ED102 –Hertz/Volts/Octave Converter
ELBY Designs - Laurie Biddulph
9 Follan Close, Kariong, NSW 2250, Australia
3 of 12
CV systems
In the Hz/V system, the VCO oscillator frequency is proportional to the control voltage whilst in the V/Octave
systems the oscillator frequency changes one octave for every one volt (1V) change in the control voltage.
Since an increase of one octave means that the frequency is doubled, each increase of one volt in the
control voltage means a doubling of VCO frequency.
The problem with the V/Octave system is that it must employ a log amp in order to double the frequency for
each one volt increase in the control voltage. Log amp circuitry can, unfortunately, be very unstable because
of its sensitivity to temperature changes. This causes so many problems that most professional musicians
automatically assume that synthesizers always have unstable pitch. When Korg developed their first Korg
synthesizer, they decided that such a circuit was entirely unsuitable for a musical instrument. So, instead
they invented their own unique, patented circuit in which the keyboard voltage (which is the VCO control
voltage) itself doubles for each one octave increase in pitch.
The graph in Figure 1 shows the relationship between the VCO
oscillator frequency (pitch) and the control voltage (keyboard
output voltage). The straight line on the graph is from a
V/Octave synthesizer in which there is a one octave change for
every one volt change in the control voltage. In contrast, the
curved line on the graph is the control voltage from a Hz/V
synthesizer in which VCO frequency is proportional to voltage.
Note that the voltage doubles for each octave rise in pitch.
Figure 1 - logarithmic versus linear cv systems
