Filters are one of the most important parts of sound creation, and they are a foundation to the whole concept of ‘subtractive’ synthesis. The filters do exactly that – they filter out part of the sound, leaving you with a reduced portion of it, which sounds very different to the whole portion. The main control on any filter is the filter frequency, or ‘cutoff’, which is the key point at which all frequencies are cut off – be it all frequencies which are above, below, in between, or outside of the cutoff point. Common filter types include:

• Low Pass – the most common type of filter, the low pass allows all frequencies below the cutoff point to pass through.
• High Pass – the opposite of the low pass filter, the high pass filter allows all frequencies above the cutoff point to pass through.
• Band Pass – allows a band on frequencies to pass through in the centre, but stops all frequencies outside of this band.
• Band Notch/Reject – so called because it looks like a notch, this filter stops a band of frequencies in the centre from passing through.

Each of these filter types can have a different number of attenuation slope, usually 6dB, 12dB, 18dB, and 24dB per-octave – with the higher number being more effective. This is because of the increase in the steepness of each slope per-octave, so a 24dB filter is twice as effective as a 12dB filter, in relation to higher frequencies being higher pitched – e.g. 2000Hz is an octave higher than 1000Hz. Likewise, having more poles in the filter attenuates the signal more, so a 4-pole filter will create a duller, more muted sound than a 2-pole filter which is less effective in reducing frequencies.

Filters also have a feature called ‘resonance’, which basically boosts the frequency the cutoff point is currently set off. The more you boost it, the most the set frequency increases, in most cases to the point where the filter will ‘self-oscillate’, meaning that it creates it’s own sine wave – the pitch of which can be controlled by changing the filter frequency. On its own, resonance is useful for giving a sound a little more high-end, but it can really create spectacular sounds when used in conjunction with an envelope or an LFO, often creating what is known as a ‘filter sweep’.

Without oscillators, your synthesizer will make no sound (well, actually it can - but that's in a future article). Oscillators produce the original sound which you can hear in your patches, which is then fed through the rest of the signal path in the synthesizer. Oscillation itself is the production of a certain type of waveform, which produces a different sound depending on the shape of the waveform. The waveform is constantly ‘run' depending on the speed/pitch of the note - so if an oscillator is set to a low enough pitch you will eventually hear gaps due to the slow speed of oscillation (see LFO below). Common oscillator waveforms are:

• Saw Wave - shaped like the teeth on a saw blade, this produces a very common sharp, biting tone.
• Square Wave - looks like a (near) perfect square, produces a reedy, hollow sound.
• Pulse Wave - a variation on the above, the pulse wave is half as wide as a square wave, and has the unique ability to have its width modulated (called ‘Pulse Width Modulation').
• Triangle Wave - unsurprisingly shaped like a triangle, this sounds somewhere in between a saw wave and a sine wave.
• Sine Wave - a smooth rising and falling shape (like a horizontal ‘S'), this produces a mild, soft tone.
• Noise - not exactly a waveform, but a source of sound produced by a certain colour of noise.

To start off with, that's all you need to know. To spice things up a bit though, depending on the number of oscillators a synthesizer has (usually 2 or 3) you can mix waveforms together! And not only that, but you can tune them differently from each other. This tuning can occur in octaves, semitones, and also in cents - which is a 100^th of a semitone, and adds a swirling ‘detuned' sound created by multiple oscillators which are cents apart from each other.

LFO, or ‘Low Frequency Oscillator' is a special kind of oscillator which oscillates at a frequencies so low you cannot hear it - unless you deliberately tune it into the standard hearing range. It is used to modulate other parts of the synthesizer, such as the pitch of an oscillator, or the frequency of the filter. LFOs still use standard waveforms just like oscillators, but because they operate so slowly the variation in time between the start and finish of the waveform is clearly noticeable - for example, with a sine wave you are able to hear the smooth ascending and descending nature of the waveshape. Used properly, this adds animation and a moving texture to the sound in your synthesizer.