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Automatic Gain Control and Echo Suppression Circuit

IP.com Disclosure Number: IPCOM000079649D
Original Publication Date: 1973-Aug-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 39K

Publishing Venue

IBM

Related People

Abramson, P: AUTHOR [+2]

Abstract

In TOUCH TONE data transmission, echoes may be detected over a voice grade telephone path at a remote receiver 50 ms to 250 ms after the last data transmission, due to unbalanced hybrids at both the sending and receiving sites "feeding back" the data stream several times. Parenthetically, echo amplitudes in the order of -12dh have been detected. The method contemplates averaging the signal level of the received data over a time interval, defined by two cycles of the lowest frequency component of the communications channel, and using the "moving" average as a threshold to inhibit all signals at the receiver of a magnitude less than the moving average. The suppression circuit is implemented digitally using the PCM encoding of signal samples.

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Automatic Gain Control and Echo Suppression Circuit

In TOUCH TONE data transmission, echoes may be detected over a voice grade telephone path at a remote receiver 50 ms to 250 ms after the last data transmission, due to unbalanced hybrids at both the sending and receiving sites "feeding back" the data stream several times. Parenthetically, echo amplitudes in the order of -12dh have been detected. The method contemplates averaging the signal level of the received data over a time interval, defined by two cycles of the lowest frequency component of the communications channel, and using the "moving" average as a threshold to inhibit all signals at the receiver of a magnitude less than the moving average. The suppression circuit is implemented digitally using the PCM encoding of signal samples.

Referring to Fig. 1A, there is shown an apparatus for practicing the method. A time varying analog signal is applied to input 1 of an analog-to-digital (A/D) converter 3, in order to produce ordinary binary coded magnitude equivalents. An equivalent to the analog operation of rectification is performed, by changing the minus signs of all negative binary numbers to the plus sign of positive binary numbers.

2N successive binary coded signal samples are obtained from the A/D converter and averaged by a filter formed by a shift register 7 and digital accumulator 11. The number N is a function of the lowest frequency f sampled and the sampling rate R, where N = R/T. This assures that at least two sampling times will be averaged.

The filter of Fig. 1A has a rectangular impulse response lasting over 2N sample times. Each binary coded output from converter 3 is simultaneously applied as one input into accumulator 11 over path 5B, and as the current input into register 7 over path 5A. Note, register 7 has a length of 2N samples. Upon a particular sample being applied to the accumulator 11, it becomes added to the sum of the previous 2N-1 sample. That p...