Compressor (Cont'd)
Types of Compressor
Most compressors work in essentially the same way: a volume-
controlling element or "gain cell" is inserted into the audio signal
path. The level of the signal at any given moment is measured
and that information is used to control the gain cell. So if the signal
gets bigger, the volume is turned down. Various types of gain cell
in common use include FETs, valves (tubes), light-dependent-
resistors (photoelectric), digital potentiometers and voltage-
controlled-amplifiers, better known as VCAs.
The twinQ
2
Compressor is a unique recreation of the sort of
photoelectric compressor used by record producer Joe Meek in
the 1960's. Using modern components for consistency and
reliability, it nonetheless reproduces faithfully the same punchy
sound that was so characteristic of the pop records of that time.
Compression Ratio
What?? OK, it's simpler than it sounds. If the input gets 10dB
louder but the output only increases by 5dB then the compression
ratio is "2 to 1". If the input goes up 10dB but the output only goes
up 1dB, then the compression ratio is "10 to 1". In a theoretically
ideal compressor, this ratio is the same for any size of signal
above the threshold but for that to be true, the gain cell and its
control circuitry must be perfectly linear over a very wide range. In
practice only compressors based on VCAs and digital
potentiometers are likely to behave in this way.
Some compressors have a control to set the ratio anywhere
between 1:1 (ie: no compression), and 20:1 (which would be
regarded as a "brick wall limiter"). In the twinQ
2
the "Slope" or
"Compression Ratio" is variable from 1:1 (ie: no compression) to
10:1. Slopes around 3:1 are gentle for vocals while higher slopes
are hard for drums and guitars. However that's not all there is to it.
Variable Ratio
In the Joemeek optical compressor the compression 'threshold'
is not clearly defined and the compression ratio varies with the
amount of compression applied. Suppose the 'SLOPE' control is
set to 5:1. For signals only just exceeding threshold, the ratio is
little more than 1:1. As the compressor is driven harder, the ratio
rises to 5:1, at least up to a point. It is a feature of the Joemeek
compressor that the compression ratio actually reduces again
during large transients and, adjusted correctly, this helps to retain
brightness that is often lost with other types of compressor. This is
why vintage optical compressors often sound more lively than
their modern counterparts.
Controls
'COMPRESS'
sets the level of signal (the "Threshold") above
which the signal starts to be compressed. Turning the
compression control clockwise lowers the compression
threshold, and drives the compressor harder.
'SLOPE'
sets the average compression ratio applied to signals
above threshold. Lower settings (anti-clockwise) have less effect.
Turning the control clockwise increases the ratio and makes the
effects of compression more dramatic. At maximum (10:1) the
Joemeek compressor effectively becomes a limiter.
'ATTACK'
sets how quickly the compressor reacts to peaks
above threshold. Turn this control anticlockwise for a quick
response. Slower (clockwise) allows the fast leading edge of
percussive sounds to pass uncompressed for a moment, before
the compressor reacts to control the gain. This example of
"changing the envelope" of a sound exaggerates the percussive
nature of drums and other instruments. Settings around
mid-position are used where the compression needs to be less
obvious. Vocals for example, require Attack times around
10msec for natural sounding results. Faster attack times (anti-
clockwise) in conjunction with large amounts of compression,
result in extreme "pumping" effects.
'RELEASE'
sets how long the compressor goes on squashing
the sound for, once the signal has dropped below threshold. If it
stopped instantly there would be very noticeable modulation or
"pumping" of the sound. So we may want it to stop compressing
less abruptly and that is what the Release control is for. Generally,
the longer the Release time, the less obvious is the compression.
Of course some "pumping" might actually be desirable as a
special effect and that is another way in which the envelope of a
sound can be modified. The twinQ
2
Release is variable from
100msec up to 3 seconds giving a wide variety of effects.
How the compressor behaves actually changes with programme
content and volume. So experiment with the controls with
different kinds of material to discover the range and depth of
effects that can be achieved. The 'COMP' in/out switch allows
comparison between compressed and uncompressed sound
(blue LED lights when active). Remember that the 'MAKE UP
GAIN' is there to restore the level of the signal after compression.
Correctly adjusted, there will be no change in volume as the
Compressor 'ON' switch is operated.
The Compressor is after the Preamplifier and the Insert Point,
and before the Meequalizer.
'COMP LINK'
This is important when the twinQ
2
is used for stereo work. When
two mono compressors are used for stereo, differing amounts of
gain reduction occur in each channel, which causes the stereo
image to wander. The 'COMP LINK' switch avoids this problem by
summing the compressor control paths of both channels and
assigning control of ("slave") channel 2's gain cell to ("master")
Channel 1. The 'COMPRESS', 'ATTACK' and 'RELEASE'
controls on the Channel 1 then operate both channels and the
corresponding controls on Channel 2 are redundant. Note
however that all other controls still operate independently on both
channels and must be set the same for correct stereo balance.
The LED lights when active.
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