Go-to Guide for Source Options - Page 6  

Chris, this whole picture information debate is over, Htpcfan and others gave perfect examples and FAQ concerning what is a good "robust" digital signal.
Now if you think DVD's are encoded with special BTB/WTW information, please check your own sources, for none of them are saying what you are?

Also: you suggest that I PM you on the subject. I don't answer to you, I'm free to
post just like anyone else. So as long as this forum will let me subscribe, I don't see I need you, suggesting the form of my communication IMO :D

Man you won't give it up. Here, go argue with Joe Kane. He say's it's there. His disc miraclulously produces it. He made buckets of money selling it to the world. tbrunet is the first I've run across to dispute it.

http://www.videoessentials.com/ve_d_alb_discnotes.php

FWIW: All information below reference black "has" serious gamma distortion, and how one calibrates to distortion is beyond my ability to comprehend :) Joe Kane and Chris need classes in "Video Level 101" and "% IRE"

Also: show me a commercial DVD that has BTB information on it, and I'll show you
a DVD that is worthless, ie "incompetent" mastering!

thomas
:cool: gamma@2.35

GetGray,

I would like to recommend to Chris, Joe Kane, ect...google the terms 'Time Base' & 'Oscilloscope'
& 'Gamma Linearity' & 'Normalization' & 'Gamma Correction' & 'APL' ect...

Chris and Joe are refereing to picture information they themselves are not able
to measure, quantify, & the logic here, just trust us! all hollywood is doing this and we
want to preserve the original coded information..FWIW not one "competent" artist would purposely crush BLACKS and clip WHITES :)

Then please :) post "Joe Kane" Thanks in advance.

thomas

I admit to being sympathetic to htpcfan's desire for crisper reference standards in video. There are some advantages to the "computer" way of doing things, especially with newer fixed-pixel technologies that don't have the fuzzy margins that an analog CRT display chain has.

However, it's worth noting that video engineers are not idiots. They didn't dedicate that many codes to below-reference and above-reference without thinking things through very carefully. They did have to deal with the fact that analog video has room for slop in the levels and they didn't have a crystal ball to predict today's modern display technologies. I'm sure if they were designing the system today they would have made adjustments.

There is a disconnect in the video standards between the reference levels and the "real" practical range. Very few people, in my experience, really understand this disconnect, though a careful reading of all the documents helps.

First off, black level. Even though 16 is specified as the "black level" in, for example, BT.601 and BT.709, practically speaking it's not the darkest level a video professional can see on a calibrated Broadcast Video Monitor (BVM). Telecine operators and video editors calibrate a BVM's black level by putting up a SMPTE color bars pattern and using the PLUGE section in the lower right to adjust the brightness. The procedure is to turn the brightness up until the below-reference bar becomes visible, then slowly turn it down just until that bar disappears visually into the background.

Note that this process does not use a light meter. It doesn't use an oscilloscope. It's a perceptual process. Furthermore, that below-reference bar is encoded at 4% below reference, which is digital level 7.

Consider that for a moment. Even disregarding the perceptual nature of the calibration process, the established process for calibrating for video actually sets the blackest perceivable level at somewhere between 7 and 16, depending on how good the calibrator's eye is at distinguishing between levels. Not 16.

This means in general that some of the levels below 16 are still visible if the calibration is done correctly. Stacey and I have done some informal tests that suggest that the actual number of visible levels below 16 on a calibrated BVM is around 4, but we need to do more work to get a more accurate number. I suspect (purely an educated guess) that it may be as much as 6 or 7 levels under the right circumstances.

And just as obviously, the levels above 235 are completely visible, because BVMs are CRT displays and have lots of inherent headroom.

A good video technician is not going to let large amounts of picture information go above 235, but all they care about is that the bulk of the histogram stays below that level - they are perfectly OK with small spikes above the reference level, which happens with specular highlights, details in bright or highly saturated objects, etc. Those are exactly the kind of details that headroom is designed to handle.

There are people who master DVDs who run the video through a "legalizer" before finalizing the master. The worst of these "legalizers" clamp the video levels (in YCbCr) to 16-235. Most don't - most just ensure that there aren't huge swaths of above-reference or below-reference values, and to do so apply a non-linear compression to the values with a smooth knee and shoulder (essentially an S-shaped gamma), and don't compress everything into the 16-235 range. These are completely unnecessary for DVD mastering but people have gotten used to using them because television transmitters have stringent range requirements.

Moreover, even with the crappy legalizers that clamp everything into the reference range, the RGB range can (and does) still stray outside the reference ranges, and it's ultimately RGB that matters. I have yet to see a DVD that didn't have above and below reference RGB values. And those values are visible on a properly calibrated CRT (such as a BVM).

Some companies (the good ones) don't apply legalizers at all for DVD mastering. There's really no point. There's nothing inherently wrong with picture information extending above or below the reference levels. That's the way the whole system was designed. In practice, very little of the image will typically extend above or below, but that's to be expected. Most real images have very little data at the very high or low end of the range. Unfortunately, even though it's not a lot of data, it can be fairly important data.

There really shouldn't be any controversy about this - we all are (I hope) trying to get the picture on our home theater screens to look as close to the picture the director and/or cinematographer approved. The director or cinematographer supervised the transfer on a calibrated BVM connected to pro equipment. That equipment doesn't clip off the above or below-reference range, and neither should consumer equipment.

Don

Don,

Y= 16-235 with a offset of "16" and an excursion of 219

CbCr= 16-240 with an offset of 128 and an excursion +/- 112

There are 256 discrete levels of "Chroma" , and NOT 256 levels of luminance!

FWIW I must be one of the idiot engineers you are refereing to! Also headroom was built into the digital payload, it's called "Peak White" and it's bound at 235.
According to you theres NO offset? I have read Poynton and I'm going to quote him when he states!

"The headroom and toeroom are NOT PICTURE INFORMATION"

*Please go argue with Charles Poynton, if you think think overshoot data is "picture information"

From what I've read so far, 75% of the original R'G'B' information is lost by the time
it get to us in DVD form! It's called compression :)

thomas

I'm with you so far. Those are the reference ranges. The encoded levels are allowed to stray outside those ranges. That's why there are more codes.

I agree. There are 254 discrete levels of luma because codes 0 and 255 are reserved. How this is relevant is beyond me.

601 calls it "peak white" but Poynton does not. I'm going to go with Poynton:

Digital Video and HDTV, Charles Poynton, pg. 22:

"Headroom allows code values that exceed reference white; therefore you should distinguish between reference white and peak white." (italics in original)

Huh? When did I say this?

I've searched through Poynton and can't find anything remotely resembling this quote. Could you provide a cite, please? Book? Page number? Web site?

All this doesn't really matter - I've looked at hundreds of DVDs at this point. I'm confident in saying that ALL DVDs contain information coded above 235. And ALL DVDs contain information coded below 16. You can scream and shout about how it's wrong (and I disagree, as I've made clear), but that's the way it is.

As for the codes below 16, many of them are invisible and were invisible on the BVM, and aren't supposed to be seen. I would still like to keep that range, partially to allow proper calibration and partially because some of them were visible on the BVM.

I have had it demonstrated to me that clipping below 16 is visible on real-world DVDs. It's subtle, but it's there. So all the theory about "why" is kind of immaterial. It actually does change the picture to clip the below-reference values.

The stuff above 235 is much simpler - ALL of it is visible. ALL of it was visible on the BVM. The director approved it. It shouldn't be clipped off.

Don

http://www.poynton.com/PDFs/SMPTE93_Gamma.pdf

On page (8) in the link above, here he describes the nonlinear transfer function of a CRT display. Note how the video input signal regardless if 7.5% or 0% setup is used,
the resulting nonlinear plot does "NOT" have any values below 0mV! The Y plane is the Light Intensity (vs) the X plane which is the input Video Signal. The chart is a reference guide for monitor calibration.

http://www.poynton.com/PDFs/Colour_interchange.pdf

At the bottom of page (5) Poynton describes the headroom and footroom are for filter overshoot, and not BTB/WTW expanded luminance steps or discrete quanta. Filter ringing is the artifact caused by fast on/off analog edge transistions.

http://www.snellwilcox.com/knowledge.../glossary/?c=b
:rolleyes: QUOTE:
Blacker than Black
Jargon for the video signal parts below reference black level, for instance, the luminance signal overshoots after a white-to-black transition.

thomas

This started out as a Reference thread providing information. Somehow it is not side tracked. Please do not post questions to this thread.
I will be cleaning this up when I get a chance and removing some of the off topic posts.
Thank you.

Alan,

Sorry about all the mis information I've been posting here and other threads, looks like Chis has a firm understanding of the SM.

thomas