intact, a display that applies HD 709 decoding to any HD inputs will unknowingly apply the wrong decode, leading to color errors. I place the blame with the DVD player.

You can check for this color problem by examining the color decoder test pattern on Avia(found under the section Special Tests: Color Decoder Check). If the player is upconverting to an HD resolution and not twisting the colorspace into 709, you will likely see color decoding errors in the Avia test pattern, with the most significant one being green depression(note that this is not a grayscale error but a color decoding error, and will not depend on you having correct grayscale: you will see this problem on the color decoder check pattern regardless!). To avoid these color decoding errors, your display may allow you to change between 709 and 601 manually. If it does not, using HDMI RGB or DVI RGB (analog RGB is a rarity, but also would avoid this problem) should eliminate this problem. If your only output option is analog component video YPbPr and your display does not have its own upscaling capabilities, then you may have to live with these color errors.

4) HTPCs

If you are using an HTPC, I will dodge the complex arena that is computer video, and merely recommend strongly that you do your best to maintain Studio levels and prevent your computer from expanding the video to PC levels. To check this, examine deep ramps on Avia PRO, or use the ramps and PLUGE patterns on DVE. Note that consumer Avia does not contain data outside the bounds of reference black/white (16-235) so you will not be able to observe the clipping of data outside these bounds using Avia. Moving to PC levels will usually clip data 1-15 and 235-255(or significant portions of this data); this data is present in the ramps and PLUGE patterns on DVE and in many patterns on Avia PRO.

Remember that proper playback of video requires Studio levels to maintain the full range of image data. This includes data outside the ‘bounds’ of the reference black/white points. Achieving this on a PC can be difficult sometimes, and it also means that your desktop and your video cannot both look correct at the same time. Expanding your video to match PC levels (0-255) will make your desktop and your video ‘agree’ and eliminate the need to recalibrate, but you’ve also negatively impacted video playback by clipping useful image data and introducing banding/contouring artifacts. This is undesirable, you should strive to maintain Studio levels if at all possible. Since this guide is directed at HT enthusiasts, I am assuming that accurate video playback is priority #1. I acknowledge that for users in other environments(multi-use, graphics etc) the problems introduced by expanding to PC levels might be an acceptable compromise for convenience. But wherever you strive for the best possible video quality, Studio levels are fundamental.

CZ
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post #2 of 160 Old 01-11-2005, 04:57 PM
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Outstanding. Chris, ask the mods if they'd consider making this thread a sticky. :)

Great ISF Job by Chad B.
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Thanks Bandit!

FYI:
I've decided to leave this thread open so that users can discuss and help each other with the material presented and what might be confusing for some at first.

If anyone feels something is *factually* incorrect, or needs elaboration I'd prefer you pm me about it and we can discuss, as I've tried to be very careful to be factually solid on this post. I Don't want to be defending this material constantly.
Thanks for the consideration!

As for how to calibrate, that is a long discussion in and of itself that I didn't include, if someone would like to add substantially to that area for those who may not be as initiated, let me know!

--Chris
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post #4 of 160 Old 01-14-2005, 04:09 PM
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Excellent post Chris. Definitely sticky-worthy.
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post #5 of 160 Old 01-14-2005, 09:06 PM
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I've created a color bars image that's a little easier to view than Stacey's,
with all four YCbCr to RGB conversion permutations in one frame.

Ron
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post #6 of 160 Old 01-14-2005, 09:51 PM
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Superb guide.. The one thing I have a problem with is digital outputs to digital projectors.

We all know that contrast is one issue with digital projectors that we like to improve upon. My feeling is that by preserving blacker than black signals you have to raise 'base' black level above lowest black that the projector can create (there is no way out of this logically)..

So in order to preserve a signal on some discs (badly authored discs IMHO) that present a final signal in the 0 - 7.5 IRE (0 - 16) range you end up watching the rest of your entire collection at a higher than lowest black signal. With this higher than best black you in turn have a drastically reduced real contrast ratio. It strikes me as odd that people will pay 1000's more for projector A over projector B due to improved CR and then not use it to its full abilities.

I dont see either choice as being necessarily 'wrong' (but I guess a ISF tech would have to have a favorite) merely one of those compromises you have to choose between. Either you have a system that displayed the best CR and black level on most of your titles (how I tune my setup) and miss some below black info, or you have a system that retains all the signal and displays below black info for a minority of titles but has a higher black level displayed for many titles mastered at 7.5 / 16 = black.

I dont know why but I have less of an issue with the white level side of the system range or clipping (probably as I have a fairly bright projector). Thinking about it I guess that people with a non dedicated room (perhaps people that allow some light in thier theater or have light walls) may mind the issue less as the raised black level may be less percievable in that kind of setup.

Perhaps there is something wrong in my thinking of this issue as I have always had this 'problem' over 0 = black or 16 = black while others have tried to explain to me that it is simple and I am wrong.. Any corrections to miss understood ideas welcome.
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My feeling is that by preserving blacker than black signals you have to raise 'base' black level above lowest black that the projector can create (there is no way out of this logically)..
"Preserving" BTB does not mean that you calibrate such that it is viewable. Be clear that you do not raise your black level calibration(thus reducing contrast) to see the BTB material. This is wrong. When you calibrate, you calibrate black (16) to be black on the display. The BTB data will *not* be normally visible. Raising the black level to test whether your system is maintaining BTB data to the display is only temporary to see whether the BTB data is being clipped or not. After checking (and making changes if needed) then you re-calibrate, and correctly calibrated you will *not* see the BTB data.

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final signal in the 0 - 7.5 IRE (0 - 16) range
These are not synonomous things at all. They have no direct relation to each other.

If you read through my explanations (I realize they are long and laborious, but there's really no painless way to try to completely re-explain things like this) you'll see that I am very clear in explaining that the ANALOG voltages (IRE) as being *totally* distinct from the digital levels.

If you read through carefully, you'll see why your statement arises from the confusion of IRE and digital levels as being intertwined:

Quote:
or you have a system that retains all the signal and displays below black info for a minority of titles but has a higher black level displayed for many titles mastered at 7.5 / 16 = black.
Expressely stated in the background explanations are rebuttals for statements like these. DVDs are not mastered in voltages(like 7.5 IRE or 0 IRE or whatever). It's impossible. There are no volts on a DVD at all. Black is defined and standardized at digital level 16 for DVD, PERIOD. *ALL* dvds place black at 16 if they are mastered correctly.
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post #8 of 160 Old 01-14-2005, 11:59 PM
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Agreed I am competely miss using IRE (7.5 for black or 0 for black) incorrectly.. Really its a case of struggling to find terms for 'slightly higher shade of grey than absolute black' etc. Lack of terminology and being lazy in my descriptions.

I did read it all once.. Will go over again and see if I can get my head round it.. Still dont understand what a plunge or blacker than black signal is doing (or what use it has) at output if you clip it at display ?? Either you should see it and it should be in the outputted image or it should be clipped out (how I have worked).. I can understand possible uses for it in the signal processing side but not at oputput.

I think its comments like highlight room and footroom that confuse me because surely these are counter to the idea that all correctly mastered DVD's stop all (outputted) image detail at studio black (16) and studio white (235).. If so what is this issue of footroom ?? That should not be seen in the final image (IMO) ?? Why would peak white (above 235) ever be outputted ?? To show 'cloud detail' just seems odd to my ears.

Anway will read and digest.
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post #9 of 160 Old 01-15-2005, 07:31 AM
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Phat Phreddy,
This is the nub of people's confusion on this, along with the idea that the 0 or 7.5 IRE choice has something -- anything -- do to with the way the producer intended the material to be seen.

Specifically, the Blacker than Black data is *NOT* intended to be seen. If your display is calibrated correctly (and its Black levels don't float around) you won't see it. It will all merge into one, perfect, identical, absolute "Black".

Yet it's still a good idea to have it there. Why? Because it "protects" the Black and Above data in the presence of additional signal processing that might happen before the pixels light up, and because it preserves the actual dynamic range that was used when the producer originally recorded the data. The sorts of signal processing that might happen include things like scaling, and gamma correction, plus, plus, plus. It's also good to have it JUST IN CASE your display has Black levels that float a bit depending upon what's currently being displayed.

Now the video stream is necessarily an incomplete representation of a continuous real world image. The producer's choice of what source light level he wants to identify as "Black" is essentially arbitrary. It is a reference point based on the low light level details he wants to remain visible, and is used as a basis to insure that quality is maintained throughout the reproduction chain for that light level and above.

But a video signal goes through the signal processing mangle many many times before your pixels light up. And each time that happens there is the chance that artifacts will be introduced because the data has been arbitrarily clipped -- has a hard filter clamped on it -- cutting off anything below what the producer chose to consider as "Black". So to protect against that happening, the video data includes information BELOW the light level the producer arbitrarily chose to represent as "Black". Such Blacker than Black data should stay in the stream all the way through to when your pixels light up. Again, if things are set up right it won't become visible. But it's presence in the video stream preserves the quality of the Black and Above data through any final signal processing that occurs.

--------------------------------------------------------------------------

Meanwhile the 0 vs 7.5 IRE choice is something the DVD content producer has no part in. He doesn't care. It's a choice that only matters for an analog video stream, and thus only matters between two devices that are connected with analog video cables. All the producer cares about is that his DIGITAL video stream will be reproduced with complete fidelity REGARDLESS of whether two analog video devices choose to use a 0 or 7.5 IRE voltage to represent "Black".

With properly working video devices, the digital data from the DVD can be represented IDENTICALLY well with either the 0 or 7.5 IRE choice for analog hookups between any two such devices, and the resulting image will be IDENTICAL in appearance. I.e., with good hardware, the choice to use 0 or 7.5 IRE levels for any particular analog video hookup is completely arbitrary and won't alter the resulting image as long as both devices agree -- which means if you make a change on one device you have to adjust the OTHER device AS WELL to keep them in agreement, i.e., you need to RE-calibrate.

But not all analog video hardware works that well.

So for example, your hardware might work best with a 0 IRE (voltage) analog video hookup between the DVD player and an external scaler but with a 7.5 IRE (voltage) analog video hookup between the external scaler and your projector. It doesn't matter. As long as the full range of the signal is properly preserved in both hookups -- including the Blacker than Black data -- the picture from your projector will be correct.

The confusion here arises because SOME devices will only work properly with one analog voltage setting and not the other, and people have got it in their heads that that means the video signal CAN'T be represented completely correctly by ANY analog video hookups except by always using just one of those two choices.

It is further complicated by the fact that some devices prefer one setting or the other based on things you would think wouldn't matter, such as whether you decide to use S-video vs. Component cables, or whether you decide to pass a 480i signal vs. a 480p signal. And choosing a voltage level your particular hardware doesn't like can have unexpected side effects as well -- such as clipping off the Blacker than Black data which is properly passed FOR THAT HARDWARE only if the other voltage level is used for analog hookups.

So you have to check your calibration levels according to each hookup choice you make and you have to make sure the Blacker than Black data (and Peak White data) is getting through.
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Still dont understand what a plunge or blacker than black signal is doing (or what use it has) at output if you clip it at display ??
This part is explained in the guide, and is also explained in the linked thread about BTB data by Stacey Spears and Don Munsil. They haven't really said anything in that thread that I didn't include, though.

Second, it's not "clipped" at the display. Rather it is calibrated so as to be not directly visible. It's affects are subtle, and used for processing, black level float, DLP dithering, etc all the reasons I explained.

Quote:
surely these are counter to the idea that all correctly mastered DVD's stop all (outputted) image detail at studio black (16) and studio white (235)
You have the wrong idea. Correctly mastered DVDs still use this footroom and headroom. Some DVDs are clamped at some point in the master and may clip headroom/footroom anyway (this isn't good). In any case, the bits certainly weren't intended to be wasted or video would just follow PC standards and use all 8-bits of the range not allowing for any footroom or headroom. This isn't the way video was designed, and for good reason. If everything was hard-limited to 16-235 with no data beyond that, that would be a strange way to waste precious bits.
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post #11 of 160 Old 01-15-2005, 11:50 PM
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All this theory is giving me a headache!

Has anyone got, or ever seen, a real world example of a DVD that actually looks different when viewed with blacker than black being passed, but not displayed, AND blacker than black not being passed at all?

(ie Identical setup in every other respect)

Bill
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post #12 of 160 Old 01-16-2005, 08:20 AM
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btiltman,
The problem is that the artifacts are different depending upon the type of processing that happens AFTER the BTB data is clipped, and if the data is clipped inside the DVD player that means the issue is also significantly dependent upon exactly where it happens -- e.g., at the video output stage, or earlier in the processing.

Some folks with "simple" projector systems (little or no signal processing) that are made well enough to have no "floating" of black levels, and who are using a DVD player that clips BTB data right at the video output stage, see no problem when BTB is clipped in their video -- once they've got black levels properly calibrated of course. There's not enough processing happening after BTB is clipped to make a difference.

But the most common symptom that people see when BTB is clipped is "black crush" to varying degrees. This is a feeling that low black detail is being lost no matter how careful you are at trying to set black levels properly. This is due to two things. First, if the black levels on your TV float up and down depending on what's being displayed (usually based on the average brightness of the image) then when the BTB range temporarily floats up to become visible there's no detail there -- it's all been rounded to one value. Second, if the image processing in your TV is sensitive to (i.e., depends upon or takes advantage of) the BTB data, then near-black details ABOVE "Black" may get incorrectly rounded to the same level, and/or the absolute "Black" level may get rounded up a bit yielding dark gray instead of black on screen.

So the most common reported symptom associated with improper clipping of BTB data is loss of detail in low light scenes on various DVDs. Usually if you've got this problem, it will be seen in most ALL of your DVDs that feature detail-rich, low light scenes.

Now "black crush" can come from other errors in processing and calibration than just improper clipping of Blacker than Black data. But if you've got visible "black crush", checking for proper passing of BTB data is one of the first things to do.

The second most common symptom of BTB clipping is "noise" in low light level scenes. Again this is due to signal processing that happens after the BTB data is clipped. But in this case, instead of improperly rounding true detail to the same level (thus blurring the detail), random variations are introduced in that detail. Think of it as a type of "ringing" as the signal processing tries to deal with the fact that the light sampling has a sharp cutoff at "Black" -- something that wouldn't occur in "real" images.

The commonality of this symptom is harder to quantify since the more usual problem of people seeing noise in low light detail is that they've mistakenly mis-calibrated their black levels to make BTB data visible when it *IS* present. BTB data is inherently lower quality since the entire video reproduction chain is geared primarily at preserving the fidelity of Black and Above data. So if you make the common newbie mistake of, "Keep turning up the Brightness control until new detail stops appearing," then you will have made the BTB data visible when it shouldn't be and it will be more noisy.

[In addition, DVD players that exhibit the Macroblocking Bug, will typically exhibit this bug as what looks very much like noise in low light level areas of the image -- particularly in "backgrounds".]

So folks who have made this newbie mistake might find that their mis-calibrated image (with BTB improperly visible because black levels are set too high) becomes LESS noisy if they mis-adjust their player or TV to CLIP the BTB data!

Given all this, it's much harder to pin down just how often noisy blacks really are related to improper clipping of BTB in modern equipment. And when it really does occur, it is likely to be very dependent on just what you are watching -- i.e., less common across different DVDs than the "black crush" symptom.

Finally there's the issue of PC vs. video style digital encoding. One way that BTB data gets clipped is if the video source device sends out PC style digital video over a DVI connection. PC style digital video encodes "Black" as digital 0, so there's no room to send BTB data below that, whereas video style digital signals encode "Black" as digital 16 and (usually) pass BTB correctly in the steps below 16.

Some older TVs and projectors only know how to handle a PC-style digital video signal via their DVI inputs because they were designed expecting that the DVI input would be used only to hook a computer to the display. And thus DVD players with digital outputs usually have some sort of user option somewhere to format the output signal that way so they can still be used with these older displays. Since such TVs and projectors aren't EXPECTING BTB data on their digital inputs, they are usually careful to avoid the most serious sorts of problems that might occur if they used signal processing that was designed to take advantage of BTB data.

But you still aren't out of the woods as regards image defects. Since the data range for the DVD video stream has been EXPANDED (at both ends) when converted to the PC style encoding, the original digital steps have to be increased in number. This means the player and TV need to decide how to extrapolate the additional steps. Often this is not done very cleverly and the result is false gray scale transitions -- steps that are too wide in one place and too narrow in another. And THAT translates into false contouring -- which could happen all at one end of the gray scale or scattered throughout the gray scale. This is not an issue with clipping of BTB per se, but if you test your video and confirm that BTB data is properly present, you can be sure you haven't accidentally set your system up to use PC-style digital video encoding with a TV that properly handles video-style encoding. And that's a good thing.
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Has anyone got, or ever seen, a real world example of a DVD that actually looks different when viewed with blacker than black being passed, but not displayed, AND blacker than black not being passed at all?
Bill, please heed my request to keep debates about this off this thread. If it continues it will have to be left as a closed-thread sticky, as this can be debated elsewhere (and has been already at length in the linked thread that I provide in my guide: http://www.avsforum.com/avs-vb/showt...hreadid=416292).

Joe Kane, Guy Kuo, Stacey Spears etc are all video experts and demand that Studio levels for video, and BTB and peak white data be maintained correctly. Joe Kane is especially stringent on this. You can feel free to disagree with them elsewhere(a losing proposition) if you'd like, just not here please.

Thank you,
--Chris
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Originally posted by ChrisWiggles
Bill, please heed my request to keep debates about this off this thread.
Thank you,
--Chris
Hi Chris,

I have no strong feeling either way on this issue so I am somewhat confused as to your response. I was asking a simple question as to whether the effect was demonstratable and, if so, what it might look like. Perhaps you misunderstood?

Bob,

Thanks for your response, that explanation really answers my question almost as well as an actual example. I appreciate your effort, thanks!

Bill
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post #15 of 160 Old 01-17-2005, 06:27 AM
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Chris, your detailed explanations have shown me what IRE really is. Thank you for taking the time to bring the rest of us up the AV learning curve!

I have some questions...

Question 1. How best to setup a TV for non-DVD sources, like an OTA HD tuner?
I have been using DVE through my DVD player, while still at the factory default settings, to set the TV's picture settings. Some digital video, mostly SD but some HD, looks very right in terms of color, while some HD seems to stray from a familiar color palette. I realize much may be deliberate -- on my set Cold Case has a blue bias, CSI has a yellow one. Others look like something's wrong -- PBS's European Travels and occasional HDNet demo channel programs seem to have a slight magenta tinge on my set. This leads to question the current calibration. How do I know what the "right" settings are for the OTA box? To do any better is specialized calibration equipment required?


Question 2 When is gamma correction a viable step to further refine black and white levels?
I had difficulty understanding how best to set up my Sony GWII (LCD RPTV, which has an inherent limitation in contrast) with my Denon 2900 via component connects. At first, I set brightness as high as possible without seeing any color bias in a 100% white area (my set's white goes cyan at the highest levels). Then, I set black using the DVE three bar pattern such that the BTB bar was slightly visible below the surrounding field. This seemed to give me crushed whites -- highlight areas appeared unnaturally large and really stood out. That was clearly wrong.

The white crush drove me to use the DVE ramp to ensure that I could discern a difference between the highest and lowest bars in the pattern. That made for a better picture. However, my recollection is that the DVE gray ramp appears asymmetric through my system. That is, the amount of "crush" is different at the lower end and higher ends of the scale. For now, there's a bit of crush at the black end.

My DVD player does have a gamma correction curve. There are ~16 points to adjust along a graph that I think shows output vs. input values. Is messing with gamma a viable next step or did I miss something fundamental with setting the black and brightness levels?
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Excellent questions, let's see if we can point you in the right directions:

Q 1) OTA stuff is difficult, because the channels can (and will) vary all over the map. As long as you've calibrated for that input type by using patterns in your box, or a signal generator, or (tentatively) a DVD player that's set to the same output levels as your box (make sure you know what you're doing here!), then that's sort of the best you can hope for. If some channels look really off, it might just be their fault, and some individual tweaking is ok.

Q 2) For setting black level and white level with LCD, look for clipping and colorshift, as you mentioned. First, the BTB bar will be calibrated to be *NOT* visible, it will look as black as the black background, and you will still see the above black bars when using the DVE pluge patterns. When using Avia, calibrate so that the above-black bars are just barely visible above black. Avia Pro also provides a BTB bar which can be handy.

For whites, you should be looking at not just a 100% white screen, because then you cannot look for clipping as well. A DLP or LCD may clip data at high white levels, and it may also colorshift. You should be just below whichever of these things occurs first. Black and white level settings may interact a little bit, so you may have to go back and forth between them to arrive at the correct setting.
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post #17 of 160 Old 01-17-2005, 05:06 PM
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>>>All this theory is giving me a headache!<<<

No theory here -- just fact. Disagreement regarding below black and above white signal preservation is a losing proposition, as Chris has already stated. Perhaps those of us who contributed to this FAQ should have been a bit more emphatic about not questioning this "set-in-stone" fact, indeed the most important part and heart of this work. Please accept my apologies for thinking that links to discussions, information and explanations by industry leaders would prevent this.


If you'll accept a rather crude analogy of the movie experience and the affects of passing/clipping black levels, maybe this will help. Let's liken the experience to a wrestling match. In one corner, representing the actual image recorded on film, we have Andre the Giant. In the other corner, representing the image on a DVD, we have Andy Kaufman. To illustrate the difference in displaying the complete signal vs the compromised signal, we'll use two different wrestling rings. My ring first.

Andre grabs Andy by the arm and swings him to the side of the ring. Andy goes flying toward the ropes and, arms flailing, meets up with his rubberized friends. Only he doesn't stop when they gently couple him, as the ropes have give and flexibility and allow him to stretch their welcome a few feet. Andy is spared the pain and is allowed to return to Andre in the middle of the ring. Gladly, Andre grabs Andy's other arm and sends him reeling to the other side of the ring where, again, he stretches the ropes and saves himself from any real pain. Now, your ring.

The Giant swings his flippant instigator toward the ropes at an amazing rate of speed. But just before Andy gets to the edge of the ring (a padded wall, instead of rubberized ropes), he cringes and tries desperately to stop his impending doom. Nonetheless, he slams into the wall with a great thud and, although there is a bit of padding, he feels the impact of the bricks on the other side of that wall. Not quite getting the message, he challenges the human building and runs toward the champ. Again his arm is grabbed and he is flung in the other direction to meet his earlier fate: lots of cringing and screaming before that loud thud.

Now, while watching Andy Kaufman get slammed into a wall may seem like a ideal wrestling event, I'm sure you'll agree that the ringed mat will make for a more interesting and enjoyable match. Like I said, crude, but it should get the point across.


Since you asked for an example, try this one. If you have a display that is capable of displaying below black signals, here's an easy, yet again crude, way to see what happens when below black signals are clipped. Use a DVD player that you know passes below black signals in one mode (as in progressive and interlaced), but not in the other. Use a calibration disc (Digital Video Essentials) and calibrate the display in the mode that correctly passes below black using the PLUGE pattern. Now play the scene in Jurassic Park III where the T-Rex and "bigger/newer-saurus" dinosaur get into a scuff. Watch the scene in the mode that passes below black and then watch it in the mode that clips below black video (the player should be able to switch between interlaced and progressive modes on the fly for a more direct comparison). You should easily see more details and better saturation when in the below black mode vs the clipped black mode, which will seem grayed out and cloudy/pasty looking -- regardless of whether the mode passing below black is progressive or interlaced.

The above experiment (though not necessarily that scene) should cement ones opinion that clipping black is definitely undesirable into a fact.

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post #18 of 160 Old 01-17-2005, 11:42 PM
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I was not 'questioning' any fact. My system is already passing blacker than black etc. and I agree entirely with what is being presented.

I merely wanted an example scene to play with that will readily demonstrate the difference as not all DVDs will benefit from the adjustment.

I will do as you suggest, except I will calibrate black for each mode separately and compare. This is purely for my own education.

I am really finding the tone of this thread to be very aggressive and will no longer refer to it. I think you should make this a closed thread as it does not encourage participation anyway.

Chris, thanks for your PM apology.

Probably catch you all around the traps.

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post #19 of 160 Old 01-18-2005, 05:49 AM
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Quote:
I merely wanted an example scene to play with that will readily demonstrate the difference as not all DVDs will benefit from the adjustment.
Here's an image I created from "The Talented Mr. Ripley". I've changed all
the below black pixels to white, so that they can be easily identified.

http://archive.avsforum.com/avs-vb/a...749&fullpage=1

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post #20 of 160 Old 01-18-2005, 08:52 AM
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Very interesting...

If I am still stuck miss understanding then just say so but if a display is perfectly calibrated would all those values not be outside of the dynamic range of the display.. Eg all that below black would be 'clipped' at the display ??

Thats why I dont understand the advantage of below black in the outputted signal.. If I am supposed to tune black so that below black is not visible this information is then clipped at display ??
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post #21 of 160 Old 01-18-2005, 09:05 AM
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I should point out that my HT experience has been solely with HTPC (and projectors) I have never even owned a DVD player etc.. My HTPC output a full 0 - 255 signal with below black data. I then tune at the display to use the dynamic range of 16 - 235.. Perhaps this lack of experience with CRT based systems or systems and sources that do clip is where / why I am having such a hard time understanding. That said I still have a problem with the following...

Say I have my setup as is.. Outputting full signal with below black and peak white with the display adjusted so that below black is not visible etc.

Then say there was a secondary system that had the 16 - 235 data and outputted this (note I say output only) using full 0 - 255 range. the display would then be calibrated to use the full dynamic range again so that below black is again not visible and peak white is again not visible.

To my mind both of those outputs should look exatly the same provided that high quality signal processing is applied and only the output to the display is effected. I understand that the signal processing if poorly implemented could introduce banding or incorect hard cutoffs (leaving muddy undefined areas) but lets just assume that signal processing IS done correctly and this is merely a function of output signal, if the display can accept PC gamma and video gamma and both can be adjusted to display the correct range fully I dont logically see the difference.
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Phat Phreddy,
Yes, if everything is working right the BTB data will be invisible -- i.e. it will look just the same as "Black". It is detail that has been recorded BELOW the light level that the DVD producer intended you to see.

But some TVs have "floating" black levels which vary according to what's actually being displayed at the moment. If you have such a TV and the BTB data floats up into the range of visibility despite your care in setting black levels properly, then your image will be damaged if the BTB data was clipped -- i.e., all rounded to the same value -- before it got to your TV. The portions that now float up into visibility will have no detail in them any longer.

And other TVs do signal processing (such as scaling or gamma correction) which takes advantage of the proper presence of BTB data in the image to do a better job adjusting the Black and Above data just above it. If the BTB data is improperly clipped -- i.e., all rounded to the same value -- these algorithms will introduce artifacts in the Black and Above data that you are SUPPOSED to see. What I think you are missing is that there are different ways to accomplish certain types of signal processing. Algorithms that know they can depend upon the proper presence of BTB data are capable of doing a better job -- but only if that data is really there!

Read any writeup on digital filtering for example and you will find a discussion of the various bad things that can happen (often called "ringing" or "aliasing") if you put an artificial hard filter on data that's supposed to represent a continuous, real-world event. What the film or video camera sees doesn't stop all of a sudden at what the cinematographer or film digitizing editor decides to identify as "Black". So you record some light levels BELOW that arbitrarily identified "Black" AS WELL just to help protect the Black and Above data in the face of various forms of subsequent processing.
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post #23 of 160 Old 01-18-2005, 10:09 AM
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Fine, thats my understanding also (though I had believed some argued that below black should be visible.. That perception was wrong)..

However.. Given the previous example of Jurrasic park (I niether own this particular DVD or anyway of presenting a signal that does not pass below black) and given, for arguements sake, a perfectly clear display device that does not float, does not do any scaling, does not postprocess the image, pretend we have the perfect transparent display :) !!

Why will it be that if both inputs (the below black passing and below black clipping) are set up correctly to have black at 0 light and below black clipped, then why will one we have the following ??
Quote:
You should easily see more details and better saturation when in the below black mode vs the clipped black mode, which will seem grayed out and cloudy/pasty looking
Given we have an equal display, once both input are calibrated (and lets assume source performs scaling correctly and only 'clips' at the very output stage) to our imaginary perfect display (that performs no scaling) what causes this ??

I realize that having perfect scaling and perfectly transparent displays is unrealistic but I am trying to seperate out the below black issue from any other effects.
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post #24 of 160 Old 01-18-2005, 10:22 AM
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Phat Phreddy,
If you have such an idealized display that makes no attempt to use BTB data in any way whatsoever then of course the BTB data won't be used and its presence or absence is irrelevant. This is tautology.

Real world displays for home theater usually don't work that way. Think of how you would implement a Brightness control on a display, for example, that wouldn't care whether adjacent pixels were black, vs. one step below black, vs. many steps below black. And how does gamma correction interact with that. Or automatic gain controls/limiters. And what of the processing that adjusts pixel fire timing in a plasma for example or other specialized, display technology specific processing. In the end, real world displays are physical devices and so what appears in any given pixel is partially a function of the signal around that pixel.
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post #25 of 160 Old 01-18-2005, 01:55 PM - Thread Starter
 
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Phreddy, again, please don't let this argument lead to thread closure.

Your questions have already been adequately addressed in my guide:

Quote:
“Why does data below black even exist? That makes no sense! What can be blacker than black!?â€
In video, headroom and footroom is important for a number of reasons. The most basic is that mastering can be less than perfect, so some “fudge-room†has always existed. However, even with ‘perfect’ mastering, data regularly extends outside reference black/reference white. Peak white data allows for highlight details to be maintained, which is common in clouds and other bright objects. BTB data helps prevent image anomalies from hard clipping of the analog waveform at black when converted to analog. BTB can also sometimes become visible as the actual black level on a CRT display floats up and down with image content because black level retention on CRTs is not perfect. The mastering engineer is viewing on a CRT display and actively changing the encoded levels so that they are imaged correctly on the display. This reverse-float compensation in black is allowed with BTB footroom. This reverse float compensation should be the only times data encoded below black is visible in the final image. If you are using a PLUGE pattern with BTB bars to calibrate, you should calibrate so that the BTB data is not visible. BTB data also helps define dithering duty patterns on DLP projectors. Lastly, BTB and peak white data is quite useful for any image processing/scaling etc applied to an image. For all these potential reasons, video engineers advocate preserving full BTB and peak white data whenever possible. Lastly, this data is helpful for any image processing that is applied on an image, such as sharpening, scaling, etc.
Please also see this thread, in which Stacey and Don cover this in their discussions:

http://www.avsforum.com/avs-vb/showt...hreadid=416292

Including this post on page two of that thread:

Quote:
It is, in fact, real-world and you can absolutely see the below-black detail, if you're using a CRT (or a fixed-pixel device that properly emulates a CRT).

First off, the below-black samples affect the waveform output from the D/A converter. In the same way that hard clipping of audio samples produces unfortunate frequency-domain artifacts that sound bad, clipping the video waveforms produces visible artifacts.

Secondly, no CRT has perfect black-level retention, and when there is a bright image on the screen, the black level goes up, revealing some of the information that is ostensibly "below black." The CRTs that telecine operators use have exactly this characteristic as well, so when the operator adjusts the shadow detail down so it's just barely visible it inevitably shifts data into the below-black range. Think of the 16 level not as a hard barrier but as a target level. The real black level floats around quite a bit.

This is easily verified (and I have done so). I have clips right here on my desk that have lots and lots of below-16 and above-235 values in them. These are not weird clips or unusual clips. I didn't seek them out as test material - they're just clips I happen to have around. I've actually never seen a video clip that stayed entirely within the 16-235 range, with the exception of test patterns. In my experience, real video ALWAYS goes outside the "nominal" range.

Similarly, I can and do demonstrate that clipping at 16 and 235 is visible. It's not just a little bit visible, it's awful. Your white detail goes all to hell. Clouds look like white plastic cutouts in the sky. Snow scenes lose all their detail. People wearing dark clothing look like silhouettes.

The real lesson here is that computer graphics should have been allowing for headroom and toeroom all along. Video engineers know much more about how to display images on a CRT than computer engineers, but they were not consulted when the computer RGB standards were created. The result is this flawed 0-255 standard, which was not really designed around showing high-end video and images in the first place.

Going forward, it's clear that the video concepts of headroom and toeroom will seep into all computer graphics interfaces, and eventually all graphics will use "video" levels or at least a format that allows for head and toeroom. Until then, we have these two standards to deal with. I wish we didn't, but there you are.

Don
If you want to continue nitpicking with imaginary situations, please move it to private messaging in deference to others who may benefit from keeping this thread open to added explanations. This has been debated for over 400 posts and 20 pages in that thread. I refuse to allow this thread to rehash that repeatedly.
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post #26 of 160 Old 01-18-2005, 02:20 PM
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Chris,

You say a few times that it is important to maintain studio RGB levels, and I follow the reasoning behind all of that.

The question that I still have, is how does the non-ISF person calibrate their HTPC/display to properly display studio RGB levels?

There are quite a few test pattern generator tools out there in HTPC land that allow you to setup your display, but presumably all of these output PC levels. I'm thinking specifically of the Philips and Nokia monitor test patterns.

Is there a way to use these, or perhaps a different generator, to properly set up sRGB?
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post #27 of 160 Old 01-18-2005, 02:21 PM
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In deference to Chris' concerns that this thread be focused on practical matters of just HOW to achieve proper gray scale and color rendition in the presence of real-world, home theater equipment, as opposed to whether or not the long-established standards for representing moving images in analog and digital video (plus the newer, evolving versions of those for high-res video) are actually "a good thing" or not, I won't be responding to any further questions -- in this thread -- on WHY BTB data should be preserved, or just how significant a problem it can be if it isn't preserved.

The theory and practice on this stuff is solid, but the math and signal processing theory that underlies it is just too much for a practical thread, and the simplified analogies and thought experiments that have been used so far to try to convince folks are obviously not doing the trick (based on at least a year's worth of threads on this topic -- not just the recent discussion in this thread).

I'd be happy to keep trying, up to a point, IN SOME OTHER THREAD, but in a perhaps forlorn effort to forestall that, let me just say:

TRUST US ON THIS!
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post #28 of 160 Old 01-18-2005, 02:28 PM - Thread Starter
 
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jvincent: I use PC test software mainly to get high-resolution patterns for this reason. They will allow you to calibrate to graphics (PC) levels only, usually. You can manually make patterns with data at the levels you desire, but the easiest way is just to use Avia or DVE. Calibrating this way will calibrate you properly to whatever your video software is doing. If you manually calibrate to Studio levels, and it turns out your video is being expanded to PC levels, then you'll be off. Just use a test DVD and you can be sure that if you calibrate to a test DVD, other DVDs will look correct.
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post #29 of 160 Old 01-18-2005, 02:35 PM
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jvincent,
If you've got your HTPC system set up to play commercial DVDs then you will have no problem checking this. Get the Digital Video Essentials (DVE) calibration disk and check that Blacker than Black portions of the appropriate test charts can be made visible *TEMPORARILY* by cranking up the Brightness on your display. For example, on the DVE pluge chart the outermost black bars on each side are encoded at a "below black" level and that data SHOULD be making it through to your display. If you can make three bars visible on each side by temporarily cranking up Brightness, then you know the data is getting to your display. Now remember to finish by re-setting the black levels PROPERLY by reducing Brightness again. When you are done, the outermost of those bars will vanish (merge back into the black background).

If on the other hand you can only make two bars visible no matter how high you crank up the Brightness then you have one of two problems. Either the BTB data isn't getting to your display -- which could happen if you are sending PC-style digital video, or alternatively if you are sending Studio-style digital video but with BTB improperly clipped -- OR your display doesn't have enough calibration range for Brightness adjustments to show them -- which could happen for example if you are sending an analog video signal at the 0 IRE level when your display only has enough calibration range to handle a 7.5 IRE signal (or the other way around). In either case you need to try to fix this problem.

Once you know that BTB data is getting to your display AT ALL, you can then use the DVE gray ramp charts to check this to a finer level -- including also confirming that Peak White data is getting to your display while also refining your black and white level settings (Brightness and Contrast respectively).
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post #30 of 160 Old 01-18-2005, 04:02 PM
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Chris and Bob,

Thanks for the replies. I don't have DVE, just AVIA (non-pro), but I can use the THX optimizer that comes with some DVDs since it has a BTB pattern.

Just as a sanity check, what I have been doing to date is using the PC tests to set up my display (CRT HDTV connected via component) such that the 1% black bar is only just barely visible and 100% white is, well white. I then set up my DVD player (TT using VMR9) using AVIA such that the left black bar is only just barely visible.

Now if I understand correctly, what I have just done is set both my display and DVD player/ HTPC to 0 IRE black levels and assuming that it still sends BTB to the display (I need to check this) I should be OK. Is this right?

UPDATE: So I just checked with the THX optimizer, and I am indeed sending BTB as seen by cranking up the brightness on the display so I guess I am good to go.

It was also worth noting that as I lowered the brightness setting in TT the BTB settings started to get clipped.
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