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{{morefootnotes|date=April 2010}}
'''Differential phase''' is a kind of [[linearity]] [[distortion]] which affects the [[hue|color hue]] in [[TV broadcasting]].

== Composite color video signal ==

'''Composite color video signal''' (CCVS) consists of three terms:
*Monochrome ([[luminance]]) signal
*Auxiliary signals ([[sync pulse]] and [[blanking (video)|blanking]] signals )
*Color signal, which is actually a subcarrier modulated by [[chrominance|chroma]] information
The first two terms are usually called [[composite video signal]] (CVS)

The modulation technique of the color subcarrier is [[quadrature amplitude modulation]] (QUAM ) both in [[PAL]] and [[NTSC]] 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]] 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 ==
 
{{Main|Colorburst}}
The colorburst is a 10 period signal of color carrier (3.58&nbsp;MHz in [[CCIR System M|System M]] and 4.43&nbsp;MHz in [[CCIR System B|System B]] and [[CCIR System G|System G]]). It is superimposed on the [[back porch]] of the CVS. The [[peak to peak]] [[amplitude]] 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.<ref>[http://www.tek.com/Measurement/App_Notes/NTSC_Video_Msmt/25W_7247_1.pdf Television Measurements: NTSC Systems]. ''Techtronix''</ref> 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 ==

{{Main|PAL}}

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.<ref>Gordon J.King:''Beginner’s Guide to color television'', Newness technical, London, 1978; ISBN 0-408-00101-1, p 65-70</ref><ref>V.F.Samoylov-B.P.Khromoy: ''Television'' (trans:Boris Kuznetsov) Mir Publishers, Moscow, 1977, p.375</ref> 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.

[[File:Phases in PAL.jpg|frame|1.Original phase, 2.Phase in the first frame, 3.Phase in the second frame, 4.Vectorial addition, 5.Original phase reconstructed (with reduced amplitude)]]

The mathematics involved is as follows:

Let '''<math>\theta</math>''' be the phase difference of the color signal with respect to colorburst and '''<math>\beta</math>''' be  the extra shift introduced by DP.

The original signal is  

: '''<math>\mbox{f(t)} =sin(\omega t+\theta)</math>'''
If there is a DP distortion, the received signal for the first frame is

: '''<math>\mbox{f(t)} =sin(\omega t+\theta+\beta)  </math>'''

In the second frame (after multiplying by -1)

:''' <math>\mbox{f(t)} =sin(\omega t+\theta-\beta)  </math>'''

The average is

: <math>\mbox{f(t)} =\frac{1}{2}(\sin(\omega t+\theta+\beta) +\sin(\omega t+\theta-\beta))= \sin(\omega t+\theta)\cdot\cos(\beta) </math>

So while the effect of '''<math>\beta</math>''' diminishes on the color hue, the amplitude of the color signal is reduced by  '''<math>cos(\beta) </math>''' which means that color saturatioın is reduced.

== See also ==
*[[Differential gain]]
{{Analogue TV transmitter topics}}

== References ==
<references/>

{{DEFAULTSORT:Differential Phase}}
[[Category:Broadcasting]]
[[Category:Television technology]]