Fig 1. Harmonic series represented in music notation
Harmonically challenged
Sound and music, by its very nature, is made up of harmonic overtones. These are a naturally occurring set of pure notes (overtones), that appear in everything that we hear, to a lesser or greater extent, each harmonic being a sine wave. The brighter a sound may be, the more upper harmonic content it contains. The quieter, or duller a sound, the less harmonic content there is. These harmonics are derived from the harmonic series, which can in turn be derived by methods of number crunching, but we’re going to stick with a more musical explanation.
The first harmonic of a note is also described as the fundamental; this is the ‘named note’ that you are describing or playing. We’ll work on the concept using the note C. The second harmonic is the note C one octave higher, with the third harmonic being the note G, a 5th above the C. We then continue rising, to another C, then an E, a G and so on. (See Fig 1)
Where we, as synthesists, become involved with the harmonic series, is by our reliance on waveforms. These are the prescribed sounds, available from the oscillator section on a subtractive synthesiser, that will be represented by a set of wave icons. We tend to rely on a familiar set of waveforms, determined in shape by the harmonic content, which would be the sum of all the included harmonic sines, if viewed in real-time on an oscilloscope.
The most common examples include the sawtooth, so called because it looks like a tooth from a carpentry saw, and contains harmonics which lessen in volume as the harmonics increase in frequency, hence it’s quite bright and loud. The square wave sounds a little hollow by comparison, as it is missing all of the even harmonics. (Fig 2)
There are other examples, such as triangle, and choice does tend to differ from synth to synth, but you’ll be hard pushed to find a subtractive synth that does not include sawtooth and square waves.
Chip away
So to the ‘subtractive’ notion of this form of synthesis. The hope is that we begin with a waveform which is relatively rich in harmonic content. Imagine the waveform as a large chunk of stone in front of a sculptor, before they start work. Using a hammer and chisel, the sculptor will begin to remove bits of the stone that they do not wish to be seen. Now apply that same principle to our harmonically-rich waveforms, and we’re going to start to filter out the sound
that we don’t wish to hear. We’ll do this using (unsurprisingly) a filter!
The most common type of filter is known as a low pass filter. The clue, as they say, is entirely in the name! The main control within the filter section is called the cutoff frequency. We told you that the word ‘frequency’ would be back, and sure enough, wherever you place this control, in terms of a frequency point, the filter will let the lower frequencies pass through, filtering out the harmonics above this point.
Many synthesisers turn this upside down, using a filter called a high pass filter. You can probably guess how this sounds, as it lets the higher harmonic elements through, filtering out the lower harmonic elements.
Another ubiquitous filter control is resonance. This control will increase the amount of resonance at the cutoff point. It is this control that amplifies the ‘squealing’ effect that’s very common with certain styles of electronic music.
Where it starts to get a little more involved is with the make-up of the filter. In days of analogue, filters were constructed from discrete components, with certain companies or styles of analogue synthesiser acquiring a legendary sound and status, largely drawn from the filter circuit. In our world of plugins, this might translate by way of description, such as a Moog ladder-style filter. In many respects, all that matters is that you like the sound, and if you have a choice of filter, experimentation is essential to work out what is best for your sound, production and track.
Amps and modulation
Just as the oscillator will flow to the filter, so the filter flows to the amplifier section. (See diagram on the left.) This is often intrinsically linked with an element described as an envelope. These are incredibly useful and used to apply what is known as modulation to an element of your synth sound. (There’s more on modulation in the box on the previous page.) In the most common scenario, you may see the letters ADSR (Attack, Decay, Sustain, Release) residing in the amplifier section, which makes perfect sense, as they control the volume (or amplitude) of your sound as it is played.
Each of the ADSR phases occurs, one after another. The attack phase dictates how long it will take for your sound to increase to its maximum volume. This is then immediately followed by the decay phase, which is how long it takes for your sound to fade to nothing, while the synth key is being held. The decay phase can be interrupted by the sustain level; this is the only phase that applies a level, as opposed to a timing. The higher the sustain level, the louder the sound’s sustain is, interrupting the decay phase in mid-flow. Only once you release your keyboard/note, will the release phase begin, specifying the amount of time it takes for your sound to fade to nothing.
Looks familiar
While we have only touched on the basics here, discussing the three most used elements of subtractive synthesis, you will find that these elements crop up in 99% of currently available synthesisers and samplers, whether hardware or software based. Getting to grips with these basics will open up a host of creative opportunity, where you will no longer be a slave to your presets!
Now it’s time to explore another form of popular synthesis as we catch the next wave…