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How it works: The electrostatic generator

Electromechanical vs. electronic generators

Compton electrostatic generator

Compton electrostatic generator

The heart of any electric organ is its tone-generating system and in the Electrone this is electromechanical rather than electronic. The frequencies required to make up its sounds are produced by movement of one part relative to another at the appropriate speed, rather than oscillatory valve or transistor circuits. From the very earliest days of commercial electric organs, it was theoretically possible to use purely electronic techniques to build tone generators. Pioneering electronic solo instruments such as the Theremin relied on valve circuitry, as did the amplification section of electric organs themselves. Nonetheless, electronic oscillators had some marked disadvantages for organ use, on account of the large number required in a single instrument. Even with the best components of the 1930s, it would have been a challenge to keep the many oscillators in tune with one another over time and variations in temperature, despite costing much more to build than equivalent electromechanical generators. Thus the first commercially successful electric organs were almost entirely electromechanical.

Magnets vs. Light beams vs. Electric fields

Many organ generator designs were based on rotating discs or cylinders that were sensed by electromagnetic, electrostatic or (rarely) optical pickups. For example, the electromagnetic generator system invented by Laurens Hammond for the Hammond organ uses toothed steel wheels termed ‘tonewheels’ rotating in front of pickup coils that are wound on magnetised steel cores. As the teeth of a tonewheel pass its pickup, they modulate the magnetic flux passing through the core and induce a small audio-frequency current in the coil. The Compton tone generator employs an entirely different physical principle to convert mechanical energy into electrical, using an electric field instead of a magnetic one.

The electrostatic generator

Fig.1. Stator waveform traces

Fig.1. Stator waveform traces

Fig.2. Rotor and stator

Fig.2. Rotor and stator

Inside each generator is a ‘stator’ consisting of an insulating plate covered on one side with a conductive metal coating. Engraved into the coating are narrow grooves that follow a wavy course around a complete circle of the stator, dividing the conductive coating into isolated bands that narrow and widen in a regular pattern. In fig. 1 at left, every second band of coating has been picked out in a different colour and numbered. These are the waveform traces of octaves 1, 2 and 3, of which the width can be seen to form 2, 4 and 8 cycles of a wave respectively. The intervening green areas labelled E are the earth traces, all connected together and earthed. Spinning very close to the stator, without actually touching it, is a rotor disc also covered with a metallic coating. Moulded into its surface are rings of raised sectors or ribs equal in number to the cycles of stator waveform beneath. These raised areas lie only a few thousandths of an inch from the stator, resulting in a significant electrical capacitance between the coatings on the two surfaces. In fig. 2 at right, the scanning pattern for waveform trace No. 1 is shown, with its two raised sectors represented by the dark areas. As the rotor revolves, the capacitance between it and trace 1 rises and falls in proportion to the area of the trace covered, while its capacitance to earth varies differentially. If Trace 1 is now polarised by a positive DC voltage with respect to earth, positive and negative charge will be induced alternately on the scanning sectors on the rotor as they pass the wide and narrow areas of the trace, resulting in an AC waveform or tone being generated. So far we have referred to the waveform-shaped areas of the stator as ‘traces’; Compton literature also uses the term ‘electrodes’ with identical meaning.

Multiple frequencies from each generator

The AC frequency generated is proportional to the rotational speed and to the number of cycles of waveform per revolution. For example, with the rotor revolving at 1200 rpm or 20 revolutions per second, polarising trace 1 will cause the generator to produce 2 x 20 = 40 Hz. With trace 2 polarised instead, the output will be 4 x 20 = 80 Hz, twice the frequency or in musical terms one octave higher. The amplitude of the tone is proportional to the DC polarising voltage, while its waveform depends on the shape of wave engraved on the stator and the width of the scanning sectors. The AC signal is collected from the rotor by capacitive coupling to a stationary ‘pickup’ electrode facing its opposite side, from which it is sent to the amplifier. Any or all waveform electrodes can be polarised at once to any desired voltage, in which case the generator will deliver the sum of all tones generated. Some models of generator have only one stator and rotor, others have two mounted back-to-back either side of a central pickup as illustrated by the 2.5-inch double sided unit below, one of the standard designs used in many models of Electrone. On the left is the bass side that produces three octaves exactly as illustrated above. At right is the treble side of the same generator, producing a further four octaves. The assembled unit is shown in the centre.

Bass rotor and stator

Bass rotor and stator

Treble stator and rotor

Treble stator and rotor

Double-sided generator

Double-sided generator

Completing the musical scale

To provide all the frequencies in the musical scale, twelve generators are used; one for each letter-name of note. Although the twelve generators are identical internally, they are fitted with different sizes of drive pulley that cause them to rotate at different speeds when driven by a common belt. The lowest note on the organ keyboard is C and at the lowest octave available on the standard Electrone generator systems this has a frequency of 32.7 Hz. Thus we find the C-generator has the largest pulley and runs slowest, at 981rpm or 16.35 rotations per second. Its lowest octave waveform trace has two cycles per revolution, therefore it produces a frequency of 2 x 16.35 = 32.7 Hz. The next note is C#, provided by the lowest octave of the C# generator using a pulley smaller in diameter by the ratio of the twelfth root of two or about 6%, this being the frequency ratio of an interval of a semitone. This ratio is repeated for each of the following generators, D, D# etc. up to B, the 12th generator. The next note above is C, one octave higher than the first; this is generated by the C generator again, but using its second trace, with twice as many cycles. The pattern repeats every octave, up to the highest octave of the B generator which is the top note of the organ.

Set of 12 generators

The number of waveforms available from each generator ranges from seven to thirty according to model. The smaller generators as shown above produce just seven octaves of sine waves, i.e. there is only one kind of tone encoded within the waveform electrodes at each note pitch. By adding various generator pitches together that are related in a particular way, a waveform more complex than a sine wave can be synthesized that still sounds like a single note. Its timbre or tonal quality can be tailored to simulate a particular organ stop by adjusting the relative strength of these additional pitches or ‘harmonics’ We will refer to Electrones using this method of voicing as ‘additive synthesis’ models.

An alternative system was used in early models and high-specification instruments, using generators equipped with both sines and complex waveforms embodying many harmonics from which typical pipe sounds can be derived. For example, the model 347 generator has four different waveforms available over five, six, seven and eight octaves respectively, making a total of 26 waveforms. By analogy with pipe-organs, we will refer to these as ‘multi-rank’ models.

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