Abstract: The invention provides an audio subsystem for use in a telephone terminal operable in handsfree mode and comprises a receive path including a receive transducer for generating audio signals from a received signal and a transmit path including a transmit transducer for converting audio signals to electrical signals for transmission to a far-end user. An acoustic echo canceller is communicatively coupled between the receive path and the transmit path and includes a subtractor circuit connected serially with the transmit transducer. The echo canceller is responsive to control signals for controlling the subtractor circuit. The subsystem comprises means for detecting a near-end talking condition including first circuit means for measuring the level of the total energy of the out-of-telephony-band components in the signal from the transmit transducer and circuit means for comparing the energy level represented by an output signal from the first measurement circuit to a first predetermined threshold.

Abstract: A circuit arrangement is provided for connecting a personal computer (PC) to a port of a key telephone system via an adapter circuit which also interfaces a digital key set to the port. The adapter circuit is connected to the computer via an RS-232 single channel data link. The PC is programmed to generate and receive a serial data stream into which are encapsulated messages relating to link control and telephone functionality. The adapter circuit is also programmed to receive and generate the serial data stream. The data rate through the adapter circuit and the telephone system and hence to a remote computer is dependent on the utilization of the attached key set by the user to make a call.

Abstract: The idle channel for a speech encoder is detected by the detection, at the encoder comparator output, of zero-level traversals caused by noise transient signals appearing at the analog input of the encoder. In response to the detection of an idle channel, the encoder is caused to output a PCM code sample corresponding to a zero amplitude analog signal. In order that a speech signal passing through zero amplitude during the sampling period not be confused with an idle channel condition, the detection circuit responds only to a plurality of zero-level traversals.

Abstract: A telephone line circuit for supplying energizing current to a telephone line and for providing a voice frequency connection between the telephone line and an associated switching facility includes a transformer having a split primary winding, consisting of n turns, for connection across the telephone line and for connection to a power supply or battery. A first current flows in the split primary winding in proportion to the telephone line load characteristic. A balance winding in the transformer is connected in series with one portion of the split primary winding. The balance winding is counter-wound relative to the split primary winding and consists of more than n turns. A circuit including a current regulator causes sufficient balance current to flow in the balance winding to substantially cancel the d-c flux associated with direct current in the split primary winding.

Abstract: A digital tone signal generator supplies PCM sample sequences of various signalling tones to TDM channels in an associated switching facility to accommodate signalling requirements. The phase of any one tone supplied to various channels is essentially different to minimize crosstalk in telephone network transmission facilities. The tone signal generator includes an address sequencer for generating a repetitive series of addresses. Each repetition preferably includes more addresses than channel occurrences per frame, the number of addresses corresponding to a dividend yielding other than a whole number quotient with a divisor corresponding to the number of samples in the tone sample sequence and integer submultiples of same other than 1. A memory responsive to the addresses includes the samples of the tone sample sequence stored at address accessible locations.