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.
