Multiplex communications – Wide area network – Packet switching
Patent
1992-03-06
1995-09-26
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
379410, H04B 323
Patent
active
054539765
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an audio signal processor circuit for treating the signals from a telephone subscriber's line when these have been received at the telephone exchange, and converted to sampled digital form.
2. Description of Related Art
In a telephone exchange, the signals on the telephone subscriber's line usually in analogue form and are converted from analogue to digital form by a sampling and coding operation performed by an audio processor circuit. The impedances which are presented by each individual telephone line at any particular time are likely to give an impedance mismatch and this can cause a 2-to-4 wire hybrid echo signal to appear. It is possible for at least part of this echo effect to be cancelled digitally in the audio processor unit. The echo cancellation operation may be static, that is, its operation will be pre-set by the manufacturer of the circuit at some compromise impedance matching value. Alternatively, the echo cancellation could be designed to work by a process of continuous adaption to minimise the echo signal.
If sinusoidal signals happen to be present on the telephone line instead of speech, any adaptive operation can lead to the occurrence of further unwanted side effects in the adaption circuit output.
It is possible to overcome some of these problems by providing an adaptive echo cancellation system which can be modified in response to the statistical properties of the signal input, but a conventional adaptive system using a conventional signal correlator would need a substantial quantity of hardware for its realisation. The correlator hardware and necessary signal processing would be likely to increase significantly the area of circuit board necessary for that subscriber at the exchange. This could lead to a substantial increase in the cost of building the exchange.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an audio processor circuit which can carry out an adaptive echo cancellation function with normal speech signals and which will not respond adversely to large sinusoidal signals, but which can be economical in the quantity of hardware needed.
According to the invention, there is provided an audio signal processor circuit for treating a digitally encoded incoming signal, the circuit comprising circuitry for sampling the encoded signal, means for calculating from the sampled values a measure of correlation function relevant to a particular time frame, the value of said function serving to control operation of an adaptive echo cancellation algorithm, in which the sampled signal portions are truncated such that only the arithmetic sign bit of each signal portion contributes to the function calculation.
The circuit further comprises means for controlling the operation of said adaptive algorithm at the end of each time frame for a new function calculation.
In one embodiment, incoming signal samples are initially treated in an extraction block arranged to determine the arithmetic sign of those signal samples, the resulting sign bits being fed in sequence to a shift register arranged to store a predetermined number of said sign bits.
BRIEF DESCRIPTION OF THE DRAWINGS
By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of a correlator circuit used in the present invention;
FIG. 2 is a block schematic diagram of the correlator of FIG. 1;
FIGS. 3 and 3A are diagrammatic representations of sensitivity of correlators, according to the present invention, to sinusoidal inputs of different frequencies;
FIG. 4 is a block schematic diagram of a preferred correlator, according to the present invention; and
FIG. 5 is a diagrammatic representation of a DC blocking filter usable with a correlator according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to prevent an adaptive echo cancellation system exhibiting instability with sinusoidal sig
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P. Geisler, "Digitaler Vorzeichenkorrelator", Oct., 1985, pp. 113-114.
Emley Robin A.
Glanville Derek N.
GPT Limited
Olms Douglas W.
Plessey Semiconductors Limited
Vu Huy D.
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