ARP2600 - Fundamentals of Music Technology - Cyborgstudio.com

SECTION EIGHT: THE VCA - 065increased, then decreased from the initial gain setting. In this way, it is similar to the effect of negativecontrol voltage on the VCF’s cutoff frequency. However, there are very few practical applications ofthis design, since it means that sound is constantly flowing through the VCA, regardless of whether akey is being played. Over time, this design disappeared. Almost without exception, modern VCA’s willonly increase gain when responding to incoming control signals.LINEAR VS. EXPONENTIALIn Figure 8-1 on page 62, one can see that the two control inputs on the VCA have a small label overthem that says control. The left jack, which is normalled to the AR generator, is also labeled LINEAR,while the right jack, normalled to the ADSR generator, is labeled EXPONENTIAL. Although manyearly synthesizers offered one or the other, the ARP 2600 was among the select few that offered usersboth response curves.Volume is typically measured in a unit called decibels or dB. One dB is the smallest change in volumethat a person can perceive. Although people can hear volumes much louder, 140 dB is considered to bethe threshold of pain, or the volume at which a sound is so loud, it is painful to hear. 1 dB is the softestsound that human ears can hear. One might think that in order to create a sound which is twice as loudas a 50 dB sound, one would just double the voltage being fed to the VCA. However, human ears do notrespond in the way that one might think. The decibel is not a linear measurement. Although the decibelscale increases smoothly upwards in a straight line (the blue line in Figure 8-3), the amount of soundenergy it takes to produce those sounds does not (the red line in Figure 8-3). In fact, a 20 dB sound isactually ten times as intense as a 10 dB sound. A 30 dB sound is actually one hundred times as intenseas a 10 dB sound, and a 40 dB sound is actually one thousand times as intense as a 10 dB sound.Perceived loudness in dBWhen it takes increasingly large amounts of volume toproduce the same amount of perceived change, this iscalled an exponential response. Human ears function inthis way. Sometimes, it is advantageous to have an amplifierwhich functions in a linear mode, and sometimesit is advantageous to have an amplifier that functions inexponential mode. The ARP 2600 gives the user bothoptions.When the linear control input of the VCA is used, volumeswill seem to change smoothly over the course ofActual amount of voltagethe envelope or whatever is controlling the VCA’s amplitude.While using the linear input, it does not take asFigure 8-3: A linear and a logarithmic curve much voltage to create a change in gain, especially atlower levels. When the exponential input is used to controlthe VCA, the changes in amplitude will be more dynamic and certain elements of an envelope willseem exaggerated (most notably the decay). If a waveform which is naturally smaller in amplitude isused (e.g. a sine or triangle wave) little change may be heard using the exponential input. The differencebetween the two control inputs can be heard on CD track 43