Communication arrangement with improved echo and noise suppression in a channel containing quantization

Abstract

A modem that matches itself to a central office codec to which it is connected and incorporates encoding of the data. An echo canceller following the central office codec subtracts an estimate of the echo introduced in the central offices. More particularly, digital data applied to the modem is multi-level encoded and mapped onto a subset of the levels recognized by the central office codec. In an illustrative embodiment, the multi-level encoding encodes the two least significant bits in the binary representation of the levels delivered by the modem. The least significant bit is encoded with a code that is more robust than the code of the next-to-least significant bit, in recognition of the fact that errors in the least significant bit are much more likely. The corresponding echo canceller in the network first estimates the echo and subtracts it from the signal applied to the digital network by the central office. Thereafter, a decoder decodes the received signal in stages. In the first stage, the least significant bit of symbols is identified based on a first error correcting code. In the second stage the remaining bits of the symbols are determined, with the aid of a second error correcting code and the results of the first stage. Block or convolutional encoding can be used.

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