Abstract: An parametric speech codec; such as a CELP, RELP, or VSELP codec; is integrated with an echo canceler to provide the functions of parametric speech encoding, decoding, and echo cancellation in a single unit. The echo canceler includes a convolution processor or transversal filter that is connected to receive the synthesized parametric components, or codebook basis functions, of respective send and receive signals being decoded and encoded by respective decoding and encoding processors. The convolution processor produces and estimated echo signal for subtraction from the send signal. In order to process the synthesized parametric components having distinct basis functions in the convolution processor, conversion means are provided for providing the receive-side parametric component to the processor, or for providing the estimated echo signal, in terms of the send-side parameter. Plural convolution processors are provided for processing respective parametric components of the desired coding scheme.

Abstract: An ATM cell carrying a voice-band signal originating in an STM network is processed in accordance with a particular signal processing function in an ATM mode at an ATM data rate without incurring a delay penalty that arises.

Abstract: A multiple-master, digital communication technique is provided which employs standard universal asynchronous receivers and transmitters ("UARTs") linked to a common data transmission bus, and a hardware-based inter-station contention scheme utilizing a bus separate from that employed for data transmission. This communication technique effectively eliminates any window of collision for data transmitted on the bus, and greatly reduces the amount of software processing required to resolve contention between stations. In addition, the hardware required to support the dedicated contention bus scheme is comprised of inexpensive, commercially available logic gates.

Abstract: An extended impulse response characteristic is obtained by connecting a plurality of adaptive echo cancelers in a tandem connection (FIG. 1 or FIG. 2). Delay units (102) are associated with receive inputs (X) of the adaptive echo cancelers (101, FIG. 1, or 201 FIG. 2) so that each canceler models a predetermined fixed portion of the extended impulse response characteristic. The relative positions of the cancelers in the tandem connection are controllably switched (via 103-0 through 103-(N-1) and (104) at a predetermined rate (via 105) to insure so-called "good" adaptation of the individual adaptive echo cancelers. In one embodiment, the adaptive echo canceler in a preferred position in the tandem connection is allowed to adapt at its normal rate while the other cancelers are inhibited (via INH) from adapting.

Abstract: Loop gain normalization is employed in adaptive filters to control weighting of the filter characteristic updates in order to converge properly to a desired filter characteristic. Filter instability is avoided during intervals that transient or other rapidly pulsating signals are received or inputted by normalizing the update gain with a representation of a so-called fast attack estimate of a prescribed characteristic of the input signal. In one embodiment the fast attack estimate is the maximum of a plurality of power estimates generated from a corresponding plurality of subsets of amplitude samples of the received signal. In another embodiment, the maximum of the magnitude representations of the input signal samples is modified to represent the fast attack power estimate of the received signal.

Abstract: Loop gain normalization is employed in adaptive filters to control weighting of the filter characteristic updates in order to converge properly to a desired filter characteristic. Filter stability and rapid high quality convergence is realized for a variety of received or inputted signals by employing both long term and fast attack estimates of a prescribed input signal characteristic to normalize the update gain. In one embodiment, both long term and fast attack input signal power estimates are generated and one of the two estimate values is selected to normalize the update gain. Specifically, the fast attack estimate is modified by a predetermined value and, then, the larger of the long term estimate and modified fast attack estimate is selected to normalize the update gain.

Abstract: In adaptive filters, under certain incoming signal conditions the tap coefficients tend to drift toward relatively large values. This condition has been countered by introducing so-called leakage into the tap coefficients which tends to drive them toward zero. Introduction of leakage is desirable under certain incoming signal conditions and not others. Indeed, during intervals that partial band energy, e.g., single frequency tones, multifrequency tones and the like, is incoming to the filter there is a need to introduce a relatively large leakage value into the tap coefficients. However, during intervals that whole band energy, e.g., speech or noise, is incoming to the filter no leak is desired or needed. This is realized by detecting intervals when partial band energy is incoming to the filter and controllably increasing the leakage value during those intervals.

Abstract: Energy in a received signal is distinguished as being whole band energy or partial band energy by comparing an average value of the received signal to a modified magnitude value of the received signal. A signal representative of the comparison result is supplied to a filter including hysteresis for generating a signal to control representative of whether whole band or partial band energy is present. Hysteresis is provided in generation of the control signal by comparing the output amplitude of the filter to a first threshold value when the control signal is a first state and then comparing the filter output amplitude to a second lower threshold value when the control signal is a second state. When the modified magnitude value exceeds the average value, the received signal includes whole band energy, otherwise the received signal includes only partial band energy.