The science of Synthesis Part 8 - Modifiers

Most synthesizers also offer additional tools for manipulating sound in the form of modulation sources and destinations. Using these tools, the response or movement of one parameter can be used to modify another totally independent parameter, hence the name 'modifiers'

The number of modifiers available, along with the destinations they can affect is entirely dependent on the synthesiser. Many synthesisers feature a number of envelope generators that allow the action of other parameters alongside the amplifier to be controlled.

For example in many synthesizers feature a number of envelope generators that allow the action of other parameters alongside the amplifier to be controlled.

For example, in many synthesisers, an envelope may be used to modify the filter's action and by doing so you can make tonal changes to the note while it plays. A typical example of this is the squelchy bass sound used in most dance music. By having a zero attack, short decay and zero sustain level on the envelope generator, a sound that starts with the filter wide one before quickly sweeping down to fully closed is produced. This movement is archetypal to most forms of dance music but does not necessarily have to be produced by envelopes. Instead, some synthesisers offer one-shot low-frequency oscillators (LFOs) which can be used in the envelope's place. For instance by using a triangle waveform LFO to modulate the amp, there is a slow rise in volume before a snowdrop again.

Low Frequency Oscillator

LFO's produce output frequencies in much the same way as VCOs. The difference is that a VCO produces an audible frequency (within the 20Hz-20kHz range) while an LFO produces a signal with a relatively low frequency that is inaudible to the human ear (in the range of 1-10Hz).

The waveforms an LFO can utilise depend entirely upon the synthesizer in question, but they commonly employ sine, saw, triangle, square and sample and hold waveforms. The sample and hold waveform is usually constructed with a randomly generated noise waveform that momentarily freezes every few samples before beginning again.

LFO's should not be underestimated because they can be used to modulate other parameters known as  'destination', to introduce additional movement into a sound. For instance, if an LFO is set to a relatively high frequency, say 5Hz, to modulate the pitch of a VCO, the pitch of the oscillator will rise and fall according to the speed and share of the LFO waveform and an effect similar to that of vibrato is generated. If a sine wave is used for the LFO, then it will essentially create an effect similar to that of a wailing police siren. Alternatively, if this same LFO is used to modulate the filter cut-off, then the filter will open and close at a speed determined by the LFO, while if it were used to modulate an oscillator's volume, it would rise and fall in volume recreating a tremolo effect.

This means that an LFO must have an amount control (sometimes known as depth) for varying how much the LFO's waveform augments the destination  rate control to control the speed of the LFO's waveform cycles, and a fade-in control in some. The fade-in control adjusts how quickly the LFO begins to affect the waveform after a key has ben pressed. An example of this is shown in figure the figure below.

LFO fade-in

The LFO on more capable synthesizers may also have access to it's own envelope. This gives control of the LFO's performance over a specified time period, allowing it not only to fade in after a key has been pressed but also to decay, sustain, and fade away gradually. It is worth noting, however, that the destinations an LFO can modulate are entirely dependent on the synthesiser being used. Some synthesizers may only allow LFOs to modulate the oscillator's pitch and the filter, while others may offer multiple demonstrations and more LFOs. Obviously the more LFOs and destinations that are available, the more creative options you will have at your disposal.

If required, further modulation can be applied with an attached controller keyboard or the synthesizer itself in the form of two modulation wheels. The first, pitch bend, is hard-wired and provides a convenient method of applying a modulating CV to the oscillator(s). By pushing the wheel away from you, you can bend the pitch (i.e. frequency) of the oscillator up. Similarly, you can bend the pitch down by pulling the wheel towards you. This wheel is normally spring loaded to return to the center position, where no bend is applied, if you let go of it, and is commonly used in synthesisers solos to give additional expression. The second wheel, modulation, is freely assignable and offers a convenient method of controlling any on-board parameters, such as the level of the LFO signal sent to the oscillator, filter or VCA or to control the filter cutoff directly. Again, whether this wheel is assignable will depend on the manufacturer of the synthesiser.

On some synthesisers the wheels are hard coded to only allow oscillator modulation (for a vibrato effect), while some others do not have a separate modulation wheel and instead the pitch bend lever can be pushed forward to produce LFO modulation.

Practical applications

While there are other forms of synthesis - which will be discussed in later posts - most synthesisers use in the production of dance music are of an analogue/subtractive nature therefore it's vital that the users grasps the concepts behind the elements of subtractive synthesis and how they can work together to produce a final timbre. With this in mind, it is sensible to experiment with a short example to aid in the undertaking of the components.

Using the synthesiser of your choice, clear all the current settings so that you start from nothing. On many synthesisers this is known as 'initialising a patch', so it may be a button labelled 'init', 'init patch' or similar. 

Begin by pressing and holding C3 on your synthesiser, or alternatively controlling the synthesiser via MIDI programming in a continual note. If not, place something heavy on C3. The whole purpose of this exercise is to hear how the sound develops, as you begin to modify the controls of the synthesiser, so the note needs to play more continually.

Select sawtooth waves for two oscillators, if there is a third oscillator that you cannot turn off, choose a triangle for this third oscillator. Next, detune one sawtooth from the other until the timbre begins to thicken. This is a tutorial to grasp the concept of synthesis, so keep detuning until you hear the oscillators separate from one another and then move back until they become one again and the timbre is thickened out. Generally speaking, detuning of 3 cents should be ample but do not be afraid to experiment - this is a learning process. If you are using a triangle wave, detune this against the two saws and listen to the results, Once you have a timbre you feel you can work with, move onto the next step.

Find the VCA envelope and start experimenting. You will need to release C3 and then press it again so you can hear the effect that the envelope is having on the timbre. Experiment with these envelopes until you have a good grasp on how they can adjust the shape of a timbre; once you are happy you have an understanding, apply a fast attack with a short decay, medium sustain and a long release. As before, for this next step you will need to keep C3 depressed.

Find the filter section, and experiment with the filter settings. Start by using a high-pass filter with the resonance set around midway and slowly turn the filter cut-off control. Note how the filter sweeps through the sound, removing the lower frequencies first, slowly progressing to the higher frequencies. Also experiment with the resonance by rotating it to move upwards and downwards and note how this affects the timbre. Do the same with the notch and band pass, etc. (if the synthesiser has these available) before finally moving to the low pass. Set the low-pass filter quite low, along with a low-resonance setting - you should now have a static buzzing timbre.

The timbre is quite monotonous, so use the filter envelope to inject some life into the sound. This envelope works on exactly the same principles as the VCA, with the exception that it will control the filter's movement. Set the filter's envelope to a long attack and decay, but use a short release and no sustain and set the filter envelope to maximum positive modulation. If the synthesiser has a filter key follow, use this as it will track the pitch of the note being played and adjust itself. Now try depressing C3 to hear how the filter envelope controls the filter, essentially sweeping through the frequencies as the note plays.

Finally, to add some excitement to the timbre, find the LFO section. Generally, the FLO will have a rotary control to adjust the rate (speed), a selector switch to chose the LFO waveform, a depth control and a modulation destination. Choose a triangle wave for the LFO waveform, Hold down C3 on the synthesiser's keyboard, turn the LFO depth control up to maximum and set the LFO destination to pitch. As before, hold down the C3 key and slowly rotate the LFO rate (speed) to hear the results. If you have access to a second LFO, try modulating the filtre cut-off with a square wave LFO, set the LFO depth to maximum and experiment with the LFO rate again.

Here concludes the seventh part of this post, if you want to know more about acoustic science please read. Rick Snoman's Dance Music Manual (Second Edition) Tools, Toys and Techniques.