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4.4 Wide-band Automatic Gain Control (WB-AGC)
Figure 35: Automatic Gain Control (AGC)
The wideband AGC works with the full-range audio sig-
nal. Its function is to compensate for differences in level
in the input signal of the processor, so that it reaches the
following stages with constant level. The time constants
of the AGC are a very important adjustment, and they
change according to the characteristics of the sound ma-
terial. Example: An optimized setting for voices will re-
quire different tempos to another one created with mu-
sic.
4.4.1 Target level
Target level is the value at which the AGC adjusts the in-
put signal, amplifying it when it is less than that value, or
attenuating it when it is higher. The output of the AGC al-
ways tends to the target level. The variation of the output
level of the AGC is displayed dynamically in a time chart
(AGC Out).
The gain range set the degree of attenuation or amplifi-
cation that the AGC can apply to the signal. For example:
if the gain range is 20dB, the AGC can compensate for
variations of up to 40dB in the input signal.

If the hold threshold falls within the gain compensation
range, the signal freezes, does not compensate. For exam-
ple: if HOLD is -40dB, target -22dB and range is 20dB; The
AGC could compensate for a -44dB signal. But if the signal
drops to -44dB the AGC freezes. The gain compensation
will occur from the hold threshold.
4.4.2 Hold
The AGC is of the triggered type. If the input signal drops
abruptly, the AGC does NOT change its gain, but
"freezes" its current value, remaining in that state until
the signal exceeds the "hold" threshold. The value does
not remain frozen indefinitely. While the signal remains
below the "Hold" threshold; the AGC level will slide to the
target level, with the slope defined by the "Return to refer-
ence" value.
Without this feature, the AGC would continually compen-
sate for the input level and in long silences would start to
raise background noise, because in the absence of signal
the AGC would increase its gain to the maximum possi-
ble. With the triggering technique this drawback is elimi-
nated.
On the other hand, it is possible to adjust it to preserve
part of the dynamic range in that music that is character-
ized by great changes between pianos and forts. That is
to say: if after a strong passage there is a subtle input of
an instrument, the AGC will be frozen in its previous level,
allowing the volume contrast.
4.4.3 WB-AGC attack time
It is the time it takes for the AGC to reduce its gain when
the input signal increases. As a general rule, it can be said
that the attack time must be slow to prevent the AGC re-
acting with transient volume increases (a laugh or a musi-
cal beat). For the voice, times of about 15 sec are ade-
quate, while for music values of 20 sec or slower work
well.
The attack time only takes effect while the signal level
moves into the "Outside Window". When the signal level
drops out from "Outside Window", the AGC goes into quick
response mode (See below for Outside Window).
Note that when the signal level increases sharply, during
the attack time the signal is contained by the multiband
compressors, which act strongly until the AGC adjusts its
gain. Depending on the processing stage settings, a very
slow AGC attack time can cause excessive signal com-
pression (especially in the voices), which generates a very
audible unwanted effect.
4.4.4 WB-AGC recovery time
When the input signal lowers its level, the AGC starts to in-
crease its gain to compensate for the level of the input sig-
nal. The time taken by the AGC to compensate for the re-
duction of the input level is called the recovery time. Re-
member: The function of the AGC is that the signal
reaches the following process stages with a stable level,
independent of the output level of the mixing console.
This recovery time takes effect while the signal level is
inside the "Outside Window" range. When the signal level
falls out of that range, the AGC switches to using the fast
response times.

Wideband AGC (WB-AGC) times interact with multi-band AGC
(MB-AGC) times, as both controls changes the signal level at
the same time. When the MB-AGC is active, the gain variation at
the output of both AGC's will be faster than that indicated by
the WB-AGC controls, depending on the range of action in
which the multiband AGC is working.
4.4.5 Outside window (fast)
Defines a range or "window" of levels that determines the
behavior of the WB-AGC depending on the input level.
While the input signal remains within that range of levels,
Audio Processing Core - SOLIDYNE 542 APC Page 33