(extract from Using audio modules in a video synth system which was written by @creatorlars)
The 2 Frequency Domains of Audio Synthesis
With audio and sound, things come in twos. You have two ears, a stereo recording has two channels, and in audio synthesis we are concerned with two frequency domains. Subsonic frequencies are below the limits of human hearing, and signals in this frequency range animate and change sound across time. Frequencies within the range of human hearing are in the audio domain and create audible sound.
In an audio synthesizer, signals from the subsonic frequency range are used to modify signals in the audio frequency range to synthesize specific timbres or amplitude shapes and to compose music through sequencing.
Subsonic ( 0 - 20 Hz)
Signals below 20 Hz are in the subsonic frequency domain. Example subsonic frequency generators include LFOs, envelope generators, sequencers, and random voltage generators.
Audio (20 Hz - 20 KHz)
Signals between 20 Hz and 20 KHz are in the audio frequency domain. Example audio frequency generators include VCOs and samplers.
The 3 Frequency Domains of Video Synthesis
With video and images, things come in threes. You have three types of photoreceptor cells in your retinas, RGB image processing happens in three channels, and in video synthesis we are concerned with three frequency domains.
In a video synthesizer, signals from the vertical frequency range (~30 Hz to ~15KHz) are used to modify signals in the horizontal frequency range (~15 KHz to ~6 MHz) to create patterns and shapes. Signals in the animation frequency range (less than the video frame rate, ~30 Hz) modify signals in the upper two frequency ranges to create motion. In order to create a a two dimensional pattern or shape on the screen, you need both a Horizontal signal generator and a Vertical signal generator in your patch. To put this shape into motion, changing or moving across multiple frames of video, you also need an Animation signal generator.
If you’re perceptive, you’ve already made these observations:
The Subsonic domain of audio synthesis overlaps the Animation domain of video synthesis.
The Audio domain of audio synthesis overlaps the Vertical domain of video synthesis.
Signals generated by audio synthesizers just barely overlap the Horizontal domain of video synthesis.
What does this mean for you? If you have audio synthesis modules, you probably have the ability to generate 2 out of the 3 ingredients required for 2D pattern and shape generation already! The missing element is a signal generator capable of high frequency signals within the Horizontal domain.
Animation (0 - 30 Hz)
Signals below frame rate (~30 Hz) are in the animation frequency domain. This domain overlaps the Subsonic domain of audio synthesis, so we can use the same subsonic signal generators like LFOs and envelope generators to create movement and motion in a video synthesizer.
Vertical (30 Hz - 15 KHz)
Signals between frame rate (~30 Hz) and the horizontal sync rate (~15KHz) are in the vertical frequency domain. This domain overlaps the Audio domain of audio synthesis, so we can use the same audio signal generators like VCOs and samplers to create the vertical elements of a pattern or shape in a video synthesizer.
Horizontal (15 KHz - 6 MHz)
Signals above the horizontal sync rate (~15 KHz) and video bandwidth (~6MHz) are in the horizontal frequency domain. Audio synthesizers don’t create signals in this domain, so we need a signal generator designed specifically for video synthesis, like the Prismatic Ray VCO.