GB1510662A - Decoder for the secam system - Google Patents

Abstract

1510662 Colour television THOMSON-CSF 28 Aug 1975 [30 Aug 1974] 35621/75 Heading H4F S.E.C.A.M. decoder comprises channels 1 and 2 for the direct and one line period delayed colour subcarrier, each channel including a limiter 3, 4, a frequency discriminator 5, 6, a low-pass filter 7, 8, a clamping device 9, 10 and a sampling and holding device 13, 14. The clamping device and sampling and holding device in each channel are operated alternately during successive horizontal blanking back porch intervals, and the outputs from the sampling and holding devices compared 15, 16 with a reference signal V o , -V o whose absolute value corresponds to the difference between the nominal levels of the signals delivered by the frequency demodulators 3, 5 and 4, 6 for the two different resting frequencies F R and F B used for the alternate transmission of the red and blue chrominance components. The resulting outputs of the comparators, e.g. differential amplifiers 15, 16 are applied as gain control signals to the frequency discriminators 5, 6. The outputs of clamping devices 9 and 10 are fed to a double switch 20 operating at half line frequency which directs the red and blue chrominance components via respective de-emphasis filters 11 and 12 to a matrix 21 which forms the red, green and blue chrominance components. The switching, clamping and sampling pulses are derived from the trailing edges of the horizontal sync. pulses L by means of bi-stable 25, monostables 22, 23 and switch 24, the phase of the bi-stable 25 being controlled by the output of a differential amplifier (polarity discriminator) 28 receiving the output of sampling and holding device 13. The reference signals fed to differential amplifiers 15 and 16 may have the reverse polarity, as shown, or the same polarity according to whether the clamping devices and sampling and holding devices in the two channels are operated in phase or in phase opposition.