VCOs vs DCOs
a detailed explanation of the oscillators design choices we made for ALT
during the two and a half years of ALT’s development, there were a few very important decisions that needed to be made.
what kind of sound source? what kind of filter? how many of them? what audio effect would fit best? how to modulate everything?
all of these decisions would ultimately define ALT, but let’s start with the core of its sound: oscillators.
the early days
the very first prototype of ALT was a simple drone box based on 6 oscillators, a low pass filter, a volume knob and that’s it. we called it DROSC (drone / oscillators - why not). we learned sound synthesis through analog hardware electronics, so naturally, we started with voltage controlled oscillators, VCOs.
DROSC
what are VCOs ?
voltage controlled oscillators are the original analog oscillators, found in the first synthesizers of the 60’s and 70’s. their principle is simple: the more voltage, the higher the frequency.
imagine a water faucet: as you open it, the water flow increases; same thing for VCOs, as you send more voltage into the oscillator, its frequency rises.
small changes in voltage can create a range of pitches.
this design creates a warm, organic sound. on the other hand, VCOs are very sensitive to temperature and power supply fluctuations, as well as components accuracy, creating pitch variations. these variations are VCOs sound signature everybody loves, but they come with a drawback: when played together, multiple VCOs never stay in tune for long.
that was not a problem for ALT in the first half of its development life. all of our 5 VCOs were “free”, unquantized, not made to be played with a keyboard, nor to play melodies. so accuracy was not really a problem, and VCO’s were perfect for us.
ALT prototype 1
let’s quantize
everything changed when one day, VG’s cousin, who is a accomplished music producer and our synth expert, suggested we add a quantizer to ALT. that would allow for the oscillators’ pitch to change step by step, quantized, always hitting notes, following scales.
suddenly, melodies would be possible. ALT could now be way more than a drone synth.
we dived into research, and found a solution: using an MCU (microcontroller unit), ADC (analog to digital signal converter) and DAC (digital to analog signal converter), we could take analog CV inputs (from the sliders and the modulation matrix), treat them digitally and send back precise analog CV signals to our oscillators, corresponding to the notes, following the standard “1 volt per octave” set by M. Bob Moog.
this took 4 months to develop, and when it finally worked, we were thrilled by this new feature. ALT completely changed, for the better. it was now capable of both linear pitch curves, and quantized melodies.
ALT prototype 2
tuning
ok, the quantizer is amazing. but now, frequency precision becomes vital.
so I guess we have to tune the VCOs?
yep. and now we understand why on vintage synths there were never more than 2 oscillators.
if you’ve never tuned a VCO, here’s the complete process. it’s long, tedious, and not so rewarding.
VCOs are super sensitive, we had to tune them very often, and for each VCO, 2 settings need tuning: the reference frequency (or root note), and the 1 volt per octave ratio.
ALT has 5 oscillators, that means 10 parameters to tune in order to keep everything harmonized. that was fine for prototypes, during development, but that would never have been viable for our customers in the long run.
introducing DCOs
we looked for a solution to our tuning instability problem, and digitally controlled oscillators (DCOs), quickly became the obvious solution.
DCOs were introduced by roland in 1982, when they release the famous juno synthesizer.
they designed them to address exactly the same issue we had with ALT: palliate the tuning instability of VCOs while still delivering analog sound quality.
by using the clock of an MCU (super accurate, super stable, whatever the environment) for frequency control and analog components for waveshaping, DCOs offer the warmth of analog with much-improved tuning reliability. this way, oscillators are always in tune, barely any pitch drift and no more tedious tuning sessions!
and since ALT’s quantizer already required a micro controller to force the oscillators to follow scales, DCOs made even more sense.
did ALT just become a digital synth?
no. DCOs’ clocks are digital, precise, but their waveforms are shaped with analog components, keeping the rich and fluctuating tone of analog circuits.
in fact, the entire audio path stays analog. only the frequency management is digital.
to tell you the truth, we could not completely let go all of our VCOs: out of the 5 oscillators of ALT, 4 of them are DCOs (OSC2-OSC5) but OSC1 is still a VCO. it is a sine, it cannot be quantized, it cannot be played with MIDI. it is free and we love it this way. it adds a little uncertainty to ALT.
as a bonus, our micro controller receives a ton of CV inputs from the pin matrix to modulate the DCOs, but when no pin is inserted, the modulation CV value that is read by the micro controller is slightly floating, recreating the typical tiny frequency fluctuations of VCOs, while keeping all 4 DCOs in tune with one another.
with 4 DCOs and one unquantized VCO, now defined as ALT core sound, melodies are precise, in tune, while the sound stays analog, rich and warm.
two more things…
just in case you really miss the ‘out of tune’ aspect of VCOs, we’re adding a detune parameter per oscillator, to easily recreate acoustic beats with multiple oscillators at near identical frequencies.
we’re also introducing 2 tunings systems to chose from: equal temperament or just intonation. this will be the topic of another journal entry.
if you want to learn more about oscillator circuit design, here are some good resources: