[Research] YIQ color Space vs. YUV color Space for NTSC

I was doing some research into the YIQ color space for composite/S-Video reasons, and I stumbled on this:

The original color space used for encoding NTSC video in 1953, which was very similar to YUV. Only a small number of TV sets that decoded YIQ were ever manufactured, and it became obsolete. All NTSC encoding today is YUV. See YUV.

So from what this is saying is CRT’s didn’t use a YIQ color space it used a YUV color space.

Wouldn’t this mean the composite and S-Video shaders should be using RGB/YUV/RGB instead of RGB/YIQ/RGB. (To be more accurate.)

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As with all of this stuff, the answer to that is “it’s complicated”. :stuck_out_tongue:

The NTSC standard is YIQ, though YUV is used for some NTSC variants, apparently. YIQ decoding was indeed not done much because the decoders are expensive, but that just means that they used a cheap filter to separate the I and Q components rather than actually decoding them (at least, as far as I can tell from reading).

With all that said, YUV and YIQ are extremely similar (the first row of their conversion matrices are identical). Switching maister’s NTSC shader to use YUV matrices makes a very slightly different image (yiq first, followed by yuv:

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I actually like the YUV image better…

It’s doing something with the red that’s noticable on mobile.

YIQ was introduced as the first encoding system for NTSC because it gave better chrominance horizontal resolution (color changes across the screen). YIQ gave about 150 color changes (120 TV lines per picture height) across for reddish orange and greenish blues and about 50 (40 TVL) for other colors which was the best that could be achieved at the time. The human eye was more sensitive to color changes involving those oranges and those blues. It was probably less than a year later that a substantial reduction in circuitry, tubes, and cost could be achieved by going to the YUV (Y/Pb/Pr) color decoding (Simplified Color Circuitry in RCA’s literature) in the TV set which used essentially the same broadcast video signal. One consequence was that color resolution became 40 TVL for all colors. Even in a A-B comparison between YUV and YIQ, the difference in color resolution was hardly noticeable.

TV Lines – Traditionally refers to the number of black dots and intervening white spaces all in a line, that can be individually distinguished.in a distance equal to the diameter of the largest circle that fits in the screen or other frame of reference, a 4:3 aspect ratio has been assumed. For analog TV (scan lines on a CRT) horizontal resolution, i.e. the aforementioned line of dots goes from left to right, is the most important resolution measurement.