Automatic Gain Control (AGC)
Also called a speech leveler, the automatic gain control (AGC) either adds or reduces gain, depending on the strength of the incoming signal. A properly adjusted AGC helps compensate for differences in level between loud and soft talkers, making it one of the few processors to actually raise the volume. Automatic gain control does this by raising the level of the input signal of low-level sources, increasing audibility over the entire presentation.
The key setting on an AGC is called the hinge point. Gain is added to signal levels below the hinge, while signals above it are attenuated. At the hinge point, the signal is at the desired output for the given sound source. This target level is the unity gain point, where no addition or subtraction of gain occurs. The hinge point volume is set at the desired output, or target level, for the given sound source. The threshold sets the level where the AGC begins to take effect.
Similar to the compressor, the attack setting adjusts the speed at which the AGC takes effect, and decay sets how long the AGC takes to release. When used to make gain adjustments for different talkers, using longer attack, hold, and release times results in smoother transitions and less false triggering. To use an AGC, the sound system must have high enough gain-before-feedback to accommodate the maximum gain setting of the AGC.
Acoustic Echo Canceller
The acoustic echo canceller (AEC) is an adaptive processing device designed specifically for use with audio and video teleconferencing systems. In these systems, a distinctly perceptible echo can be caused by several sources of delay in the signal chain, and can be extremely
disruptive to the smooth flow of conversation.
Acoustic echo occurs when audio received from the remote site reaches active microphones at the near site, and is thus transmitted back to the remote site along with sound from the near site talkers. To address this type of echo, an AEC monitors the incoming audio signal from remote sites and compares it to the signal that is about to be transmitted. If the echo canceller detects the presence of the incoming audio within the outgoing signal, it attempts to remove it
electronically by subtracting it from the return audio feed.
The echo canceller attempts to prevent the incoming audio from other sites from being sent back to them, which means that it improves audio for the remote site, not the one where the unit is installed. To address echoes locally, the distant site’s teleconferencing system would require an AEC as well.
In recent years, powerful processors and advanced cancellation algorithms have made acoustic echo cancellers more effective and less expensive. Thus, most teleconferencing systems now come with AEC capability. These vary in their power and effectiveness,depending on whether they are designed for single user laptop systems or complex multi-user meeting rooms. In larger rooms with multiple microphones and loudspeakers, it is desirable to have a separate echo canceller for each mic in the teleconferencing system, providing optimum echo reduction.
Finally, please note that acoustic echo cancellers are single-purpose devices. They are not designed to address audio issues such as acoustic reverberation within a reflective room. In fact, excess reflective sound makes it difficult for the echo canceller to work properly.
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