Multi-stage echo canceller

Details Multi-stage echo canceller Multi-stage echo canceller

H - Electricity

04

B

379/64, 340/72

H04B 15/00 (2006.01) H04B 3/23 (2006.01)

Patent

CA 1238381

MULTI-STAGE ECHO CANCELLER Abstract of the Disclosure In an echo canceller for use in full duplex, two-wire, digital transmission systems, satisfactory operation at higher bit rates (>144 Kb/second and above) is achieved by at least two adaptive filter stages operating in tandem. In preferred embodiments, each stage comprises a random access memory (RAM), a digital/analogue converter and differencing means. In each stage, each sequence of bits in the transmitted signal address the RAM which outputs a corresponding digital word to the digital/analogue converter. The corresponding analogue signal from the digital/analogue converter is subtracted from the received signal. The difference signal so obtained is used to update the digital word in the RAM so as to approximate the echo more closely when the particular sequence recurs. The second stage digital/analogue converter has an output range much smaller than that of the first stage, preferably about equal to the minimum gradation of the first stage digital/analogue converter. Consequently the first stage performs coarse cancellation of the echo and the second stage then further reduces the residual. To enable such further reduction, the updating of the RAM in the first stage is "frozen" once that stage has converged. - i -

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