Digital Signal Processor: Receiver for Touch-Tone Service

This paper describes the design of a single-package, all-digital receiver for TOUCH-TONE® service implemented by using a digital signal processor (DSP) integrated circuit.1 The receiver is particularly suited for systems that operate on signals that have been encoded into a digital pulse-code-modulation (PCM) format. TOUCH-TONE service is a voice frequency signaling system in which any one of 16 digits may be transmitted by simultaneously sending two tones. The frequency of one of the tones may be either 697, 770, 852, or 941 Hz (called the low group) and the frequency of the other tone may be either 1209, 1336, 1477, or 1633 Hz (called the high group). The receiver must tolerate frequency shifts in the transmitter, operate over a wide dynamic range in the presence of noise, and be insensitive to speech (digit simulation).2 The program contained in the DSP was designed to emulate the signal processing functions of a central office grade receiver. Measurements of performance confirm the equivalency. 1573