Learning Synthesis with Vacuum

Now that Pro Tools ships with a great sounding, simple to program synthesizer plug-in, users have a wonderful way to dive into the world of subtractive synthesis. Part of the new AIR Creative Collection, Vacuum is a two-oscillator tube modeled analog synthesizer perfect for getting your feet wet in synth programming. This week at the corner I will give you a crash course in subtractive synthesis programming via Vacuum.

Subtractive Synth Basics

In its most basic form, subtractive synthesis is essentially the process of taking harmonically rich waveforms (usually generated by simple oscillators) and removing (or subtracting) some of their frequency content via filters (most commonly hi and low-pass filters). For example, by feeding a harmonically rich sawtooth wave into a lowpass filter, we can remove or reduce the higher partials to better approximate the timbre of a physical instrument (like a bowed string). By combining multiple oscillation sources with a variety of different filter and envelope options, one can generate a near infinite number of unique waveforms, some of which may emulate the waveforms of other instruments. Generally we associate subtractive synthesis with the ‘classic analog synth’ sound, a Moog Voyager is an excellent example of a subtractive synthesizer.

Synthesis in a Vacuum

While there are probably more than a hundred subtractive synth plug-ins on the market today, Vacuum strips things down to the basics with a vintage inspired, single-page interface and old-school tube sound. Vacuum is a mono-synth, much like the Moog Voyager or Little Phatty. This means you can only play one note at a time, perfect for leads and bass sounds, but you’ll want to look elsewhere for lush polyphonic pads and strings.

One of the best ways to learn any type of synthesis is to work your way backwards from a preset patch, critically examining each parameter and asking yourself how it might affect the resulting sound. While I am happy to admit that I do not have a degree in synthesis or consider myself an expert in any way, I do believe it is important for a producer to know how to ‘get sounds’ and learning the basics of synthesis key. Nine times out of ten I can find close to what I am looking for in a factory preset and with a little tweaking of a few key parameters get to the sound in my head.

Deconstructing a simple bass patch

After placing Vacuum on a mono instrument track, I have called up the ‘Bass > Woody Chap’ preset from the factory presets menu. This is a simple bass patch with straightforward oscillator, filter and envelope settings that serves as a great introduction to the concept of subtractive synthesis.

Start with the Oscillators

A synth’s sound begins with its oscillators. Remember that sound waves are made up of periodic variations in atmospheric pressure, oscillating up and down like waves in the ocean. A synthesizer’s oscillators serve as its core tone generators and will create the basic blocks of sound that we will carve our patch from.

Vacuum has two vacuum tube oscillators (labeled ‘VTO ONE’ and ‘VTO TWO’) that generate four different wave ‘shapes’ (Triangle, Noise, Saw, and Pulse Wave). The shape setting is continuous so you can create settings like 65% triangle and 35% saw. These two oscillators are combined together in the mixer to create a more complex waveform that is then fed into the filters. The rate of oscillation (and thus pitch) is determined by the note you play on your MIDI keyboard, but the octave is defined by the ‘Range’ control, with the ‘Fine’ control giving you an additional 7 semitones of pitch control in .01 semitone increments.

Notice that the Woody Chap patch uses both oscillators set to ‘SAW’ shape but separate range settings, with one an octave below the other. Play a note and change the ‘VTO1’ volume in the mixer to the right, notice the higher octave component drift in and out. At this point I could add a small amount of pitch shift to the ‘fine’ control, maybe only a 1/10 of a semitone, to achieve a subtle chorusing effect.

Move through the filters

A synth’s filters are essentially simple EQs that shape the output of the oscillator section, thus shaping the timbre of the sound. Most synths feature a low-pass filter (or LPF) with a resonance control. A low-pass filter will attenuate the hi-frequencies (beginning at the ‘cutoff’ frequency) while the resonance control will add a gain peak at the cut-off frequency. It is probably easiest to understand just by listening. Play a note and sweep the cutoff control of the LPF, increase the resonance and sweep through again. The sound you hear as a result of sweeping a resonant low-pass filter is very similar to the sound of a guitar through a wah-wah pedal. Vacuum features both high-pass (HPF) and low-pass filters (LPF). High-pass filters do the opposite of low-pass filters, attenuating the low or bass frequencies.

This patch uses no high-pass filter cutoff and a low-pass filter cutoff of 24%, aggressively restricting the higher frequency partials from the oscillator’s saw waves (remember this is a bass patch). There is a fair amount of resonance added to the LPF, so try sweeping the cutoff for a cool effect. The ‘SLOPE’ control sets the steepness of the filter and is measured in dB per octave. The envelope tracking on the LPF is positively correlated and set fairly high, meaning the filter’s cutoff will respond significantly to the envelope controls, we’ll talk about envelopes in the next section.

Enter the envelope

Most synths feature some sort of envelope that controls how the sound evolves over time, once a note is played. Think about a bowed instrument, like a violin. When bowed, the violin doesn’t immediately achieve full amplitude as it takes time before the bow causes the string to oscillate at full power. Furthermore, the tone of the instrument may change over the course of oscillation. The envelope parameters of a synth act to simulate the same concept, allowing a note to evolve over time. Vacuum features two envelope controls that by default act on or ‘modulate’ the filter and amplitude components of the instrument. The filter envelope ‘ENV ONE’ modulates the filter’s cutoff frequency while the amp envelope ‘ENV TWO’ modulates the sound’s volume.

Vacuum’s envelopes are built on the ADSR model (Attack, Decay, Sustain, Release). Each time a MIDI note is played Vacuum goes through the ADSR cycle, modulating the filter and amplitude components of the sound. ‘Attack’ defines the time it takes for modulation to reach its highest point. ‘Decay’ reflects the amount of time it takes for modulation to die down to the ‘Sustain’ level. ‘Sustain’ represents the level at which the envelope stops while the current note is held. ‘Release’ represents the time it takes for modulation to drop back to zero after the note is released. Check out the diagram for a visual representation of ADSR.

The example patch has a fairly straightforward filter envelope, where the attack (A) is set at 0ms and the decay (D) at around 80ms. What this is going to achieve is a short filter burst, moving the cutoff frequency of the LPF higher for a fraction of a second, creating a little brightness at the head of each note. To help yourself understand this, change the decay to 0ms and listen, now change it back. Notice a difference in tone? The amp envelope (ENV TWO) is set for a standard, instant-on sound with an infinite sustain. This is achieved with an attack time of 0ms and a sustain of 100%. Because sustain is 100% the decay parameter doesn’t have any effect on the amplitude. Try moving the attack time to 300ms, notice how the sound is much softer as it takes time to reach full amplitude. Set the release to 1 second and notice that the note rings out even after you have released the note. Practice understanding ADSR, knowing how to manipulate the envelopes of a patch is key to getting the sounds you want from factory presets.

Modulation Magic

Most synths allow other parameters to be modulated, outside of the envelope modulation of filter and amplitude. For example, I may want to simulate vibrato by using an additional low-frequency oscillator (LFO) to modulate the pitch of my sound generating oscillators. Many synths pride themselves on complex modulation matrices, with unlimited routing options. While this is cool for getting super tweaky, Vacuum features all the basic modulation routings you’d expect to find on a decent mono-synth. In the synth world modulation is all about source, destination and depth, or “who is modifying what and by how much.” In my example of simulating vibrato, I would make the source ‘LFO’ and the destination ‘Pitch’ using the ‘depth’ control to define the width of the vibrato. Modulation routing can be one of the tougher concepts to understand in synth programming so the best way to get a sense of it is to reverse engineer some of the factory presets.

The woody chap patch uses very little in the way of modulation routing, aside from a basic mapping of mod wheel to low-pass filter cutoff. Remember the depth controls the amount of modulation; in this case the depth controls the amount that the mod wheel opens the low-pass filter’s cutoff.

Unique to Vacuum

Beyond its basic synthesis components, Vacuum has a few unique features worth mentioning.

‘Age’ simulates the characteristics of older synths that may have unstable oscillators (drift) and worn out contacts (dirt).

‘VTA’ or vacuum tube amplifier acts as a colored master volume control. Use the shape control to add additional tube saturation to the final output. Remember, Vacuum is designed to simulate the characteristics of an analog synthesizer, so you can drive the oscillators and filters to achieve cool saturation effects. Just make sure to monitor the master volume output as to not clip the output in Pro Tools.

In Closing

Obviously this wasn’t a comprehensive tutorial on Vacuum, but more of an introduction. Hopefully I have inspired you to crack the manual or start exploring and tweaking Vacuum’s sounds on your own. A good foundation in subtractive synthesis will not only help you get closer to the sounds in your head, but also prepare you for more complex forms of synthesis down the road.