COLOR SUBCARRIER

The frequency that carries the color information in the baseband composite video signal. In NTSC the color subcarrier is 3,579,545 Hz, ± 10 Hertz. This number is usually rounded off in text to 3.58 MHz. Unlike the black and white television standard, where the vertical and horizontal frequencies are derived from the 60 Hz power line frequency, the color subcarrier became the clock reference for the color system. The horizontal and vertical frequencies are now derived from the color subcarrier rather than the power line rate. When we shifted to the color system in 1954 the horizontal and vertical rates changed. The amount of change was small enough so that black and white sets would still synchronize to the updated rate. A color sync burst of 3.58 MHz is added to the beginning of the horizontal lines to synchronize the color decoding circuits in the display device with the source video. The phase of the color information in the video is changed 180° for every adjacent line. The change occurs automatically as part of the horizontal timing being derived from the color subcarrier. It has the overall effect of canceling the display of the color subcarrier in the picture. It does, however, produce "dot crawl" at color transitions. It takes two complete pictures for the phase relationship between the subcarrier and horizontal signal to return to zero. (This presents a minor problem for laserdisc players when displaying a still frame. The color phase has to be electronically reversed every other time a single frame is repeated because a still frame is only half of a color cycle.) See Horizontal Scanning Frequency and Vertical Scanning Frequency.