Differential Phase in Colour Television Systems and its Correction in an Ntsc Receiver

摘要

In colour television systems, the colour information is carried by a chrominance carrier whose amplitude represents the saturation and phase, the hue. Any change in the phase of the subcarrier will result in change in the hue of the picture. In NTSC system, various phase correction schemes are available which improve the overall hue of the received picture. However, they do not correct the differential phase, which is an error in the sub-carrier phase introduced by the changes in the luminance level. The PAL system self corrects the hue errors as well as the differential phase with added complexity. Differential phase is introduced in the television transmission chain due to various causes such as non-linear behaviour of active devices, by distribution systems where the signal is transmitted via two or more channels, and as a result of incidental phase modulation at the RF stages in the transmitters etc. This paper makes a study of all the causes of differential phase. A new modified NTSC coding and decoding scheme is suggested, where, a reference is injected in the video signal to detect the amount of differential phase undergone by the chrominance signal. A correction scheme which corrects the differential phase using the information derived from the reference is also suggested.