Fostex 3070 - Page 9

4. OPERATION
The following is a simplified explanation of the operating
technique behind the 3070 compressor/limiter with expander/noise
gate function.
With no compression or gain reduction, the ratio of input
level to output level, other parameters being equal, would be
linear. See Figure 2.
Fig. 2
One of the most outstanding features of the 3070 is the
control element. Gain reduction is accomplished by a VCA circuit,
which is controlled by pulse width modulation.
In the VCA circuit there is an electronic switch which opens
and closes at a rate of 200KHz. By varying the length of time
this electronic switch is open or closed during each cycle,
signal energy is reduced without distorting the program.
An important benefit of this type of circuit is lower dis¬
tortion. Continuous low frequency response, especially, is much
lower in distortion than typical units when the attack time is
set fast. For this reason you have more latitude in the use of
the release control, without worrying about gross pumping and
breathing effects.
Finally, the detect circuit which controls the VCA operation
is externally accessible on the 3070. There really are no rules
or theory of operation here, you are in the realm of creative
audio effects. Personal taste is the determining factor.
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