Compressor - Joemeek TwinQ User Manual

8

Compressor

The hardest device to understand, yet one of the most useful, the
PhotoOptical Compressor is what gives Joemeek products their unique
character. Its job is to make quiet sounds louder and loud sounds quieter,
or in other words to reduce the dynamic range of the programme material.
It's a bit like manually riding the volume control, except the compressor
does it automatically, responding far quicker and more accurately than you
ever could by hand. The compressor is applied in several ways:
1. Make Sounds Stand Out
Because compressors make loud sounds quieter, you can boost the volume
of the quiet bits without the loud bits getting even louder. That means you
can raise the average level of an instrument or vocal in the mix, which has
the effect of lifting it and bringing it forwards. This can actually improve
vocals for example, bringing them out in front of a mix, making them sound
denser, more even, and more confident!
2. Crank Up The Volume
Raising the average volume of whole mixes means they can be heard in
noisy environments, such as vehicles and factories. Boosting the average
level is what makes radio stations sound LOUD and the same technique is
used on TV commercials too, which is why they always seem annoyingly
louder than the movie you were trying to watch!
3. Protection
Fast response times are generally used to control brief transients. In other
words if an occasional peak sticks its head above a maximum permitted
level, the compressor clobbers it; this is known as limiting and a compressor
designed solely for this purpose is known as a Limiter. Limiters are primarily
used to protect recorders and monitor systems from overload, radio trans-
mitters from overmodulation, etc. The Joemeek compressor is not primarily
intended for this purpose as the Attack is not really fast enough to satisfy
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 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 bright-
ness that is often lost with other types of compressor. This is why vintage
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 COMPRESS 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
9
radio station requirements, although it is generally good enough to protect
recorders and monitors, where the effect of transients is less critical.
Normally you should not hear a limiter operating but if it is driven hard
constantly, it can render a mix somewhat flat and lifeless.
4. Accommodation
The dynamic range of the human ear is phenomenal, extending from
the threshold of hearing (eg: a pin dropping onto soft carpet) to threshold
of pain (eg: standing next to a jet aircraft) - some 120dBA in all. By
contrast, vinyl, cassette tape and radio broadcasts all have a dynamic
range of about half that. Since the advent of the CD, the dynamic range
of the medium is far less of an issue and compressors are used more
to give a certain "feel" to a production. AM and FM radio however, is still
very much compressed to fit its restricted dynamic range.
5. Modification
A compressor can change the dynamics, or "envelope" of the track and
it is here that the Joemeek Compressor excels!
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 potenti-
ometers and voltage-controlled-amplifiers, better known as VCAs.
The twinQ Compressor is a unique recreation of the sort of photo-
electric compressor used by record producer Joe Meek in the 1960's.
Using modern components for consistency and reliability, it none-
theless reproduces faithfully the same punchy sound that was so
characteristic of the pop records of that time.
more dramatic. At maximum (10:1) the Joemeek compressor effec-
tively 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 exag-
gerates 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 Release is
variable from 100mS 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 Comp in/out switch is operated.
The Compressor is after the Preamplifier and the Insert Point, and
before the Meequalizer.