DTMF tones are easy to decode — at least now. We’ve got DSP chips to do the job all in one package, or we could roll a simple circuit from phase-locked loops. But how did the phone company do ...

[Rogal] wrote a cell phone application called ToneTool that generates audio tone sequences. It can be used to output DTMF and SelCall sequences which are used by telephone systems and radio ...

The dialing technology in an analog telephone. Each digit has a physical button that transmits a different audio tone when depressed. Known as the dual-tone multifrequency (DTMF) system ...