This is a VERY simplified treatment of some typical broadcast ANALOG audio signal processors. Multi-band or split-band units are not covered here. But this is how I understand these terms, based on my knowledge of the technology used at radio stations I visited during much of the 1960s. Please add to this by correcting or adding information I may have misunderstood and/or overlooked.
I bring this up because there are SO many YT videos (many of questionable accuracy) that purport to define and describe use of compressors, but relatively few that delve into AGCs or peak limiters.
Automatic Gain Control (AGC)
increases gain for parts of a signal below a certain threshold;
decreases gain for parts of a signal above a certain threshold;
effect: reduction in dynamic range of signal, but allowing a more constant output level.
Examples: Gates STA-LEVEL, Gates LEVEL DEVIL, CBS Labs Audimax
Compressor
does not affect those parts of a signal below a certain threshold;
decreases gain for those parts of a signal above a certain threshold;
effect: reduction in dynamic range of a signal, but allows the average level of the overall signal to be increased following compression, giving it more “punch”.
Examples: can't recall any.
Peak Limiter
drastically and quickly reduces gain of signal peaks that would cause over-modulation of a radio transmitter;
has no effect on signals below limiter’s threshold; has a compression ratio of well over 100:1 and very fast attack times.
Example: CBS Labs Volumax
The AGC or compressor were typically used in tandem, with the AGC or compressor situated at the studio BEFORE signal was applied to studio-to-transmitter link, and the (peak) limiter usually resided at the transmitter site.
I bring this up because there are SO many YT videos (many of questionable accuracy) that purport to define and describe use of compressors, but relatively few that delve into AGCs or peak limiters.
Automatic Gain Control (AGC)
increases gain for parts of a signal below a certain threshold;
decreases gain for parts of a signal above a certain threshold;
effect: reduction in dynamic range of signal, but allowing a more constant output level.
Examples: Gates STA-LEVEL, Gates LEVEL DEVIL, CBS Labs Audimax
Compressor
does not affect those parts of a signal below a certain threshold;
decreases gain for those parts of a signal above a certain threshold;
effect: reduction in dynamic range of a signal, but allows the average level of the overall signal to be increased following compression, giving it more “punch”.
Examples: can't recall any.
Peak Limiter
drastically and quickly reduces gain of signal peaks that would cause over-modulation of a radio transmitter;
has no effect on signals below limiter’s threshold; has a compression ratio of well over 100:1 and very fast attack times.
Example: CBS Labs Volumax
The AGC or compressor were typically used in tandem, with the AGC or compressor situated at the studio BEFORE signal was applied to studio-to-transmitter link, and the (peak) limiter usually resided at the transmitter site.