An evaluation of two simple methods for detecting tones over telephone lines

An important practical application of signal processing theory is the problem of complex tone detection. Within the telephone plant there often arises a need for a simple, yet efficient, method for detecting the various tones which are used in telephone communication. Two such methods are discussed in this paper. One method uses measurements of the short-time signal energy and makes the decision as to whether or not there is a particular tone present on the line based on the periodicity of the envelope of the signal. This method has application in determining if the energy on the line is periodic or aperiodic where sample examination time is not limited. The second method uses measurements of the short-time zero crossings of the signal. A parallel processing scheme is used to determine if a particular tone is present based on the detailed statistical properties of each of the tones. This method has application in determining if specific frequencies are present, especially when the examination time of the sample is limited. Using a large number of dialed-up connections, both systems were evaluated as to accuracy and speed. Results are presented which show the properties of the two tone detection methods.