by MC » Wed Oct 21, 2015 11:54 am
Most VCOs are v/oct (volts per octave) - in a one volt per octave system, a one volt increment doubles the frequency (octave change). These are exponential VCOs.
A cheaper simpler VCO can be built to operate on v/hz (volts per hertz). Doubling the voltage doubles the frequency (octave change). These are linear VCOs. But linear VCOs have limits.
The original Taurus was built with v/hz VCOs. To stay faithful to the original design (and its huge bass sound), the Minitaur and Taurus III are built with v/hz VCOs.
In a v/hz VCO, you double the voltage to raise the frequency one octave. There are limits to the voltage - you can't exceed the power supply rails in the synth, and you can't have too low a voltage or error signals inherent in the opamps/transistors will throw off the pitch (can't be calibrated out). The power rails limit the upper frequency, and the opamp error signals limit the lower frequency. For a bass synth the lowest frequencies are designed to make the error signals insignificant (you don't want pitch drift, do you? Didn't think so).
Using a v/hz VCO, assume that 1/2 volt establishes the lowest frequency. To raise it an octave, it is doubled to 1 volt. A second octave, double 1 volt to 2 volts. A third octave, double to 4 volts. A fourth octave, double to 8 volts. A fifth octave, double to 16 volts STOP our power supply rail is 15 volts, cannot go higher than that.
So we got four whole octaves with a v/hz system. It doesn't take long to reach a limit.
Contrast that with a one volt per octave VCO with 1/2 volt establishing the lowest frequency. To raise it an octave, we add one volt resulting in 1.5 volts. A second octave, add another one volt to raise to 2.5 volts. Third octave, 3.5 volts. Fourth octave, 4.5 volts. Fifth octave 5.5 volts (still under power rails!). We can add another nine octaves up to 14.5 volts and still be under the power supply rails, and we got 14 octaves total. This is a hypothetical case for illustration, most synth v/oct VCOs have a ten octave range; higher ranges need some error correction of their own (read: more $$$), and a ten octave range has been plenty since the 1960s.
Linear v/hz VCOs also are not easy to modulate. A pitch bend on 1/2 volt may yield a pitch change of 1/4 octave at one note, but as you play other notes the pitch change using the same 1/2 volt won't be the same! That is why the original Taurus did not have an LFO or pitch modulation of any kind (I do not know how it is accomplished in the T3/Minitaur). Exponential v/oct VCOs are much easier to modulate, which is why they are more prevalent in synths.
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