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# gamma on 0-255 v gamma on 16-235 - Page 5

Quote:
Originally Posted by lcaillo

Your questions have nothing to do with my point that saying % IRE is incorrect useage. No, I have no relation to Chris. I may be just as much of a pedantic ###hole when it comes to proper use of technical terms, but I hardly know him. You might want to check your references on the proper definition of IRE, BTW.

Discussing IRE in digital video is obscurring the matter. IRE is a voltage in an analog signal. It may be analogous, but it just is not necessary to mix conventions.

Quote:
Originally Posted by alluringreality

What both hwjohn and lcaillo seem to have intended to point out is that the IRE convention doesn't appear to use a percentage symbol. Hopefully the following would illustrate the point.

50 percent = 0.50

50 percent IRE ≠ 50 IRE

100% IRE = 1 IRE

It's somewhat a minor item, but if you're going to argue a point you might at least try to understand what other people are saying. It certainly would be great if I too was perfect all the time, but please attempt to get the details correct or at least be open to being corrected if you want to quibble over somewhat pointless items.

It doesn't matter how you explain it to him. When you live in your own little world, it's perfectly OK to make up terminology as you go along.

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http://www.tek.com/Measurement/App_N...25W_7009_2.pdf
The black-level setup or “pedestal’’ for EIA RS-170A is specified at +7.5 IRE
Consequently, since the peak white voltage is not increased with setup, the black-to-white amplitude range is reduced by 7.5%.

7.5 IRE = 7.5%

Quote:
Originally Posted by lcaillo

Mathematically, percent means to divide by 100

I think Tektronix "divided" by 100 to get the number 7.5. NO? I was simply typing the hypothetical for ANY value... i.e. % value for IRE. One could unambiguously figure "it's" analog voltage or digital level for any bit depth.

Can your % stimulus convention do the same?

What is 100% stimulus as it correlates to PC level?
Quote:
Originally Posted by hwjohn

Have you even looked at the disc? IRE only pertains to analog signals.

That is flat out WRONG!

Acorrding to Greg Rogers:

"the IRE value and the % amplitude value will be the same for today's consumer digital video standards"

"If I say you need a 75 IRE digital signal, you can unambiguously figure out exactly what that is in 8-bit, 10-bit, 12-bit digital video using video-levels (16-235 for 8 bits) or PC-levels (0-255)"
When it comes to basic display calibration the complication of IRE is that black can be represented as either 0 or 7.5 IRE. By using percent stimulation that item never arises and the user has one less thing to possibly learn about. For most general users, I fail to see much useful that using IRE instead of percent stimulation would add to their understanding in being able to do a basic calibration. Simplicity can be helpful in understanding basic concepts, and this point can be illustrated by your own inability to calculate light levels based upon digital signals and gamma even though you were attempting to bring up far more complex topics.

Quote:
Originally Posted by tbrunet

What is 100% stimulus as it correlates to PC level?

The answer you're looking for is 255. Now tell me if 100% stim is instead called 100 IRE, how would that more useful for anyone not dealing with analog signals? I already explained how not using the term IRE might simplify learning, so what is the benefit of IRE outside of its use with analog signals?
[quote=tbrunet;13480146]That is flat out WRONG!

Acorrding to Greg Rogers:

"the IRE value and the % amplitude value will be the same for today's consumer digital video standards"

"If I say you need a 75 IRE digital signal, you can unambiguously figure out exactly what that is in 8-bit, 10-bit, 12-bit digital video using video-levels (16-235 for 8 bits) or PC-levels (0-255)"[/QUOTE

I have no problem with using IRE when referring to digital signals if you want to. It is less correct, but most understand the meaning, but like alluring asked, what is the reason? In an industry with so much confusion, why open up the possibility of increasing confusion. And you won't see Greg saying %IRE, I am sure. The point here is that you were corrected on incorrect useage and now defend that incorrect use of the terms. OK, but what does it gain?
Quote:
Originally Posted by alluringreality

The answer you're looking for is 255.

Correct but you failed to answer my earlier question?

What digital level is 109% stim as it correlates to PC level? said "peak white"

Quote:
Originally Posted by hwjohn

with 109% corresponding to 254

I see you and hwjohn are unable to grasp that IRE units are actually a more powerful tool the % stim.

I also failed to note where you corrected Greg Rogers for his use of "75IRE digital level". Your pedantic nature seems a little selective
Quote:
Originally Posted by tbrunet

What digital level is 109% stim as it correlates to PC level? said "peak white"

When going from video to computer levels, the above white values are clipped. There's no 8-bit PC level that correlates to above white video.

So now that I've answered two of your questions AGAIN I'M ASKING... How is IRE "more powerful" like you're advocating? On what basis do you make such a statement? From an introductory standpoint I think IRE is possibly more confusing due to the 0 and 7.5 IRE item. Outside of it's use with analog levels, what does IRE bring to the table for someone trying to learn to calibrate their display? A majority of people are not going to be dealing with analog voltages, so I merely think it's simpler to avoid using the IRE term and I miss how it adds value.

If I've "failed to grasp" something then please fill me in as simply and concisely as you're able to do. Based on your posting history I question if you're looking to add anything to any discussion. In spite of my "pedantic nature", it took me a while to remember how people write don't feed the troll in the HD forums area. Here, check this out I made up my own symbol to pretend I'm witty at the end of my post -> ii!ii
Quote:
Originally Posted by tbrunet

That is flat out WRONG!

Well maybe we should just use IRE to measure everything. Maybe my cup should be 40 IRE full, and the speed limit should be 65 IRE. The bottom line is that IRE units were devised for analog signals, to represent a voltage. Why should we confuse things by labeling everything as IRE just because we can? I could come up with a hundred arbitrary "units" to measure things with. What purpose does it serve? None. The truth is that IRE is no more powerful than % stim. According to your precious quote, they are actually one and the same "for today's consumer digital video standards."

Quote:

Acorrding to Greg Rogers:

"the IRE value and the % amplitude value will be the same for today's consumer digital video standards"

"If I say you need a 75 IRE digital signal, you can unambiguously figure out exactly what that is in 8-bit, 10-bit, 12-bit digital video using video-levels (16-235 for 8 bits) or PC-levels (0-255)"

This seems to be your lone leg to stand on. I don't know Greg, but I respect him as much as the next person. Again, we could apply IRE to anything under the sun, but I disagree that it is "unambigous." To the contrary, IRE is about as ambiguous as it can be; it means absolutely nothing without some context. If I tell you that something is 1 foot, you immediately understand what that means because a "foot" is a physical measure of length. To the contrary, IRE by itself doesn't describe anything in particular, hence the last part of that quote:

Quote:

"If I say you need a 75 IRE digital signal, you can unambiguously figure out exactly what that is in 8-bit, 10-bit, 12-bit digital video using video-levels (16-235 for 8 bits) or PC-levels (0-255)"

Without all that additional information, IRE means nothing. I don't know where this quote comes from, but I have a feeling that just like IRE, you have also taken it out of context.
Quote:
Originally Posted by hwjohn

Without all that additional information, IRE means nothing.

The inability of your DVD player to output a format compliant specified voltage has no bearing on whether or not a IRE can be represented in the digital payload. We live in an analog world where “real world” images are analog. The combination of ADC and DAC should produce unity gain and no DC offset.
Quote:
Originally Posted by Guy Kuo

This leads me to another misconception which keeps circulating on the forums --- the idea that black is encoded onto a DVD as being at a specific IRE level independent of the DVD player.

That is not a misconception.

0 IRE = 0 mV = BT.601 Y’ code 16
100 IRE = 700 mV = BT.601 Y’ code 235

IRE is a measure of the grayscale or luma (aka luminance). Just because consumer devices (DVD player) exhibit broken by design characteristics, these artifacts will not reverse engineer themselves into my calibrated analog test equipment, thus making my scope obsolete. Wheter or not your DVD player is adding setup via some LUT is irrelevant. To imply one can't encoded a O-100IRE test pattern onto a DVD is a misconception.

http://www.ebu.ch/CMSimages/en/tec_t..._tcm6-4775.pdf
Testing for conformity with ITU-R Recommendations BT.601 and BT.656

"Recommendation BT.601 states only that the form of signal coding is uniformly-quantized PCM with eight bits per sample. For the luminance signal there are 220 quantization levels, with black corresponding to level 16 and peak white to level 235"

Over 100 IRE Luma signals will cause Gamut errors
Since there was so much debate on this topic and his quotes were being referenced in places, I decided to email Charles Poynton. He has given me permission to post our correspondence here. I started to ask him what the thought about "%IRE," but thought better of his time...

Quote:

Me: Dr. Poynton,
First and foremost, I would like to thank you for making publicly available the work on your website. It has proven to be a valuable resource for me as I study video systems as a hobby. Thank you.

My name is Casey and I am a member of a popular internet forum (you may have heard of it), www.avsforum.com. Your work is often cited in the more academic discussions that take place on AVS, and is widely regarded as authoritative. Recently, a debate has been started regarding blacker-than-black and whiter-than-white signals. It seems that respected forum members are in disagreement on the topic of video material being present above digital 235 and below digital 16 on commercially mastered DVD's. Often times these debates are solved by quoting your work, but this particular debate finds the two sides interpreting the same quotes from you in two different ways. Both factions also seem to interpret the Rec. 601 and Rec. 709 specifications differently.

I won't go into the particulars of the debate as I'm sure you are very busy and have better things to do, but I was wondering if you would be so kind as to clarify a question for us:

Should commercially mastered DVD's contain actual video material (not noise) below 16 or above 235 if they have been mastered according to standards?

This question is of importance to us so that we might calibrate our displays correctly. I realize that this question is quite likely to be trivial for you, and I apologize in advance for taking your time with it. I would understand if you do not have the time to respond; however, any light you could shed on the subject would be greatly appreciated.

Dr. Poynton's first response:
Quote:

C. Poynton: Casey -

>Should commercially mastered DVD's contain actual video material (not noise) below 16 or above 235 if they have been mastered according to standards?

Studio standards do not, at the moment, completely define what is supposed to happen outside the interface code range 16 through 235. Studio standards are in the process of being revised to incorporate elements that are necessary in the flat-panel world, and improved documentation of signal treatment in the footroom and headroom regions is likely to be part of the work.

In 1984, at the dawn of digital video, provisions were made for footroom and headroom for several reasons. The main reason was - and remains - the accommodation of filter undershoots and overshoots. To maximize image quality, undershoots and overshoots should be preserved through the signal chain as long as possible even if they are eventually clipped.

Generally, transient (short duration) excursions into the footroom and headroom regions are allowed, and can be expected in studio mastered material. Consider "transient" to encompass 4 or 5 samples in the footroom region.

During the last 10 years, the headroom region has been increasingly used to accommodate specular highlights. In the headroom region, specular highlights may be conveyed using interface codes above 235, and their durations can be longer than 5 samples - but should not last longer than about 16 samples.

Ideally, the display transfer function should follow its inverse gamma curve from reference white (interface 235) up to peak white (interface 254). Inverse gamma should be a 2.4-power for studio viewing conditions (100 nit display, 64 lux ambient). A substantially brighter display of a substantially brighter ambient calls for a lower value of gamma; for example, for typical desktop computer environments the 2.2 value of sRGB is appropriate.
Details - perhaps more than you want! - are available in "Digital Video and HDTV Algorithms and Interfaces".

Feel free to copy this note to .

Cheers,

- C

Just to make absolutely sure I was interpreting his email correctly, I asked for some clarification.

Quote:

Me: Dr. Poynton,
Thanks so much for taking time to respond. We were ultimately trying to figure out if our digital displays should be calibrated to show WTW data through 254, or if we should try to force the display to clip at 235 (while still preserving headroom for processing until we reach the display). From your comments regarding gamma and the fact that recent trends are to place highlights above 235, I suppose the answer to our question is that we should not force our displays to clip at 235, and instead ensure that our displays follow D65 and a proper gamma response through 254. Please correct me if that conclusion is incorrect.

Again, thanks so much for taking time to respond. I'll be sure to check out your book. As an engineer myself, I never consider too many details a bad thing.

Regards,
Casey

Quote:

C. Poynton: Casey -

>... ensure that our displays follow D65 and a proper gamma response through 254

That's my recommendation, for best picture quality.

Be aware that display vendors are, today, stuck quoting "brightness" (luminance, in cd.m^-2, or "nits") at REFERENCE white, but if they follow the true power law up to 254, that's 20% brighter but they can't quote it. In other words, if they clip at 235, they can effectively quote a higher brightness value on the spec sheet. My conclusion is that the CEA should establish a policy that permits them to quote the PEAK luminance, but that's not the case at the moment.

Now I know that some of you are going to have a hard time with this. I would kindly ask that you not email or call Dr. Poynton and call him an idiot, or ask him if he is related to Chris. Let's not try to bend his comments around as they are very specific.

Hopefully this will shed some light on the subject...
Quote:
Originally Posted by tbrunet

That is not a misconception.

0 IRE = 0 mV = BT.601 Y' code 16
100 IRE = 700 mV = BT.601 Y' code 235

IRE is a measure of the grayscale or luma (aka luminance). Just because consumer devices (DVD player) exhibit broken by design characteristics...

Let me make sure I understand this. Your contention is that DVD players that adhere to North American NTSC where:

Black = Y' 16 = 53.57mV = 7.5 IRE

are "broken by design"?
Nope, I'm saying it's a fact that:

"One can encode O mV (0 IRE) for quantization level 16"
"One can encode 700mV (100 IRE) for quantization level 235"

per ITU-R Recommendations BT.601. Whether your DVD player is capable of outputting the correct 0mV for 0 IRE OR adding 7.5% correctly is the issue.
Quote:
Originally Posted by hwjohn

..email or call Dr. Poynton and call him an idiot, or ask him if he is related to Chris.

Just for the record, according to Chris Wiggles, Poynton conflates dynamic range, bit depth, and contrast ratio.

Three (key) ingredients which are specifically addressed in Dr. Poynton's emailed response to your query.
Quote:
Originally Posted by tbrunet

Nope, I'm saying it's a fact that:

"One can encode O mV (0 IRE) for quantization level 16"
"One can encode 700mV (100 IRE) for quantization level 235"

per ITU-R Recommendations BT.601. Whether your DVD player is capable of outputting the correct 0mV for 0 IRE OR adding 7.5% correctly is the issue.

This is precisely the reason we don't use IRE for everything... you have even confused yourself!

0 mV does not necessarily correspond to digital 16... in a display chain that uses NTSC setup, digital 16 from disc will produce a > 0 mV signal, which you must correctly align your display's black level with.

If you are going to use IRE to describe the digital levels on disc, you can't automatically assume that the "digital IRE" is going to produce the same "analog IRE" at an analog output. Why? Because IRE doesn't mean anything unless you give it a specific context.
Quote:
Originally Posted by tbrunet

Just for the record, according to Chris Wiggles, Poynton conflates dynamic range, bit depth, and contrast ratio.

Three (key) ingredients which are specifically addressed in Dr. Poynton's emailed response to your query.

LOL, enough of this non-sense. No wonder people call you a troll.
Just in case you are implying I'm making things up or don't understand the subject matter!

Why are we so focused on Contrast Ratio (Post #78)
http://www.avsforum.com/avs-vb/showt...=866036&page=8

Originally Posted by Knuck
I did not start this thread to reveal any ground breaking concepts nor was it my intent to revolutionize your or anyone elses understanding or perspective of CR. Have you spoken to any industry professionals who actual design displays?
Quote:
Originally Posted by ChrisWiggles

I have Poynton's book sitting next to me and I've read it many times. And I've discussed that very passage, in the context of some other passages, in a great deal of detail. Quite frankly, if you really want to get into that passage, I will say that it, taken by itself is a very misleading passage, and appears to conflate CSF limitations with contrast limits erroneously
Quote:
Originally Posted by hwjohn

Hopefully this will shed some light on the subject...

Well thats fine as long as we can now all agree that me and Chris were right in the first place and Notsocool is a loudmouthed half-wit.
Quote:
Originally Posted by Mr.D

... and Notsocool is a loudmouthed half-wit.

Something tells me he isn't alone...
Quote:
Originally Posted by hwjohn

Something tells me he isn't alone...

Well I find most of the other participants in this thread can be reasonable enough on occasion despite their differences.
Quote:
Originally Posted by lcaillo

And you won't see Greg saying %IRE, I am sure.

No, % amplitude or IRE, but not % IRE (which doesn't make much sense). (I also don't like the term % stimulus.)
I will admit the error, but it's not like I don't comprehend what an IRE represents in the first place. Greg if you will, can you please address whether or not one is able to measure the % amplitude or IRE level that is encoded on a given DVD.

Does "digital level" 16 correlate to 0mV per ITU-R BT.601?
Quote:
Originally Posted by tbrunet

I will admit the error, but it's not like I don't comprehend what an IRE represents in the first place. Greg if you will, can you please address whether or not one is able to measure the % amplitude or IRE level that is encoded on a given DVD.

I don't know what you mean by "measure"? Are you talking about reading the digital video data from a DVD using a PC or something like that?

Quote:
Originally Posted by tbrunet

Does "digital level" 16 correlate to 0mV per ITU-R BT.601?

Digital level 16 correlates to black (for DVDs). Whether or not that produces a blanking level (0 IRE = 0 mV) or a black setup (pedestal) level (7.5 IRE = 52.5 mV or 53.6 mV depending on the video standard) depends on which video output (YPbPr, Y/C, or composite) you are talking about, and what standards the player adheres to (or tries to adhere to).
Quote:
Originally Posted by gregr

I don't know what you mean by "measure"? Are you talking about reading the digital video data from a DVD using a PC or something like that?

Thank you for replying to my request Greg.

When the DVD disc is played back. No 7.5% setup added to the amplitude. If the D/A conversion process is trasparent and calibrated, can I use my oscilloscope that is properly terminated for (75 ohms) will I measure (0mV) for black and (700 mV) for reference white test patterns?
Quote:
Originally Posted by tbrunet

Thank you for replying to my request Greg.

When the DVD disc is played back. No 7.5% setup added to the amplitude. If the D/A conversion process is trasparent and calibrated, can I use my oscilloscope that is properly terminated for (75 ohms) will I measure (0mV) for black and (700 mV) for reference white test patterns?

When a DVD is played back, if there is no 7.5 IRE setup on the particular video output that you are measuring, then you should measure the blanking level for black (should be less than 100 mV DC, but it is usually AC coupled). When you measure the reference white level it should be either 700 mV or 714.3 mV above blanking, depending on the video standard in use (which also depends on which video output you measure). If you make the measurement on a video monitor or a video measurement set (such as a Tek VM700) that is calibrated in IRE (rather than volts on an oscilloscope) you must be particularly careful because many of these instruments are calibrated for NTSC IRE only (or mV for PAL) and you will get wrong IRE values for other standards. There are also errors in the test files of some automated video measurement sets (such as the VM700) because of changes in the video standards that have never been fixed in those instruments.
No offense but my question was rhetorical. If I don’t measure 0mV (aka blanking level) for the quantization level Y’=16 , Cb=128, Cr=128, then either the D/A is not (unity) calibrated or the test pattern is non standard. Greg I also understand the concept of AC or DC coupling.

Nominal excursion for ITU-R BT.601 has blanking level = 0mV and reference white 0-700mV respectively.

Another scope in my shop is a Tektronix Component waveform monitor that has the vertical scale graduated so that blanking is 0mV and reference white 700mV.

The first scope I referred to was standard Tektronix (2246) 100MHz o-scope.
Quote:
Originally Posted by gregr

When a DVD is played back, if there is no 7.5 IRE setup on the particular video output that you are measuring, then you should measure the blanking level for black (should be less than 100 mV DC, but it is usually AC coupled).

Not if I'm measuring the Y component for a YPrPb signal. I did not think it was necessary to mention that I was NOT referring to a composite signal.

Blanking = 0mV = Y' digital level 16
Quote:
Originally Posted by tbrunet

No offense but my question was rhetorical.

Rhetorical??? "Greg if you will, can you please address whether or not one is able to measure the % amplitude or IRE level that is encoded on a given DVD."

What's rhetorical about a direct question to me?

Quote:
Originally Posted by tbrunet

If I don't measure 0mV (aka blanking level) for the quantization level Y'=16 , Cb=128, Cr=128, then either the D/A is not (unity) calibrated or the test pattern is non standard. Greg I also understand the concept of AC or DC coupling.

Nominal excursion for ITU-R BT.601 has blanking level = 0mV and reference white 0-700mV respectively.

Rec.601 is one standard. It is not NTSC, it is not Betacam, it is not MII, it is not PAL. DVD players use several standards in the same player for different outputs. There are also consumer DVD players (CEA standards) and DVD players made for professional standards. There are DVD players for the US, Europe, and Japan. So black is not always at the blanking level. Reference white is not always 700 mV above blanking.
See figure 1A
http://www.tek.com/Measurement/App_N...5W_15609_0.pdf

The scope graticule is marked 0 mV for blanking and 700mV for reference white amplitude. Would you change these marked values?

When I measure all calibrated YPbPr test signals they align exactly with the ideal amplitude. Whether or not a given DVD player can produce it was not my point. I was simply illustrating that I can indeed measure the disc encoded levels.
Quote:
Originally Posted by tbrunet

When I measure all calibrated YPbPr test signals they align exactly with the ideal amplitude.

So if you measure the YPbPr signals from a device (including a professional DVD player) that complies with the Betacam standard and the black level isn't at blanking and the reference white isn't 700 mV above blanking, it isn't calibrated correctly?

Quote:
Originally Posted by tbrunet

Whether or not a given DVD player can produce it was not my point. I was simply illustrating that I can indeed measure the disc encoded levels.

You aren't measuring the disc encoded levels at all. You are measuring the analog output levels of a DVD player.
Quote:
Originally Posted by gregr

..from a device (including a professional DVD player) that complies with the Betacam standard and the black level isn't at blanking and the reference white isn't 700 mV

I'm not sure who was talking about "Betacam" standard. How about SMPTE standard?
Quote:

You aren't measuring the disc encoded levels at all. You are measuring the analog output levels of a DVD player.

I said at least three times already for a hypothetical ideal A/D to D/A unity.

Component video systems, standardized by the Society of Motion Picture and Television Engineers (SMPTE), employ no setup and have a 10:3 video/sync ratio. Figure 2 in the url:

http://www.commsdesign.com/main/mult...05/0005hd2.htm

Traditional SDTV component systems only have a negative sync of the same amplitude (300mV). Note that the blank (or black, since no setup occurs) to white maximum video amplitude is 700mV. Because no color is superimposed, 700mV is also the maximum excursion required for a digital-to-analog converter (DAC) to generate SMPTE-compliant component video.

Ever heard of SMPTE?
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