Synthesis • advanced techniques
Other modules • Synthesis would be fairly dull if we were limited to mixing together and filtering a few standard waveforms. However, there are many other components and controls on even a basic synthesiser. • On most synths, the number and structure of these different modules is normally fixed, unless you are working with a modular hardware or software synth. So for example, the Subtractor synth in Reason has just two oscillators, whereas a modular soft-synth like Analog Box 2 or Reaktor allows any number of oscillators to be wired up however you like. • Regardless of what synth you are using though, it is still helpful to be aware of the signal flow of how that synth is structured ‘underneath’ and have a good understanding of the most common modules. • So first, we’re going to go through each of these, before going on to look at the various ways all these components can be used to modulate each other.
Filters • Essentially creatively applied EQ, common types of filter include: • Low pass (LP) - leaves low frequencies, filters out highs. • High pass (HP) - leaves high frequencies, filters out lows. • Band pass (BP) - filters both high and low frequencies leaving only frequencies around the target frequency. • Notch - removes frequencies around target, leaving high and low frequencies untouched. • Other important parameters: • Resonance - a boost or cut around the target frequency. Responsible for the sweeping filter sounds that feature in a lot of electronic music. • 6 pole, 12 pole, 24 pole etc... term referring to the steepness of the filter. The value refers to the number of dB dropped over an octave after the target frequency. As with Q value, higher value means a steeper curve.
Noise generators • Noise generators output random noise, which can then be shaped by filters and other devices. • Useful for adding texture to sounds and for creative sound design. Especially useful for creating synthetic percussion sounds. • Sometimes exist as modules in their own right, but often one of the oscillators on a given synth has the option of being set as a noise generator.
LFOs • LFO stands for Low Frequency Oscillator. • Just like a regular oscillator it generates signals with different waveform shapes. • However, these signals never enter the audio stream and are instead used to control or modulate other parts of the synthesizer in various ways. • They also oscillate at much lower frequencies, usually in the range of 0-20Hz. • The generated signal is called a control voltage.
Sample-and-hold • A sample-and-hold circuit describes a device which quantises a signal to fixed values. • When combined with a random noise generator, it can be used to generate random values when triggered, which in turn can then be used to control other parameters, for example the pitch of an oscillator.
Envelopes • When triggered, envelopes generate a control voltage according to a number of variable parameters. As with LFOs, this signal can then be used to modulate the synth in various ways. • Most synths feature at least an amp envelope to control the envelope of the sound generated. • Parameters include: • Attack (ms) • Decay (ms) • Sustain level (dB) • Release (ms)
Modulation • Modulation refers to the many different ways in which control voltages and sometimes audio signals are used to control or modulate one another. There is usually some form of depth control to set the degree to which the modulation signal can affect the target or carrier signal. • Depending on the synth, you will have different options available as to how and which signals can be routed to control each other. For example, on one synth, the envelope controls may be fixed to control volume, whereas on another you may be able to switch it to control pitch or filter frequency. • Modulation occurring at low frequencies, eg, modulation from an LFO, tends to be perceived more as an effect, leaving the original timbre relatively intact. • Modulation occurring at high frequencies is more likely to result in timbral changes to the sound and the generation of extra harmonics. These extra harmonics may also affect the perceived pitch of the sound in some cases. • Modulating control voltages can either be unipolar or bipolar. Unipolar signals range from zero upwards, where as bipolar signals can be both positive or negative, oscillating around a zero point.These extra harmonics may also affect the perceived pitch of the sound in some cases. • An amp envelope controlling volume would be a good example of a unipolar signal, where as a vibrato/pitch modulation effect would require an LFO generating a bipolar signal.
Amplitude modulation (AM) • In amplitude modulation, a unipolar signal is used to modulate the volume of a carrier signal. • At low frequencies, this produces tremolo effects; at higher frequencies it will have a noticeable timbral effect on the sound. • Generates sideband frequencies either side of the carrier.
Ring modulation (RM) • Describes a circuit which multiplies two bipolar signals together. These signals would usually both be audio signals. • Essentially results in a form of amplitude modulation where the volume of one signal alters the volume of another (As apposed to regular mixing, where the two signals are simply added together). • However, since the audio signal is effectively being modulated by another audio signal, rather than a low frequency control voltage, a type of frequency mixing results, often creating quite extreme effects. • Like AM, RM results in the generation of extra harmonics either side of the central frequency called sidebands. However, unlike AM, the original carrier signal cancels itself out, resulting in a more ‘affected’ sound. • Responsible for creating Dalek voices!
Pulse Width Modulation • A square wave can be thought off as a pulse with equal-length positive (peaks) and negative values (troughs). • Pulse Width Modulation allows the ratio between these distances to be changed, thus affecting the harmonic content of the waveform. • Even on synths without this feature, it is easy to recreate by multiplying two identical oscillators together and adjusting the phase of one relative to the other. • The resulting cancelling out between the two oscillators when they are multiplied together, allows for the pulse width to be varied by varying the phase between the two.
Pitch modulation • The vibrato effect created when the modulation wheel is turned up on a keyboard, is the result of an LFO modulating the pitch of the sound. The modulation wheel varies the modulation depth, meaning the amount by which the modulated parameter is effected. • The modulation rate is controlled by the speed of the LFO. • Even if the wheel is down all the way, it can be assumed that the LFO is still running in the background, albeit without any audible effect.
Frequency Modulation(FM synthesis) • An oscillator is used to modulate the frequency of another oscillator. • Essentially pitch modulation above 20Hz. • Like AM and RM, this results in sidebands. The technique has also led to its own branch of synthesis, with dedicated FM synthesizers appearing in the 1980s such as the Yamaha DX7. • In the world of FM synthesis, oscillators tend to get called operators instead. The more operators available on an FM synthesizer, the more complex the sounds which can be produced. • Dedicated FM synthesizers can be programmed with different algorithms which affect the configuration of these operators. So for example, one oscillator may modulate a second oscillator which then modulates the carrier. • FM synthesis is very good for creating metallic, bell-like sounds due to the enharmonic nature of the generated sidebands.