Voltage controlled oscillators (VCOs) are the heart of any synthesiser.
The quality of the VCOs ultimately determines the performance of the
synthesiser.
The
two principal requirements of a synthesiser VCO are stability and good
tracking. Stability means that if the control voltage applied to the
VCO remains constant, then the frequency of the VCO should also remain
constant and not drift. Tracking means that the VCO must follow the
prescribed logarithmic 1 Volt/Octave characteristic as closely as possible.
In particular, where several VCOs are used they should all have similar
characteristics. These parameters are particularly important in a chording
instrument where a number of VCOs are used simultaneously.
In a synthesiser
using only one VCO slight drift or deviation from the 1 Volt/Octave
characteristic might not be noticed since the ear is not particularly
good at judging absolute frequency, unless a person has `perfect pitch'.
In any chording instrument however, even slight mistuning is immediately
apparent due to the formation of beat notes.VCO 1 & VCO 2 use a sawtooth-based design substantially borrowed from
Electronotes and has the following features:-
- 4x adjustable Logarithmic
FM Inputs
- 1x fixed 1V/Octave FM input
- 2x Linear FM Input
- 1x Sync Input
- 2x PWM Input
- Sawtooth Output:
5V p-p (centred around 0V)
- Square Output:
5V p-p (centred around 0V)
- Triangle Output:
5V p-p (centred around 0V)
- Sine Output:
5V p-p (centred around 0V)
- COARSE/FINE tuning range: ~0.3Hz to ~3.9kHz
- FINE tuning range: +/-0.8 octave
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