Subtractive synthesis is one of the oldest types of synthesis introduced with the birth of analog (modular) synthesizers back in 1960`s and 1970`s. That’s the reason why this type of synthesis is often called “analog“. For instance, most of analog synthesizers use this method to generate sound.
Analog Synthesis
The following is an example of subtractive synthesis and components signal path as it might occur in an electronic instrument. Specifically, 3 main components comprise it.
1. Oscillator
2. Filter
3. Amplifier
- The oscillator creates a sound that can be processed in a number of ways by a filter. Consequently, it’s sent to an amplifier where the volume of the sound can be controlled. Two main parameters of the oscillator named waveform and frequency (pitch), are available for alteration and controle. Waveform is the specific structure of the soundwave that the oscillator is emitting. Usually there are 4 main types (Sine, Saw, Square and Triangle.) The Sine waveform is the most simple sound which consists of a fundamental tone. In addition, it has no harmonics which means that there is not much to filter out when the sound reaches the filter component. On the other hand, there is a Saw waveform that is very rich in terms of harmonics and has a very buzzy sound to it. Pitch parameter is another word for frequency or the speed of the waveform. While the frequency is often referred in Hertz, the pitch is represented in notes and octaves on the keyboard.
- Filter works by removing the parts of the frequency spectrum. It firstly defines the cutoff point represented in Hz, and depending of the chosen filter type (Low Pass, Hi Pass, Band Pass…) removes frequencies above, below or around the cutoff point. Besides cutoff, resonance or emphasis is also involved in the filter. Resonance works by boosting the frequencies near the cutoff point in order to emphasise what the filter is doing.
- Amplifier controls the volume of the sound and this leads to envelope modulation.
LFO & Envelope Generator
Indeed, there are numerous ways with which we can modulate the parameters of these components. The most common way would be to use the envelope generator and low frequency oscillator (LFO) for amplifier and filter components.
Envelope generator is basically multiplying values with a range from 0 to 1. Usually 4 points points called Attack, Decay, Sustain and Release change it over time. Envelope generator can also control the volume. However, it is also applicable to any other synthesizer parameters.
LFO (Low frequency oscillator) is similar to the oscillator but instead of transmitting sounds it transmit values like the envelope generator. The LFO also usually work in much lower frequency than the oscillator. Just like with the normal oscillator with the LFO we can control the waveform and the frequency.
Additional Resource & Source Texts
http://courses.cs.washington.edu/courses/cse490s/11au/Readings/SynthesisChapt4a.pdf
