Pushbutton Calling with a Two-Group Voice-Frequency Code

In recent years, customer signaling from a telephone set by means of pushbuttons has appeared increasingly promising. An early plan has been reported 1 in which the operation of a pushbutton produces four effects: (a) the generation of a damped oscillatory wave at one of ten digit-identifying frequencies within the voice range; (b) the generation of a similar damped wave at one of eight party-identifying frequencies, also in the voice range; (c) a stepwise reduction of the direct current drawn by the set and (d) the temporary disablement of the speech transmitter. Actions (c) and (d) provide protection against talk-off, i.e., false signaling due to speech or noise at the transmitter. This plan contemplated only the transmission of information to the central office, but in the long run it would be advantageous to be able to signal "end-to-end," over any established connection that will transmit speech. With this added objective several new considerations are introduced. Clearly, the signals should not contain an out-of-band component such as the dc step. Again, there may be a need for more than ten distinct signals, and these ma}' be difficult to provide in a scheme based on a one-out-of-Ar code, for a new frequency must be added for cach new signal. On the other hand, party identification would not be a feature of end-to-end signaling. Lastly, sustained rather than damped signals are strongly preferred for end-to-end signaling, in order to main235