VOGAD
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In electronics systems, a VOGAD or voice-operated gain-adjusting device is a type of automatic gain control or compressor for microphone amplification. It is usually used in radio transmitters to prevent overmodulation.
In its simplest form, a limiter can consist of a pair of back-to-back clamp diodes, which simply shunt excess signal amplitude to ground when the diode conduction threshold is exceeded. This approach will simply clip off the top of large signals, leading to high levels of distortion.
While clipping limiters are often used as a form of last-ditch protection against overmodulation, a properly designed VOGAD circuit actively controls the amount of gain to optimise the modulation depth in real time. As well as preventing overmodulation, it boosts the level of quiet signals so that undermodulation is also avoided. Undermodulation can lead to poor signal penetration in noisy conditions, consequently VOGAD is particularly important for voice applications such as radiotelephones.
A good VOGAD circuit must have a very fast attack time, so that an initial loud voice signal does not cause a sudden burst of excessive modulation. In practice the attack time will be a few milliseconds, so a clipping limiter is still sometimes needed to catch the signal on these short peaks. A much longer decay time is usually employed, so that the gain does not get boosted too quickly during the normal pauses in natural speech. Too short a decay time leads to the phenomenon of "breathing" where the background noise level gets boosted at each gap in the speech. VOGAD circuits are normally adjusted so that, at low levels of input, the signal is not fully boosted, but instead follow a linear boost curve. This works well with noise cancelling microphones.
