| 摘要 |
In order to deal with echoes having exceptionally long tails, such as are generated by 2B1Q signals in ISDN systems, an echo canceller comprises a first echo cancelling means, conveniently a transversal filter, operative over a first number of baud periods, and filter means, conveniently an infinite impulse response filter, operative over a later number of baud periods. The filter means responds to each symbol of the transmitted signal to generate a pulse response which decays exponentially according to a predetermined function. The echo canceller further comprises means for subtracting the outputs of the first echo cancelling means and the filter means, respectively, from the received signal. The filter means may have more than one tap, enabling the exponential decay function to be adjusted to give a closer approximation to the shape of the echo tail. 21
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