. . "This circuit functions in the following way: the head-end modem transmits, for example, between the protocol word frames, a series of identifiable data, for example 101010 . . . etc, whose bit frequency is such that after demodulation in the demodulator 27 they generate a signal having a frequency of 19.2 kHz, which signal is compared in the comparator 22 with the local signal generated at the out" . .