XTA apa-4e8, Руководство оператора

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Page 68 APA-4E8 Operator’s Manual
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Peak Limiter
The main limitation with traditional dynamics control is the inability of the processing to react
truly instantaneously to the signal. One of the most significant advantages of digital signal
processing over analogue is the ability to delay the audio signal precisely and without
extensive complex hardware. The entire domain of digital signal processing is based around
the combination of delaying, multiplying, and accumulating numbers (representing samples
of audio) to implement all the filters and dynamics processing we have come to expect today.
In the case of dynamics processing, being able to delay a signal allows the processor module
to delay the main signal in relation to the sidechain (the signal being monitored relative to the
threshold), so that it can compensate for peaks prior to the arrival of the main signal.
Consider the situation of a monitor engineer listening to a band perform. Having no access to
dynamics processors, he has had to resort to manually ‘riding the faders’ in an attempt to
keep control of the levels. Should the level of one of the channels on his desk reach an
unacceptably high level, he will turn it down appropriately.
There is a hidden sidechain
in operation even in this
case. The main signal path
is fed through the monitor
desk and the gain
controlled by adjusting the
fader. The sidechain is
formed by the feedback
path between the
engineer’s ears checking
the level and his brain
instructing his hand to turn
the fader down if the
volume goes over the
threshold he has chosen.
In this case, the delay between the signal actually going over the threshold, the engineer
registering the situation, and then turning the signal down will be in the order of several
hundred milliseconds at best. This will only be true if he is not distracted — in reality, it may
be several seconds before any gain reduction is imposed on the signal to bring it under
control.
For an analogue dynamics processor, the
situation is much better. Controlling the
gain electronically, and not relying on a
human sidechain feedback mechanism, it
can react much more quickly.
The red waveform represents the input to
the dynamics module, with the dotted line
showing the threshold for gain control to
occur. There are several peaks towards the
start of this signal that are above the
threshold, and so the dynamics processing
should react to these as appropriate. (In
this case reduce the gain).