Nippon Telegraph and Telephone Public Corporation

Victor Company of Japan, Limited

Kunihiro Kato

Yukinobu Ishigaki

Teruo Muraoka

Makoto Yoshitoshi

Ryoichi Matsuda

Tatsuhiro Naganawa

Ryoichi Okayasu

Masaki Hagihara

Takeo Nasu

A three-terminal power supply circuit for telephone set has an input terminal, an output terminal and a common terminal. The input terminal receives a DC voltage superposed with a sending or receiving signal transmitted to or from a telephone station through a subscriber line. The output terminal delivers DC power to a telephone circuit. Between the input and output terminals a constant current circuit and a discharge protection circuit are connected to each other in series. Between the output terminal and the common terminal a constant voltage circuit is connected. Between the input terminal and the common terminal an electronic switch is connected through the constant current circuit. The electronic switch is closed only when an instantaneous voltage applied between the input terminal and the common terminal is lowered to a predetermined value or a lower value. When it is closed, the electronic switch causes a short circuit in the constant voltage circuit through discharge protection circuit. This lowers the cutoff voltage applied on the input side of the power supply circuit, and both a sending signal and a receiving signal can be transmitted without being distorted.

Three-terminal power supply circuit for telephone set

A phase tracked loop frequency demodulator having a phase comparator receptive of a frequency modulated signal and a peak-shifting filter for imparting a constant phase shift of 90.degree. over the full range of its instantaneous frequencies in response to a feedback control signal to permit the phase comparator to compare the non-phase shifted and phase shifted signals. The carrier frequency components of the output from the comparator are eliminated by means of a second filter to derive a demodulated signal which is used as the control signal. In order to eliminate high frequency limitations of the peak-shifting filter, this filter is comprised of a variable reactance element and a complementary reactance element so connected thereto to form a resonant circuit which possesses a quadratic frequency response characteristic with a resonant peak at the upper or lower frequency end of the passband.

Phase tracked loop frequency demodulator

In the loudspeaking telephone circuit, the speech signal is used as a control signal for detecting the extent of decreasing the acoustic coupling between the loudspeaker and the microphone of the telephone set and of the coupling between the transmitting and receiving circuits of an anti-sidetone circuit from an expected worst condition for which the telephone circuit has been designed thereby automatically decreasing the value of the variable loss utilized for preventing howling by an amount corresponding to said decrease. This circuit provides satisfactory speech communication with a smaller loss inserted than a prior art voice switch thus greatly improving unnatural sound as caused by the voice switch.

Loudspeaking telephone circuit

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Yokohama, Japan

Kunihiro Kato