| 摘要 |
A converter, for converting a dual-tone multi-frequency (DTMF) signal produced by a TOUCH-TONE telephone to a 2-out-of-7 coded representation of the "dialed" number, includes an oscillator having a frequency much higher than the frequencies of the tones, a high band circuit for high band tone frequencies, and a low band circuit for low band tone frequencies. Each circuit includes a filter for passing the respective band of tone frequencies from an input terminal, a limiter for translating a received tone sine wave passed by the filter to a corresponding square wave, a counter, a switch enabled by the square wave output of the limiter for coupling the output of the oscillator to the input of the counter during a cycle of the square wave from the limiter, and a decoder for translating the resulting count in the counter to a signal on an output line representing one of the tone frequencies.
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