Differential phase is a kind of linearity distortion

Distortion

A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

 which affects the color hue

Hue

Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...

 in TV broadcasting.

Composite color video signal

Composite color video signal (CCVS) consists of three terms:

• Monochrome (luminance
  Luminance
  Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
  
  ) signal
• Auxiliary signals (sync pulse and blanking signals )
• Color signal, which is actually a subcarrier modulated by chroma
  Chrominance
  Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
  
   information

The first two terms are usually called composite video signal (CVS)

The modulation technique of the color subcarrier is quadrature amplitude modulation

Quadrature amplitude modulation

Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...

 (QUAM ) both in PAL

PAL

PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

 and NTSC

NTSC

NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

 systems. The amplitude of the color signal represents the saturation of the color and the phase lag of the color signal with respect to a certain reference which is called colorburst

Colorburst

Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...

 represents the hue; i.e., each phase lag is assigned for a different color hue. So, in order to reproduce the original color in the receiver, the phase difference between the colorburst and the color signal must be kept constant throughout the broadcasting.

The colorburst

The colorburst is a 10 period signal of color carrier (3.58 MHz in system M

System M

System M, sometimes called 525 line, is the analog broadcast television system used in the United States since July 1, 1941, and also in most of the Americas and Caribbean, South Korea, and Taiwan. In addition, Japan uses System J, which is nearly identical to System M. The systems were given...

 and 4.43 MHz. In system B

System B

System B is the analog broadcast television system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. - Specifications :Some of the important specs are listed below.Frame...

 and G) It is superimposed on the back porch of the CVS. The peak to peak amplitude

Amplitude

Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...

is about 300 mV. That means that, when the color signal has a low luminance, its DC component is comparable to that of the colorburst. All dark colors have more or less the same DC component as the colorburst. But light colors have a higher luminance and hence a higher DC component.

Differential phase distortion

During broadcasting, the inherent non lineraity in electronic devices may cause a level dependent phase shift. Differential phase distortion (DP or dP) is the shift of color signal phase with respect to the colorburst phase. Since DC levels of the light colors and the colorburst are different, this type of distortion is more pronounced in lighter scenes. Especially a slight change in skin color may be irritating to the viewers. (too yellow or too red skin color dependeing on whether shift is positive or negative)

PAL system

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 consequative frame. So in one frame the phase of the colorburst leads and in the second frame the phase of the color signal leads. However if there is DP distortion in the system, the shift caused by DP has always the same polarity, thus the overall shift is more than the original in one frame and less than the original in the second frame by the same amount. The average of two frames yields the original phase difference and thus the color.
The mathematics involved is as follows:

Let be the phase difference of the color signal with respect to colorburst and be the extra shift introduced by DP.

The original signal is

If there is a DP distortion, the received signal for the first frame is

In the second frame (after multiplying by -1)

The average is

So while the effect of diminishes on the color hue, the amplitude of the color signal is reduced by which means that color saturatioın is reduced.