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 feet 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 into almost every modem and is simply a subroutine in the instruction program for an embedded
microprocessor. ==Advisory positions==