How does an analogue synthesizer work?

In this article, I’m going to answer your question, about how does an analogue synthesizer work, but I’m going to do it in a very specific way.

If you are interested in synthesizers more generally then check out my article on how does a synthesizer work. This covers other types of synthesisers, digital synths, FM wave-table and other stuff but in a more general, less specific way.

This article is just about how analogue synthesizers work and specifically I’m going to show you how a typical analogue synth works in detail. Not at component level but at module level. It will be done in such a way that you can find your way around other analogue synths once you know what you are doing.

Analog synthesis is also known as subtractive synthesis. You start with something like square waves and then use the filter to remove frequencies from the original sound wave.
This differs dramatically from digital synthesizers. Rather than subtractive many digital synthesisers are additive.

The Sequential Circuits Pro 1 analogue synthesizer

This was the first proper synth I owned. By proper, I mean bought from a shop. Previous to this I had built some synthesizer projects from magazines being an electronics magazine enthusiast but the Pro 1 was a proper synth used by proper synth gods like Vince Clarke from Yazoo who used it almost entirely for their first album “Upstairs at Eric’s”. Check it out if you want to hear the Pro 1 at its best.

The picture above shows the front panel of the Pro 1 front panel. It’s actually the U-he Repro 1 VST plug-in recreation or the Pro 1 and incidentally, this is incredibly good and the next best thing if you can’t get a Pro 1!

The Sequential Circuits Pro 1 creates its sound entirely from analogue circuits. Although digital circuitry is used for the control of some of its features like the sequencer and keyboard, the sound creation circuitry is entirely analogue.

The Voltage controlled oscillator

The first thing on an analogue synthesizer is the oscillator. It’s the sound that we hear to start with. The Pro 1 has two audio oscillators. Some analogue synthesizers only have one like the Yamaha CS5, Roland SH101 or Korg MS10. Having two gives you many more sonic possibilities as we’ll see later.

So the oscillator produces a sound that you can hear. It’s pretty flat and static as it sounds. You will usually find that the oscillator will produce different waveforms. As you select the different ones you should hear different tones. The ramp wave or sawtooth wave is a waveform rich in harmonics. This will become more apparent when we move on to the filter section.

If you have a triangle wave listen to that and compare it to the sawtooth. Next, select the square wave and listen to the sound of this waveform. You will probably have an associated control which lets you alter the pulse width of the square wave. Listen to the sound as you sweep it across the range. As you manually move the control you create a timbral movement.

The picture above shows Oscillator B of the Pro 1. You can choose any of the three waveforms and you can also switch it to low frequency and disconnect the keyboard control.

The Voltage controlled filter

The sound waves from both of the voltage-controlled oscillators go to the voltage-controlled filter.

This is common to all analogue synthesisers. Each analogue synthesiser has a slightly different sound. This comes for the most part from the particular sound of the filter. Once you become more experienced with analogue synthesisers you will start to recognise the sound of particular synthesizer filters.

The Pro 1 filter is a 24 dB type. Some have 12dB ones which sound more electronic. Moog filters have always had a particularly desirable sound. It is warm and probably down to the slight overdrive that occurs. The two most important controls on a filter are frequency and resonance, sometimes known as the Q control.

Trying out the filter

Select a sawtooth wave from an oscillator and set the resonance to zero.

Twist the filter frequency control and you should hear the filter acting on the sawtooth wave. It will go from bright and fizzy to dull and almost vanish. Turn the resonance up to about a quarter and do the same as you just did with the filter frequency. Keep increasing the resonance while twisting the frequency control and you will be hearing the traditional analogue filter squelch sound.

On some filters, as you increase the resonance control you will get to a point where the filter self oscillates and produces a whistling sound. The frequency of this whistle is dependent on the frequency setting of the filter.

The Pro 1 filter will self oscillate and it does come on useful for some sounds as we’ll see later.

The reason I said to select a sawtooth is that it has a lot of harmonics and the harmonics are what the filter filters. Try sweeping the filter but this time use the oscillator square wave. A square wave has plenty of harmonics but different ones from a sawtooth. You will hear the sonic differences between sweeping the filter with a square wave and a sawtooth wave.

Now try it with the triangle wave if your synthesizer oscillator produces one of those. The filter effect won’t be as pronounced. That’s because a triangle wave doesn’t have as many harmonics so there’s less for the filter to take out.

The envelope generator

The Pro 1 has two individual envelope generators. You may only have one on yours or you may have a second one with limited controls. Maybe just attack and decay but with a Pro 1, you get two fully functional ADSR envelopes.

The first envelope is used to control the voltage-controlled amplifier and as such, it is hardwired to that specifically. The ADSR refers to attack, decay, sustain and release. It may seem complicated to start with but once you play around and understand what you are doing it soon becomes second nature over time. Set all the AMPLIFIER controls to midway or 5 on the Pro 1.

Attack

This is the first portion of a sound envelope. If you have set it to midway or 5, when you press a note on the keyboard you will hear the sound slowly fade up to full volume every time you press a key. Turn the attack to zero and press a key. The sound is then heard immediately. Turn the attack control to 10. This is the slowest attack time you can get. Each time you press a key you will hear it slowly fade up.

Decay

Turn the attack control back to zero. Now when you press a key the sound is there immediately and as you keep the key pressed you will be hearing the decay portion of the sound envelope. The decay is the time it takes for the sound to get to the sustain level so turn the sustain level down to zero to hear the decay effect better.

Turn the decay to zero and you should hear a tiny blip of sound. As the attack is zero, the keypress instantly plays the sound but then decays it to nothing almost immediately. Turn the decay up slowly as you repeatedly play a note and you will hear the time taking longer and longer to decay away. Turn it to maximum and that’s the sound of the longest decay your synthesizer envelope generator can produce

Sustain level

This is a level control and not a time as the attack, decay and release are. Set the decay control to midway 5 and set the sustain level to 5. Now hold down a note. After the decay has finished (you now know what you are listening for after the decay experiment) the sound will reach a plateau as you continue to hold the note. This is the sustain level.

When you play the keyboard and hold a note this is the level the envelope generator gets to once the attack and decay have finished. If you now turn the decay control to its maximum it won’t do anything. Do you understand why?

If the attack is zero, when you play a note, the notes volume goes to full almost immediately. It then decays to the sustain level but if that’s already at maximum there’s nowhere for it to go. Hopefully, that makes sense now. Try it out and you’ll get the hang of it.

Release

The above envelope experiment is the best way to demonstrate the last control of release time.

The release time is the time the sound takes to die away from the sustain level to zero. So set the attack to zero, decay to maximum, the sustain level to maximum and the release time to zero.

Play a note and it will sound as soon as you press the key and stop immediately as you lift the key.

Increase the release time and keep repeatedly playing a note and you will hear what happens.

Again, turn it to maximum and each time you stop playing a note it should take a while to die away.

That’s how each control of the sound envelope works. The sound envelope plays quite a large part when you step sequence parts. Depending on the speed of the sequence you can tweak the ADSR for maximum effect.

How does an analogue synthesizer work – Two oscillators

Back to the oscillators, the Pro One has two and using the frequency controls of the oscillators you can tune them together. Or slightly apart. When they are slightly apart they produce a detuned almost chorus effect.

This is a typically powerful synthesizer sound, now tweak the filter. Try switching the octave of one oscillator down an octave or two.

You may have noticed a switch on the Pro 1 VCO labelled sync. This innocent little switch can have quite an effect. With it turned on, the frequency of the two oscillators is locked together. However, if you try to alter the frequency of oscillator 2 it stays in tune with oscillator one but you will be stretching the wave shape, with it getting more extreme the further you tune it from oscillator one.

This produces all sorts of extra harmonics that you can clearly hear with the filter fully open, try switching the oscillator waveforms. I told you it’s possible to do lots more with two oscillators

How does an analogue synthesizer work – Voltage control

You may have heard the term voltage controlled. Voltage-controlled oscillator, voltage-controlled filter. Voltage-controlled amplifier. What’s this all about then?

Well, let us look at the voltage-controlled oscillator first. Its pitch can be voltage-controlled. When you turn the voltage-controlled oscillator frequency control you’re just changing the voltage that alters its frequency. So what is so good about this? Well, an oscillator produces tones that are just fluctuating voltages. So the pitch of the voltage-controlled oscillator can be controlled by the output of another oscillator. You probably heard this effect.

How does an analogue synthesizer work – LFO

The LFO or low-frequency oscillator is specifically made to produce a voltage output to control other modules. Not to produce any sound itself.

As it only produces a frequency up to a few hertz even if you heard it, it’s below the range of human hearing so you would just hear a clicking sound.

An LFO controlling the pitch of a voltage-controlled oscillator can produce a mild vibrato with something like a slow triangle wave with a small amount of depth. Turn up the depth and you get siren-type sounds, switch the LFO to a square wave and you will hear the two separate tones as a square wave switches between two values. Turn up the depth and the pitch of the two notes gets further apart.

How does an analogue synthesizer work – Modulation matrix

On the Pro 1, this is where the magic happens. It’s where all the modulation voltages get routed around

It can also be hard to get your head around if you don’t fully understand the concept so if you are a beginner don’t just jump into this bit and expect to become proficient.

The modulation sections of the Pro 1 are particularly well thought out and gives you plenty of options.

You may have heard about modular analogue synthesizers. With a modular synthesiser, the oscillators, filters and envelope generators aren’t connected at all. You have to make the connections yourself with patch cords. This does give you the option to connect them together exactly how you want. This does make them more difficult to use than a Pro 1 but it does make them far more versatile.

The modulation matrix on a Pro 1 tries to emulate some of the flexibility that a fully modular system gives you by routing the instrument’s signals.

How does an analogue synthesizer work – Sources and destinations

Some things on the Pro 1 are sources of control voltages and some are destinations of control voltages. Some are both. The modulation matrix links them all together. They can go directly or through the modulation wheel.

You’ll find the modulation matrix on the left side of the Pro 1 panel. It’s labelled modulation. On the left of the oblong are the three “from” modulation sources. From top to bottom, these are FIL ENV, OSC B and LFO. To the right of these three controls are three switches one for each to decide whether they go direct or through the control wheel. On the right are five switches. These are OSC A FREQ, OSC A PW, OSC B FREQ, OSC B PW and FILTER. Each of these five switches can switch between a central OFF and DIR or WH.

So how do I use it?

Start with the three amount controls set to zero and three switches set to direct. Set the five TO switches to the central OFF position.

Now we’re ready to go. In the LFO box set the frequency to 7 and switch to a triangle wave. Now switch the top OSC A FREQ to the bottom DIR position.

Now turn up the bottom AMOUNT marked LFO and you will hear it modulate the pitch of OSC A. If you switch the third TO switch marked OSC B FREQ to DIR, the LFO will now be modulating both OSC A and OSC B’s frequency, with the AMOUNT controlling the depth.

If you switch both the OSC A FREQ and OSC B FREQ to the central OFF and instead switch OSC A PW to DIRECT. The LFO will now be modulating the pulse width of oscillator A. You will have to have the oscillator output switched to a square wave or you won’t hear it. Alter the manual pulse width control to either end and you will hear the cumulative effect.

The modulation wheel

You can also route through the modulation wheel and use that to bring the modulation effects in under the control of the wheel and two different sources can control one destination.

Also, you can get really clever about this by using two LFOs to modulate one thing or have the filter envelope modulate the pitch of oscillator B and have the pitch of oscillator B modulate the pitch of oscillator A. You can get some pretty indescribable sounds from this and can create some things like the LFO vibrato speeding up as you hold a note and then slowing down as you release the note.

To be honest I could probably write two articles alone just on the modulation options the Pro 1 gives you.

How does an analogue synthesizer work – Summing up.

Anyone who has an interest in electronic music would have wondered at some time how do analog synthesizers work?

Hopefully, you have a better understanding of how does an analogue synth work. It’s not as easy as something like a guitar but like most things, it is a lot easier to understand if you are following along with an analogue synthesiser to try stuff out.

I’ve explained a lot of things using the Sequential Circuits Pro 1 as an example. This is one of the best analogue synthesisers I’ve ever owned. Recently Behringer released a Pro 1 clone so if you want to you can still get this fantastic synthesiser.