Amati Communications Corporation

Mark B Flowers

John A Bingham

John Matthew Cioffi

Jacky S Chow

Michael D Agah

A transmission system includes transmit and receive paths coupled via a hybrid circuit to a bidirectional transmission path. The transmit path includes a digital-to-analog converter (DAC) having a given range and precision. The transmit path signal is limited and truncated to this range and precision before the DAC. In one arrangement including an echo canceller coupled between the transmit and receive paths, a difference of the transmit path signal before and after limiting and truncation is used to produce an estimated echo response which is subtracted from the receive path signal. In another arrangement, all of the inputs to the echo canceller are derived from the transmit path after the limiting and truncation. Instead, or in addition, the amplitudes of transmit path signal samples can be shaped in conjunction with the limiting. The system can in particular be an ADSL transmission system using multicarrier modulation. In such a system the amplitudes of transmit path signal samples can be scaled to reduce clipping, with an indication of the scaling being transmitted to a remote receiver.

Mitigating clipping and quantization effects in digital transmission

An oversampling technique that improves signal-to-quantization noise ratio without requiring substantial amounts of processing power is disclosed. The oversampling technique determines the oversample values by successively scaling the previous value using a scaling factor. The effect of the invention is the randomize the quantization error for each of the oversample values so that the quantization error becomes uncorrelated from one oversample value to the next. When such decorrelation occurs, the quantization noise is spread over the enlarged frequency band associated with the oversampling. The invention is applicable to any transmission scheme which utilizes oversampling and digital-to-analog conversion.

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Sunnyvale, CA, United States of America

Mark B Flowers