the blog of smokyfrog, deranged windbag and thinker of thoughts: May 2015

As I get lured back into eurocrack land, it's possibly time I finished this off. Can't remember when I started it but several years ago. Presumably it was intended for something but a (very) quick scan shows I probably tossed it off in a few spare moments. I don't know if it's useful or not, see what you think:

analogue modular synths: a starter's guide

Getting started with a modular can be a daunting thing and if you want to make some noises quickly, hopefully this little, quickly-penned guide will help describe the important connections, the modules, the acronyms. It is neither complete nor exhaustive but could grow as (if) more ideas and tips occur.

So what's a modular synth anyway? A scary panel full of knobs and wires, typically, but it's only when you break them down into individual synthesizer modules with their specific functions and connections that it starts to make sense. Maybe there will be a better answer to this later or maybe it'll dissolve into philosophical pap, who knows?

Anyway, a modular synth is one in which no connections are made between the various synth components, the exception to this being some modulars with MIDI interfaces and pre-wired connections - usually for the VCOs and envelopes.

A semi-modular synth (e.g. Korg MS20) is one where the most common patching is made internally so you can make a noise without cables. There are patch points too, so you can change the signal flow to a greater or lesser extent depending on the synth.

The most common terms
VCO

Voltage Controlled Oscillator - sometimes you'll hear of a DCO too, which is a Digitally Controlled Oscillator. This is still analogue but with a digital clock controlling tuning and probably not relevant to modulars^¹. The voltage control bit just means that voltage is used to set the pitch so you can play it conventionally. Other oscillator terms that come to mind include VCDO (Voltage Controlled Digital Oscillator) as found in the Digisound modular.

The VCO, often called simply the oscillator, is the heart of an analogue synth and typically features a number of waveforms - ideally you'll have sawtooth, square, triangle and, if you're lucky, sine. Of these the square wave may be the only one whose pulse width is variable but on some synths the other waveforms might allow this too. We'll look at what you can do with pulse width a little later. Hopefully all waveforms should be available simultaneously so you can pipe them around your modular freely.

An important point to remember early on is that modulars typically have two types of signal - audio and control voltage. Some modulars even suggest these should be treated separately while in other modulars that idea is frowned upon. Although oscillators, noise generators etc. are audio signals, they should still be candidates for sticking into voltage control inputs, as we'll see.

To start with, if you connect any of these waveforms straight to your mixer or soundcard with no voltage control present, you'll hear a constant tone. The sawtooth is the richest, most full-sounding waveform and very popular in subtractive synthesis (i.e. synthesis where you use a filter to remove harmonics). The square wave is more hollow-sounding and contains only the odd-numbered harmonics. If there's a pulse width control, adjusting this will change the sound, gradually becoming a thin, nasal-like pulse. Tonally the transition is roughly like a morph between a clarinet and oboe. Your VCO should have a voltage input for pulse width and this can be used to simulate the action of moving the knobs using a voltage source such as an LFO or EG.

Drones are all very well but sooner or later you'll want to play the VCO from a keyboard or sequencer. For this a CV (Control Voltage) input is required. A VCO will typically have a couple of voltage inputs at least, and if not a mixer module will come in handy to mix voltages together. Initially you'll connect the CV signal taken either from a MIDI interface or from a CV keyboard - a CV derived from note pitch. Most modulars these days are 1V/Octave meaning that for every volt supplied the oscillator increases in pitch by one octave. If your modular has a voltmeter you can even see this and get a feel for what's going on.

Your VCO will probably have some CV inputs with an adjacent knob for "CV amount". While not usually desirable for conventional keyboard pitch, this is useful for other modulation types - for example if you are modulating pitch by an LFO to obtain vibrato. The mod amount knob or CV amount knob, whatever, is used to bring in the vibrato once the LFO patch connection is made. When you start getting more advanced you might use an envelope and VCA to fade in the modulation automatically...

A VCO might have octave switches, possibly labelled as in organ pipe footages where a 16' pipe is an octave below an 8' pipe. Some have a simpler octave up or down switch and some have none at all, or a large frequency control, hopefully with a fine tune also. An oscillator can also be synchronised to another - check if you have a sync input. If so, connecting the output of a second VCO (a square wave is typically ideal for synchronisation due to the hard edge of its waveform but experiment with others) will cause the synced oscillator to restart in line with the master. Here a diagram would help but for now try it out and then experiment by changing the pitch of the synced oscillator manually. If you like the sound, you'll probably already be thinking about connecting a modulation source to the oscillator's pitch to simulate your knob turning automatically. If so you're getting the idea why modulars are so darn addictive!

The Voltage Controlled Filter is a key component of subtractive synthesis and many different types are available to modular owners. A typical filter works by removing some of the harmonics from your VCO waveforms and also emphasising some of them at the cutoff point. The two main controls to achieve this are cutoff frequency and resonance, the latter also known as Q or emphasis - and probably even wackier things too. Filters are a big subject so we'll look at them in more detail later but for now it's enough to think of some of the most common sorts, which are low pass, high pass and band pass. The names indicate the type of filtering done - the low pass filter gradually filters out all but the lowest harmonics. If you have a filter with these three modes it's sometimes referred to as a multimode filter. Try out each and see what the differences are.

If starting with a pure waveform such as a sinewave you shouldn't hear much difference since a sine contains only the fundamental and no harmonics. However, depending on the filter design and the resonance amount, you may hear something change. This is because analogue filters tend to introduce distortion - often almost imperceptible, but when the input signal level is high the resulting overdrive can be interesting. Sometimes you might even use an attenuator to boost your signal in order to deliberately overdrive inputs. The Minimoog Model D has an external input to the filter and if you routed its own headphone output back into the filter, a classic Moog overdrive was the result. Don't try this on a Voyager unless you're prepared for disappointment though! OK, I digress.

Most filters include voltage control over the cutoff frequency - so are prime candidates for envelope, LFO or keyboard voltage control, to name the obvious three usually included in hardwired synths. Keyboard voltage is just another term for the pitch CV used to drive an oscillator in tune. Some filters allow voltage control over resonance and some even give you the individual outputs of each filter pole. Ah, poles. Let's get to them later, no point getting bogged down in details yet.

Voltage Controlled Amplifier - often used as the final output stage. In common with the filter, the VCA's output is typically shaped by an envelope - so look for a CV input to make the connection. Some VCAs have a gain control that raises their level without needing an envelope - this should be fully down if the envelope is used or there will be a scratching of heads. As with many modules there should be several inputs for signals, several for control voltages and at least one output, possibly a couple of duplicated outputs. There may also be linear or exponential inputs for the control voltages. We'll look at the difference in a bit.

Low Frequency Oscillator, sometimes also known as a Modulation Generator (MG), by Korg anyway. It is very like a VCO, in fact some are also voltage controlled but the simplest ones don't offer this. It's an oscillator but if you connect to a sound system, the chances are all you'll hear is a series of pops. It's low, see. At its highest frequencies you should hear a tone but this depends on the range of the LFO. The lower and slower it goes, the longer you have to gradually open a filter or sweep oscillator pitch. The faster it goes the more strange, atonal stuff you get as you reach audio-level modulation.

An LFO should have the same waveforms as a VCO, some might even include sample and hold - or this could be available as a separate module (we'll talk about it later as if it was). Connect the various waveforms to your VCO and hear the difference. There's typically a visual aid to the speed in the form of an LED. You want lots of LEDs in your modular, no harm admitting it. If you are lucky enough to have a voltage input, you can use other modulation sources, maybe a second LFO, to vary the LFO speed.

ADSR

Attack, Decay, Sustain, Release - the traditional 4-stage envelope used to shape the level, the filter etc. Other envelopes are possible, such as the simpler AR or AD envelopes sometimes seen and even complex multi-stage envelopes can be found or VC-ADSR where voltage is used to modify each envelope stage - lots of fun!

I think the name is pretty self-explanatory and each stage easy to understand without me going on about it. One thing that often confuses people is that an envelope is essentially a slew generator - it takes a certain time to reach its maximum voltage (attack time), then if you keep the gate open takes time to decay to the sustain level. Finally when you release the key, the gate ends and the release phase kicks in, here it takes time to reach zero. As these voltages are held in capacitors, a new gate played during the release phase does not start from zero (a sin committed by lots of software envelopes) but from the current voltage. It would take extra circuitry to discharge the voltage at this point - but almost nobody does this because it sounds shit when you do it. Not that this is, in any way, a hobby horse of mine.

Envelopes are triggered by a Gate signal - this is a voltage that remains on until a key is released. They may also be triggered by a regular pulse, say a square wave from an LFO or the clock signal from a sequencer. The gate signal is the traditional output of a CV keyboard or MIDI/CV convertor. Other terms you might hear are single or multi triggering - this is usually the property of the keyboard itself. Single triggering (e.g. the Minimoog Model D) allows smooth legato playing, envelopes are not retriggered if you play legato. A multi triggering synth (e.g. the ARP Odyssey) retriggers the envelopes for every new note, regardless of the nuances of playing. Both have their values of course and it's only flowery hippy types who always play legato, isn't it?

Envelope Generator - another name for ADSR, typically.

Patches, patch cords, patching, Apaches

You'll have heard the term patch to mean a complete synth sound or program and patch cords are just the bits of wire used to connect diffferent modules together. With a modular synth you can make connections that never occurred to anyone else and therefore never appear on a fixed architecture synth. Whether you can remember what you did later is another matter but that's part of the adventure. Digital cameras are handy to "store" your patches made with a modular.

Ring Modulator

A module that takes two signals and outputs their sum and difference combined. I think that explanation manages to make the ring mod sound boring - but it isn't. You should experiment with sending two sine waves (it's easier to hear what's happening with sines) at different frequencies and hear the bell-like tones that result. Later you'll send more complex waveforms but for now, the ring mod is not just used for Dalek voices, OK?

What, you really want to create a Dalek voice? You'll need a way to get your voice in and combine it with a sine wave, its speed adjusted until it sounds Daleky. I may flesh out this explanation if necessary.

Envelope Follower

It strikes me that mention of external processing brings up the Envelope Follower. This creates an envelope shape from an audio signal, so it's very useful for external stuff. Vocoders use them to control band pass filters, the more of these the more you can understand what's being said. Not sure how I got onto that.

Control Voltage - voltage sources include envelopes, LFOs, VCOs, sequencers, keyboards, joysticks, armadillos and aardvarks. Voltage is where it's at and you should be aware of the voltage requirements for each module, each input. If combining different modulars, there could be large differences between the expected voltages, some might trigger an envelope on a change between 0 and +5V, another envelope might sit quietly until it is poked with +10V, for example.

Pulse Width Modulation

You remember twiddling the pulse width control manually? Well, with a voltage input you can get an LFO to do it for you. Or another source. If your VCO has a variable sawtooth wave, this gives an entirely different sound to varying the width of a square wave. Check it out. Faster PWM is commonly used as a source of rich, string-like patches.

Portamento/Glide/Slew

These terms are often used interchangeably and the process is similar to basic low pass filtering. Feed in a fast, complex LFO and a slew generator will smooth it off, send in notes from a keyboard and hear the differences between notes blur, you get the idea.

Linear/Exponential

these are terms that are often used to describe the shape of an envelope stage but apply more widely too. If an envelope has a linear attack it rises steadily and predictably. An exponential curve sounds and feels more natural and some synths even offer variations on the shape, positive or negative curves and so on. For fast sequencer pieces, an exponential curve on the filter envelope decay is instantly recognisable and if you have a linear decay you sometimes have to combine to two envelopes with two speeds of decay to simulate this. Again, a diagram is in order.

Sequencer

An analogue sequencer is a joy to behold, especially for the LED-fixated (i.e. all of us). It consists of at least one row of knobs, each sending a voltage. An input clock is used to step through the sequence and each voltage is sent as the step is reached - simple. Sequencers vary but some allow different directions to be selected for each row, steps to be skipped, input voltages to transpose the sequence and so on. Some sequencers can be clocked at audio speeds, generating an audio waveform - think of this like an LFO running very fast and into the audio range. I can either write a short amount about analogue sequencers or a lot - better I write a short amount I think.

CV Mixer

You know what an audio mixer is, a CV mixer might well be the same thing depending on the modular manufacturer. Mixing voltages lets you feed more complex signals into a single input plus you can get voltage controlled mixers too in which a voltage source controls the levels. This in turn opens up the possibilities for automated mixing, panning, morphing between audio sources or voltage sources. Sky, limit, that sort of thing.

Noise

Noise - assuming you've watched X-Factor, you know what noise is. It appears in different colours: white, red, brown and so on. White noise contains an even mix of all frequencies while pink noise has more emphasis on lower frequencies. Brown is lower still, blue higher and so on. Noise makes a great modulation source as well as an audio source for whooshes, surf, spacey wind and whatnot. It's an ideal source for a sample and hold module to sample and hold...

Oh, and sampled noise is rarely as good because it's not random and quite often you can hear the loop.

Sample and Hold

If you feed noise into a sample and hold module, oh you need to clock it too so the module knows how often to sample, you get a random voltage output. You can feed it different signals too, standard waveforms and mixed waveforms, if total randomness isn't your thing.

Multis or Multiples

Some modulars have banana jacks where you can stack connections on top of each other. The rest have multiples, a line of jacks in series that allow you to trigger multiple envelopes from a single source and generally connect more inputs and outputs than you usually could. One issue that is worth noting is multiples are not active or buffered. So if you're using them in applications where precision is important, such as oscillator CV input, you might notice some of that analogue magic, reduced range. It depends on your modular, the number of connections, the impedence and probably other stuff like solar prominences, atmospheric neutrinoes and celebrity tweets.

Joystick

Distortion/Waveshaping

Quantization

BBD modules

Bit Crunching

Spring Reverb

Ribbon Controllers

Fixed Filter Bank

Theremin

Voltage/Gate, Trigger Delays

MIDI

A serial interface invented in 1756 designed so that musicians could mime effectively on stage. Superceded in 1822 by the Laptop.

(and here's my most recent module)

1Because of their greater stability a DCO is often seen as 'less analogue' although it served well enough in synths such as Roland's Juno series or the Waldorf Pulse. Multiple DCOs stacked together don't sound as sweet as multiple VCOs though.