Roland System 700 Analog Synthesizer Modules
3. The Voltage Controlled Amplifier (VCA)
The Voltage Controlled Amplifier is classified as a "modifier." Its function is to modify the audio signal by amplifying that signal. As such it is a relatively simple module on the synthesizer with relatively few controls.
As can be seen by the diagram above, the VCA is inserted into the signal path: the signal enters, is modified, and then exits. Specific types of voltages can be brought into the control voltage inputs. These voltages adjust the amount of amplification that the signal receives. Shown on the following drawing are the physical features of the Roland VCA.
As with the VCF the audio signal inputs lie at the top of the module and on the main module both VCAs are hardwired to receive their inputs from the VCF-1 and VCF-2 outputs. To connect to these modules, all one needs to do is raise the slider beneath the appropriate input. Other signals may be input by inserting a patch cord into the jack above the slider. Signals must be input via patch cords to the VCAs on the auxilliary module.
The output from the VCA lies beneath the inputs. LED lights exist to monitor the strength of the signal: green = signal present, red = distortion (circuit overloaded).
There are only 2 true manual controls on the Roland VCA: the Gain control and the Mode Selector switch. The Gain control adjusts the initial level of amplification and the Mode Selector switch is used to place the VCA into a Linear or Exponential response mode. The difference between these two modes can best be understood by referring to the graphs below.
In the Linear mode control voltages applied cause the signal to be amplified in a direct, linear fashion. In the Exponential mode the result is a curve in which the output signal steeps quite rapidly near the end.
The significance of these differences is most apparent after an understanding of the ADSR, but for the present the basic thing to understand is that the Exponential mode is most useful for percussive type envelopes (i.e. a "wood block" sound) where you want the sound to have a very high attack volume and then die very quickly. The Linear mode is used for most other sounds. In experimenting with these two modes you should note that when in the Linear mode, the Gain control must be at zero amplitude to avoid having the sound continuously present (whether you depress a key or not). However, in the Exponential mode the Gain can be (and, in some cases, should be) slightly turned up. This is due to the nature of the curve when the VCA is set in that mode (It amplifies less at the beginning, more at the end).
At the bottom of the module are the control voltage inputs. As you might notice by looking at the main module, the device almost exclusively used to control a VCA is the ADSR. A selector switch below the slider allows for the choice of a hardwire connection to either ADSR-1 or ADSR-2. On the auxilliary module, all control inputs must be connected via patch cords.
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