Editing Automatic gain control (section)

===Vogad===

A voice-operated gain-adjusting device<ref>[http://www.its.bldrdoc.gov/fs-1037/dir-039/_5820.htm Vogad at Federal Standard 1037C]</ref> or volume-operated gain-adjusting device<ref>
{{cite journal
 | journal = Popular Mechanics
 | title = Roar and Whisper Equalled by Radio Voice Leveler
 | page = 236
 | date = Feb 1939
 | url = https://books.google.com/books?id=f9sDAAAAMBAJ&pg=PA236
 }}</ref> 
(vogad) is a type of AGC or [[Audio level compression|compressor]] for [[microphone]] amplification. It is usually used in radio transmitters to prevent [[overmodulation]] and to reduce the [[dynamic range]] of the signal which allows increasing average transmitted power. In [[telephony]], this device takes a wide variety of input amplitudes and produces a generally consistent output amplitude.

In its simplest form, a limiter can consist of a pair of back-to-back [[Clamper (electronics)|clamp diode]]s, 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 [[Clipper (electronics)|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 [[radiotelephone]]s.

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 (noise reduction)|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 cancellation|noise cancelling]] microphones.