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1.
To re-equalise the program material to provide a consistent tonal balance and
sonic signature;
2.
To prevent excessive limiting by the following peak limiter stages.
Because of the RMS based level detectors the multi-band AGC can re-equalise the
sound in a more natural manner than the peak limiter stages which use peak detectors.
As the human ear works on average loudness rather than peak level the re-equalised
audio will sound more natural when dynamic range reduction is per- formed by RMS
based level detectors.
Because the peak to average ratio of the program material can be quite wide it is still
necessary to control the peaks of the audio with the multi-band limiters but unlike most
other audio processors the bulk of the work has been performed by the multi-band AGC
and the limiters can be fed with a more controlled level allowing them to operate in
their sweet spot.
Over the course of the next few pages we have included several scope shots clearly
illustrating the input and output of the single band AGC together with the AGC control
signal. The effect of the control signal is clearly evident on the output audio waveform.
These scope shots help to visually illustrate the concepts under discussion. The multi-band
AGC stage is designed so that a 0VU input level to the processor will drive the multiband
AGC to the midway resting level of 0dB.
Individual drive controls are provided as a way of equalizing the audio before
processing. This can be used to add a touch of more presence or bass. Keep in mind
that the multiband AGC by its very nature will tend to compensate any cut or boost you
make here.
The attack and decay times of the AGC have a range of 1-10 and this corresponds to
time constants of 100mS to 30S. We suggest an attack somewhere in the region of 3-4
and a decay setting of 1 or 2 positions higher than that.
Effects of time constant speeds
The AGC stages in the DSPXtra-AM Encore are gated. This slows down the release time
of the multi-band AGC when the program material drops below a certain level, which
prevents noise suck up and gain hunting from occurring during quiet periods or lulls in
the audio.
AGC attacking and decaying with
faster time constants.
AGC attacking and decaying with
slower time constants.