Chrominance - Television Standards

Television Standards

In analog television, chrominance is encoded into a video signal using a subcarrier frequency. Depending on the video standard, the chrominance subcarrier may be either quadrature-amplitude-modulated (NTSC and PAL) or frequency-modulated (SECAM).

In the PAL system, the color subcarrier is 4.43 MHz above the video carrier, while in the NTSC system it is 3.58 MHz above the video carrier. The NTSC and PAL standards are the most commonly used, although there are other video standards that employ different subcarrier frequencies. For example, PAL-M (Brazil) uses a 3.58 MHz subcarrier, and SECAM uses two different frequencies, 4.250 MHz and 4.40625 MHz above the video carrier.

The presence of chrominance in a video signal is indicated by a color burst signal transmitted on the back porch, just after horizontal synchronization and before each line of video starts. If the color burst signal were visible on a television screen, it would appear as a vertical strip of a very dark olive color. In NTSC and PAL, hue is represented by a phase shift of the chrominance signal relative to the color burst, while saturation is determined by the amplitude of the subcarrier. In SECAM (R'-Y') and (B'-Y') signals are transmitted alternately and phase does not matter.

Chrominance is represented by the U-V color plane in PAL and SECAM video signals, and by the I-Q color plane in NTSC.