28 Aug 2004 AGC-100 page 8 of 11
6.0 THEORY OF OPERATION
The AGC-100 can proportionately change its gain setting in response to program level. The
AGC-100 operates in an adaptive-slope mode, which approaches 2:1 (dB) correction at lower
signal input levels, and approaches constant-level output (infinite correction) at higher input
levels, thus preserving good dynamic range at normal levels, but preventing significant over-
modulation if the input levels exceed normal.
The basic action of the AGC-100 is quick attack (gain reduction), with delayed, gradual release
(gain increase).
Quick attack reduces the gain rapidly enough that the human ear does not perceive any
excessive levels from too-strong signals. Supplementing gain reduction is instantaneous hard-
limiting to prevent any sudden transients from causing severe overmodulation. (This limiting is
seldom active, due to the fast attack of the AGC.)
Delayed, slow release preserves nearly all the normal dynamic range of program material,
while effectively reducing undesired increases of background sounds ("pumping") during lulls
in the foreground program.
More detail of the action of the AGC-100 is presented in the following paragraphs.
When operating in the ON mode, signal input levels which would normally result in an output
above 0 VU cause a rapid gain reduction (approx. 5 mS) to normal output levels. Clean gain
reduction to prevent over-modulation is limited only by the dynamic range of the AGC-100's
input stage, which can handle over +20 dBm.
Following any automatic gain reduction, there is a ten-second period during which the gain will
not increase. The delay applies to gain increases only; any program peak above normal will
rapidly reduce the gain to nearly 0 VU at any time, and will re-initiate the 10-second delay.
This delay significantly improves the smoothness of the program level by maintaining constant
gain between speech syllables and paragraphs, music rests, and downbeats.
Following this delay, if the signal level drops from its earlier peak, and remains somewhat low,
the gain will gradually increase, at a rate of approximately 1 dB per second, until a new
program peak at 0 VU halts the increase. In order to prevent excessive increases in
background noise during sustained relatively-quiet passages, the maximum upward correction
is limited to 12 dB gain increase above the Setup level.
The AGC-100 also pauses when the program pauses. This process avoids increases of
background sounds during program pauses (sudden level drops over 20 dB) by holding the
gain constant at the last setting prior to the pause. If the program returns within 10 seconds,
the AGC action resumes where it left off. However, if the pause exceeds 10 seconds, the AGC
silently returns to the Setup gain setting (no relative correction). The AGC then waits for the
program to return, at which time the AGC automatically returns to the active mode, rapidly
adjusting the gain upward or downward as appropriate.
A brief exception to the slow release occurs whenever the AGC-100 first enters the active
mode, whether manually switched to ON from OFF / SETUP, or, if already in the ON switch
position, automatically, when the program returns following an extended pause. The release
