Abstract: A hybrid circuit includes a capacitive charge-transfer arrangement, comprising parallel capacitors and series switches,for coupling signals in one or both directions between a four-wire line and a two-wire line. The switches are constituted by VMOS power FETs. In one embodiment, the charge-transfer arrangement is provided in the receive path to give the desired properties of electrical isolation and unbalanced to balanced conversion of receive path signals. Signals from the two-wire line are coupled to a transmit path of the four-wire line via an attenuating and high-pass filtering arrangement and a differential amplifier, thereby providing good common mode rejection of high voltage signals, especially at power line frequencies. In another embodiment, the charge-transfer arrangement is provided, commonly for the receive and transmit paths, for coupling the two-wire line to the remainder of the hybrid circuit.

Abstract: A tone detector in the form of a digital correlator is disclosed in which PCM signal samples are delayed in a RAM for the correlation period. Using PROMs, the products of the current and delayed samples with samples of a sine signal at the frequency to be detected are produced, and their difference is cumulatively summed to produce a first accumulated signal. A second accumulated signal is similarly produced using samples of a cosine signal at the frequency to be detected, and an output signal indicating the presence or absence of the tone is produced from the sum of the squares of the two accumulated signals. The use of memories enables the detector to be readily multiplexed for detection of a plurality of tones on each of a plurality of channels.