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US 4,064,379 AGranted
Logarithmic echo canceller
AssigneeCommunications Satellite Corporation
InventorOtakar Anthony Horna
Life of this patent
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AssigneeCommunications Satellite Corporation
InventorOtakar Anthony Horna
Estimating the term…
A logarithmic echo canceller is disclosed which simplifies the hardware of the canceller by (a) using a logarithmic format for both the sampled voice signal x.sub.i and the coefficient h.sub.i, (b) using sophisticated algorithms for performing the different computational tasks, and (c) using analog techniques for echo subtraction and error signal generation. The incoming signal is sampled in a sample and hold circuit. The sign of the sampled input signal is sensed and its absolute value is converted directly in a special analog-to-digital converter into an A-law companding code. The whole digitized sample x.sub.i is multiplexed and stored in a plurality of shift registers. An average or pseudo rms value of the samples is computed, and the samples are compared in a comparator with the stored average or pseudo rms value, and if the sample is greater than the average or pseudo rms value, a signal enables updating of the existing value of h.sub.i stored in the H-register. The average or pseudo rms value of the incoming signal is also converted in a digital-to-analog converter to a reference voltage used to bias an analog center clipper and the error detector or comparator. A bandpass filter, pre-emphasis circuit and a low pass filter with de-emphasis circuit reduce the harmonic distortion caused by the center clipper. The sample x.sub.i and h.sub.i stored in the X-register and H-register are fed into a multiplier. In order that the A-law encoded x.sub.i and h.sub.i values may be used directly without further A-law/logarithmic conversion, the multiplication is performed in three different ways. In the case when both exponents are non-zero, addition of exponents and mantissas in two adders takes place. When both exponents are zero, the mantissas are directly multiplied. When one of the exponents is non-zero and the other is zero, both mantissas are directly multiplied in a multiplier, and the result is shifted in a shift register according to the value of the exponent. The result of the multiplication is then added to or subtracted from the contents of an accumulator. The output of the accumulator is converted in a digital-to-analog converter into an analog voltage which is subtracted in a differential amplifier from the echo signal which is present in the send out branch of the two-wire to four-wire hybrid.
A digital echo canceller for use in long distance telephone circuits of the type having means for storing a fixed number of most recent samples x.sub.1, . . . , x.sub.i, . . . , x.sub.n of a received signal, means for storing a corresponding fixed number of estimated impulse response coefficients h.sub.1, . . . , h.sub.i, . . . , h.sub.n, digital convolution means responsive to said stored samples and said stored coefficients for generating an approximate echo signal, means for subtracting said approximate echo signal from a real echo signal thereby producing a difference signal corresponding to residual echo, and cross-correlation means responsive to said residual echo and said stored samples for updating said stored coefficients respectively, the improvement wherein said stored samples and said stored coefficients are encoded in a logarithmic format having sign, exponent and mantissa, said digital convolution means comprises means for multiplying by the addition of the logarithms of said stored samples and said stored coefficients, means for accumulating the products generated by said multiplying means, and means for converting the value of the accumulated products to an analog signal representing said approximate echo signal, said analog signal being analogically subtracted in said means for subtracting.
A pseudo rms value circuit comprising:
Dependencies are inferred from the claim text.