The Cat 21 module is a rather nice object in itself, and is actually the modular electronic 'heart'
of the system. It contains the discrete (simple transistors and other passive components) four
band noise reduction circuit. Those of us who are used to wielding a soldering iron might think
'ah, poor show - no component identification' on the PCB, but these modules were actually
assembled with state-of-the-art equipment that used a moving programmed laser spot to
indicate where the operator had to 'stuff in' each component.

Gallery

Dolby A is of course a very clever and subtle form of noise reduction that works very well
and produces very few audible 'artifacts'. It takes the well known technique of dynamically
expanding the level of signals when recording, and then automatically compressing the signals
(in reverse as it were) on playback. This expansion and compression (known as companding)
can be used to reduce tape noise, but it also usually sounds quite 'obvious'. Ray Dolby's
genius was to make his processing work with the way the ear actually works by exploiting the
ear's natural psycoaccoustic responses and reaction times. Because noise is (usually) at a
lower level than the wanted signals, he decided to only process low level signals which are
quite difficult to hear anyway, the rest is left untouched . This processing was also
simultaneously carried out in 4 carefully selected frequency areas of different bandwidths
(bass, lower midrange, midrange and treble). The result was that when correctly adjusted,
Dolby A could do quite a bit of 'spring cleaning' to a tape recorder and made the reproduced
audio seem much 'cleaner'. In addition, unlike the domestic B version, it was not nearly as
intolerant of incorrect calibration and record levels, and did not produce obvious problems
with frequency response and level 'pumping'. (Indeed non-Dolby playback of an A encoded
tape could sound rather good!)

For some years I used Dolby A with a 1/4 track Akai GX77 'domestic' tape deck with
remarkably acceptable (fantastic actually) results, even at 3 3/4 I.P.S.