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Exponential converter

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A type of amplifier circuit used in most analog synthesizers to process control voltage inputs to voltage controlled oscillators, voltage controlled filters, and other frequency-specific processing functions. The nature of most VCO and VCF circuits is that they respond to their control voltage input in a linear fashion (from an electrical engineering standpoint); that is, they have a volts/Hz response to the control voltage. Unfortunately, this does not correspond to the way human ears perceive pitch. The musical scale is not linear; the frequency interval from each note to the next higher note is greater than the interval to the next lower note. The interval doubles each octave, so that for instance, when the familiar concert A is at 440 Hz, the A an octave above it is at 880 Hz, the A two octaves above is at 1760 Hz, etc. So, when dealing with control voltages that control frequencies within a synth, volts/octave is easier to work with; if it is desired to raise a control voltage input by one octave in a volts/octave system, it can be done by adding one volt to the voltage. Voltage addition circuits are easy and inexpensive to build. In a volts/Hz synth, which does not contain exponential converters, to raise a control voltage by one octave it is necessary to double the voltage. Voltage multiplying circuits are relatively expensive and difficult to implement accurately -- a voltage multiplier is essentially a VCA.

Because Volts/octave is the preferred characteristic in a synth, the exponential converter circuit is used to pre-process the control voltage inputs to the VCOs and VCFs to get the desired response. The exponential converter, as the name might suggest, produces an output voltage that is exponentially proportional to the input voltage. (The typical amplifier produces an output voltage which is the input voltage multiplied by a constant gain factor.) Analog exponential converters that track accurately require some care in the design and build, and in some older analog synths these circuits rely on components which were produced in the 1970s and are rare now. Also, the most common analog exponential converter uses a transistor in a way that is inherently temperature-dependent, which means that the circuit must be compensated for ambient temperature variations (see tempco). These circuits are often sources of tuning instability in VCOs.

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