A method of generating all of the notes and pitches that a synth is capable of producing, using a small number of fixed-frequency oscillators, as an alternative to VCOs. One such architecture, used by many of the 1970s-era string synthesizers, is known as "top-octave division". Its origins lie in the transistor organs of the 1960s, such as Vox, Gem and Farfisa, who used this approach to supply notes to each octave below the top. The failure of multiple octaves of the same note is a typical indicator of a problem with a specific top octave oscillator. By tuning the top oscillator all octaves below are brought into exactly the same tuning for that note. A word of caution, to the novice technician, the tuning pots of 40 year old top octave boards can be very fragile and difficult to find.
The twelve top oscillators are found in transistor organs on individual boards, but in string machines they may be grouped together. They usually lack voltage control inputs and are not calibrated for 1 volt per octave response, unlike the careful circuits designed for Voltage Controlled Oscillators. All of the musical notes for the entire span of the keyboard come from one set of 12 oscillators, one circuit for each of the 12 notes of the chromatic scale. These are tuned to the highest notes of the keyboard. Octave divider circuit is used to produce all of the notes in the lower octaves. A variant to this idea is the Polymoog which used a complex variation to this approach.
An "Ultrasonic divider" is an architecture that takes this concept to an extreme; a single master oscillator runs at well above audio frequency (in the megahertz range), and divider circuits divide down this master frequency into the audio range to produce all of the note frequencies. This type of circuitry has been used by many sophisticated electronic organs. It is no coincidence that one of the most famous string machines of the 1970s, the Solina, emerged out of the designs of a Dutch organ firm, Eminent and was licensed to ARP.
Divide-down architectures are often criticized for producing note and frequency intervals that are "too perfect"; for instance, using the top-octave method, not only will all D's or E's or F#'s, etc, be in absolutely perfect frequency ratios, but they will all be in perfect phase with each other. The result is a sound that, unless some modulation is applied, rapidly fatigues the ear due to the lack of any variation. (There are exceptions to this view: search Lamonte Young). Ultrasonic divider circuits play all notes in phase, since they all derive from one master oscillator, and they sometimes have problems with the divide ratios (which must all be integers) not producing the correct frequencies for certain notes. The resulting sound can be simultaneously cloying and harsh, a combination which some regard as unlistenable. Part of the charm of the Solina string machine was the incredibly lush and complex chorus circuit which animated its sound. The brilliant designer Jürgen Haible conducted a rather precise study of the Solina chorus as part of his emulation of what he called the Solina's triple chorus which he concluded owed its distinctive sound to phase shifts in modulation and the related frequency cancellations.