Pairs of voice-frequency tones activated by pushbuttons are used for transmitting digital information from telephone stations to central offices. The signal receiver may be exposed to speech during interdigital periods. To minimize the problem of digit simulation by speech, the signal frequencies were chosen to avoid harmonic relationships, and the receiver employs several tools to reduce the problem further. Comprehensive field trial results indicate that this system will operate satisfactorily over all types of standard customer lines.
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