Reference Manual
CHAPTER 20. LIVE AUDIO EFFECT REFERENCE 254
dynamic processing applications including limiting and loudness maximization. Compressor
II's design is a lot more sophisticated and capable than Compressor I's. It includes frequency-
selective compression using a sidechain EQ, variable look-ahead times and two response
modes, Peak and RMS. Compressor I is still a valuable sound design tool in spite of, or rather
because of, its simplicity and roughness, which can add interesting avors to your sounds.
A compressor can only react to an input signal once it occurs. Since it also needs to apply
an attack/release envelope, the compression is always a bit too late. A digital compressor
can solve this problem by simply delaying the input signal a little bit. Compressor II offers
three different predelay times: zero ms, one ms and ten ms. The results may sound pretty
different depending on this setting. Keep in mind that using ten ms of predelay makes the
output appear signicantly later. You may have to delay other tracks with a Simple Delay in
order to stay in sync.
Compressor II can either react to short peaks within a signal or to something that is more
related to how people perceive loudness. The parameter for this is the Peak/RMS switch. If
you intend to use Compressor II as a limiter in the master section, Peak is probably better
since it reacts more to the actual signal level, while RMS is usually more musical. But as
always if it comes to compression, trust your ears and not the meter!
The most exotic feature of Compressor II is the sidechain EQ. The sidechain is the part of
the signal that is used to control the compressor. Normally the sidechain signal is the same
as the input signal. However, it can make sense to apply some ltering here. Imagine a bass
drum, a snare and some chords in the background. The bass drum has a pretty high level
and it will normally determine how the compressor reacts. If you now turn the side chain EQ
on and set its Frequency to 100 Hz and the Gain to -15 dB, the bass drum will not inuence
the compression anymore and the behavior of the compressor will be very different. You
could also set the Frequency to around 1 kHz and turn up the gain to make the compressor
more responsive to the snare. Since the EQ is only in the sidechain and not part of the
normal signal path, it will not change the sound of the input signal. It only changes how the
compressor reacts to different frequencies of the input.
Unlike Compressor I, Compressor II has a built-in compensation stage that counteracts the
gain loss due to the compression and makes it much easier to adjust the other parameters.
Careful adjustment of attack and release times is essential when it comes to compression of
rhythmical sources. If you are not used to working with compressors, play a drum loop and
spend some time adjusting Attack, Release, Threshold and Gain. It can be very exciting!