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RAM Audio LMS244 Digital Controller
The top row of icons gives access to each output. The icon showing characters in red corresponds to
the selected output: all values of the different parameters (limit, ratio, gain, knee, attack and release
times, etc...) appear on the faders. The graph on the left shows the dynamic curve, expressing the
ratio between the input and output signal levels in dBu.
C.R.I. (Continuous Ratio Increment) Compressor / Limiter
Through the use of compressors and limiters it is possible to modify the dynamics of a signal. The use
of compressors reduces the dynamic range from a certain threshold whilst limiters do not allow the
signal to exceed a preset level.
The behaviour of compressors and limiters is described by their Dynamic Curve in which the output
signal level is linked to the input signal level.
These curves are shaped as follows:
Compressor
Limiter
dBu
dBu
threshold
limit
dBu
dBu
Input Level
Input Level
In the case of the Compressor, the output signal is in direct relationship to the input signal whilst below
the threshold level. Once exceeding this level the output is reduced by the selected ratio. In the
diagram the ratio is 1:2 which means that once the signal exceeds the threshold, for every 2 dB of
input increase, there will be 1 dB of increase in the output.
The limiter behaves as a compressor with a 1:0 ratio, and does not allow the output level to exceed
the threshold limit.
In both situations the behaviour is governed by attack and release time constants which control
response in relation to time.
The attack time corresponds to the time the compressor/limiter takes to act and allows the transients
of a signal to pass without lessening them. Very short attack times lead to a loss of transients in the
signal loosing the "punch" sensation. On the contrary, long attack times allow too much signal level to
pass which leads to some loss in the use of the compressor/limiter.
The release time is the time period between the reduction of the signal down to the threshold and the
releasing of the effect. Short release times allow the original signal level to be restored too quickly
which is then reduced again when faced with the arrival of the next higher signal level. This effect
induces the so-called "pumping" and in extreme cases even causes distortion producing an
unpleasant sound. In the opposite case of excessively long release times, it is not possible retrieve
signal level and there will be a loss of dynamics.
The adjustment of these times is very important for the final sound quality. As a basic rule of selection
of times, these should depend on the frequency range submitted by the input signal. For low and sub-
low frequencies, attack times should be selected between 16 and 10 milliseconds, for mid frequencies
between10 and 5 milliseconds and for high frequencies less than 5 milliseconds. In all three cases the
release times should be between 10 and 20 times greater than the attack times. If the signal to be
compressed or limited corresponds to the full-range bandwidth, then it is necessary to keep in mind
that almost all the signal energy is situated in the low frequencies and for this reason attack times of
around 10 milliseconds should be selected.
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