Midas PRO6, Owner'S Manual

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Appendix A: Application Notes
PRO6 Live Audio System
Operator Manual
Attack
The attack control adjusts the time taken for the compressor to respond to an
over-threshold signal. The shape of the attack can be selected from one of the five
mode combinations mentioned above, making the compressor easily adaptable for a
wide number of creative and corrective applications.
Release
The release control adjusts the time the compressor takes to recover after the
programme material falls back below threshold. Both attack and release also respond
to changes in programme level that remain over-threshold. For example, a signal that
reduces in level but remains above threshold will still trigger a release, but in this case
it will only be a partial release. This is because the compressor will still be required to
generate gain reduction, but now, as appropriate for the new lower signal level.
Knee
Most compression sounds more natural in soft knee mode. Soft knee compression blurs
the distinction between over-threshold and under-threshold signals, such that signals
that are a long way below threshold remain unaffected by compression, and signals that
near the threshold get compressed, but at greatly reduced ratios. When signals are
just over-threshold the compressor ratios are still somewhat reduced; it is only when
signals go well over threshold that the full ratio compression is applied. When using a
harder knee setting the compressors operate in a more clinical way with a more defined
transition between under-threshold and over-threshold; this is better suited to limiting
style compression.
Gain
The gain control provides adjustment of the make up gain so that the level of the
outgoing compressed signal can be matched to the incoming uncompressed signal. It
can also be used to drive the clipper hard (see “Soft clip level” on page 312) to produce
more pronounced effects.
Side chain filter
A band pass filter is provided that acts on the side chain signals. This can be used to
make the compression frequency selective. The controls for this are frequency,
adjustable from 50Hz to 15kHz, and bandwidth selectable as wide medium or narrow.
Additionally, there is a listen function that places the filtered side chain onto the solo
bus and a side chain filter in to activate or eliminate the filter action.
Soft clip level
When compression is used creatively with slower attack times, it is possible to generate
very large peak signals that can eat up headroom. When soft clip is activated the
compressor output (post-make up gain) is fed through a final fast acting soft clipper
circuit. This adds progressive gain reduction to any signals that exceed a threshold set
by the clip level control. When set sparingly, this can recover 3dB to 6dB of headroom
without introducing any undesirable audio artefacts. (In the past, limiters have been
employed for this function but their time constants are invariably intrusive on the
programme.) The high-speed nature of the soft clipping introduces no time related
artefacts. This is because it only acts during the transient peaks, when it produces
predominantly third harmonic distortion, which is very musical in nature.