Differential gain （DG） and differential phase （DP） are two specifications that designers of composite video systems use everyday. We will define them here just to be sure we are all speaking the same language， and to ensure understanding of the rationale of the test technique used by National. Composite video encodes brightness （luminance）， timing （sync）， and color （chrominance） into one channel. Luminance is the voltage offset from a reference， or “black”， level. Sync appears at a level defined as “blacker than black.” Chrominance is encoded as a high-frequency （with respect to the luminance signal） subcarrier. The average value （midpoint） of the chrominance is the luminance. The color has two “dimensions”： amplitude which determines the saturation， and phase relative to a reference chrominance burst which encodes the hue. For example， pink has the same relative phase as red， but of a lower amplitude， hence a less saturated red. Red in NTSC is shifted 103.7? from the reference， green 241.3?。 DG and DP are measured at one of two chrominance subcarrier frequencies. NTSC （National Television Systems Committee， 1953） uses a 3.579545MHz color subcarrier. Phase Alternation Line （PAL） alternates the phase of the reference burst with every scan line. The PAL subcarrier frequency is 4.433619MHz