Abstract: In methods and apparatus for computing measures of echo of a far end signal in a near end signal, the evolution over time of frequency spectra of the near end and far end signals are compared to compute a measure of the echo. A far end spectrum of the far end signal and a near end spectrum of the near end signal are determined for each of a plurality of successive time intervals. A respective measure of correlation is determined for each of a plurality of spectrum pairs, each spectrum pair comprising a respective near end spectrum and a respective far end spectrum, the far end spectrum corresponding to a time interval which lags a time interval corresponding to the near end spectrum by a respective time lag. The measures of correlation are compared to determine a maximum measure of correlation which can be used as a measure of echo. The echo measure computation technique is particularly suitable for use in echo suppressors for digital cellular telephony systems, but has other applications.

Abstract: In methods and apparatus for generating an LPC-encoded noise signal from an LPC-encoded speech signal, excitation parameters of the LPC-encoded speech signal for a current frame are randomized upon detection of speech in the current frame. For frames in which no speech is detected, LPC coefficients and frame energy for the current frame may be replaced with respective averages calculated over the current frame and a plurality of preceding frames. For frames in which speech is detected, LPC coefficients for the current frame may be replaced with respective averages of LPC coefficients used to replace LPC coefficients of an immediately previous frame. The comfort noise generation technique is particularly suitable for use in echo suppressors for digital cellular radio systems.

Abstract: In methods and apparatus for distinguishing stationary signals from non-stationary signals, a set of Linear Predictive Coding (LPC) coefficients characterizing spectral properties of the signal for each of a plurality of successive time intervals, including a current time interval, is determined. The LPC coefficients are averaged over a plurality of successive time intervals preceding the current time interval, and a cross-correlation of the LPC coefficients for the current time interval with the averaged LPC coefficients is determined. The signal is declared to be stationary in the current time interval when the cross-correlation exceeds a threshold value, and is declared to be non-stationary in the current time interval when the cross-correlation is less than the threshold value. The methods and apparatus are particularly applicable to detection of transitions between an absence of speech state, characterized by a stationary signal, and a presence-of-speech state characterized by a non-stationary signal.

Abstract: In a TDMA mobile-to-mobile connection, the end-to-end audio signal quality as well as system performance can be improved by providing digital signal processors the capability to automatically switch configuration such that each digital signal processor in a mobile-to-mobile communication connection can automatically identify a TDMA mobile-to-mobile connection and bypass the speech encoding and decoding processes within the digital signal processors. The two digital signal processors are virtually connected at the channel codecs.

Abstract: In methods and apparatus for processing a speech signals comprising a plurality of successive signal intervals, each signal interval containing no speech sounds is classified as a noise interval, and LPC coefficients are calculated for each noise interval based on the samples of that noise interval and on the samples of a plurality of preceding signal intervals. When noise intervals encoded using LPC coefficients calculated as described above are reconstructed, the subjectively annoying "swishing" or "waterfall" effects encountered in conventional LPC speech processing systems are reduced or eliminated.

Abstract: In methods and apparatus for estimating the frequency response of telecommunications channels, signals carried on the channels are divided into pluralities of signal segments of limited duration, unvoiced signal segments are identified, and spectral components of the unvoiced signal segments are measured. The measured spectral components may be integrated over time to derive time-averaged measurements of the spectral components. The time-averaged measurements may be compared to corresponding components of expected unvoiced signal spectra to improve the accuracy of the frequency response estimates. Filters may be inserted in the channels to adjust the frequency responses, the filters having filter characteristics selected in response to the measured spectral components. The methods and apparatus can be used to compensate for poor bass frequency response in voice channels.

Abstract: A speech bandwidth extension method and apparatus analyzes narrowband speech sampled at 8 kHz using LPC analysis to determine its spectral shape and inverse filtering to extract its excitation signal. The excitation signal is interpolated to a sampling rate of 16 kHz and analyzed for pitch control and power level. A white noise generated wideband signal is then filtered to provide a synthesized wideband excitation signal. The narrowband shape is determined and compared to templates in respective vector quantizer codebooks, to select respective highband shape and gain. The synthesized wideband excitation signal is then filtered to provide a highband signal which is, in turn, added to the narrowband signal, interpolated to the 16 kHz sample rate, to produce an artificial wideband signal. The apparatus may be implemented on a digital signal processor chip.

Abstract: A dual tone multi-frequency (DTMF) receiver for use in detecting digitally transmitted signals in the telephone industry. The receiver derives linear predictive coefficients for the digital signals in the data frame. This information is used to compute frequency response magnitudes at the DTMF frequencies. A plurality of magnitude comparisons are then performed to verify the presence of true DTMF signals and concurrently the frequency of these signals is obtained.