Method for distinguishing SECAM colour television signals

摘要

In a multi-system colour television receiver, SECAM colour television signals are distinguished by means of a discriminator which initiates the automatic switchover to SECAM mode if a SECAM signal is unambiguously present. The colour subcarrier is extracted by means of a gate circuit (8) from the back porch of each horizontal synchronisation pulse occurring. If the signal is a SECAM system signal, the extracted colour burst signal alternately has the frequency of 4.25 MHz and 4.40625 MHz with each line passing through. A band-pass filter (16), the pass band of which has a centre frequency which corresponds to a harmonic frequency of a colour subcarrier of the SECAM system, outputs an output signal to a frequency discriminator (18) which then outputs an output signal to indicate that the SECAM format is present when the filtered burst signal occurs with half the line frequency of the SECAM system. <IMAGE>