Fascinating article. See also https://web.archive.org/web/20120314023131if_/http://www.alcatel-lucent.com/bstj/vol39-1960/articles/bstj39-1-235.pdf https://web

Fascinating article. See also https://web.archive.org/web/20120314023131if_/http://www.alcatel-lucent.com/bstj/vol39-1960/articles/bstj39-1-235.pdf https://web.archive.org/web/20120314023131if_/http://www.alc... for an early technical article describing the implementation details of the familiar DTMF "touch tone" dialing system, noting that the precise details differ from the final implementation — in particular, the high group frequencies increased from 1,094/1,209/1,336/1,477 Hz to 1,209/1,336/1,477/1,633 Hz, possibly to mitigate the "pulling" effect described on pp. 251–252 (though I can find no reference for the rationale).