Abstract: A speech communication apparatus of the present invention includes, in addition to an echo canceller for canceling an acoustic echo generated in a hands-free speech space, a chirp signal generating unit and a training unit. The chirp signal generating unit generates a chirp signal adequate for initial training of the echo canceller. The training control unit enables the chirp signal generating unit to generate a chirp signal, when a predetermined condition for starting hands-free speaking is satisfied, and a chirp tone corresponding to the chirp signal to be output as a volume-amplified tone from the hands-free speaker. The echo canceller performs initial training of the echo canceller based on the chirp tone.

Abstract: Sidetone in a four-wire telephone station set that is coupled to a two-wire telephone line through a four-to-two wire hybrid circuit is automatically and cancel the phase "to a predetermined value", set by incrementally adjusting a balancing impedance connected to the circuit until an optimum impedance match between the telephone set and line is achieved. Under software control, during idle intervals between dialed digits when a call is initiated, a microcontroller mutes the telephone transmitter and receiver on the telephone side of the circuit and substitutes for these components a tone signal generator to produce a sidetone signal that is applied to a full-wave rectifier and therefrom to a peak voltage detector to generate a sidetone envelope. Individual ones of several impedance balancing networks are then sequentially connected to the telephone side of the circuit and the sidetone envelope for each balancing impedance is digitally encoded and stored.

Abstract: A telephone set includes an anti-sidetone circuit formed by a feedback loop having a filter in the feedback path. The feedback loop output is substantially equal to the signal produced at the set connection terminals by the microphone. Preferably the filter is composed of two sub-filters connected in cascade, formed of switched capacitors in an integrated circuit. An EEPROM control circuit can be included to permit easy programming during manufacture, to match a selected transmission line impedance.

Abstract: An acoustic echo canceler and a side-tone echo canceler provided in a hands-free telephone suppress an acoustic echo and a side-tone echo respectively with few error by providing automatic gain controllers and/or limiters in the hands-free telephone so that the acoustic echo canceler and the side-tone echo canceler operate in linear, and a directional characteristic of a microphone used in the hands-free telephone for reducing the acoustic echo is controlled by a microphone direction controller so that the microphone operates as an omnidirectional microphone when a level of a received signal of the hands-free telephone is less than a designated level and as a bidirectional microphone when the level exceeds the designated level, further gains of an output of the microphone increases in a low frequency range.

Abstract: An adaptive echo cancellation apparatus disclosed herein has three channels and a channel selection circuit. The first channel serves to block an outgoing digital signal from flowing to a transceiver. The second channel is used to directly couple the outgoing digital signal to the transceiver. The third channel serves to subtract a predicted echo signal, estimated by using the incoming digital signal, from the outgoing digital signal and to couple an error signal, representative of the difference therebetween, to the transceiver. The adaptive channel selection circuit is used to select one of the channels depending on the presence of the valid outgoing and incoming digital signals in order to effectively reduce the estimation operation. Further, the third channel is provided with a filter wherein the filter coefficients are adaptively updated by using the signs of a predetermined number of samples, thereby achieving a considerable reduction in the computational complexity.

Abstract: A direct-path echo cancellation circuit includes a loudspeaker (12), first and second microphones (14, 16), and an in-phase signal cancellation circuit (21, 43). The first and second microphones (14, 16) are positioned adjacent the loudspeaker (12) in order to receive in-phase direct-path echo components (31, 32) of an output signal from the loudspeaker (12). The in-phase signal cancellation circuit (21 or 43) combines the in-phase direct-path echo components of the output signal in order to cancel the direct-path echo components. Direct-path echo cancellation helps to reduce howling and/or oscillation in a full-duplex speakerphone system. Also, direct-path echo cancellation provides more dynamic range to an input signal which is provided to a speakerphone signal processing system (26).

Abstract: A multi-channel echo canceler for canceling echo generated as a result of propagation of 2-channel received signals, along a spatial acoustical path via a first and a second loudspeaker, from a transmitted signal containing a speech source signal received through a first and a second microphone and the echo. The echo canceler is capable of highly accurate delay time difference estimation and also of quick detection of the movement or change of the talker. Right after the movement or change of the talker, the filter coefficient set updating is stopped, or step size is updated. Deterioration of the echo cancellation performance does not occur that might otherwise result from delay of the detection of the movement or change of the talker or reduction of estimated delay time difference.

Abstract: A simultaneous interpretation system includes a plurality of simultaneous interpretation stations interconnected with a multi-conductor bus which carries up to thirteen audio lines. One audio line designated FLOOR connects to a microphone into which a source language is spoken. An interpreter located in a booth operates station controls which interact with the system. The interpreter may input select FLOOR or any one of twelve LANGUAGE lines for input to a headset and may output-select any one of the twelve LANGUAGE lines for output via a microphone. A three position microphone switch provides selective routing of the interpreter's voice from the microphone to an output-selected LANGUAGE line during "active" simultaneous interpretation, or back to the headset of the interpreter when not "active." Each booth houses two interpreters working as a team, with parallel interconnected controls, and each interpreter hears the other interpreter when their microphones are both not in the "active" mode.

Abstract: An electrical telephone speech network is disclosed in which the transmitting and the receiving amplifiers are serially connected so as to reduce the minimum working current of the whole speech network. As a result, a telephone set in the network can operate properly with a much lower line current, and more extension telephones can be connected parallelly with the speech network. Further, a bias switching circuit is matched with the speech network. Thus, in pulse dialing mode, the line voltage drop of the network loop during "make" status is automatically decreased so that a pulse dialing signal received by the exchange can be more accurate. At the same time, in DTMF dialing mode, the voltage drop of the telephone set is automatically increased, for the purpose of providing a larger AC signal dynamic range to enable a telephone set to deliver a large DTMF signal and operate with a lower line current.

Abstract: A speaker telephone comprising a microphone outputting an electrical signal, and a loudspeaker is disclosed. An amplifier has its output coupled to the loudspeaker. This amplifier has non-inverting and inverting inputs. A switch having two positions couples the microphone to the amplifier or couples the tuning signal source to one of the inputs of the amplifier. A tuning signal source and control circuit output a tuning signal. Initiation means initiates a tuning sequence by causing the switch to be in the second position and causing the tuning signal source and control circuit to generate tuning signals. A coupling device is connected to one of the inputs of the amplifier and adopted to be connected to a telephone circuit. A hybrid voltage generator circuit is responsive to the output of the microphone when the switch couples the microphone to the amplifier to generate at its output a cancellation signal which prevents transmitted voice signals from appearing at the loudspeaker.

Abstract: An intrinsically safe telephone as provided for use in hazardous areas. The telephone includes a single line instrument located in the hazardous area which, in turn, is coupled by a two-wire transmission path to a shunt diode barrier and a telephone line interface located in a safe environment. The telephone line interface reduces power of incoming ringing and audio signals. The telephone line interface further includes an off-hook indicating circuit which senses when the single line instrument is off-hook. The intrinsically safe telephone is useful in all hazardous environments as classified by the National Electrical Code and requires no explosion-proof housing.

Abstract: A conversion circuit for connecting a two-wire circuit and a four-wire circuit to each other comprises a transformer having a primary side and a secondary side, the primary side being connected to a two-wire circuit having a variable impedance, a first amplifying circuit having a first input terminal connected to the secondary side of the transformer for receiving an audio frequency signal from the two-wire circuit and a second input terminal, the first amplifying circuit having an output terminal connected to a reception-side line of a four-wire circuit, a second amplifying circuit having an input terminal connected to a transmission-side line of the four-wire circuit and an output terminal connected to the secondary side of the transformer for sending out an audio signal on the transmission-side line to the two-wire circuit, a plurality of impedance matching circuits having respective impedances each determined so as to simulate one state of the variable impedance of the two-wire circuit, and a switch for s

Abstract: A side tone preventive circuit for telephones which prevents the output of the transmitter on the speaker's side from being transmitted to the receiver on the same side. The balance of the impedance bridge in a bridge circuit for separating a transmission signal and a reception signal is automatically adjusted in accordance with the circuit impedance at the location at which a telephone is installed. An impedance measuring circuit measures the impedance of the telephone circuit, and a balance adjustment circuit the impedance of which is adjustable is provided in the bridge circuit. The impedance of the telephone circuit is measured at the time of installation of a telephone, and the impedance of the balance adjustment circuit is adjusted in correspondence with the result of measurement. Thus, the impedance balance of the bridge circuit is adjusted in correspondence with the circuit impedance, thereby effectively preventing a side tone.

Abstract: The present invention disclose a switching system including an exchange coupled to a plurality of central office lines and PBX's telephone lines, and a plurality of telephone sets. Each telephone set includes a side-tone adjusting circuit which adjusts a side-tone in response to a balanced impedance to obtain a suitable side-tone. When the exchange receives a speaking path selection request, representing one of the lines, from the telephone set, it determines one of a plurality of predetermined impedance information and makes a speaking path between the selected line and the telephone set. The telephone set selects one of the balanced impedances in response to the determined impedance information.

Abstract: A dynamically configurable telephone 10 has an intelligent controller 42 which implements a communication protocol to receive commands and parameters and to transmit status indicative of particular phone features. Upon receipt of the parameters, the controller translates the parameters into configuration settings for the telephone and also stores the parameters in a non-volatile memory 44 if such is installed. The telephone accommodates many different country-specific requirements without hardware or firmware changes being made to the phone itself. The controller returns solicited or unsolicited status messages, or events, to a PABX 14 over a bit serial digital data link 12. An event can be indicative of error conditions, such as data packet errors, timeouts, or data storage errors. An event can also be a reply to a previously received command, such as configuration status, internal test results, a phone serial number or a controller firmware revision level.

Abstract: A key telephone system comprised of a key service unit connected to a plurality of local sets via four-wire conductors for providing full duplex signal communication between the key service unit and sets. Signals are carried by a first pair of each of the conductors for unidirectional transmission of signals from each set to the key service unit, and the remaining pair of conductors carries signals transmitted by the key service unit for reception by one or more of the sets. Four-to-two wire signal conversion and line impedance balancing is effected by one or more trunk circuits of the key service unit in the event one or more of the local sets are connected via the key service unit for communication between the sets and the outside telephone lines.

Abstract: An intrinsically safe telephone as provided for use in hazardous areas. The telephone includes a single line instrument located in the hazardous area which, in turn, is coupled by a two-wire transmission path to a shunt diode barrier and a telephone line interface located in a safe environment. The telephone line interface reduces power of incoming ringing and audio signals. The telephone line interface further includes an off hook indicating circuit which senses when the single line instrument is off hook. The telephone line interface further includes a bootstrap power supply for powering the single line instrument whenever the single line instrument is off hook. The intrinsically safe telephone is line powered and requires no auxiliary power in the hazardous environment. The intrinsically safe telephone is useful in all hazardous environments as classified by the National Electrical Code and requires no explosion-proof housing.

Abstract: A speech control is provided for a telephone station and provides, dependent upon speech direction, the transmitting channel or the receiving channel being attenuated or the receiving channel being attenuated or the receiving channel or the transmitting channel being de-attenuated with a structure which is as simple as possible to manufacture with, inexpensive, non-critical components. A potentiometer is provided in each channel for setting the volume and the potentiometers are simultaneously and symmetrically operated with a single operating element. The potentiometers, dependent on speech direction, determined by a speech analyzer, are simultaneously shorted or simultaneously activated. The shorting in the transmission channel has a de-attenuating effect and has an attenuating effect in the receiving channel. When the potentiometers are in the activated condition, they cause the opposite effect.

Abstract: A full duplex microphone communications system permitting simultaneous bi-directional communications for a two conductor shielded microphone cable. A talkback signal is injected by an encoder at one end using a balanced input to transmit a common-mode signal on the two conductors of the microphone cable using the shield as a ground reference. This signal is then decoded at the other end of the cable by decoding circuitry which covers the common-mode talkback signal.

Abstract: This invention is a technique for providing isolation between the two channels of a handsfree telephone subset without resorting to gain switching or gain variations in the transmission path. The invention involves separating the frequency spectrum into two portions and allowing the stronger signal to capture a major portion of that available spectrum. Switched capacitor filters are used to control the degree of frequency bandwidth allocated to each channel while the filters are clocked from a common source assuring no possibility of overlap in the frequency spectrum. The affect of varying the bandwidth is much less noticeable than varying the amplitude and, therefore, creates much less user irritation.