Differential phase

Differential phase is a kind of linearity distortion which affects the color hue in TV broadcasting.

Composite color video signal (CCVS) consists of three terms: The first two terms are usually called composite video signal (CVS) The modulation technique of the color subcarrier is quadrature amplitude modulation (QAM) both in PAL and NTSC systems.

That means that, when the color signal has a low luminance, its DC component is comparable to that of the colorburst.

During broadcasting, the inherent non linearity in electronic devices may cause a level dependent phase shift.

To solve the problem of differential phase distortion in PAL system, the polarity of both the colorburst and the color signal are reversed in each consecutive lines.

The average of two lines yields the original phase difference and thus the color.