A Touch-Tone Receiver-Generator With Digital Channel Filters

In Ref. 1 we describe a family of filters with several advantages over existing filters, which can be used to generate and detect single tones. Here, we describe how such filters can be used in the construction of a Touch-Tone® receiver. The standard Touch-Tone receiver is described in Ref. 2; many other receivers have been proposed in the literature; one which is completely digital is described in Refs. 3 and 4, and an analog receiver with a digitally controlled center frequency is described in Ref. 5. The basic Touch-Tone telephone must generate tones to identify the ten basic possible inputs (1, 2, · · ·, 9, 0) or, in the case of augmented telephones, 12 to 16 possible inputs (including, for example, * and # ) . This is done by arranging the input buttons in a grid of four rows and three or four columns. Associated with each row is one of four "low" frequencies (697, 770, 852, or 941 Hz), and associated with each column is one of three or four high frequencies (1209, 1336, 1477, or 1633 Hz). When a button is pushed, one low and one high frequency are simultaneously generated, corresponding to the row and column in which the button is situated. In the central office, a detector decodes the incoming pair of tones to determine which button was pushed. 455