Time compression receiver for detecting the signalling tones in a DTMF (dual tone multi-frequency) format present on eight receiver input lines. A multiplexer repeatedly samples the input signals present on the eight input lines. These analog samples are converted to digital samples and stored in a memory. Stored samples are read out for each input line in order while new samples are being written into the memory. Samples are read out at twice the rate they are written in. The samples are converted to analog signals and passed through a low pass filter. The frequencies present in the resulting signal are speeded up by a factor of sixteen over the frequencies of the signalling tones present in the input signal from which it was derived. The signal frequencies are passed through reject filters, comparator and limiter circuits, a bank of tone filters, and level detectors to check for each possible resulting frequency and to produce an indication of the presence or absence of each.

A half-duplex digitally-controlled telephone communication apparatus for permitting half-duplex communication between local and remote subscribers. Analog speech of one subscriber to be transmitted to the other subscriber is converted to a digital format, processed by real-time speech processing apparatus, stored in a delay buffer, and processed in modem apparatus for transmission to the other subscriber. In accordance with the present invention, the delay buffer provides a delay of a sufficient duration to compensate for inherent delays in the system in detecting the presence (or absence) of speech signals and serves to prevent the loss to the remote subscriber of portions of speech of the local subscriber, especially the onset of speech or rapid utterances of the local subscriber. The apparatus of the invention further includes apparatus for preventing the transfer of the communication channel or line from the local subscriber to the remote subscriber during normal pauses in the speech of the local subscriber.

A system for suppressing positive feedback in acoustical-electrical devices having acoustic or electric feedback paths or a combination of both. Electrical signals from a microphone are transmitted through a sampling switch to a signal storage device. The transfer rate and duration are controlled by a pulse generator driven by an oscillator to time domain process the signals. The discrete signals output from the signal storage device are smoothed by a low pass filter and used to drive follow on circuitry, e.g., a speaker. A plurality of switches, signal storage devices, oscillators and pulse generators can be arranged in serial or parallel to provide various processing combinations. Several embodiments of the system are employed in a bidirectional telephone unit having separate incoming and outgoing signal channels to enable full duplex mode operation. In one embodiment, the incoming signals only are time domain processed and used to generate signals for controlling a voltage control gain block in the outgoing channel. In another embodiment, both the incoming and outgoing signals are time domain processed. In another embodiment, both the incoming and outgoing signals are time domain processed and voltage gain controlled by the signals in the opposite channel.

A system for control of speech direction in duplex telephone circuits (e.g. hands-free telephones), derives a switching control signal proportional to both the immediate transmit-channel signal value, and a Regulating Value which is the ratio of the Receive-Channel Signal to the Transmit-Channel Signal under no-local-sound-source condition. Thus the Regulating Value is proportional to the local acoustic feedaround (Overhearing) signal between local loudspeaker to microphone. The Regulating Value responds to faster variations (caused by speech) to cause direction-switching, but merely compensates for slower variations (caused by movement of people or microphone in the local room). Switching (Breakthrough) of the microphone to On and the loudspeaker to Off also depends on the magnitude of the closed-channel (non-connected) microphone signal exceeding that of the open channel.

A maximum value tracing circuit employed in echo suppressors and the like for selecting the peak values of digitized voice signals. The digitized value of a signal previously stored in a register is continuously compared with the present digitized value. The larger value of the compared signals is stored in the register after application of each clear signal to the register. First and second maximum value tracing circuits of the type described may be employed in combination with a comparator-selector circuit for comparing the outputs of the first and second maximum value extraction circuits to produce a comparison output signal representative of the larger of the two compared outputs derived from the first and second maximum value extraction circuits, which output signal is utiized for providing reliable, high speed echo suppression.