issues with color spaces in HD vs SD
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good article here:
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http://www.glennchan.info/articles/technical/hd-versus-sd-color-space/hd-versus-sd-color-space.htm
some good points:
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There is no single HD or SD color space as they are actually many different HD and SD color spaces. We'll begin by looking at the two most common video standards in use today- ITU-R BT. Rec. 601 and Rec. 709- as the most common HD and SD formats derive from them. The two main differences between the Rec. 601 and Rec. 709 standards are [A] the luma coefficients (and corresponding scale factors) and [B] the primaries.
In practice, the difference is subtle and is usually not noticed. This is partly because:
- We aren't very sensitive to (small) color inaccuracies. This is especially so when there is no side-by-side comparison.
- Real world images tend not to contain highly saturated colors (a random distribution of R'G'B' values would contain much more highly saturated colors). These errors are strongest for highly saturated colors.
- Most film and video material do not contain large patches of color, which make it easier to discriminate between colors.
- In consumer applications, consumer TVs tend to be (wildly) color inaccurate to begin with.
When the NTSC standards were developed, the designers envisioned a wide gamut system where the primaries are much more saturated/pure than what we have now. The more saturated primaries allow a greater range of highly saturated colors to be reproduced. It is theoretically ideal to make the primaries as saturated/pure as possible so that the widest range of colors can be reproduced.
One downside to wide gamut systems is that overall luminance of the display is lower. Luminance can be increased by making the primaries less saturated. For this reason, consumer TV manufacturers ignored the NTSC standard in order to make displays brighter. Early consumer TVs were fairly dim so this might have been a reasonable compromise.
Later on, SMPTE created the "SMPTE C" standard realizing that there needed to be a production standard. These primaries were derived from the Conrac CRT monitors that were commonly used for reference monitoring at the time. Eventually after that, the EBU created their own standard to reflect changes in CRT phosphors. The EBU primaries are the standard for PAL countries and Japan.
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later
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