A high efficiency speech coding apparatus which can eliminate repetitions of high efficiency speech coding and decoding processes of a digital speech signal upon inter-office transit switching without employing a special exchange and a transit switching system which can prevent degradation in quality of speech information and increase in encode and decode processing delay upon transit switching are disclosed. The apparatus repetitively inserts a synchronizing signal into an output speech signal to an exchange and supervises an input speech signal from the exchange to detect a synchronizing signal in order to determine whether the destination of connection of the exchange is a subscriber line or another trunk circuit connected by way of another high efficiency coding apparatus. When a subscriber line is connected, code conversion and inverse code conversion between a high efficiency coded speech signal and a speech signal for the subscriber line are performed using a coding section and a decoding section, respectively. When another trunk circuit is connected, an invalid signal to raise the bit rate of the speech signal to allow transparent transmission through the exchange is added and removed bypassing the coding section and the decoding section.

An automatic linear compression circuit for a telephone "handsfree" loudspeaker is provided. The circuit consists of a variable attenuator the level of attenuation of which is controlled by the input signal level. The input signal is passed through a positive peak detector to obtain a time averaged signal corresponding to the average positive peak values of the input signal and this time averaged signal is compared against a time increasing reference voltage. As long as the reference voltage is below the value of the time averaged signal a positive attenuator control signal is derived. This positive control signal keeps the attenuator portion of the circuit operative. In the attenuator the input voltage is sampled to a first capacitor. The first capacitor is subsequently connected to and disconnected from a second capacitor a number of times as long as the positive attenuator control signal is present. Each time this is done the voltage on the first capacitor is reduced by a factor depending on the capacitances of the two capacitors. After a predetermined time the voltage remaining on the first capacitor is passed through buffers and another capacitor to provide an attenuated output. The circuit is simple and flexible, permitting wide ranges in the operating parameters to be achieved simply by including one or more variable capacitors.

An analog echo suppression circuit is used with a telephony 2-wire circuit to 4-wire circuit interface. The 2-wire circuit carries a first signal and the 4-wire circuit carries a second signal. Each circuit has an input port and an output port. The interface produces an echo signal on the output of the 2-wire circuit when the second signal is received at the input port of the 2-wire circuit. A first communication channel connects the output of the 4-wire circuit to the input of the 2-wire circuit. A second communication channel connects the output of the 2-wire circuit to the input of the 4-wire circuit and series analog switches or transmission gates can interrupt the second channel. A comparator circuit receives the first and second signals from the first and second channels and has first and second outputs. First and second controller devices are operatively connected to the comparator circuit and provide signals such that, if an echo signal is present, the one analog switch interrupts the second channel and another analog switch grounds the input to the 4-wire circuit.

Known is a telecommunications system comprising a near end and a far end subscriber unit communicating with each other via a transmission channel, at least the near end subscriber being arranged for handsfree communication. In the known system, the near end subscriber unit comprises a duplex controller controlling the near end subscriber unit to three stable modes, a transmit mode, a receive mode, and an idle mode. The known subsciber unit operates in half-duplex mode and has speech and noise detectors for the transmit and receive paths, and a speech level comparator controlling the mode switching. At mode switching, suitable gain adaptation is done in the transmit and receive path so as to prevent howling in handsfree mode. The known duplex controller does not operate satisfactorily if the near end speaker is at a relatively large distance from the microphone. A telecommunications system is provided that satisfactorily operates when the near end speaker is a relatively large distance from the microphone. Time constants of the system are influenced when the far end speaker stops talking and the near end speaker takes over such that no speech loss occurs at the far end.

A ballast circuit includes an output isolation transformer having a primary winding and first and second secondary terminals coupled to opposing ballast lamp terminals for additively applying potentials on the primary winding and the first and second secondary winding potentials across the lamp and limiting ground fault voltages. The circuit can include a closed loop feedback path from a load to a feedback rectifier for promoting linear operation of an input rectifier.