Selection of Call Progress Tones for Digital Systems

The precise tone system based on the four basic frequencies 350, 440, 480, and 620 Hz is extensively used in switching systems to indicate readiness to dial, ringing of the called subscriber, lines or trunks busy, and other conditions. In digital switching systems it is normally advantageous to generate and process these tones in a digital form. It is shown that by judicious selection of the tone frequencies, considerable simplification of digital tone generators may be achieved. Modification of the precise tone frequencies within ± 0.5 percent reduces the number of samples which must be stored in ROM's from 1650 to 326. Another set of frequencies is given which requires only 228 samples and simplifies the tone generator readout logic.