Abstract: A system and method to reduce echoes in voice communications are described. Input data containing return loss values of a communication trunk within a communications network is received, the return loss values being measured at each frequency of a plurality of frequencies within a predetermined frequency band. An optimum transhybrid balance impedance is calculated from the input data. Finally, the optimum transhybrid balance impedance is transmitted to a user for further processing.

Abstract: A test system measures performance of telephone network echo cancellers using a primary criterion of estimated user annoyance due to audible returned echo. The invention generates live telephone calls, uses real speech samples as stimulus signals and provides tail-circuit emulation using actual measured telephone tail-circuit impulse responses. These features provide better ‘real-life’ test conditions for the echo canceller system under test than current ITU standard test methods. Two methods are employed for echo canceller performance evaluation via metrics of estimated user annoyance due to echo. Energy-based method employs point-by-point comparison of talker speech and talker echo signal energy envelopes and uses variable energy thresholds for estimation of echo audibility. A perceptual-model based method uses a Perceptual Speech Distortion Metric (PSDM), such as ITU P.861, in an unique configuration to estimate user annoyance due to audible echo.

Abstract: The present invention relates to an improved method and apparatus for echo cancellation in a communication system utilizing a bidirectional transmission medium. The invention significantly reduces computational overhead associated with echo cancellation by using sub-Nyquist sampling in the echo path. In particular, the invention relates to a method and apparatus for echo cancellation in a communication system utilizing different signaling or baud rates in the transmit and receive directions, whereby the computational overhead of the echo cancellation is significantly reduced as compared to traditional methods. In a preferred embodiment herein, the present invention reduces by one-half the computational overhead associated with echo cancellation in a data communications system utilizing symmetrical information rates at asymmetrical signal rates.

Abstract: Provided is a method and apparatus for inband testing of an echo canceller. The apparatus includes a signal generator for generating an excitation signal including a preamble portion and a test portion. The echo canceller receives the excitation signal at a decoder. The decoder differentiates the preamble from the test portion and obtains control information by decoding the preamble portion. A controller configures the echo canceller during testing according to the control information obtained from the preamble portion. A strength of the present invention is that the preamble portion allows precise control of the echo canceller during testing by synchronizing the test control to the location of the preamble. This allows exacting control of the echo canceller during testing to within a single sample time of the excitation signal, e.g., 125 microseconds.

Abstract: Data is transferred by delaying an audio signal less than fifty milliseconds to produce a shadow signal and combining the shadow signal with the audio signal. The shadow represents the data to be transferred A telephone constructed in accordance with the invention has at least two, separate machine states, each of which includes operating in half duplex mode and two, separate machine states, each of which includes operating in full duplex mode. The state is selected based upon one or more shadows detected at the line input of the telephone.

Abstract: An apparatus having a band-separating filter bank for separating a digital signal into a plurality of sub-band signals, to be processed or transmitted, and a band-combining filter bank for subsequently combining the resultant sub-band signals into a single digital signal, wherein each of the band-separating filter bank and band-combining filter bank incorporates a FIR low pass filter having an asymmetric impulse response, as the prototype filter of the filter bank. A significant reduction can thereby be achieved in the amount of overall group delay that results from the processing performed by these filter banks.

Abstract: A method for the performance testing of an echo canceller including the steps of transmitting a test signal to the echo canceller, recording an echo energy signal output from the echo canceller as a result of the test signal, measuring the energy and duration of the echo energy signal and calculating a performance score for the echo canceller based on the measured energy and duration of the echo energy signal, wherein the test signal transmitted to the echo canceller has a plurality of discrete frequency band segments of white noise representing sub-bands of an overall bandwidth ranging from about 0 kHz to about 3.5 kHz. The test signal preferably has four discrete frequency band segments of white noise, wherein each discrete frequency band segment of the test signal is repeated in succession, whereby the test signal has a total of eight segments.

Abstract: In order to identify a teleservice during the setting up of a connection in a telephone network (PSTN) from a calling point (TXA) to a target subscriber (TNB), a signal recognition device (22) is temporarily applied to the requested connection on the basis of the call request (20) in a switching center of the network, preferably in a target switching center (VSB) associated with the target subscriber, said signal recognition device being sufficient at least for emitting a service call signal indicating a teleservice, via the calling point (TXA). When a service call signal is received (23) by the signal recognition device, said signal is evaluated in terms of the service indicated, and the connection (24-26) is further treated according to the service thus indicated.

Abstract: A fault associated with a communication medium is analyzed by correlating a first set of information indicative of echo associated with the communication medium with a second set of information indicative of echo. The fault is characterized based on the correlation between the first set of information and the second set of information. The first set of information and the second set of information can include echo canceller coefficients generated by an echo canceller coupled to the communication medium.

Abstract: Method and system are provided for optimization and transmission of digital signals through twisted pairs of telephone cables. Crosstalk reduction is obtained due to cancellation of electromagnetic fields generated by correlated twisted pairs by establishing mutual pair-to-pair coherence at each tone of a transmission spectrum and introducing a requested phase shift between the tones propagating in different pairs.

Abstract: In one embodiment, a test tone is placed in an outbound path of a full-duplex transmission system. An echo cancellation capability is disabled, enabling an echo signal of the test tone to pass through the inbound path of the full-duplex transmission system, which echo signal is analyzed to determine the condition of the full-duplex paths. In another embodiment a standard DSP generates DTMF tones which are looped-back and tested by the DSP, under control of a CPU. The DSP is a standard DSP avoiding the need for any hardware/software modification of the DSP to perform the test mode. The tests are performed the local level, i.e. at the subscriber's location, eliminating the need for central office intervention, for example.

Abstract: A system and method provide ASR talkoff suppression in a prompt-and collect voice transaction system in which a prompt signal is sent and an input signal is received. A comparison is made of a characteristic of the prompt signal and the input signal. Processing of the input signal to reduce the likelihood of an ASR response thereto is performed if the result of the comparison satisfies a predetermined criterion.

Abstract: An apparatus for separating oxygen from an oxygen-containing gas and facilitating a chemical reaction with the separated oxygen includes a mixed conducting membrane, a porous body, and a material for catalyzing the reaction. The mixed conducting membrane has respective oxidation and reduction surfaces and is made of a non-porous, gas-impermeable, solid material capable of simultaneously conducting oxygen ions and electrons. At least the membrane and the catalyzing material are non-reactive with each other or are physically separated from each other during oxygen separation and the chemical reaction. The apparatus can be embodied by tubes, and a plurality of such tubes can form part of a reaction vessel in which various chemical reactions can occur benefiting from the apparatus design.

Abstract: A communications system includes first and second subscriber lines, a first device, and a second device. The first device has a first negotiation unit coupled to the first subscriber line. The second device has a second negotiation unit coupled to the second subscriber line. The first negotiation unit is adapted to send a first negotiation signal on the first subscriber line. The first negotiation signal induces through cross-talk a second negotiation signal on the second subscriber line. The second negotiation unit is adapted to modify an operating parameter of the second device based on the second negotiation signal. A method for negotiating operating parameters in a communications system is provided. A first negotiation signal is transmitted from a first device coupled to a first subscriber line. The first negotiation signal is received in a second device coupled to a second subscriber line independent of the first subscriber line.

Abstract: System and method for controlling an ancillary telecommunications device such as an echo canceller within a telecommunications system. The system and method include and involve a switching system that is configured to receive a call from a calling station and to route said call within said telecommunications system, and an ancillary telecommunications device that is configured to control a call processing parameter to affect the call. The switching system and the ancillary telecommunications device are coupled to each other via a channelized call data network. The channelized call data network is configured to communicate data corresponding to the call over at least one data channel. The ancillary telecommunications device may control the call processing parameter to affect the call based on a continuity test issued by the switching system to the ancillary telecommunications device prior to the call.

Abstract: A service chip for use with a computer. The chip includes a CPU interface, a transceiver interface, an adaptive echo cancellation filter, a monitor, and first and second data synthesizers. The CPU interface receives a transmit sample sequence from a modem sample generator executing on a central processor of the computer, and presents a receive sample sequence to a modem sample receiver executing on the central processor. The transceiver interface presents data to a line interface, which includes digital-to-analog and analog-to-digital converters for converting samples to/from analog signals for transmission on a telephone line. The filter adapts in response to an echo correlation between data transmitted over a transmit channel of the modem and data received on a receive channel of the transceiver interface. The monitor monitors the transmit sample sequence for a data starvation condition.

Abstract: A first pseudo-echo signal approximating an echo of a reception signal superposed on a transmission signal is produced from a delayed reception signal corresponding to a past time according to filer coefficients, and a first residual signal is produced from the first pseudo-echo signal and a delayed transmission signal. Also, the filer coefficients are renewed to reduce the first residual signal. Therefore, when a single-talk state based on a far-end caller's voice of the reception signal is changed to a double-talk state by adding a near-end caller' voice to the transmission signal, the first filter coefficients deteriorate. Also, a second pseudo-echo signal is produced from a reception signal currently received according to the first filter coefficients renewed.

Abstract: Two signals, which are mutually correlated, are subjected to a principal component analysis and converted into two signals being put in an orthogonal relation, thereby generating two signals being non-correlated. Those two signals are reproduced by speakers, and the voice generated from the speakers are collected by microphones. The cross spectra of a signal as the result of subtracting an echo canceling signal from a voice collected by each microphone, and a voice before it is generated from the speaker, are obtained. Those cross spectra are ensemble-averaged for a predetermined period of time, and inverse Fourier transformed, thereby producing impulse response estimation errors of each filter. Impulse responses of those filters are updated so as to cancel those impulse response estimation errors.

Abstract: By comparing the measured data of a transmission line to a model of the transmission line comprising models of each individual portion of the line, an estimation of the loop can be determined. Specifically, the transmission line can be modeled by a model of the transmit filter, a model of the receive filter, a model of the analog hybrid circuitry and an ABCD matrix model of the loop.

Abstract: A test signal is injected on the line (L) and the corresponding return signal produced by the line itself as an effect of the echo is detected. A signal having lenticular envelope is used as test signal and is generated preferably starting from a first sinusoid with a first given frequency modulated in amplitude with a second sinusoid having a second given frequency, lower than the aforesaid first given frequency.

Abstract: In one embodiment, a test tone is placed in an outbound path of a full-duplex transmission system. An echo cancellation capability is disabled, enabling an echo signal of the test tone to pass through the inbound path of the full-duplex transmission system, which echo signal is analyzed to determine the condition of the full-duplex paths. In another embodiment a standard DSP generates DTMF tones which are looped-back and tested by the DSP, under control of a CPU. The DSP is a standard DSP avoiding the need for any hardware/software modification of the DSP to perform the test mode. The tests are performed the local level, i.e. at the subscriber's location, eliminating the need for central office intervention, for example.

Abstract: In a technique for canceling noise from a microphone communications path, an electrical equivalence circuit which is positioned close to and electrically matched to the microphone produces a signal free reference signal. The microphone converts speech to a voice signal. An analog multiplexer alternately switches from the microphone to the electrical equivalence circuit to produce a multiplexed analog signal composed of the voice signal from the microphone and the signal free reference signal from the electrical equivalence circuit. A wire connects the output of the analog multiplexer switch to an A/D converter. The wire picks up noise from the surrounding environment. The A/D converter converts the multiplexed signal having the noise to a plurality of voice samples taken from the voice signal portion of the multiplexed signal and a plurality of noise samples taken from the signal free reference signal portion of the multiplexed signal. A noise cancellation unit applies a noise suppression algorithm (e.g.

Abstract: A method for reducing disturbing effects of coupling between a first transmission/reception device and a second transmission/reception device that are each connected to a subscriber line. According to the method, a signal received on a reception path of the first device is delayed by a delay equal to p times the transmission period. A coupling signal relating to a transmission path of a second device and the reception path of the first device is estimated based on a signal transmitted over the transmission path of the second device, and the delayed signal is ridded of the estimated coupling signal. Additionally, a device for transmitting/receiving a signal is provided. The device includes a memory coupled to a reception path for temporarily storing p symbols, a subtraction circuit, and a coupling estimation block.

Abstract: A method and apparatus for aligning a received echo signal. The method operates by determining a memory address pointer value that directs which transmit data is to be transmitted. The pointer is selected such that an appropriate delay is created, resulting in the associated received echo having a certain timing alignment within the received block of data. The method thus includes predetermining a pointer or pointer offset value; generating blocks of digital transmit data; transferring the transmit digital data to a converter using the determined pointer or pointer offset value; converting the transmit digital data to a transmit analog signal and transmitting the analog signal; sampling a received signal and forming blocks of received digital data, wherein the received signal includes a transmit echo; and wherein the pointer offset results in block alignment between the transmit echo from a transmit block and a receive block.

Abstract: A method and apparatus for testing and measuring transmission line parameters, such as the post-dial delay, the line rate, the noise level and the signal level, among others. The method comprises the step of enabling an echo canceler present in the transmission line and afterwards measuring the parameters of the line.

Abstract: A digital signal processor-based test set is configured and programmed to enable a craftsperson to monitor, receive and process signals of a variety of formats that may be present on a line under test, including user feature analog modem signals. Such user feature signals may include caller-identification signals, visual message waiting indicator signals, and analog display services interface signals. The test set employs a signal processing routine that operates in two modes: 1- terminal mode—in which the test set functions in the place of customer premises equipment (CPE); and 2- line monitor mode—in which the test set is connected to the line under test and monitors data signals being transmitted to and received from an existing CPE. In both modes the test set has the ability to not only display the data being sent to and from the CPE, but also parametric information about the data.

Abstract: A system and method for determining the impedance of a telephone transmission line are disclosed. A test signal is transmitted through a telephone switch to the telephone line and corresponding echo signals are monitored. The test and echo signals are correlated to derive an impulse response. A transfer function for the system is calculated from the impulse response. Then, a transfer function for the transmission line is derived using the system transfer function and the transfer function of the telephone switch. Finally, the transmission line impedance is calculated from the transmission line transfer function. Once the transmission line impedance is known, an impedance matching circuit for the telephone switch can be modified to match the transmission line and the switch.

Abstract: The present invention provides a cheap handsfree unit which can prevent the howling phenomenon and reduce the input sound from the loud-speaker into the microphone in both the analog and digital modes. A reception side comprises a signal level adjusting circuit constituted by a switch circuit and attenuators, and a transmission side comprises a signal level adjusting circuit constituted by a switch circuit and attenuators, to introduce the attenuation of 15 dB or 30 dB in response to the analog or digital mode. A voice switch is constituted so that the analog reception signal and the analog transmission signal are rectified respectively by rectifiers, the levels of both signals are compared by a comparator, and either switch or switch having a higher input signal level becomes conductive as a result of the comparison. The attenuation is introduced on the basis of the communication mode, only to either the analog reception signal or analog transmission signal having a smaller signal intensity.

Abstract: A modem monitor for monitoring full duplex communications comprises one receiver for each data stream and an echo canceler for canceling "far" echo. Prior to each receiver decoding the data stream it is designed to monitor, it replicates an echo expected to be found in its received data stream from a sample of each of the other predecoded data streams. The echo canceler then subtracts the expected echo from the received data stream to remove all far echoes caused by the other data streams. The echo canceled data stream is then decoded. The monitor can also include a data communications network interface and a sample rate converter operating with the PSTN for converting from the network transmission rate to the data transmission rate.

Abstract: Modems using a telephone line for high speed communications between them are arranged to monitor crosstalk from other existing communications systems with which they may mutually interfere, and adjust the power spectral densities (PSDs) of their transmitted signals to reduce overlap between the PSDs of the different systems, thereby reducing near end crosstalk. Communications can thereby be optimized for whatever crosstalk conditions may exist. The modems can have a master-slave relationship for communicating buffered frames in a half-duplex manner using a collision avoidance protocol for computer network access. Analysis of monitored crosstalk PSD information can be performed by each modem, by the master modem, or by a separate computer on the network. A digital signal processor used in each modem for receiving signals can be configured to be used at other times for the monitoring of crosstalk.

Abstract: A telephone communications switching system incorporating echo cancellers that may be replaced by other echo cancellers, and a method for replacing such echo cancellers are disclosed. The system comprises a control module for receiving and switching call data; and active and redundant echo cancellers. Under normal operation, an active echo canceller cancels echo in a call. A processor on the control module monitors the system and in the response to a sensed trigger event activates the redundant echo canceller and switches call data at the input of the active echo canceller to inputs of both the active and redundant echo cancellers. The processor then waits a determined amount of time; and thereafter disconnects the output of the active echo canceller from the control module and connects the output of the redundant echo canceller to the control module.

Abstract: The present invention is directed to an adaptor for measuring an echo return loss in a communication system. The adaptor includes two transformers, each having a primary winding and two secondary windings. The secondary windings are cross-connected to form first and second out of phase circuits. The first circuit includes impedance elements which match an impedance of an external device. When the external device is attached to the second circuit, the first and second circuits are schematically equivalent, albeit out of phase. When an oscillation signal is input to one transformer, the matched impedance and out-of-phase connection of the secondary windings cancel the signal such that only an echo signal is output by the other transformer.

Abstract: A digital signal processor that is used for echo cancellation computations is disclosed, which includes parallel multiplier/adders, an input signal sample memory, and a plurality of accumulators for temporarily storing the results of the computations performed. Consequently, not all of the computational results have to be saved in an external data memory, which minimizes data transfer bottlenecks and significantly increases the computational efficiency of the digital signal processor. The adaptive filter coefficient updates can thereby be computed much faster, and the adaptive filter coefficients can be copied much faster from one filter to the other. The computational complexity of the digital signal processor is significantly less than that of prior digital signal processors.

Abstract: An echo cancelling system for communication systems. The system includes an adaptive filter that processes the speech signal re-constructed by a voice decoder to generate an echo estimate. The echo estimate is subtracted from the echo containing signal propagating in the communication link to remove or at least reduce the echo corruption. The tap coefficients of the adaptive filter are established on the basis of an error signal that is representative of difference between the echo estimate and the actual echo and on the basis of a signal element that is a component of the re-constructed speech signal. The signal element is highly decorrelated to enable the adaptive filter to converge faster and manifest an improved echo return loss enhancement.

Abstract: An improved method and apparatus for canceling echo is disclosed. The echo canceller of the present invention compensates for the existence of quantization error in the echo signal that does not exist in the reference signal by introducing the quantization effects into the reference signal. The quantization effects are typically companding quantization errors caused by the A-law and .mu.-law quantizers prevalent in digital communications equipment.

Abstract: A modem for transmission and reception of data samples over a transmission line includes a transmission line break detection device having a correlation meter for providing a measure of the degree of correlation between transmitted and received data samples. Such device further includes a threshold meter for triggering a line break alarm when the correlation exceeds a threshold value. The correlation relates to a limited number of data samples occurring during a portion of an echo period for reception of an echo of transmitted data samples.

Abstract: A telephone system coder/decoder (CODEC) having improved access for testing CODEC components and a method of testing a CODEC in which a test signal is not contaminated by passage through CODEC filters. Input test pads are positioned between the receive channel filters and the connection of the receive channel to the hybrid balance network, and an output test pad is positioned between the transmit channel filters and the connection of the transmit channel to the hybrid balance network. Test signals selectively applied to the test pads or to CODEC terminals indicate operation of receive programmable gain section, hybrid balance network frequency response, hybrid balance network echo cancellation, and the transmit buffer, buffer splitter and transmit programmable gain section in the transmit channel. The test signals do not go through the transmit and receive filters.

Abstract: A method of detecting narrow-band signals in an echo canceller to control the coefficient update process, echo suppression process, and the disablement of the echo canceller is provided. The method includes the steps of searching for the presence of a generic tone, searching for DTMF tones, searching for a 2100 Hz tone and, if a generic tone is found, searching for a dial tone. If a generic tone is detected on the far end signal, a tone flag is set which triggers the disablement of the echo canceller coefficient update process and the enablement of the echo suppressor process. If a 2100 Hz tone is detected, the echo canceller is disabled. If a DTMF signal is detected, DTMF flags are set which are used by the echo suppressor for suppressing the near end echo. Finally, if a dial tone is detected, dial tone flags are set which cause the disablement of the echo suppressor.

Abstract: A method and apparatus for detecting and correcting for variations in the flat delay, which permits smooth echo canceller operation even under dynamic delay conditions. In a delay monitor, the received signal is compared against delayed versions of the echo estimate to determine whether a shift in the echo delay has occurred. If such a shift in delay has occurred then a signal indicating the amount of the shift is to the adaptive filter of the echo canceller and the filter taps are updated in accordance with the delay shift value.

Abstract: An echo canceller is disclosed which generates an echo replica based on a transmit signal to cancel an echo. The echo canceller includes a linear echo canceller circuit for generating an echo replica of the transmit signal in the absence of distortion. The echo canceller further includes a waveform-distortion compensation circuit, coupled with the linear echo canceller circuit, for generating an echo replica for compensating waveform distortion occurred in an echo response when the transmit signal is distorted. In the echo canceller, the echo is canceled by use of the echo replica generated in the linear echo canceller circuit in the absence of distortion in the transmit signal, and is canceled by use of the echo replica generated in the waveform-distortion compensation circuit when the transmit signal is distorted.

Abstract: A processor controlled test set for testing special service circuits of a telecommunication system. A microprocessor controls the overall operation of the test set, while a digital signal processor provides high speed timing signals to the various test circuits for generating the wave forms used in testing, as well as analyzes the test result signals that are converted into digital signals. A calibration of the test generator signals as well as the signal measuring path is carried out prior to the test sequence. The digital signal processor also provides gain control over a talking path to maintain stability thereof. An I/O circuit of the test set provides plural communication paths between remote equipment and the test set to initiate and carry out various tests. Processors in the I/O module are effective to convert the various protocols of the serial data, by way of software, to digital bit streams usable by the test set.

Abstract: In a modem, after scrambling data which are time-divided into main data and secondary data, a 2 W-4 W conversion circuit sends the data to the two-wire line. An estimated echo component is subtracted from a signal which is available from the 2 W-4 W conversion circuit. Following this, the signal from the 2 W-4 W conversion circuit is demodulated, descrambled and separated into main data and secondary data. A generator polynomial for scrambling is different from a generator polynomial for descrambling. The modem comprises a line-trouble-detect part which samples data of the secondary channel, by multipoint sampling, to obtain the secondary data and detects the number of inversion points in the descrambled secondary channel data per unit time period. The line-trouble-detect part outputs a line-trouble signal which indicates that there is a trouble on the line if the number of detected inversion points is more than a predetermined number.

Abstract: A system and method for detecting convergence in an echo canceller prevent false convergence from occurring as a result of receiving only tones in an input signal. An adaptive filter is used in an echo canceller to estimate an echo signal produced by an unknown echo channel. The estimated echo signal is subtracted from the return signal to remove the echo produced by the unknown echo channel. To prevent the echo canceller from falsely converging on a signal which contains only tones a system and method are provided for detecting the presence of tones in the absence of other frequencies. Filter taps of the adaptive filter are filtered to produce a filtered signal. The amount of energy in the filtered signal is compared to the amount of energy in the unfiltered filter taps to determine whether only tones have been present. If only tones have been present, the echo canceller is not allowed to reduce the adaptation step size of the adaptive filter.

Abstract: In an echo cancelling arrangement, an estimated echo signal is produced by an adaptive filter from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal, to compensate for ERL (echo return loss) via a hybrid circuit. Short-term averages of signal levels on the receive path and the incoming and outgoing transmit paths are produced by exponential averaging. An ERLE (echo return loss enhancement) due to the subtraction of the estimated echo signal is calculated from the incoming and outgoing transmit path averages and, when this exceeds a threshold value, enables averaging of a ratio of average levels on the receive path to average levels on the incoming transmit path. The resulting long-term average constitutes a determination of the actual ERL.

Abstract: Method and apparatus for cancelling in-band interference in real time. A cancellation circuit is provided where one demodulator circuit provides a demodulated output having interference voltage owing to interference between carrier signals, between a carrier signal and noise, or a combination of both. Cancellation of interference is particularly directed to narrowband applications. Because the interference is within the band of the desired signal, it cannot be filtered without materially harming the quality of the message. A separation circuit is provided for removing the interference voltage component from the message signal. Outputs are provided to a second demodulator for creating a phase-shifted near replica signal of the dominant carrier signal without an interference voltage component. The separation circuit includes an isolation circuit.

Abstract: Echo cancellation can be enabled to cancel echoes on an outbound transmission path based on original signals on an inbound transmission path. A controller of echo canceler enablement monitors digitized control information contained in call origination messages associated with the establishment of communications. A determination of the type of digitized information to be carried by a two-way communication channel is made. The controller causes the echo canceler to be enabled or disabled on a per-call basis based on the type of digitized information. A stand-alone echo canceler provides automatic enablement/disablement on a per-call basis based on the type of digitized information to be carried on a communication channel as determined by monitoring associated control messages.

Abstract: Speech path switching control apparatus and a method for making maintenance of speech possible even upon occurrence of a fault in echo cancellation. The speech path switching control apparatus includes a time switch for receiving PCM voice data from a space switch through a switch network link and performing time slot interchange with trunk lines, an echo canceller connected to the time switch through a plurality of sub-highways to cancel echoes, an access switching processor for discriminating whether echo cancellation is required for a trunk call allocated with a particular one of the trunk lines and outputting the resultant discrimination signal, and a time switch control processor for controlling the time switch and the echo canceller in response to the discrimination signal from the access switching processor. If the echo cancellation is not required or the echo canceller is abnormal, there is formed a speech path which is not passed through the echo canceller.

Abstract: Transmission system comprising a sub-band echo canceller which includes a sub-band coding arrangement.The invention relates to a transmission system comprising a sub-band echo canceller which includes a canceller that can be switched between a receive path (r) and a send path (s).To reduce the computational complexity as well as the signal delays also with sub-band coding or sub-band decoding, the sub-band coding arrangement comprises a synthesis filter bank and an analysis filter bank which are provided for sub-band coding and also for sub-band echo cancellation.Advantageous applications for the echo canceller are subscriber terminal units, for example, hands-free facilities for car telephones, videophones or video conference units which have an echo path formed by at least one microphone and at least one loudspeaker.

Abstract: A method and apparatus is provided for characterizing a differential circuit. A measurement system (200) is used to introduce input signals to the differential circuit and to measure corresponding output signals. Particularly, an input differential wave is introduced into the differential circuit (1010) while correspondingly measuring a differential output wave (1020) and a first common mode output wave (1030). Similarly, an input common mode wave is introduced (1040) while measuring a second differential output wave (1050) and a second common mode output wave (1060).

Abstract: An echo-canceling system and method uses an echo estimate to modify the error signal which is obtained by the difference between an echo signal and the echo estimate. The modified error signal, rather than the error signal itself, is transmitted. By using the echo estimate as a parameter for modifying the error signal, improved hands-free performance may be obtained in a vehicular radiotelephone, such as a cellular telephone. Preferably, modification of the electrical error signal is performed by a center clipper which removes a portion of the electrical error signal which is below a variable threshold, wherein the variable threshold is determined by the echo estimate electrical signal. The variable threshold may be produced from the echo estimate electrical signal using an envelope detector.