dtmf circuit

Posted on Feb 5, 2014

This article describes how to build a general purpose DTMF decoder with a low-price chip from MITEL. The circuit supports DTMF squelch based on 3 digits station ID (full 999 combination). It also supports decoding of 4 additional commands - station ID plus one more digit which can be used to perform remote control on external devices (on/off or si

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ngle shoot trigger operation ). One expansion port is included to provide future development. Core of the whole equipment is the MT8870 DTMF decoder. This is a state of art single chip DTMF receiver incorporating "switched capacitor filter" technology and advanced digital counting / averaging algorithm for period measurement. Conventional method of frequency decoding usually based on frequency counting or phase locking technique. These methods are time consuming and inaccurate when handling unsteady complex signal. Switched capacitor filter technology based on a completely different approach. It works on the theory that any A. C. signal are rapid changes of potential energy with repect to specific time factor. A. C. components, provided that they bear the same frequency characteristic, will have the same timing on instantaneous potential variation. For example, a signal of frequency 1 Hz will experience peak positive value every one second. - the duration of 1 sec. will be constant among any A. C. signal bearing frequency 1 Hz, even though they have different wave forms. Suppose a circuit is designed to sample the potential value of a signal at preset intervals ( say, at exactly 90 degree phase shift timing for a specific frequency ). If the total sum of all samples always experience zero while signal really present. We can conclude that an A. C. component of that specific frequency has been identified. Each fixed timing...

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