Pushbutton calling with a two-group voice-frequency code

@article{Schenker1960PushbuttonCW,
  title={Pushbutton calling with a two-group voice-frequency code},
  author={Leo Schenker},
  journal={Bell System Technical Journal},
  year={1960},
  volume={39},
  pages={235-255}
}
  • L. Schenker
  • Published 1960
  • Physics
  • Bell System Technical Journal
A customer voice-frequency pushbutton signaling scheme involving a new two-group signal code, termed “four-by-four”, is described. It is shown that the use of this code, with judiciously chosen frequencies, permits the detection of bona fide signals after transmission over any ordinary voice connection and facilitates discrimination against false signals resulting from speech. Apparatus for generating the tones is described, and the principles of reception are discussed. 
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