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Old 14th December 2007, 19:19   #1  |  Link
adoniscik
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What is the gamut of video?

I want to compare the SD and HD gamuts. I understand that the former uses ITU-R BT.601 while the latter uses ITU-R BT.709 primaries.
  1. How wide is the NTSC output (TV) gamut? Do manufacturers set the primaries arbitrarily depending on the display technology (e.g., CRT vs. LCD) or are they fixed? If so, to what, SMPTE-C?
  2. How wide is the NTSC input (DVD) gamut?
  3. What if the DVD is viewed through a computer instead of CRT? Logically, if DVDs are assumed to use rec.709 primaries, then you should perform a color space conversion from BT.709 to the color space of your monitor to get accurate color. The gamut is defined by BT.709.
Aren't "primaries" a relic from the CRT era? Digital videos are encoded in luma-chroma color spaces rather than RGB. You could conceivably build a multi-primary display (and some have).

Addendum: color management for film

Last edited by adoniscik; 14th December 2007 at 21:17.
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Old 14th December 2007, 21:29   #2  |  Link
hwjohn
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Quote:
Originally Posted by adoniscik View Post
I want to compare the SD and HD gamuts. I understand that the former uses ITU-R BT.601 while the latter uses ITU-R BT.709 primaries.
  1. How wide is the NTSC output (TV) gamut? Do manufacturers set the primaries arbitrarily depending on the display technology (e.g., CRT vs. LCD) or are they fixed? If so, to what, SMPTE-C?
  1. For NTSC, or Standard definition, the primaries are typically SMPTE-C. TV manufacturers have widely varying primaries in practice. In a perfect world, SDTVs would have SMPTE-C primaries and HDTVs would have Rec. 709 primaries. This is hardly ever the case. Very few TVs have the ability to adjust the primaries. For those that do, the difference between SMPTE-C and Rec. 709 is often smaller than the adjustments that can be made to the primaries.

    Quote:
  2. How wide is the NTSC input (DVD) gamut? I read that DVDs use rec.709 primaries (by recommendation and by default in the absence of metadata). How much sense does this make if SD displays are assumed to be rec.601, or at least SMPTE-C? Does it not matter in practice since SMPTE-C is close to rec.709?
  3. There seems to be a lot of confusion about this. Keep in mind that primaries and decoding matrix are two entirely different things. It is my understanding that most (if not all) DVD's are mastered with SMPTE-C primaries. In practice, SMPTE-C is pretty close to Rec. 709 and most people don't worry with it. That isn't to say you can't see the difference. The main issue is that you can't normally adjust the display devices primaries anyway, and the error in the TV primaries is typically larger than the difference between SMPTE-C and Rec. 709 anyway.

    There has been some recent discussion about what primaries/decoding matrix that DVDs use. A lot of confusion comes from the fact that MPEG2 defaults to 709, while DVD video defaults to SMPTE-C. That seems to be a source of confusion because DVD Video really is MPEG2. Take a look at the HCenc and colorimetry thread in this forum for more info. See the table at the bottom of the post.

    http://forum.doom9.org/showthread.php?t=132295

    Quote:
  4. What if the DVD is viewed through a computer instead of CRT? Logically, if DVDs are assumed to use rec.709 primaries, then you should perform a color space conversion from BT.709 to the color space of your monitor to get accurate color. The gamut is defined by BT.709.
To get all of this correct is not trivial in practice. You have to know what the original material was mastered to (primary wise), and you also have to know which matrix was used to encode from RGB --> YPbPr. If you then know what primaries your intended display device is using, then you at least know what your playback software/device should be doing. It is entirely possible to "twist" primaries from Rec. 709 to SMPTE-C, but not vice-versa (the SMPTE-C gamut is inside the Rec. 709 gamut). You can also translate the 601 matrix to 709 (this does work both ways). Once you know what the material was mastered in and encoded with, then you can determine which transformations should be done. It is generally difficult to determine which transformations actually are being done through the playback chain.

Quote:
Aren't "primaries" a relic from the CRT era? Digital videos are encoded in luma-chroma color spaces rather than RGB. You could conceivably build a multi-primary display (and some have).
I'm not sure what you mean by a "relic." The three primaries are used to make every other color within the displays gamut, so they are pretty much a necessity. Every TV I know of decodes YPbPr to RGB for display. YPbPr is used on disc because it is a more efficient way to store the data. RGB contains some redundant data that doesn't need to be stored.

A quick look at the link you provided looks like they are trying to use 5 primaries instead of 3. This would allow you adjust the gamut at 5 points instead of 3, producing a gamut inside a pentagon instead of a triangle. Sure that might be useful, but it would increase the cost of TVs and would require an entirely new system (RGBCY).

Addendum: color management for film[/QUOTE]
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Old 14th December 2007, 21:38   #3  |  Link
adoniscik
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Thanks, John. If the SD and HD primaries are not that different, does it not follow that they should have similar gamuts? Is this consistent with the observation that HDTVs are capable of displaying more saturated colors? Is this simply because they use newer technology? If so, how does this agree with the assumption that the primaries are specified by ITU-R BT.709. In other words, how can you increase the gamut (get more saturated colors) without moving the primaries? Perhaps it is a perceptual phenomenon; the increased contrast (deep blacks and bright whites) lends the impression of a wider gamut?

Last edited by adoniscik; 14th December 2007 at 21:40.
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Old 15th December 2007, 05:46   #4  |  Link
hwjohn
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Quote:
Originally Posted by adoniscik View Post
Thanks, John. If the SD and HD primaries are not that different, does it not follow that they should have similar gamuts? Is this consistent with the observation that HDTVs are capable of displaying more saturated colors? Is this simply because they use newer technology? If so, how does this agree with the assumption that the primaries are specified by ITU-R BT.709. In other words, how can you increase the gamut (get more saturated colors) without moving the primaries? Perhaps it is a perceptual phenomenon; the increased contrast (deep blacks and bright whites) lends the impression of a wider gamut?
The SD and HD gamuts aren't that different, but there is a visible difference. The difference is relatively small in comparision to the typical error seen in TV primaries.

As for the observation that HDTVs are capable of displaying more saturated colors. Take an HDTV with perfect 709 primaries, and an SDTV with perfect SMPTE-C primaries. The HDTV can reproduce more saturated colors because, as you stated, its primaries are "wider." However, it is not a night and day difference in saturation because the two standards aren't that far apart, relatively speaking.

Another issue is probably where the "HDTV has more saturated colors" thing comes from. If you look at the trend of todays HDTVs, a lot of them have wildly oversaturated primaries. A lot of people think that this is done for marketing purposes. Most consumers actually don't like a realistic, correctly calibrated picture. They like oversaturated colors and high contrast ratios.

As an example, I have a Sony A3000 RPTV (LCoS) and my brother has an XBR2, which is a higher end predecessor to the A3000 (also LCoS). I have measured both TV's primaries. The A3000 is relatively close (better than 95% of sets ou there) to the 709 standard. The XBR2 has wildly oversaturated green. The green I measured was almost on the edge of the CIE chart. Red and Blue weren't as bad, but they were still very oversaturated.
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