Abstract: Disclosed is a PCM coder-decoder having a construction such that a digital filter contained originally in the PCM coder-decoder is utilized on the time division basis in order to fold back a digital reception signal to a digital signal transmission side and thus to accomplish interruption, communication exchange between three parties, gain control, fold-over test of the PCM signal, and so forth, in addition to the coding and decoding functions inherent to the PCM coder-decoder.

Abstract: In a PCM encoder, in order to reduce noise in an idle channel or in the absence of any voice signal, a detector circuit is provided which detects the idle channel, and a circuit is connected to the detector circuit which fixes the polarity bit of the PCM signal produced by the PCM encoder when the detector circuit has detected the idle channel.

Abstract: A PCM encoder for converting a voice signal into a eight-bit code by approximating the .mu.-low characteristic where .mu.=255 with 15 segments comprises a capacitor array circuit including eight capacitors for determining lowermost voltages of the segments, a resistor string circuit for producing step voltage in each of the segments, a comparator circuit for comparing the output voltage of the capacitor array circuit with a reference voltage, and a successive approximation register circuit for controlling switch groups provided in the capacitor array circuit and the resistor string circuit. The resistor string circuit is provided with taps for deriving voltages corresponding to (2n-1)/33 (where n=1-16) of a voltage applied across the resistor string. A PCM encoder which conforms to the .mu.-low with high fidelity and is capable of quantizing mid-tread at the first segment is disclosed.

Abstract: A PCM decoder for converting to an analog voice signal an 8-bit PCM signal the first bit of which is a polarity specifying bit, the PCM decoder comprising a capacitor array having binary-weighted capacitors and a resistor string circuit having plural resistors for dividing a reference voltage to obtain different tap voltages, wherein the tap voltages corresponding to the four lower bits of the PCM signal are derived from the resistor string circuit and the combination of the reference voltage and each of the tap voltages, made according to the contents of the second, third and fourth bits of the PCM signal is applied to the corresponding one of the capacitors in the capacitor array circuit whereby the capacitor array circuit delivers an analog voltage signal corresponding to the received signal, the resistor string circuit having two groups of intermediate taps so that the conversion characteristic for obtaining voltages in the signalling frame may be different from that in the non-signalling frame.

Abstract: In a reference voltage generation circuit consisting of a capacitor, a reference voltage source for generating a predetermined voltage for charging the capacitor and a voltage polarity switching circuit for selectively inverting or not inverting the charged voltage of the capacitor to produce an output voltage at an output terminal, and capable of being fabricated in a semiconductor integrated circuit, the improvement wherein the voltage polarity switching circuit includes a second capacitor for compensating for an error between positive and negative reference voltages due to the stray capacitors.

Abstract: A non-uniform weighting circuitry which is effective for enhancing speed and accuracy in the operations of encoders and decoders comprises in a cascade connection a constant current switch, a variable attenuator, a polarity changing circuit and a uniform weighting circuit. An impedance converting means is provided to make at least one of input and output terminals of the variable attenuator to be of low impedance, whereby spike-like noises generated by a switching element constituting a part of the variable attenuator is reduced.