H - Electricity – 04 – M
Patent
H - Electricity
04
M
354/149, 379/64,
H04M 1/74 (2006.01) H04B 3/20 (2006.01) H04B 3/23 (2006.01) H04M 3/18 (2006.01)
Patent
CA 1081380
ABSTRACT OF THE DISCLOSURE 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 xi and the coefficient hi, (b) using sophisticated algorithms for performing the different computational tasks, and (c) using analog techniques for echo subtraction and error signal gen- eration. 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 convertor into an A-law companding code. The whole digitized sample xi 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 hi 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 n 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 xi and hi stored in the X-register and H-register arc fed into a multiplier. In order that the A-law encoded xi and hi 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 branch of the two-wire to four-wire hybrid.
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