Adaptive digital echo cancellation circuit
First Claim
1. An adaptive digital echo cancellation circuit for a telephone line circuit comprising:
- finite impulse response digital filter means having a plurality of delay stages and having coupled thereto a plurality of received digital words, for obtaining for each sample interval of N words the product of N of said digital words and a plurality of updated coefficients each associated with one of said digital filter stages to derive an output representative of the value of an undesired signal portion of a plurality of transmitted digital words;
summation circuit means having coupled thereto a plurality of transmitted digital words having superimposed thereon an undesired signal portion, and the output of said digital filter representative of the value of said undesired signal portion, and having an output comprised substantially of said transmitted digital words;
correlator means coupled to said digital filter means for obtaining an N term average correlation for each sample interval of N words of digital signals representative of N of said transmitted digital words and N of said received digital words to derive a control output representative of the instantaneous undesired signal present in said received transmitted digital words to vary said coefficients each N word sample interval in accordance with said control output; and
nonlinear circuit means for deriving said digital signals respectively representative of said transmitted and received digital words, such that said representative digital signals are of a bit length less than the bit length of said respective transmitted and received digital words.
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Abstract
The present invention discloses an adaptive digital echo cancellation circuit including a finite impulse response digital filter, wherein the digital filter coefficients are adapted by continuous updating to compensate for telephone subscriber line echo conditions to enable the digital filter to continuously simulate the instantaneous subscriber line echo. The continuous coefficient update is provided by a correlator which includes provision for introducing non-linearities into the PCM transmitted and received signals, in parallel with the speech path, to derive a simplified digital representation of the PCM speech, thereby reducing the signal processing hardware required by the correlator to derive the updated filter coefficients.
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Citations
19 Claims
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1. An adaptive digital echo cancellation circuit for a telephone line circuit comprising:
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finite impulse response digital filter means having a plurality of delay stages and having coupled thereto a plurality of received digital words, for obtaining for each sample interval of N words the product of N of said digital words and a plurality of updated coefficients each associated with one of said digital filter stages to derive an output representative of the value of an undesired signal portion of a plurality of transmitted digital words; summation circuit means having coupled thereto a plurality of transmitted digital words having superimposed thereon an undesired signal portion, and the output of said digital filter representative of the value of said undesired signal portion, and having an output comprised substantially of said transmitted digital words; correlator means coupled to said digital filter means for obtaining an N term average correlation for each sample interval of N words of digital signals representative of N of said transmitted digital words and N of said received digital words to derive a control output representative of the instantaneous undesired signal present in said received transmitted digital words to vary said coefficients each N word sample interval in accordance with said control output; and nonlinear circuit means for deriving said digital signals respectively representative of said transmitted and received digital words, such that said representative digital signals are of a bit length less than the bit length of said respective transmitted and received digital words. - View Dependent Claims (2, 9, 10, 11)
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- 3. An adaptive digital echo cancellation circuit in accordance with claim l, wherein said digital words comprise PCM transmission signals.
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12. A telephone subscriber line circuit comprising:
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two-to-four wire converter means coupled to a near-end analog subscriber line; analog-to-digital converter means for converting near-end analog signals coupled thereto from said two-to-four wire converter to near-end digital transmission signals comprised of a plurality of digital words digital-to-analog converter means for converting far-end digital signals comprised of a plurality of digital words from a far-end subscriber to analog return signals, and having an output coupled to said two-to-four wire converter; finite impulse response digital filter means having a plurality of delay stages and having coupled thereto said plurality of far-end digital words, for obtaining for each sample interval of N words the product of N of said far-end digital words and a plurality of updated coefficients each associated with one of said digital filter stages to derive an output representative of the value of an undesired signal portion of said near-end digital words; summation circuit means coupled thereto said near-end digital words, having superimposed thereon an undesired signal portion, and also having coupled thereto the output of said digital filter representative of the value of said undesired signal portion, and said summation circuit having an output comprised substantially of said near-end digital words; correlator means coupled to said filter for obtaining an N term average correlation for each sample interval of N words of digital signals representative of N of said near-end digital words and N of said far-end digital words to derive a control output representative of the instantaneous undesired signal present in said near-end digital words to update said coefficients every each N word sample interval in accordance with said control output; nonlinear circuit means for deriving said digital signals respectively representative of said near-end digital words and said far-end digital words, such that said representative digital signals are of a bit-length less than the bit-length of the respective near-end and far-end digital words. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19)
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Specification