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Source video at sRGB levels can have illegal luminance levels?

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Rick Anvican
Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 12:10:38 am

Hi all,
If I have this source footage (H264) which used sRGB levels originally, would it be abnormal if there are occasional peaks of luminance that are out of range (e.g. on average for every 30 frames about 4 have such peaks)? Black rests at 0 IRE but the highest white could reach 106 IRE according to Vegas' luminance waveform (colors are within legal range though). Peaks occur during fade-to-white transitions in the source footage.

Please clarify. Thanks in advance.
RickAVC


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John Rofrano
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 12:27:48 am

If you're asking if video cameras shoot illegal values the answer is, "YES, all the time!" That's why you always have to color correct if going to broadcast.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Rick Anvican
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 12:43:04 am
Last Edited By Rick Anvican on Mar 17, 2014 at 12:44:00 am

Hi John,
Yes I understand that footage from most video cameras will use illegal values, but for my situation, the footage was sourced from a DigiBeta master tape of a 90s broadcast and this was converted to H264.
Despite the digital master using sRGB levels, the illegal luminance peaks at fade-to-white transitions were present on it which carried over to the H264 converted version. Do mind that full black was at 16:16:16 RGB. I'm just wondering how that's possible - is it due to tolerances in the equipment or some other factor?

RickAVC


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Norman Black
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 12:57:08 am

I think "illegal" is an unfortunate term. "out of range" is better I think since legal video signal can exist outside of 16-235 or 0-100 IRE (assuming 16 => 0 IRE and 235 => 100 IRE)

Going back to the analog days this slop at the top and bottom works with the way you do analog circuity, and leaves some slop room for on the fly processing that might have been done in live TV. A little over/under shoot room for effects. Analog is not precise.

In a "perfectly" calibrated video monitor your super back/white video signal just get clamped.

In our digital world the 16-235 swing is trying to mimic classic analog video signal in some ways.

Really only 0 and 255 (for AVC i think) are truly illegal values.


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Rick Anvican
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 2:42:45 am

Hi Norman,

[Norman Black] "Really only 0 and 255 (for AVC i think) are truly illegal values."
Would those bits be reserved with similar purposes like they were in analog PAL signals where they stored technical information?

Obviously DigiBeta is digital, but like you said if the digital video world tries to mimic the analog video signal, does that mean that such underexposure and overexposure "buffers" (e.g. 1-15 under, 236-254 over [LUMA]) exist, like the out-of-range values are recorded but not visible?

A majority of people may have moved on to the digital world, but I'm still living with some working ancient technology.


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Norman Black
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 3:41:47 am

[Rick Anvican] "[Norman Black] "Really only 0 and 255 (for AVC i think) are truly illegal values."
Would those bits be reserved with similar purposes like they were in analog PAL signals where they stored technical information?"


From what I have read are reserved for synchronization (timing references).


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Rick Anvican
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 5:13:23 am

So I guess the sRGB range really is just a guide that broadcasters, editors and other users adhere to, since it is specified within limits of the computer RGB scope. Is it probable that out-of-range occurrences are due to encoding processes of codecs, like lossy compression? I doubt that Vegas converted sRGB to computer RGB levels upon import of the H264 footage, as the original black levels were retained.


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John Rofrano
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 12:21:28 pm

[Rick Anvican] "So I guess the sRGB range really is just a guide that broadcasters, editors and other users adhere to, since it is specified within limits of the computer RGB scope."
As Norman was explaining, there are other signals carried for sync and audio, etc., and broadcasters were just trying to give themselves enough room to not interfere. This is why if you broadcast a video with out of range values the picture may roll or have other interference like audio buzzing when fading to white because these signals are being stepped on by the video signal.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Norman Black
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 5:33:13 pm
Last Edited By Norman Black on Mar 17, 2014 at 5:43:41 pm

[Rick Anvican] "So I guess the sRGB range really is just a guide that broadcasters, editors and other users adhere to,"

It all goes back to analog Black and white TV and we are carrying things forward. In a digital broadcast system there are no interference problems, loss of signal lock or signal over saturation.

In a clean, designed from scratch system, much of the confusion we have would not exist. It works fine but is not clean, IMO.

29.97fps exists because color TV was patched in, backwards compatible to BW TV and in analog they needed to avoid interference.

Studio swing, aka 16-235 digital, exists because of the way analog is done. In analog your useable min/max are never at your min/max signal levels.

Digital mimics the analog swing because for a time the two systems were mixed. In digital having an absolute low (0) and max (255) is trivial unlike analog.

In digital transmission you no longer have to worry about interference.

Digital eliminates all the quirks of the old analog system, but we keep the standards, probably in part because that is the way people have been working for over 50 years. It is what they know. It is not broken, just quirky.

Personally I really don't like the 16-235 thing. We are only 8-bit and are giving up precious video levels when we have so few. Because people are stuck in a legacy mentality, a solution is not to fully utilize 8-bit but to go 10-bit. That 10-bit is just like 8-bit and does not use all 1023 levels. Where 0 is illegal in 8-bit in 10-bit that extends to 0-3. 235 goes to 940 and so on.

Digital still photography thankfully has not been saddled by legacy, since legacy there was printed photos and not transmissions.


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Rick Anvican
Re: Source video at sRGB levels can have illegal luminance levels?
on Mar 17, 2014 at 11:33:33 pm

[John Rofrano] " This is why if you broadcast a video with out of range values the picture may roll or have other interference like audio buzzing when fading to white because these signals are being stepped on by the video signal."
Right, haven't seen these symptoms personally, but I mean NTSC and PAL have been around a long time, they're both composite video systems with very limited bandwidth where sound and video signals overlap each other, and like Norman said the old analog standards lurk with us today, even people still refer to NTSC and PAL in HDTV considering that HDTV is 1080 lines, just the framerate is the main characteristic that is retained. For once I thought we had 16 million colors on hand...


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Jim Lillis
Re: Source video at sRGB levels can have illegal luminance levels?
on Dec 6, 2014 at 6:35:12 pm

Pardon the intrusion here, I know this is an older thread, but for my fyi...

Isn't 16 = to 7.5 IRE ?

If it moves . . . Shoot it!


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John Rofrano
Re: Source video at sRGB levels can have illegal luminance levels?
on Dec 6, 2014 at 7:38:45 pm

[Jim Lillis] "Isn't 16 = to 7.5 IRE ?"
Yes and No. 7.5 IRE is an analog setting and 16 is a digital setting. 16 can equal 7.5 IRE or it might not equal 7.5 IRE because 7.5 IRE has to do with how the analog equipment will interpret the signal which the digital equipment has no knowledge of. Some NLE's may try and emulate what 7.5 IRE looks like but the digital setting is still 16.

Read Glenn Chan's excellent article "The 7.5 IRE Setup Problem" for more information.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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