9
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 threeQ the “Slope” or “Compression Ratio” is fixed at 5:1.
This is a good compromise between being gentle enough for vocals and
hard enough 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 com-
pression applied. 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 com-
pression 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 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.
‘ATTACK’ sets how quickly the compressor reacts to peaks above
threshold. Turn this control anti-clockwise 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. Faster
attack times (anti-clockwise) are used where the compression needs
to be less obvious. For vocals the Attack needs to be around 1msec
for a natural sounding result.
‘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 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 threeQ Release is
variable from 100mS up to 3 seconds giving a wide variety of effects.
How the compressor behaves actually changes with programme con-
tent 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 com-
pressed and uncompressed sound (green LED lights when active).
The Compressor is after the Preamplifier and the Insert Point, and
before the Meequalizer.
8Compressor
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
radio station requirements, although it is generally good enough to pro-
tect recorders and monitors, where the effect of transients is less criti-
cal. 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 thresh-
old 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-con-
trolling 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 threeQ 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
nonetheless reproduces faithfully the same punchy sound that was
so characteristic of the pop records of that time.