COMPRESSION BASICS – THREE EXAMPLES OF COMPRESSORS
Soft knee (over-easy) compressors
These are slightly different in design. Some compression is applied to all signals regardless of level. However, the compression ratio is very small with low level signals rising as the signal level increases. A single control sets the severity of the compression effect by determining the relationship between the compression ratio and the signal level. Most engineers agree that this type of compressor tends to give a warmer sound than those which possess a fixed threshold and has the added advantage of being easy
dbx 165A compressor limiter
to set up and operate. Some soft knee models possess attack and release controls whereas others operate completely automatically.
dbx 165A, which is a very flexible compressor of the over-easy
or soft knee variety. It also possesses a stereo coupler (link switch) which allows for
combination with a second unit in the event stereo compression is required.
The Urei 1176 LN
The Urei 1176 LN is perhaps the most popular of all studio compressors
It is available both as a plug-in and as an external hardware unit. It has
four ratio settings: 4:1, 8:1, 12:1, and 20:1. (The last ratio should be considered a
limiting setting). It also has selectable meter readings. By setting the meter switch
to GR, gain reduction is indicated. By switching to either +4 or +8, the output level
can be monitored at two different levels. (In the recording studio, +4dBm = 0VU
but in radio broadcasting +8dBm = 0VU). Although this unit is termed a limiter,
the lower ratio settings allow it to be used in gentler compression modes.
The Teletronix LA2 (and LA2A)
No discourse on compressors can be complete without mentioning one of
the originals – the LA2, which is still being used in many state-of-the-art recording
studios. (The LA2A was a later but almost identical model). There is also a software
plug-in available which emulates the sound of the LA2A. These compressor/limiters are favorites amongst engineers simply because of their warm tube-driven
sound which is quite different to that obtained from their transistorized descendants.
These ancient beauties have only three controls on the front panel: the gain
reduction control, output level control and meter switch (indicating either gain
reduction or output level). On the back of the unit is a two-position switch which
provides either a 3:1 compression ratio followed by 20:1 limiting, or alternatively
just 3:1 compression. The first option gives peak limiting plus slight compression.
The preset attack time is 10 milliseconds, but the release time is governed by
the content of the program material, so that repeated peaks cause reduction only
once and not for each individual peak. The gain returns halfway to normal in 60
ms, and the other half takes somewhere in between 0.5 and 5 seconds, once again
depending on the program material.
Compressors and limiters are useful tools, but they should be
used only where necessary. Over-compression can make a signal sound
very thin and destroy its natural characteristics. Most of the time they
are purely correctional devices but they can be used creatively.