Apparatus for distinguishing between speech and certain other signals

Abstract

Apparatus for distinguishing betwen speech and, for example, data and noise signals, for example for use in a non-speech squelch circuit in a broadcast receiver, utilises the fact that an autocorrelogram of a speech signal exhibits peaks which wander backwards and forwards along the delay axis, this not being the case for autocorrelograms of most other signals. Autocorrelograms of an input signal applied to an input terminal (1) and hard-limited by means of a comparator (5) are formed under the control of a computer (14) by means of a shift register (3), a multiplexer (10) an inverting EXOR-gate (7), an up/down counter (19) and a memory (41). For each clock pulse fed to the clock input (30) of the shift register the output (11) of the multiplexer, and hence one input of the EXOR-gate, is switched to each of a succession of taps (8) on the shift register. Thus the signal on the first tap (6) of the register is multiplied in succession by the signals on the other taps (8) and the results are used to increment or decrement corresponding quantities in the memory (41) by means of the up/down counter (19). Autocorrelograms relating to successive periods of time are formed in this way and the computer determines the positions of the peaks (if any) in each and by comparison of these positions determines whether they are stationary or wandering.