The system determines the base frequency of a speech signal which has been filtered so that the base frequency is no longer present. The signal is delayed by several successive time intervals and each time delayed signal compared with the momentary value of the input signal. The delay interval of the first delayed signal to differ from the input signal by less than a given amount is used as the period of the required base frequency. - Pref. the system uses successive delay stages, the output of each of which is supplied in turn to a comparator (7) via switches (5), controlled by a clock generator (4). The comparator (7) is also coupled to the input (8) for the speech signal and causes a counter (6) indexed by the clock generator pulses to be halted when the difference between one of the delay stage outputs and the input signal is below a given threshold.
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