Standard Communications TVM Series Owner's Manual page 58

Option Installation
Principles of Operation
With the AGC switch OFF, the AGC2000 has no effect on signal levels, and ad-
justments may be made manually with the front panel controls. When ON, the
gain of the four signals (LEFT, RIGHT, SAP, and AUX) are automatically con-
trolled. Left and Right signal power levels are sensed with a true RMS sensor
and summed. The average is applied to gain-control amplifiers for the stereo
pair. The two additional channels are monitored with true RMS sensing, with in-
dependent gain control applied to separate gain-control amplifiers. RMS power
sensing is performed throughout, because RMS sensing more closely conforms
to the human ear mechanisms that either peak or average deviation sensing.
The power input is compared to the reference level determined by the Unity Gain
jumper (P7). Input signals above that level are reduced in amplitude (over time)
while signals below that level are increased in amplitude (over time) in a ratio of
4:1 input to output. (See Table 5-4.) For example, if the reference level is set to
–12 dB, an input signal of –2 dB will be 10 dB above the reference level. The
AGC reduces this difference to one fourth of that value, or 2.5 dB. The output will
be –9.5 dB, or 2.5 dB above the reference value.
The system has a 'quiet' detector that freezes gain changes and holds for 30
to 60 minutes when the level drops sharply, i.e., more than 18 dB. This prevents
gain increases from outputting background noise when no signal is present.
To prevent overmodulation, the unit includes a limiter circuit with extremely fast
time constants. It aggressively slows the gain increase in a smooth manner to
prevent distortion byproducts. The last 2 dB of output (0 to –2 dB) are reserved
for the limiting process. Hard clipping can occur only when input buffers are
overdriven (above +18 dBm).
The gain adjustments contain specific lag times to provide the listener with an
added perception of wide dynamic range. Sudden loud passages are reduced in
gain in a short but not instantaneous time that leaves the sensation of extreme
loudness without overdriving the system. Softer passages are increased in gain
over a longer period to provide consistent output levels without noticeable fluc-
tuations in quiet passages. Gain control without these limiting and gain lag time
characteristics may sound harsh, flat or overly compressed.
Users need to select a unity gain point that allows headroom for their maximum
RMS power excursions. The default setting of –12 dB tends to force the average
audio level to this reference level, and is most likely to be the correct setting un-
der almost all conditions.
In normal operation, the LEDs should never show modulation over 100% be-
cause of the soft clipping. If the LED bar graph shows consistently low levels that
are never limited, consider increasing the reference level setting to increase av-
erage power level to the Stereo Generator to improve signal-to-noise and bal-
ance levels between the modulated channel and others in the system.
TVM450/S/TVM550/S/TVM550II/TVM550IIS Owner's Manual Page 5-21