Setting Up The Processing
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The last gating option available to the wideband AGC is window gating. Window gating is separate from the
silence gating that we have been discussing in that it does not work on the amplitude of the audio, rather the
peak to average ratio of the waveform and how the waveform is changing over time. The window gating feature
if enabled freezes the gain over a pre-defined range and will only let gain control commence once the waveform
has fallen outside of this pre-determined range. This is useful because it does not apply gain correction to audio
waveforms that are only changing by small amounts. The images below clearly illustrate the effect of window
gating on the control signal that controls the output waveform. The size of the window can be set to 1dB, 2dB or
3dB with an 'OFF' option to turn the window gating off. Window gating has the extra advantage in that it enables
us to use faster time constants than what would have been possible without it. Faster time constants have the
disadvantage in that the constant re-adjustment of the waveform can become audible. With a higher window
gating setting we are able to reduce the audibility of the faster time constants by freezing out these changes
when the waveform is within our window. Once the waveforms falls outside our window the faster time constant
will track it quickly and effortlessly and then the window gating will kick back in.
No silence or window gating. Gain riding is
clearly evident
Silence gating enabled. Less gain riding is
evident
Silence and window gating enabled. Gain riding
is virtually removed.
The effect of the gating controls on program material
AGC attacking and decaying with faster time
constants
AGC attacking and decaying with faster time
constants but with a 2dB window gating setting
The effect of window gating
There are several hidden automatic controls that help with the operation of the AGC. One example of a hidden
control is a speed-up mechanism to stop AGC overload when you have set attack times too slow to deal with the
overload. If you see strange meter activity that seems to defy your settings then it could be down to one of these
hidden automatic controls.
Bass enhancement
The frequency contouring effect of multi-band audio processors often leaves the bass lacking a little. The sum-
mation of the bands tends to give a boost to the presence frequencies and leaves the bass sounding a little thin.
This effect can be compensated somewhat by enhancing the bass prior to multi-band processing.
The DSPX has two types of bass enhancement filter. A low frequency shelving boost filter and a peaking bass
equaliser.
The shelving filter has a 12dB/octave slope and can be adjusted to provide between 0 and 12dB of bass boost.
Use this control with caution as too much low frequency boost can cause loss of mid-bass because the low bass,
often inaudible on many receivers dominates the gain reduction of the BAND 1 AGC and limiter. A setting of 6dB
is a good compromise and starting point.
The peaking bass equalizer is a pseudo parametric style bass equalizer control that allows you to sweet tune
the bass. Four frequencies, amplitudes and Q's are provided giving you 64 different bass curves to select from.
