PART 2: Having surfaced from the depths of the previous instalment, Steve Howell takes a deep breath and plunges back into his explanation of some of the modulation possibilities afforded by modular analogue synths... This is the second article in a three‑part series.
There's more to synths than oscillators — you also need controllers, and any decent modular synth offers various controllers and control processors not necessarily found on more portable instruments. Of course, all modular synths come with LFOs. Or do they? Some use ordinary audio oscillators to fulfil the task of low frequency modulation. Each oscillator typically has a range control for setting the basic oscillator frequency (32', 16', 8' etc.), of which 'Low' is one of the options. By selecting this, the audio oscillator is plunged into subsonic depths for control purposes. On a typical modular synth, all signal levels are in the region of several volts (+/‑5 volts, or more), regardless of whether the signal is audio or control. This gives some of these synths an extraordinary signal‑to‑noise ratio, and also permits modules normally designated for audio generation or processing to act as controllers, allowing totally free patching of modules.
Of a modular synth's eight or so oscillators, therefore, in a typical patch, only six might be used for the actual sound, while two are reserved for control purposes, as LFOs. Those of you who are used to seeing 'LFO' on the front panel of your synth may find this strange, but owners of MiniMoogs are no doubt familiar with it, having to sacrifice Oscillator 3 for vibrato and the like. This was inherited from the larger Moogs. On a modular synth, the fact that these oscillators are voltage‑controlled means that all manner of modulation possibilities are available to you, where the LFO might be controlled by an envelope generator, another LFO, the keyboard position, and so on.
Analogue FM Synthesis
Putting low frequency control to one side for the moment, the use of audio oscillators for control purposes means that you can modulate another device while the modulating oscillator is operating in the audio frequency range. This technique is known as cross‑modulation. What it is, in fact, is an early manifestation of the FM synthesis that Yamaha were to use on their DX instruments, where the idea was to use audio signals to modulate one another in order to create harmonically enriched waveforms. In all fairness, Yamaha's version of it was much more controllable, but the net results are startlingly similar.
As an example, take the output of one oscillator (Osc A) and feed it into the control input of another (Osc B). As the modulation level at Osc B's input is increased, so its tonal quality becomes progressively nastier. In fact, in the absence of a ring modulator, cross‑modulation can be used very effectively to create clangorous bell sounds and the like. But be warned! Because the tuning stability on old analogue synths leaves much to be desired, the oscillators don't track too well, and a sound that is perfectly tuned on C3 may well have disintegrated by the time you get up to F3. There's not much you can do about it on the original synth, but today, by sampling, you can take the perfect analogue cross‑modulation effect and then play it trouble‑free across the keyboard (notwithstanding the pitch transposition artefacts the sampler may introduce over pronounced intervals).
Another trick is to modulate a slightly resonant filter with an audio oscillator. With the oscillator set to 2' settings, generating a sine (or triangle) wave, and the merest hint of modulation, you can create digitalesque 'tingles' that are simply gorgeous, sounding more like a PPG than an analogue instrument. Add to this a touch of envelope modulation, and you can create an interesting, slightly metallic 'voice box' effect.
Continuing the FM analogy, modular synthesis allows you to create many of the sounds the DX7 was later to produce by way of 'shaping' this cross‑modulation. As mentioned, because audio and control signals operate at the same levels on modular synths, the Voltage Controlled Amplifiers (VCAs) can also be used to process control signals, so, by patching an audio oscillator via a VCA whose output level is being controlled by an envelope generator, you can send 'bursts' of cross‑modulation to a module. Applied to an oscillator, you would get a discordant attack; apply the same modulation to a VCF in judicious amounts, and you could introduce a short 'rasp' of modulation to the sound, which, if done well, can quite accurately allow you to introduce the 'spit' of brass sounds, thereby livening up an otherwise dull synth brass patch. Of course, similar techniques can be used to add 'scrape' to strings, 'blow' to flutes and so on.
One problem with all of this is that the tracking instability between one or more oscillators leads to certain tuning anomalies when using cross‑modulation techniques, thereby rendering the sound useful only over a short range. However, by employing oscillator sync, this can be overcome and a certain pitch stability across the keyboard range achieved while cross‑modulating (see separate panel 'All this and the oscillator sync too' for more information on oscillator sync). However, while this is fine and dandy for oscillator cross‑modulation effects, because the filter cutoff frequency cannot be synced to an oscillator in this way, tracking on filter cross‑modulation effects is a bit more unpredictable. Again, a sampler may be the cure for this particular problem, capturing a snapshot of your favourite filter cross‑modulation effect.
Low Frequency Modulation
On a well‑equipped modular synth, you can mix low frequency waveforms (either by means of a wealth of dedicated LFOs or several of the audio oscillators set to Low) to create no end of alternative control waveforms. By blending, for example, a slowly rising sawtooth and a faster square wave, you can create many interesting arpeggio effects. The fact that it is difficult (if not impossible) to synchronise the different LFO rates means that the arpeggio effect will shift, giving rise to many interesting rhythms and syncopations. However, if you use a pair of audio oscillators in their low frequency mode, and link them using the sync function, such effects can be successfully synchronised, allowing you to create many interesting alternative control shapes. It is this ability to mix control waveforms in such diverse ways that makes modular synthesis so rewarding and flexible.
Control Processors
Let's now have a look at some of the controllers found almost exclusively on larger analogue systems. One favourite of mine is the Sample and Hold module. The premise of this device is that it 'samples' an incoming voltage, and holds it for a specified period of time. The sample and hold circuit, which is used to process the keyboard voltage output, holds the sampled voltage indefinitely, while the Sample and Hold (S/H) module has an internal clock. More often than not, the S/module is used to process the random voltage output of a noise generator, to provide unpredictable voltage steps (see accompanying 'Sample and Hold' panel).
However, by using a rising sawtooth wave, a rising and falling triangle wave, or the output of an envelope generator as the 'sample' input, you can generate atonal arpeggios. You could also route a pitchbend lever or wheel to give stepped pitch bends or glissando. Furthermore, the internal S/clock can be overidden with an external clock source. Often, this external source is the square wave output of one of the LFOs (or audio oscillators set to Low) but take the Gate output from the keyboard, route it to the clock input of the S/module, and things start to become interesting!
By using a random noise generator as the 'sample' input and the gate output from the keyboard as the clock, each note you play will generate a completely random voltage. When applied to the filter, each note will have a totally random tonal quality. If you then sequence this effect, you may create synchronised, random timbre riffs that can add a lot of interest to a track. You could also apply it to one (or more) of the oscillators in minute amounts so that each note is slightly out of tune. In ensemble sounds, this creates the effect of no two players hitting exactly the same note. With synced oscillators, and a wide modulation amount on the oscillator being synchronised, the sync sweep effect can be totally random. This can be applied to pulse width modulation as well, so that each note has a different pulse width, or you can use a combination of any of these suggestions — providing you have enough patch cords!
By taking the output of the S/and using it to control the the frequency of the external clock, not only will the pitch and/or tone of the sound be random, but the time between them will also be random. It's hardly a sophisticated algorithm for generating melodies, but it can be an interesting way to create a totally random background in a free‑form, tempo‑less piece of electronic music, especially with a well‑chosen sound processed through one of today's digital multi‑effects.
Other Processors
Another processor you're likely to find on a modular synth is the lag time processor (LTP). What this does is process a stepped input and introduce a 'slur' between the steps at its output. This has many uses. Firstly, you can route the output of the keyboard through the LTP, to produce portamento or glide effects between notes. I realise that most synths already have a glide function; but by routing the keyboard to one (or more) of the oscillators via the lag time processor(s), some oscillators will glide at one rate, while others will glide at another. This can be useful in certain 'ensemble' situations where different instruments slide to the new note at different rates. If driving your synth from a MIDI‑CV converter, you can process the CV output of the converter to introduce portamento.
Another use is to process the output of the S/module, so that instead of jumping in discrete steps, the voltage slurs from one value to the next. This may be used as an interesting form of vibrato. By setting the S/H's clock rate to a suitable vibrato rate (7Hz or thereabouts), and routing the output of the S/via the LTP to the oscillator(s), you can create a more natural vibrato, where the modulation is not as cyclic as that found when using a repetitive triangle wave. When attempting to simulate 'ethnic' instruments, where the vibrato technique may not be Royal Academy‑trained (e.g. pan pipes), this can be most effective. This technique is also good for ensemble parts, such as strings and pads, and can add considerable 'movement' to a sound.
The inverter is another element on a modular synth. As the name suggests, this turns the incoming control signal upside down, but has more uses than you might think. For example, sending an LFO to one or more oscillators directly, and to others via the inverter, creates a thicker ensemble sound. Similarly, you can send an envelope to modulate the pitch of a synced oscillator directly, while sending the same envelope to the filter via the inverter. An alternative source of interesting sounds is to use two filters in parallel, modulate one directly and one via the inverter, so that they sweep in opposite directions. With care, interesting vocal articulations can be achieved.
One other element of the larger modular synths is the trigger delay. Basically, this takes the gate output from the keyboard and delays it, before passing it on to the envelope generators connected to it. In this way, you can layer sounds and have them triggering at different times. This can be useful when creating ensemble sounds. A string section, for example, will rarely play exactly in time, so by setting up and layering two similar string patches, and then holding one back very slightly by way of the trigger delay, you can re‑create this effect. If you have the facility to layer more than one string sound, and have a sufficient number of trigger delay modules, the effect can be heightened further.
Some trigger delay units can have their delay time voltage‑controlled. If you modulate the delay time from an LFO, you can create varying startup delays. By using the output of an S/module which is being clocked by the keyboard gate, you can bring about totally random trigger delays, and this can help emulate ensemble parts, because just as ensemble instruments do not play quite in time, their delay is also never consistent. Of course, in these examples, the delay times are quite small, as is the modulation amount of the delay time.
As you can see, when you begin to combine these modules together, you can create highly detailed sounds, and you should be able to see why a modular synth, particularly a large one, is so powerful. Granted, when going into this kind of detail, you're not going to knock up a complex patch in minutes (some programmers report spending days getting 'the ultimate sound'!), but the effort you put into it can be very rewarding. In the concluding part of this examination of modular synths, we will look at sequencers, scale programmers, function generators and more.
All This And The Oscillator Sync Too
The original idea behind oscillator sync was (and I quote the original Moog brochures) to "permit beat free intervals and chords throughout the audio range". You see, however well your synth is tuned up, it is almost impossible to use two oscillators simultaneously without some form of beating, and while you may tune one note to be perfect, the tracking anomalies of analogue oscillators almost guarantees that it will be beating a few semitones up or down. Oscillator sync was introduced so that you could mix several oscillators without this problem. This may seem contrary to the spirit of things, given that analogue processing can produce such a big, fat sound, but, believe it or not, there are times when you wish to create solo sounds such as oboe, flute, voice and so on, where you need to mix oscillators but lose the beating. However, the deliberate abuse of oscillator sync gave rise to the distinctive 'tearing' sync sound that analogue oscillators can produce so easily.
Oscillator sync is created where two oscillators can be phase‑locked together. This in itself can give rise to a whole host of new waveshapes, which, with simple fixed intervals, can sound extremely digital, quite belying the fact that the sound has originated on an analogue synth. However, if you sweep the 'locked' oscillator alone, you create a dramatic sound not unlike flanging on acid! Interestingly, the so‑called 'classic' MiniMoog and ARP 2600 had no such feature (unless you paid for an expensive hardware modification), but the ARP Odyssey did, and it was to become a standard feature on 2‑oscillator synths thereafter. Interestingly, the recent 3‑oscillator SE1 analogue synth allows you to sync oscillators 2 and 3 to oscillator 1. Typically, the effect is created by sweeping the synced oscillator with a spare envelope generator and/or LFO (or, on a modular synth, with whatever else you can muster). Best results are obtained when an interval of at least an octave is set between the two oscillators.