On the recognition of audible tones and speech on a telephone line

The paper deals with different ways to discriminate audible tones and speech on a telephone line. Comparing the amplitudes and measuring the zero-crossing distances of successive half periods turn out to be suitable. A series of measurements has been made to compare the two methods. The influence of tolerance, of analog band-limitation, and of the amplitude of superposed speech on the recognition of audible tones is examined. The results show that the evaluation of zero- crossing distances is the most appropriate method.