21
Using the 20/20 AD
20/20 AD User Guide
When RELEASE is set to 0, the compressor acts like an amplifier with a non-
linear transfer function. For example, setting GAIN to 6dB and THRESHOLD to
-14dB produces a close approximation of a generic tape saturation curve. This
mode is also useful for overload protection of some types of percussive
instruments.
Note: This function is not available in 4-channel mode.
When recording live music onto 16-bit media, the recording engineer tries to
maximize dynamic range by keeping the signal as close as possible to full scale
without actually clipping. Peak levels are typically set a few dB below full scale,
because losing a few dB of dynamic range is far preferable to clipping.
The digital compressor eliminates this tradeoff. To use it, first set the input levels
to give yourself a comfortable margin between peak level and clipping. For
example, to give yourself 4dB of margin, push 1db SCALE and PK HOLD and
adjust the input level until the peak reads -4dB (the LED labeled 18).
Now, turn on the compressor . Set GAIN to 4dB. This increases the signal level
by 4dB (restoring 0dB peak level ). Set THRESHOLD to 0dB to automatically
select the maximum practical ratio.
When an input signal exceeds the peak level by 4dB or less, the compressor gain
is automatically reduced just enough to assure that the compressor output never
exceeds 0dB. After the overload has passed, the compressor gradually brings
the gain back up to 4dB. The RELEASE knob determines the speed of this
recovery.
Note: This automatic gain reduction can add considerable distortion to the
leading edge of the overload signal. With THRESHOLD set at 0dB (or at any
value less than or equal to the GAIN setting), gain reduction will not start until
the signal is about to clip — too late to prevent leading edge distortion. If this
distortion is unacceptable, try lowering the THRESHOLD setting so that
compression starts before the signal clips. If, for example, THRESHOLD is
lowered to -8dB, compressor gain reduction will be gradual, instead of being
reduced all at once at -4dB.
It is important to remember that the lower you set THRESHOLD, the sooner
compression will start, and the more likely you are to color signals which were
not in any danger of clipping.
Non-linear
Transfer Functions
Overload Protection
