System for the secret transmission of audio signals and television receiver for receiving such signals

An encoding arrangement for the secret transmission of television audio signals comprising in a cascade arrangement from an input terminal:

• a first filter,
• a first modulator coupled to a first fixed-frequency modulation signal generator,
• a second filter,
• a second modulator coupled via a first change-over switch to out outputs of a first dual-frequency modulation signal generator supplying at these two outputs two modulation signals having two different frequencies, said change-over switch being controlled by a first pseudo-random generator for selecting one or the other of these two modulation signals and
• a third filter, characterized in that the first dual frequency modulation signal generator supplying two signals comprises two phase-locked loop oscillators each comprising a frequency divider in the loop and both being controlled by the same reference signal generator, in that this reference signal generator itself comprises an oscillator controlled by the field synchronizing signal of the television transmitter, and in that this signal as well as the reference signal are applied to a synchronizing circuit of the first pseudo-random generator for controlling the instants of change of the modulation signal.