Automatic Gain Control (AGC)
Purpose
The Automatic Gain Control controls signal dynamics so that, despite variations in the input level, the average output level from the device is at a pre-set level. The automatic gain control is typically used to mimic the action of a sound mixer "riding the faders". The AGC can compensate for loud or soft spoken speakers by automatically raising or lowering the gain. The AGC contains "threshold" and "recovery" circuitry which makes it particularly suited for applications where speech is input.
Inputs
The Automatic Gain Control has 2 input nodes. The top input is the Audio input, and the bottom is the Side Chain input (note that the standard version of the AGC has only one node, the side chain input is wired to be the same as the input). The gain computation for the device is based on the RMS level measured at the Side Chain input. Once the gain is computed, it is applied to the Audio input signal.
Controls
Below Threshold Indicator
When lit, indicates that the Side Chain level has fallen below the Threshold Level. Depending on the Gain Recovery switch, the gain either then remains constant or returns to unity at a rate specified by the Recovery Time control.
Bypass
Bypasses the AGC gain stage. The Below Threshold Indicator and Gain Change Meter continue to operate as though the device were active.
Note: When bypass is engaged, the AGC continues to compute a gain change. And although this computed gain is not applied to the signal passing through the device, the Gain Change Meter and the Below Threshold Indicator continue to function as if Bypass is not engaged. Careful examination of the Schematic should clarify this point.
Gain Change Meter
This meter indicates the amount of gain (or attenuation) being applied to the input signal.
Note: Although the meter can only indicate gain reductions of 18dB or less, the AGC is capable of producing greater reductions.
Gain Recovery
When engaged, causes the gain to return to unity once the signal has fallen below the Threshold Level.
Maximum Gain
This is the maximum amount of gain that the AGC provides. This prohibits the AGC from applying so much gain to a quiet source that problems such as feedback arise.
Note: There is no limit to the amount of attenuation the AGC can produce.
Overload
Lights to indicate an internal overload in the AGC. This condition is only possible if the Maximum Gain is set greater than +0dB.
Note: An overload becomes more likely as the Response Time is increased.
Recovery Time
Determines how fast the gain returns to unity once the input signal has fallen below the Threshold Level.
Note: Gain Recovery must be engaged!
Response Time
Determines how fast the AGC adjusts the gain in response to a change in the Side Chain signal level.
Target Level
This is the desired average RMS level of the output signal. The AGC will apply an appropriate gain to the input signal in order to achieve this desired output level.
Threshold Level
The threshold level and associated logic is intended to detect pauses in speech and suspend operation of the AGC during these pauses. If this feature were not provided the AGC would eventually turn up a silent input to the Maximum Gain setting.
When the RMS input level falls below the threshold level and Gain Recovery is switched off, the AGC enters a "frozen" state. In this state the gain through the device will no longer be updated based on the input signal level. With Gain Recovery enabled, the gain applied to the input signal gradually returns to unity at the rate determined by the Recovery Time setting.
Tip: For most applications the threshold level should be set so that the quietest valid source level is above the point where the AGC would enter the "frozen" state by at least 6 dB. An input with no source (such as an open microphone channel) being fed to the AGC should cause the AGC to either remain "frozen" or return to unity.
When properly set, only a true source (not background noise) will cause the AGC to become active (as shown by the Below Threshold Indicator).
Schematic
Advanced Topic
The Automatic Gain Control composite device is constructed of an Auto Gain Algorithm and some additional Level and Meter devices.
Related Topics
