In sound mixing consoles[edit]
Some mixing consoles come equipped with VCAs in each channel for console automation. The fader, which traditionally controls the audio signal directly, becomes a DC control voltage for the VCA. The maximum voltage available to a fader can be controlled by one or more master faders called VCA groups. The VCA master fader then controls the overall level of all of the channels assigned to it.[2] Typically VCA groups are used to control various parts of the mix; vocals, guitars, drums or percussion. The VCA master fader allows a portion of a mix to be raised or lowered without affecting the blend of the instruments in that part of the mix.
A benefit of VCA sub-group is that since it is directly affecting the gain level of each channel, changes to a VCA sub-group level affect not only the channel level but also all of the levels sent to any post-fader mixes. With traditional audio sub-groups, the sub-group master fader only affects the level going into the main mix and does not affect the level going into the post-fader mixes. Consider the case of an instrument feeding a sub-group and a post-fader mix. If you completely lower the sub-group master fader, you would no longer hear the instrument itself, but you would still hear it as part of the post-fader mix, perhaps a reverb or chorus effect.[3]
VCA mixers are known to last longer than non-VCA mixers. Because the VCA controls the audio level instead of the physical fader, decay of the fader mechanism over time does not cause a degradation in audio quality.
VCAs were invented by David E. Blackmer, the founder of dbx, who used them to make dynamic range compressors. The first console using VCAs was the Allison Research computer-automated recording system designed by Paul C. Buff in 1973.[4] Another early VCA capability on a sound mixer was the series of MCI JH500 studio recording desks introduced in 1975.[5] The first VCA mixer for live sound was the PM3000 introduced by Yamaha in 1985.
Digital variable-gain amplifier[edit]
A digitally controlled amplifier (DCA) is a variable-gain amplifier that is digitally controlled.
The digitally controlled amplifier uses a stepped approach giving the circuit graduated increments of gain selection. This can be done in several fashions, but certain elements remain in any design.
At its most basic form, a toggle switch strapped across the feedback resistor can provide two discrete gain settings. While this is not a computer-controlled function, it describes the core function. With eight switches and eight resistors in the feedback loop, each switch can enable a particular resistor to control the amplifier's feedback. If each switch was converted to a relay, a microcontroller could be used to activate the relays to attain the desired amount of gain.
Relays can be replaced with Field Effect Transistors of an appropriate type to reduce the mechanical nature of the design. Other devices such as the CD4053 bi-directional CMOS analog multiplexer integrated circuit and digital potentiometers (combined resistor string and MUXes) can serve well as the switching function.
To minimize the number of switches and resistors, combinations of resistance values can be utilized by activating multiple switches.