You may have heard these terms but don't know what they mean? Or you're not sure of the pros and cons of each type of oscillator? Perhaps people are trying to pull the wool over your eyes by describing DCOs as "entirely analogue oscillators, just controlled digitally" which is misleading at best. Let's dig into it deeper!
The original type of analogue synthesiser oscillator, and some would say the most desirable.
A VCO is an entirely analogue device. You feed it a control voltage (CV for short). The higher this voltage, the higher the pitch of the oscillator. Most commonly, oscillators are set up to follow the 1-volt-per-octave standard pioneered by Moog. An increase of 1 volt increases the pitch by one octave. An increase of 1/12th a volt increases the pitch by one semitone.
+ Power supply >----+ +-----------------> Sawtooth output
| |
Q1 |V |
CV input >--------| |
|\ | Comparator
| | |\
+--------+-----------| |---+
| | |/ |
----- \| Q2 |
C1 ----- |---------------+
| V|
| |
Ground >------------+--------+
Current flows from the power supply through transistor Q1. From there it slowly charges capacitor C1.
V | / o | / l | / t | / <- Slow charging of C1, voltage rises gradually a | / g | / e |/ +-------- Time ->
A comparator circuit looks at the voltage on C1. When it nears the maximum, the comparator turns on transistor Q2 briefly. This rapidly discharges C1 back to 0 volts.
Trigger voltage
|
v
V | /|
o | / |
l | / |
t | / | <- Rapid discharging of C1, voltage drops to 0 almost instantly
a | / |
g | / |
e |/ |
+--------
Time ->
From there, the cycle repeats indefinitely. You now have a sawtooth wave!
V | /| /| /| /| o | / | / | / | / | l | / | / | / | / | t | / | / | / | / | a | / | / | / | / | g | / | / | / | / | e |/ |/ |/ |/ | +-------------------------------- Time ->
Changing the control voltage will alter how much current is allowed through transistor Q1. A lower* control voltage allows less current into C1, taking longer to charge the capacitor.
V | _/| _/| o | _/ | _/ | l | _/ | _/ | t | _/ | _/ | a | _/ | _/ | _ g | _/ | _/ | _/ e |/ |/ |/ +-------------------------------- Time ->
It now takes longer to complete a cycle, resulting in a lower pitch (frequency).
Likewise, a higher* control voltage allows more current through transistor Q1, charging C1 faster.
V | /| /| /| /| /| /| o | || || || || || || l | / | / | / | / | / | / | t | | | | | | | | | | | | | a | / | / | / | / | / | / | / g | | | | | | | | | | | | | | e |/ |/ |/ |/ |/ |/ |/ +-------------------------------- Time ->
Now the cycle completes faster, resulting in a higher pitch (frequency).
Pulse and triangle waveforms are derived from the basic sawtooth waveform using a wave-shaping circuit.
* Parts have been omitted from the circuit above for simplicity. As shown, a lower control voltage will cause a higher pitch.
If you have a polysynth which uses VCOs, this is the type of oscillator you'll have. Not to be confused with a digitally-controlled oscillator (DCO).
Generating a control voltage for a monosynth is easy. Each key is connected to a voltage divider circuit. When the key is pressed, the appropriate voltage for the note is sent through to the oscillator(s).
Polysynths are much harder. A 6-voice polysynth has 6 individual synthesisers inside. Each synthesiser needs its own control voltage to know what note to play.
The way you assign notes to individual synthesisers (called voices) looks something like this:
This just isn't practical with analogue circuitry, so you need a computer. Almost all polysynths (even those from the 70s) have a basic computer inside for this.
The computer scans the keyboard for any keys pressed. It then uses a digital-to-analogue (D/A) converter to send the right control voltage to the right voice.
+---------------+ +-------------------+
+->| D/A converter |---CV-->| Synthesiser voice |
| +---------------+ +-------------------+
|
| +---------------+ +-------------------+
+->| D/A converter |---CV-->| Synthesiser voice |
| +---------------+ +-------------------+
|
+----------+ +----------+ | +---------------+ +-------------------+
| Keyboard |-->| Computer |-+->| D/A converter |---CV-->| Synthesiser voice |
+----------+ +----------+ | +---------------+ +-------------------+
|
| +---------------+ +-------------------+
+->| D/A converter |---CV-->| Synthesiser voice |
| +---------------+ +-------------------+
|
| +---------------+ +-------------------+
+->| D/A converter |---CV-->| Synthesiser voice |
| +---------------+ +-------------------+
|
| +---------------+ +-------------------+
+->| D/A converter |---CV-->| Synthesiser voice |
+---------------+ +-------------------+
In all other respects, each synthesiser voice is identical to a normal fully-analogue VCO. It has all the usual pros and cons of a VCO.
One advantage is automatic tuning. When you press the "tune" button on a polysynth, the computer "listens" to each oscillator in turn. The software adjusts the control voltage for each oscillator until it's perfectly in tune.
That way you can enjoy the slight tuning drift of real VCOs, but not have it drift so far it's unplayable.
And of course, including a computer means it can have MIDI support.
Not to be confused with a digitally-controlled VCO, this is a completely different way to build an oscillator.
Tuning drift was not desirable in the 80s. A more high-tech oscillator that's always perfectly in-tune was in demand.
Instead of using an analogue circuit at the core of the oscillator, the frequency is produced digitally. This is then fed into wave-shaping circuitry to produce a sawtooth and pulse wave.
A DCO has two parts, the core oscillator and the wave-shaper.
Core oscillator:
+----------------------------------+ +----------------+
| Crystal oscillator (several MHz) | | Computer |
+----------------------------------+ +----------------+
| |
V V
Reset +----------------------------------+ +----------------+
+--->| Digital counter | | Pitch register |
| +----------------------------------+ +----------------+
| | |
| | +-----------------+
| | |
| V V
| +----------------------------------+
| | Digital comparator |
| +----------------------------------+
| |
+---------------------+
|
V
Trigger pulse output
The crystal oscillator runs at a fixed frequency of several MHz, thousands of times higher than the audible range. Being a fixed frequency oscillator, its frequency is extremely stable.
Each cycle of the oscillator causes the digital counter to count upwards. 1, 2, 3, etc.
The pitch register holds a number set by the computer. This number determines the pitch of the oscillator. It's normally quite a high number.
The digital comparator compares the two numbers - the counter and the pitch register. Each time the crystal oscillator ticks, the digital counter moves up by one. Eventually, it reaches the value stored in the pitch register. When this happens, the digital comparator produces a pulse on its output.
This pulse is sent in two directions. Firstly it goes to the wave-shaper part of the oscillator. Secondly, it goes into the reset input of the digital counter. This resets the count back to 0 so the cycle starts over.
Let's say the crystal oscillator is 8 MHz (8000 kHz). To produce a 1 kHz tone, the computer sets the pitch register to 8000. It will take 8000 ticks of the crystal oscillator to produce a pulse on the output. Therefore the output pulses at 1 kHz.
With a high number in the pitch register:
Counter starts at zero | | +-- Counter is still counting up | | | | +--- Counter reaches the pitch register number and resets, producing | | | an output pulse v v v | | | | <-- Cycle repeats again | | | | | | | | | +------------------------- Time ->
With a low number in the pitch register:
Counter starts at zero | | +--- Counter quickly reaches the number in the pitch register, | | resetting earlier than before v v | | | | | | | | | | | | | | | | <-- Output pulses are more frequent, higher pitch | | | | | | | | +------------------------- Time ->
Now we need to turn these narrow pulses into a more useful sawtooth wave.
Wave-shaping part:
+ Power supply >----+ +-----------------> Sawtooth output
| |
Q1 |V |
Volume CV >-------| |
|\ |
| |
+--------+
| |
----- \|
C1 ----- |-----< Pulses from core oscillator
| V|
| |
Ground >------------+--------+
This part looks very similar to a VCO. The capacitor slowly charges through transistor Q1. But instead of reaching its maximum voltage and being discharged by a comparator, it's manually discharged by a pulse from the core oscillator.
+---- Capacitor charges gradually
|
| +---- Pulse from the core oscillator resets to 0 volts
| |
v v
V | _/| _/|
o | _/ | _/ |
l | _/ | _/ |
t | _/ | _/ |
a | _/ | _/ | _
g | _/ | _/ | _/
e |/ |/ |/
+--------------------------------
Time ->
If the volume control voltage is held at a fixed level, then the capacitor always charges at the same speed. This has the undesirable effect of lowering the volume as the pitch increases:
V | o | l | t | a | _/| _/| _/| _/| _/| g | _/ | _/ | _/ | _/ | _/ | _ |/ |/ |/ |/ |/ |/ +-------------------------------- Time ->
The capacitor simply doesn't have as long to charge up before it's discharged again by the pulse from the core oscillator.
To work around this, the volume control voltage also needs to be varied with pitch. Changing the volume CV allows more current through transistor Q1, in turn charging the capacitor more quickly. It's now able to reach its proper voltage at the higher pitch:
V | /| /| /| /| o | / | / | / | / | l | / | / | / | / | t | / | / | / | / | a | / | / | / | / | g | / | / | / | / | e |/ |/ |/ |/ | +-------------------------------- Time ->
Unlike the control voltage in a VCO, the volume CV doesn't need to be anywhere near as accurate. The human ear is very sensitive to variations in pitch, but not very sensitive to variations in volume. All the extra circuitry required to ensure the capacitor charges at the same rate regardless of temperature is unnecessary.
Another advantage? You can now change the volume of the sawtooth output by varying the volume CV. This is very useful in a synthesiser with two or more oscillators per voice. You can alter the volume balance between oscillators without needing a separate VCA (voltage-controlled amplifier) circuit.
For the core oscillator, many vintage synthesisers use the common 8253 programmable interval timer chip (also used in the IBM PC). This chip combines 3 digital counters, 3 pitch registers and 3 digital comparators. So just two of these chips can produce the pulses needed for a 6 voice synthesiser - probably the reason the famous Juno 6 has 6 voices rather than 5 or 8.
This type of oscillator is often confusingly called a DCO, but it's very different.
The analogue circuitry is completely removed, the waveform is generated using digital circuitry instead. This makes it possible to integrate several complete oscillators onto a single chip.
+----------------------------------+ +----------------+ }
| Crystal oscillator (several MHz) | | Computer | }
+----------------------------------+ +----------------+ }
| | }
V V }
Reset +----------------------------------+ +----------------+ }
+--->| Digital counter | | Pitch register | }
| +----------------------------------+ +----------------+ } Core
| | | } oscillator
| | +-----------------+ }
| | | }
| V V }
| +----------------------------------+ }
| | Digital comparator | }
| +----------------------------------+ }
| | }
+---------------------+ }
|
Trigger pulses ----------------------
|
V }
+----------------------------------+ }
| Digital counter | }
+----------------------------------+ }
| }
V } Wave
+----------------------------------+ } shaper
| Digital wave-shaper | }
+----------------------------------+ }
| }
V }
+----------------------------------+ }
| Digital-to-analogue converter | }
+----------------------------------+ }
|
V
Audio output
The core oscillator is identical to a DCO. But now the trigger pulses are fed into a very different wave-shaper circuit.
The trigger pulses cause a second digital counter in the wave-shaper circuit to count upwards until it overflows and goes back to 0.
The counter is fed into a digital-to-analogue converter, which makes a stepped sawtooth waveform.
+-- Each step happens when the first counter resets, increasing the
| second counter by 1
|
| +-- Second counter overflows, goes back to 0
| |
| v
V | | _ _
o | | _| | _| |
l | | _| | _| |
t | | _| | _| |
a | v _| | _| |
g | _| | _| | _
e |__| |_| |_|
+--------------------------------
Time ->
The lower the number in the pitch register, the more often the first counter resets. This means less time between steps, and the second counter will reach the top faster and overflow to 0. This makes a higher-pitched sawtooth wave.
V | o | || || || || l | | | | | | | | | t | | | | | | | | | a | | | | | | | | | g | | | | | | | | | e |_| |_| |_| |_| | +-------------------------------- Time ->
If there are plenty of bits in the second counter (and the digital-to-analogue converter), the steps are so small they're removed by the synth's filter and a perfect sawtooth is produced. This is practically the same as a DCO - an analogue circuit (the filter) takes a digitally-originated signal and produces a smooth wave.
V | /| /| /| /| o | / | / | / | / | l | / | / | / | / | t | / | / | / | / | a | / | / | / | / | g | / | / | / | / | e |/ |/ |/ |/ | +-------------------------------- Time ->
If there are only a few bits in the second counter (and the digital-to-analogue converter), the steps are large and it looks less like a sawtooth wave. This is when it starts sounding digital. The Korg Poly 800 is notorious for this, with only 4 bits (16 steps) for the sawtooth!
V | __ __ __ o | | | | | | | l | __| | __| | __| | t | | | | | | | a | _| | __| | __| | g || | | | | | e || |_| |_| |_ +-------------------------------- Time ->
Some early samplers use a special high-frequency VCO to produce the clock signal for all voices. They do this to allow a global LFO and the bender to modulate pitch without the internal computer's involvement. This VCO doesn't suffer from the drift problems of a conventional synthesiser VCO, but it does introduce some phase noise. This phase noise modulates each sample being played, and may contribute to the fat sound of those machines.
The same technique could be applied to any other sampler or synthesiser using a DCO. Replace the crystal oscillator circuit with a high-frequency VCO of similar design, and simply don't modulate its pitch. Should be equally applicable to vintage synths with DCOs and modern synthesisers with a big ASIC for the entire synth engine. Whether it would make an audible difference is left to experimentation.