Lexicon 300, Owner'S Manual

Page: 87 / 155

4-31
The Algorithms and their Parameters
General Description
Compressor
The Compressor algorithm is a true digital compressor which will run in either
Dual Mono or Cascade Setups. In Dual Mono Setups, it configures to mono in
mono out; in Cascade Setups it configures as a true stereo effect. The
compressor can be described as an upwards averaging compressor.
Digital
compressors, like analog compressors, decrease audio above a given thresh-
old. Unlike analog compressors, they increase gain below the threshold. The
result from either analog or digital compression is exactly the same — less
dynamic range
.
As shown in the diagram below, the audio path takes two routes. One path goes
through a predelay mechanism which delays the audio a maximum of 48 ms. The
other path sends control information (dependent on the settings of slope,
threshold, attack and release) to “digital VCA’s” labeled MAX COMP GAIN and
EXP GAIN. As the signal crosses the threshold point, both Compressor Gain
and Expansion Gain vary constantly. Compressor Gain determines the maxi-
mum amount of gain increase below the threshold; Expansion Gain determines
the maximum amount of gain attenuation below
the threshold.
Generally, digital compression requires lower threshold settings than analog
compression. The reason for this is that there is no such thing as headroom in
a digital system — dBfs (Digital Full scale [0VU]) is the maximum level audio.
Audio dynamics below full scale, however, can be manipulated and modified.
Example
1.
Press SETUP SELECT and turn the Softknob to select Cascade Setup: 313
STEREO COMP. This preset loads the stereo compressor into both ma-
chines but mixes Machine B to full dry. (no point in compressing twice!)
2. Press ENTER to load the preset.
3. Press EFFECT EDIT. Note the front panel level meters and try to establish
the average input level. A simple approach might be to look at the peaks and
valleys in signal amplitude and estimate the middle ground. If this all seams
too complicated, just select THRS and set it to -17dB.
4. Select GAIN and set it to Max (9 dB). At this point there should be no effect
on the dynamics because you’ve not yet adjusted the slope parameter.
5. Select SLP and increase it to 2.0 : 1 (a 2 to 1 Compression Slope). Now you
should hear a change. If you still do not hear a net effect, readjust THRS until
something becomes audible. Remember, as you adjust the threshold
clockwise (closer to digital full scale) more and more audio will be gain
increased — overload can occur!!
The whole purpose of a digital compressor is to maintain peaks while compress-
ing lower level audio signals upwards. If you think about it in terms of the most
significant and least significant bits, you would never want to reduce the most
significant bits — you want to increase the least sigificant bits.
Adding some pre-delay gives the control mechanism time to react before the
audio reaches the digital VCA. Of course, the more predelay you add, the more
“out of sync” the audio will become. Attack constants should be kept to short
values (7,15, or 30ms). A good starting point for release time is 91 or 114 ms.