Digital tone detector

Abstract

Individual tones in a tone burst of limited duration are discriminated by repeatedly counting clock pulses during a definite selected number of tone cycles to produce a count summation. The selected number of cycles in each summation is so chosen that a clock frequency of relatively small value will produce a large enough count to insure clear discrimination between the input tones to be distinguished. At the same time, the invention provides a large enough number of distinct count summations to permit rigorous validation of each received tone. The available number of count summations is greatly increased by permitting successive summations to include several tone cycles in common. In particular, for decoding multi-frequency codes of the type generated by conventional touch dialing telephone equipment, the high and low frequency tone groups are typically detected separately by counting clock pulses of about 10 kHz during at least four successive cycles of the received tone, making such a count summation available throughout the tone burst every cycle after the first three. Each summation is tested for validity, as by comparing it with its predecessor and also with the expected count ranges for the tones of the group, producing condition signals which control the later decoding of the counts. The detection procedure combines high reliability with the possibility of using a clock frequency low enough to facilitate multiplexing of the equipment.