Electrokinetica The Electro-mechanical Museum

Introduction to the 1950s Electrones

A widening range of organs

2.5-inch generator

2.5-inch generator

In its early years, the Electrone was a high-cost instrument produced in small numbers. As with many kinds of electrical and electronic equipment, demand for keenly-priced electric organs soared in the 1950s, leading Compton to re-engineer the Electrone in 1952 for a larger and more diverse market. Over time, the established electrostatic generator designs would be developed for improved performance, however at this stage there was a need for a low-cost generator assembly that would allow maximum flexibility of voicing yet fit inside the new self-contained consoles.

The additive synthesis system

The lower-priced Electrone models that first appeared in 1952 under the ‘Melotone’ guise use an additive synthesis system to give a wide range of tone colours from the smallest practical generators. These have 2.5 inch diameter rotors with only seven sinusoidal waveforms available, from which the entire tonal specification is synthesised. Many of the subsequent models employed additive synthesis, including the popular models 357, 363, CH/2 and all the smaller organs in which the size and cost of multi-rank generators were not warranted. The cornerstone of this system is the relationship between the musical scale and the harmonics present in the sound of a real instrument.

Theory of operation

When a note is played on an acoustical instrument, it emits a sound consisting of the fundamental frequency, which defines the pitch of the note, and a series of harmonics, i.e. multiples of that frequency. Harmonics arise because whatever structure in the instrument is responsible for vibrating or resonating at the fundamental frequency usually also vibrates in different modes at the same time. For example, a string will emit the fundamental, where one standing wave occupies the whole length of the string, the second harmonic where two waves occupy the same length, the third harmonic (three waves) etc. The relative strength of the individual harmonics determines the timbre or nature of the sound. A flute, for example, produces only a few harmonics at significant intensity, whereas a violin gives out a comparatively long series of harmonics of much higher relative power. The same is true of the various stops on a pipe organ, each of which selects a rank of pipes of a particular physical design that gives it a characteristic harmonic spectrum and hence a particular tone.

Basic pipe waveform

Basic pipe waveform

Harmonic spectrum

Harmonic spectrum

Synthetic equivalent and its equation

Synthetic equivalent and its equation

A practical additive synthesizer

The additive synthesis system enables simple generators to produce diverse timbres by generating sine waves at all the harmonic frequencies and adding them at suitable relative amplitudes. If every harmonic (up to perhaps the 20th) were to be separately generated using the electrostatic system, a large number of generators would be needed. However, a reasonable approximation to the set of harmonics can be made using very few generators by 'borrowing' other notes of the scale that have similar frequencies. The third harmonic, for example, is represented by the note one twelfth above, which is extremely close in frequency. In an ideal system the first few dozen harmonics would all be available, but when only one even-tempered scale is generated, many harmonics must be omitted because no notes are sufficiently close to represent them. In the Electrone, the voicing circuit offers the following ‘palette’ of harmonics; 1st, 2nd, 3rd, 4th, 5th, 6th, 8th, 10th, 12th and 16th.

-----

All content © copyright Electrokinetica 2007-2017 except where otherwise stated • Valid XHTML 1.0Valid CSS