Classic VCO Intonation Scaling
Investigating Tuning Performance of VCOs over Multiple Octaves
One of the notable performance characteristics of Voltage Controlled Oscillators (VCOs), is their tuning performance over a large range of octaves. With classic Mono and Poly synths, you can often get the oscillators tuned well within a sweet spot on the keyboard (usually somewhere between C3 and C5. However, as you reach the lower and upper limits of the keybed, you will often notice that oscillators tend to scale away from nominal tuning and have a sort of intonation behavior.
For instance, you may tune a given oscillator to be perfectly in tune at the note C4. But if you measure that oscillator playing a C2, it may be +5 cents sharp, and then if you go up several octaves to C6, you may find it to be -5 cents flat. This type of osc frequency scaling varies from synth to synth. In vintage synths from the 1970s and 1980s, if you measure each oscillator over a five octave range, you will often see a variance of 10-20 cents over that five octave range, with only an octave or so of the keybed being very well in tune (+/-3 cents from nominal)
We measured various classic Mono and Poly synths from the 70s and 80s (Including multiple MemoryMoogs, OBXas, OBX, Jupiter 4, Jupiter 8, PolySix, CS-80, MiniMoog, ARP, etc) and found this osc scaling behavior to be extremely widespread/common.
