Abstract: A system and method for performing echo cancellation in a communications network including a bi-directional two-wire communication line, and a four-wire communication line having a transmit path and a receive path. The system includes a filter coupled to the receive path for reducing the echo component. The filter uses a number of filter coefficients and selectively employs either a full LMS coefficient calculation technique to compute the filter coefficients during a first period of time to achieve quick convergence, or a sign-sign LMS coefficient calculation technique to compute the filter coefficients during a second period of time to achieve improved stability and convergence. The filter uses a power estimation of the transmit path signal to direct the filter as to which coefficient calculation technique to use.

Abstract: A system which performs automatic gain control on received audio data. The system comprises an analog adjustable gain amplifier coupled to a digital gain control loop which is preferably a digital signal processor, with an A/D converter and D/A converter interposed between the amplifier and control loop. The invention thus advantageously amplifies the analog input signal before being converted into a digital signal. This results in better performance than prior art system which amplify an already converted to digital signal which may have already been clipped before the signal could be attenuated. The combined digital/analog method provides improved detection of silence to avoid amplifying background noise. The gain control loop comprises a long-term energy averager and gain calculator as well as a short-term energy averager and gain calculator which receive the digital audio output signal.

Abstract: A system which performs automatic gain control on received audio data. The system comprises an adjustable gain amplifier coupled to a gain control loop which is preferably a digital signal processor. The gain control loop comprises a zero-crossing detector which receives the audio output signal and generates a zero crossing output upon detection of a zero crossing. The gain control loop further comprises a gain adjustment scheduler which receives the zero crossing output and a gain adjustment signal. The gain adjustment scheduler advantageously adjusts the gain of the output signal in incremental steps near zero crossings so as to minimize distortion and the possibility of clipping of the input signal. The gain control loop further comprises a voice activity detector which detects substantial voice activity, wherein the automatic gain control system amplifies the audio input signal only during the substantial voice activity.

Abstract: An improved dual tone multifrequency (DTMF) or multitone signal detector which more efficiently detects DTMF signals. For each received frame, the present invention initially performs only a portion of the Goertzel DFT algorithm for each of the possible tone frequencies and then analyzes the interim accumulated energies. The DTMF detector then selects one or more frequencies from each of the tone groups which have the largest energy values, and the detector only completes calculation of the Goertzel DFT algorithm on those selected one or more frequencies. In other words, the DTMF detector discontinues calculation of the Goertzel DFT algorithm on one or more frequencies which have low accumulated interim energies. This reduces the computational requirements and provides improved performance.

Abstract: A communications device is presented which is able to detect a ring signal without the need for dedicated ring signal detection circuitry. The communications device includes a hookswitch coupled between a telephone line and a communications circuit. In an open mode of the hookswitch, an electrical resistance of at least 25 megohms exists between two switch terminals of the hookswitch, and the communications circuit is essentially disconnected from the telephone line at d.c. A capacitor positioned between the switch terminals of the hookswitch couples a.c. signals from the telephone line to the communications circuit when the hookswitch is in the open mode. A DSP of the communications circuit performs ring signal detection when the hookswitch is in the open mode using electrical power provided by a power supply. The input impedance of the communications circuit is substantially equivalent to a series combination of an inductance and a resistance.

Abstract: An improved dual tone multifrequency (DTMF) or multitone signal detector which more efficiently and reliably detects DTMF signals. The present invention performs twist computations only when the signal has become stable, thus achieving more accuracy and reliability. The DTMF detector receives a plurality of digital samples of a received signal and calculates energy values for each of the plurality of different uncorrelated frequencies. The DSP then determines maximum values of the energy values for each of the two or more frequency groups, referred to as M(1) and M(2), to detect the plurality of tones in the received signal. The DTMF detector then performs improved twist computation and thresholding. The DTMF detector first determines if the M(1) and M(2) values of the respective frame being examined have the same indices.

Abstract: An improved dual tone multifrequency (DTMF) signal detector which uses the Goertzel DFT algorithm and which utilizes variable or differing frame widths that are frequency dependent for improved detection and reduced error. The DTMF detector includes a codec receiver which receives signals from the transmission media, and a digital signal processor (DSP) coupled to the codec. The DSP receives the digital samples and preferably applies the Goertzel DFT algorithm using differing frame lengths according to the present invention. The DTMF detector utilizes a different frame width for different tones of the possible tone frequencies according to the present invention. Thus the calculation uses a different frame length N for different ones of the uncorrelated frequencies, wherein the different frame lengths comprise at least a subset N of the number of digital samples. The different frame lengths N are designed to optimally align the calculated frequency spectrum at each of the different uncorrelated frequencies.

Abstract: An improved dual tone multifrequency (DTMF) or multitone signal detector which uses both static and dynamic thresholding techniques to provide an increased functional dynamic range, improved speech immunity for improved detection and reduced error, and simplified signal to noise ratio control. The DTMF detector receives signals from the transmission media, calculates energy values for the different frequencies, and then determines maximum values of the energy values for each of the frequency groups, referred to as M(1) and M(2). The DTMF detector then performs both static and dynamic thresholding according to the present invention to ensure valid tone detection. The static thresholding compares each of the M(1) and M(2) values with a static threshold value to ensure that the maximum energy values have a minimum energy to warrant detection.