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Automatic Gain Control

Automatic Gain Control or AGC, is a special case of a compressor with its threshold set at a very low level; moderate to slow attack times, long release times, and low ratios. It is intended to take signals of indeterminate levels up to a target level while maintaining program dynamics. Most AGCs incorporate some sort of silence detection to prevent the loss of gain reduction during silent periods. This is the single feature that separates an AGC from an ordinary compressor/limiter.

Use AGC to normalize the level of a CD player used for background music, foreground music, or music on hold, to remove some of the variation in level of a paging mic.

Ctrl Output. This module has a control signal output. The control signal reflects the dynamic gain reduction being applied by the module and changes in real time. This control signal can be used to easily create linked multi channel dynamics modules. See Creating Multi-channel Dynamic Modules for more information.

The AGC has the following controls and switches:

  • Detector Threshold. When the signal level is below Threshold, the input/output ratio is 1:1. When the signal level is above Threshold, the input/output ratio follows the Ratio control setting. Set this threshold to a level just above the noise floor of your input signal.

  • Ratio. The ratio of above-threshold input signal level change to output signal level change, measured in dB.

  • Target Output Level. The desired output signal level.

  • Speed. The recovery (return to unity gain) time of the AGC. Use Slow for music, Fast for speech. For “Fast” it is a 10 ms attack and 1 second release time. For “Slow” it is 20 ms attack and 2.6 second release.

  • Contour. Pressing the Contour button inserts a single pole 150Hz high pass filter into the side chain. This is useful to prevent the AGC from responding too strongly to signals with significant bass content. For example, without this feature, on a certain song, each kick drum hit could cause the AGC to attenuate the output level significantly. This is sometimes referred to as the "hole punching effect", because it sounds as if each bass hit is punching a hole in the audio. Turning on the Contour setting minimizes this problem.

  • Active. Activates the AGC so that the gain modifications are applied to the audio signal path.

  • Mono vs. Stereo. The controls for mono vs. stereo AGCs are identical. The input and output meters for the stereo version show (2) channels, vs (1) for the mono version. For the stereo version, the AGC gain control is directed by the loudest of the 2 inputs. Both outputs are affected identically.

  • Meters. Peak detecting meters are provided for the input and output of the AGC. The input meter can be useful in setting the detector threshold. The output meter can be used to verify the correct operation of the AGC.