- A telephone signaling system which utilizes tones which are a mixture of two frequencies. Each mixture identifies any of the 12 dialed keys, 0--9, *, and #, as found on most touch-tone phones. The tone heard when a number 5 is pressed on such a phone, for instance, is a combination of two such frequencies, specifically 770 Hz and 1336 Hz.
- A communications method utilizing multi-frequency signaling (1).
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