International Standard Electric Corporation

David Nyman

James A Murray

In a digital system, especially where PCM coded speech is conveyed over twisted pair cables optimized for analogue speech, a pulse is received distorted, the distortion being especially bad on the trailing edge of the pulse. This decays slowly enough to cause intersymbol interference with the next pulse. This is overcome by applying the signal from an amplifier (11), FIG. 4, to one input of a subtractor (12) via a delay and attenuation circuit (13-15) and to the other input of the subtractor directly. The delay (13) is such as to optimize the peak of the received pulse, and the attentuation level is calculated to cancel out the tail of the received pulse so that it is zero at the center of the sampling point of the next bit.

Adaptive equalizer

An adaptive equalizer for a twisted pair telephone line comprises a differential input/output amplifier, with the line connected to its positive input via a resistor and DC blocking capacitors. The junction of these capacitors is grounded via an adjustable impedance. The amplifier has a feedback loop with another adjustable impedance also connected to the negative input of the amplifier. Line impedance variations vary the impedance of both of the adjustable impedances, which, include diodes to separately compensate for the line's attenuation and frequency characteristics.

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New Barnet, United Kingdom

David Nyman