Dual-tone multiple-frequency-signal generating apparatus

Abstract

A dual-tone multiple-frequency (DTMF)-signal generating apparatus of the present invention is provided with a high-group frequency divider and a low-group frequency divider for dividing a reference frequency signal into high-group frequencies and low-group frequencies, respectively. The respective outputs from the high-group divider and the low-group divider are supplied to a high-group-sine-wave-signal generator and a low-group-sine-wave-signal generator. The high- and low-group-sine-wave-signal generators comprise a plurality of bi-stable circuits and include shift registers for sequentially shifting a divided input signal within a predetermined period, a plurality of switches which selectively pass high or low level signal corresponding to the outputs of the respective bi-stable circuits, and a plurality of capacitive elements in which the terminals on one side thereof are supplied with the high or low level passed by the plurality of switches, and the terminals on the other side thereof are commonly connected, the high or low level being applied to the comon connection for every cycle of the shift registers to produce the high- and low-group-sine-wave signals from the common connection. The respective sine wave signals are level converting by a high-group level converter comprising a capacitor and a switch and by a low-group level converter comprising a capacitor and a switch. The level converted high- and low-group-sine-wave signals are combined by a mixer comprising buffer amplifiers, resistors and a transistor, thereby obtaining a DTMF signal.