Adaptive Equalizer
In the early 1960s the highest speed for modems on telephone lines was 2400 bits per second. Higher speeds were not possible because of intersymbol interference. Each dialed connection would have a different distorting effect on the series of pulses sent to convey digital information, smearing successive pulses and entangling them, resulting in errors in detection.
Lucky invented a way to adaptively undo the smearing effects by automatic adjustment of a variable filter, using a tapped delay line with adjustable gains at each tap. The equalizer used a steepest descent algorithm to minimize distortion. It was initially trained by a series of known pulses, but during actual data transmission it relied upon decision-directed adjustment, which assumed that decisions about pulses were largely correct and could be used for reconstruction of the ongoing error.
The first adaptive equalizer, in 1964, used 13 adjustable gains, each set by 8 relays. The rack of equipment was about 5 foot high. Its use immediately made possible data transmission at 9600 bits per second – four times the highest previously attainable speed.
Shortly thereafter, the relays were replaced with transistor switches, then in succeeding years the equalizer was implemented with a special purpose integrated circuit. Today adaptive equalization is built in to almost every modem and is simply a subroutine in the instruction program for an embedded microprocessor.
Read more about this topic: Robert W. Lucky
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