Plant variability and uncertainty are formidable adversaries. An anecdote which
serves as a reminder of this fact can be found in Harold Black’s retrospective on
his invention of the feedback amplifier [30]. At one point, he describes the operating
procedure for his newly invented feedforward amplifier: “. . . every hour on the
hour—twenty four hours a day—somebody had to adjust the filament current to its
correct value. In doing this, they were permitted plus or minus 0.5 to 1 dB variation
in the amplifier gain, whereas, for my purpose the gain had to be absolutely perfect.
In addition, every six hours it became necessary to adjust the battery voltage,
because the amplifier gain would be out of hand. There were other complications
too. . . ”. Despite his subsequent discovery of the feedback principle and the tireless
efforts of many researchers, the problem of plant variability and uncertainty is still
with us