Subtractive Synthesis: Under a Microscope

            As I discussed last week, Subtractive Synthesis consists of a VCO that generates a signal, a VCF that filters out some of the waveform’s frequencies, and a VCA that amplifies the signal. However, there are other components that help shape the sound, and also, control each of the parameters.

The CV lasts as long as the envelope generator
tells it to. The Trigger is a quick pulse,
initiating when the key is pressed. The gate
lasts as long as the key is being pressed.

            In analog synthesis there are three basic control parameters: the control voltage, the gate signal, and the trigger signal. A control voltage is a variable voltage that is commonly used to determine pitch, or the frequency of the key pressed, and it can also be used to control any other number of parameters through external patching or internal patching (depending on what type of synthesizer you have). A gate signal is a steady voltage that denotes a key being pressed, starting when the key is pressed and ending when it is released. Lastly, a trigger signal is a quick pulse of voltage that is used to start an event such as a key press.

            When these signals are initiated either by a keyboard or other source, it is sent to an envelope generator that determines the path of the signals. Gate and trigger tell an envelope generator when to initiate. Commonly, attack, decay, sustain, and release (ADSR), of the envelope generator starts the attack upon the initial gate and ends when the trigger message is completed allowing the envelope generator to finish the release.

            Another feature that many Subtractive synthesizers have is the LFO (Low Frequency Oscillator). This feature is not common on many older Subtractive synthesizers due to the fact that their oscillator’s frequency range went quite low, lower than the range of human hearing allowing them to function much like a LFO (one oscillator was just designated to act as the LFO). LFO’s when added to the mix, lend certain qualities to the sound such as vibrato when it controls the VCO frequency, and tremolo when it controls the volume of the VCA. LFO’s can also be put on the VCF to control any of the parameters to create a richer sound.

            Other controls such as pitch bend (it bends the pitch up and down, but don’t quote me) and modulation (is assignable, and can control whatever you assign to it) use a variable voltage wheel to alter the parameters. These functions are usually located next to the keyboard, if it has one, and allows the user to adjust a wide range of performance parameters without having to go tweaking knobs on the synthesizer while they are playing.

            Some keyboards also have various other performance modifiers such as aftertouch that allows the user to change an assignable parameter by pressing the keys harder or down again without having to fully release them. Then there are other modifiers that come separately that we will not get too in-depth about such as a sustain pedal, expression pedal, and a breath controller, that interestingly enough allows you to mimic the expression of wood wind instruments.

            Again, this is not a complete rundown of Subtractive Synthesis, but rather a beginning or introduction. Each synthesizer is different, but they all share most of the features that I have described. For a more in-depth look, I will post a video of myself going over my Paia Fatman in the coming weeks (so keep following).

A basic block diagram depicting the signal flow
of a Subtractive synthesizer. With the the keyboard
sending the three control messages, a VCO,  VCF,
 VCA, and multiple envelope generators and LFO's. 

...My mistake in "Player Piano" was my failure as a futurist. I did not foresee transistors, and so imagined that super computers would have to be huge, with bulky vacuum tubes taking up a lot of space. -Kurt Vonnegut in "Letters"