His DTMF decoder box uses a CM8870 DTMF decoder chip, which you might assume is all you need to get the job done.

Winston-Salem, NC&#8212CML Microcircuits' CMX867A, a V.22 multi-standard modem IC for telephone-based information systems, includes an enhanced DTMF decoder, thus affording a more compact design and ...

Hence he’s taken a commodity joypad and interfaced a MT8870 DTMF decoder to its switch lines with a simple transistor buffer, and is able to pull the resulting information out in the host ...