Focusrite Red 1, Manual

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four channel microphone preamplifier
p a g e 7
compressor/limiter
Signals with wide dynamic range demand greater
attention from the listener, and require listening
conditions with low background noise. Consequently,
in areas with high background noise, such as a
restaurant, it is hard to listen to signals with a wide
dynamic range - only the loud parts are heard, with the
quiet parts being lost in the ambient noise.
Compressing the signal reduces the dynamic range and
so makes it easier to hear in such situations.
Similarly, the dynamic range of the signal can exceed
that of the medium used to carry it:
• 16-bit digital recordings (such as DAT) have a
theoretical maximum dynamic range of 96 dB. It is
essential that you do not exceed this limit
• Analogue tape has a dynamic range in the order of 60
dB (though noise reduction can add between 15 and
30 dB). It is not always necessary to limit dynamic
range when recording onto analogue tape, as the
tape saturates naturally when recording loud signals,
which in some cases can be useful
• FM radio has a dynamic range of 40 to 50 dB
• AM radio has a dynamic range of 20 to 30 dB
In all of these cases, you can use a compressor to restrict
the dynamic range of the signal to that of the medium.
The Controls
• The ratio and threshold controls set the
amount of compression applied to the signal
• The attack and release controls set the duration of
the compression
• The make-up gain sets the output volume of the
compressed signal.
Ratio
The ratio control determines how much compression is
applied to the signal. The ratio (such as 2:1) refers to
the ratio of change in input level to the change in
output level. So, a ratio of 2:1 means that for every 2
dB change in the input level, the output level changes
by 1 dB, as shown in the following diagram:
Setting the Ratio
As you increase the ratio, the sound becomes tighter
and the effect of the compression becomes more
noticeable. A lower ratio has a softer slope, which
preserves more of the original dynamic range, since an
increase in input level still results in a significant
increase in output level.
Threshold
The threshold determines when the compressor starts
to compress the signal. By setting a threshold, you do
not compress all of the input signal - instead, you
compress the signal only when it is louder than the
threshold, as shown in the following diagram:
Setting the Threshold
By setting a threshold, you determine that quieter
passages maintain their natural dynamic range, and
only loud passages (that go above the threshold) are
compressed.
10
0
0 10 20
2:1 Ratio
1:1 1:1 Out Out
In In
20
0
2
0
10:1 Ratio
10 20
In
Out
-24
+12
Threshold
Range