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Aura8ip / Jan 2012
A u r a 8 i p P r o G U I
In the Aura8ip the L-R Output control adjusts the stereo part of the audio signal
(its depth, width, and spectral balance) without affecting the mono, or “dead center”
part (such as live voice). This permits very unique sonic signatures to be created that
cannot be achieved any other way.
Because there is an L-R Output mix control for each band, certain parts of the audio
spectrum may have their stereo width and depth enhanced (or reduced) independently
of the other bands.
The L-R Output controls also have a MONO position. When a band’s
L-R Output control is set to MONO, there is no L-R stereo width
or depth signal added by that band. A popular use of this feature is
to remove subsonic “mud” from the stereo difference signal. Bass
energy in most contemporary music has equal amplitude, in-phase
components in the left and right channels, so any L-R signal that
might creep into the low frequencies is probably not bass at all, but noise or other
undesired signals. By setting the Band 1 L-R Output (L-R Out) control to MONO,
extraneous signals that might otherwise have contaminated the bass frequencies are
eliminated.
The setting of the L-R Output controls also adjusts the amount of stereo en-
hancement present in the output mix. Adjusting the L-R Output controls to
positive numbers increases the amount of stereo separation for frequencies con-
tained in the bands whose controls are set above 0dB. A useful setting of the
L-R Output controls for tasteful stereo enhancement would be Band 1 = 0dB,
Band 2 = +3dB, Band 3 = +1.5dB. Of course reducing these controls has the opposite
effect, reducing stereo separation.
Makeup Gain
All of Aura8ip’s processing is accomplished by feed-forward control algorithms.
Feed-forward control has the advantage that it doesn’t rely on errors
in the compressor’s output signals (as do feed-back algorithms) in
order to dynamically control the gain. Rather than measuring mis-
takes in the output level and then trying to correct them by a (now
very late) control signal, feed-forward control prevents errors in the
output signal by carefully measuring changes in the input signal’s
levels and then calculating the precise amount of gain control needed
to achieve the perfect output.
Because the compressor’s output levels are controlled by changes in the input
levels, when the input levels go up, the compressor will push the output level down
by whatever amount of gain reduction is called for. This causes the output levels of
feed-forward compressors to need to be “made up” after processing, and this is ac-
complished with a control called Makeup Gain.
The amount of Makeup Gain needed is a function of how much gain reduction
was called for due to an increase in input level. In our design we leave around 6dB
of additional headroom to accommodate the compressor attack times and such, so if
20dB of gain reduction is being done by a band’s AGC/Compressor, then the correct
amount of Makeup Gain will be 20dB plus that 6dB, or 26dB.
The less gain reduction being called for, the less Makeup Gain that is required to
bring the signal back up to normal levels afterwards.
