The Technical Stuff
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As you can see from the illustration above, at a low ratio, a compressor has relatively less effect on the
incoming signal; at higher ratios, it has more effect. When a channel is operating in compression
mode (that is, when its front-panel switch is set to COMPRESS), the 2-LA-2 uses a compression ratio
of approximately 4:1; however, as we will see shortly, this is program dependent, so that the actual
ratio changes according to the frequency content of the incoming signal.
While the terms “compression” and “limiting” are often used interchangeably, the general definition of
compression is gain reduction at ratios below 10:1; when higher ratios (of 10:1 or greater) are used,
the process is instead called
limiting
. Limiters abruptly prevent signals above the threshold level from
exceeding a certain maximum value. At very high ratios of 20:1 or greater (some limiters even offer a
theoretical infinite ratio of Infinity:1), “brick wall” limiting kicks in—that is, any change in input, no
matter how great, results in virtually no increase in output level. Infinity:1 is the ratio used by the 2-
LA-2 when a channel is operating in limiting mode (that is, when its front-panel switch is set to
LIMIT); however, the 2-LA-2 is designed so that the same warm sonic characteristics are retained,
even though more severe gain reduction is applied.
As an aside, an
expander
is the opposite of a compressor: a device which
increases
the dynamic range
of a signal. For example, a 10 dB change in the input signal might result in a 20 dB change in the
output signal, thus “expanding” the dynamic range.
Knee
A compressor's
knee
determines whether the device will reach maximum gain reduction quickly or
slowly. A gradual transition (“soft knee”) from no response to full gain reduction will provide a gentler,
smoother sound, while a more rapid transition (“hard knee”) will give an abrupt “slam” to the signal.
The 2-LA-2 utilizes soft knee compression and limiting, which is generally preferred for most musical
applications; hard knee compression or limiting is more often used in applications where
instrumentation (such as broadcast transmitter towers) must be protected from transient signal
overloads.
