VCO Harmonic Jitter
Measurements and modulation replication of raw frequency jitter from VCOs
Take measurements of any VCO oscillator, and you'll notice they all have something in common - A relatively high frequency, but low amount of frequency modulation (or Jitter). We tested many synthesizers, including MiniMoog, Behringer Model D, IK Uno, Roland SE-02, Oberheim OBX, OBXA, Oberheim SEM, Prophet 5, Prophet 10, Yamaha CS-80, MemoryMoog, Korg Polysix and others. The oscillators on all these units exhibit the same type of frequency jitter, although the exact frequency of the jitter and amount does vary from board to board.
The total amount of this frequency jitter is usually fairly low (under 1-2 cents total swing from the target pitch). At the fundamental, the effect is hardly noticable, but the effect is more significant in the harmonic series that follows. Once you get up into the upper harmonics, there is a significant amount of this jitter and the effect washes out some of the clarity of upper hamonics, creating a softer sound. Since there is amplitude falloff in the harmonic series, the effect is subtle, but it may be one of the reasons why people always refer to VCO synths as sounding more warm or soft or organic sounding.
Conversely, with DCO and Digital Oscillators, they are inherently stable. There is relatively no pitch modulation. The frequency clocks are ultra presice, and that results in the upper harmonic series being rendered with almost no motion. This creates an effect where the upper harmonics are very clear and have a more "piercing sound", rather than being more blurred or washed out. This may be why many people say that DCOs and Digital synths sound "more harsh" or "more cold".
Below is a video, showing the first test of some modern VCO mono-synths, compared to the Prophet Rev2, which is a DCO based synthesizer. We have also tested other DCO synths (Novation Bass Station 2, Poly Evolver), and since doign this test, took recording from various classic mono and poly synths, listed above. All of the VCO synths exhibit this behavior with the oscillators.