Circuit for sensing cable effects for automatic equalization

DETAILED DESCRIPTION What is claimed is: 1.
An automatic equalizer comprising: a variable equalizer having an input port to receive a digital signal from a transmission channel, an output port to provide an equalized digital signal, and a control port to which is applied a control signal that controls an equalization function for the variable equalizer; and a sensing circuit having an input port to receive the equalized digital signal and an output port to provide the control signal, the sensing circuit detecting under-shoot or over-shoot of transitions in the equalized digital signal and varying the control signal accordingly.
2.
The automatic equalizer as recited in claim 1 wherein the sensing circuit comprises: means for modifying the equalized digital signal at the input port to an absolute value signal having a constant average voltage; means for generating from the absolute value signal a slope signal that is proportional to the slope of the absolute value signal; and means for convening the slope signal to the control signal.
3.
The automatic equalizer as recited in claim 2 wherein the modifying means comprises: an automatic gain control circuit having the equalized digital signal as an input, the gain of the automatic gain control circuit being controlled by a gain control voltage; an absolute value circuit coupled to an output of the automatic gain control circuit for converting the equalized digital signal output from the automatic gain control circuit to the absolute value signal; and a lowpass filter having the absolute value signal as an input and providing the gain control voltage as an output.
4.
The automatic equalizer circuit as recited in claim 3 wherein the generating means comprises a differentiator circuit having the absolute value signal as an input and providing a differentiated absolute value signal as an output, the differentiated absolute value signal being the slope signal.
5.
The automatic equalizer circuit as recited in claim 4 wherein the converting means comprises: a limiter circuit having the slope signal as an input and providing a clipped slope signal as an output, the limiter circuit clipping either positive or negative peaks in the slope signal representing over-shoot or under-shoot; and means for integrating the clipped slope signal to produce the control signal