8.2.8 Module:
AGC
AGC (Automatic Gain Control) is designed for an automatic and subtle gain readjustment if the signal
level varies.
As opposed to the compressor which responds quickly to short-term level modifications, the AGC will
only become active if the signal level is above or below the set target level over an extended time.
The use of AGC can be of great benefit for many applications, but always make sure to exactly know
the functionality and carefully set the parameters. AGC has substantial impact on the audio signal.
Always consider carefully whether or not you need this function.
Functions in detail:
The control amplifier used for the AGC function references to the adjustable
Target Level
. Gain is increased when the signal level is lower than target level. On the other hand, the
signal is attenuated when the signal level is higher than target level.
If the signal falls below
Freeze Threshold
(adjustable -20dB … -40dB), AGC
Freeze
function freezes
gain to the current value. Signals below freeze threshold are supposed to be signal gaps or
background noise. Freezing the current gain value prevents unintentional high levels or overdriving
when the “normal” signal is applied again.
Basically, the AGC function uses very slow time constants to ensure an subtle adaptation of the level.
The
Control
Velocity
parameter controls the time constants used for the gain changes. The value is
expressed in dB per second.
Maximum Gain
can be set from 5dB … 30dB. This gives you perfect control on the maximum impact
the AGC module can have on your audio signal.
Target Level
setting range -20 dB to +20 dB
Freeze Threshold
setting range -40 dB to -20 dB
Control Velocity
setting range 0.3 dB/s to 1.5 dB/s
Maximum Gain
setting range 5dB … 30dB
Note: During speech intervals the level is far below the
Target Level
, i.e. the AGC would increase gain
normally. The
Freeze
function prevents an unintentional increase of gain in signal gaps.
But, if the speaker microphone receives loud background noise in speech intervals, the
Freeze
function will probably not be able to distinguish between background noise and a low speech pass.
Always set
FreezeThreshold
carefully. If set too low, gain might be increased and bring up background
noises during a speech interval. If set too high, a low speech pass will not be increased in gain.
Under these conditions make a test as to whether or not the use of the AGC function is practicable, or
use the
Expander Module
before
the AGC module for a suppression of background noise.
VIP
digital
Manual 4.0 Yellowtec 2004 Rev. 26.07.2004
Page 48
