Specifications

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Dealing with Automatic Gain Control
Most consumer-grade camcorders and some industrial models have an
Automatic Gain Control (AGC), which adjusts the audio level up or down as
necessary. The AGC circuit is designed to compensate for the fact that the sound
source is often far from the microphone on the camera. An external microphone is
usually placed much closer to the source, however, and therefore a much higher
signal level is fed into the camera’s input. The Automatic Gain Control responds by
rapidly reducing the audio level during the peaks of speech, and boosting the level
up very high during pauses. The sound of the AGC’s action is often described as
“pumping”, “breathing”, or “whooshing”, and is usually undesirable.
It is impossible to defeat or bypass the AGC on most camcorders.
The only other options are:
• Use the camcorder’s line-level input, if it has one. On most camcorders, the line
input is not affected by the AGC. But since a microphone signal is not strong
enough to drive the line input directly, the mic must be connected to a mixer or
microphone preamplifier that has a line level output. Using a mixer also allows
for more precise adjustment and metering of signal levels, in addition to its basic
function of combining the signals from multiple microphones into one feed.
•
Keep the external mic signal level very low. This causes the AGC to relax
and let the audio level rise to maximum. This tends to amplify the hiss of the
camera’s audio circuitry, but reduces the pumping action. The
microphone’s output signal can be reduced by moving it farther away from
the source, or by using a device called an attenuator or pad, which
decreases the signal level by a fixed amount. Some attenuators offer a
choice of settings, such as -15 dB, -20 dB, and -25 dB.
Connecting microphones to camcorders