| 摘要 |
<p>ECHO CANCELLER In a fully digital telephone system in which the subscriber to exchange loops are of the twisted pair type it is necessary to be able to convey PCM in either one of two directions. To do this it is necessary to cancel the interference being caused by imperfections in the hybrid (4). This interference is removed by applying a defined fraction of the outgoing signal from the encoder (2) in the GO path via a sample and hold circuit (11) and an echo simulator (12). The latter includes coefficient generating circuits controlled by the signal in the RETURN path and by the outgoing signal. These coefficients are subtracted from the RETURN path signal to substantially eliminate this interference. Intersymbol interference is eliminated by a feedback equaliser (14) which controls the sample and hold circuit (13) in the RETURN path to force to zero an adaptable weighted sum of symbol spaced estimates of the overall transmitted symbol time response from the transmitter to detector, specifically shaped by both analogue and digital filters.</p> |
