Patent Place is a regular feature that will describe patents of particular interest to the electronics or electro-music enthusiast, especially where they relate to or offer improvements over existing devices or design techniques, present some totally new idea, or offer scope for further experimentation by the hobbyist.

Many attempts have been made to duplicate the sound of rotating speaker systems for organs using solid-state circuitry to mimic the amplitude and phase modulation effects produced by rotating directional sound sources. Most take the form of signal processors that drive a single amplifier and speaker system, but the invention described by the Roland Corporation in patent specification 1522850 uses a multiple speaker arrangement and modulates the apparent location of the sound source in order to produce a more complete simulation of the rotating speaker sound.

Three speakers are used, equally spaced along a straight line facing the listener. Each speaker has its own power amplifier and voltage controlled amplifier so that the signal level of each can be separately modulated. The centre channel is fed with the input signal directly, while the other two receive a frequency modulated version from a voltage controlled delay element (3 in Figure 1). The VCA's and delay are controlled by signals from the modulation section, which consists of a 0.2 to 10Hz sine wave oscillator (2) and two phase shift networks (P1 & P2). Since the modulating signals for the outer channels are 180° apart, and the signal for the centre channel is 90° from each, the modulated signals from the three speakers give the effect of a rotating sound source (Figure 2) with an overall tremolo effect. The delay element is modulated by the same signal as the centre channel VCA, causing the pitch of the tone to be increased as the sound moves towards the listener and decreased as it moves away, simulating Doppler shift and producing vibrato.

A simplified system where the centre signal is mixed with the outer signals for reproduction via two speakers is also described. Both systems were used in the Roland Revo tone cabinets where in three models the speakers were arranged in a single 'Leslie' - type enclosure.

In patent specifications 1510416, the Sansui Electric Company Ltd describe a mechanical reverberation system using a number of tunable vibration elements rather than a spring or plate-type delay device.

The vibration chords are made of magnetic material and are held under tension over two moving iron transducers. A driver amplifier takes the input signal and feeds it to the input transducer (see Figure 3). The chords are simultaneously driven according to the input signal, and the output transducer picks up the vibrations for amplification. The resultant signal is mixed with the original to yield what Sansui call a "chordally emphatic, musically rich reverberation effect". Only the chords having resonant frequencies which correspond to components of the input signal continue to vibrate after the input stops: the other chords stop immediately.

As Figure 4 shows, the chords are held at one end by pegs which can be rotated to tune the vibration frequencies. They are also dependent upon the material of the chords so the resonances can be separated to give a more even reverb effect.

An arrangement is also proposed which uses a similar arrangement of transducers and piano wires fixed at one end only as the vibration elements. In addition alternatives using piezoelectric or electrostatic methods to achieve the same effect are also covered by the patent.

The sounds produced by such devices would be similar to that of a 12/6-string double-neck electric guitar with the volume of one neck turned up while the other is played, or the sympathetic vibration effect of a piano played with the sustain pedal held down. Whether such an effect would be useful for a compound musical signal is doubtful, but it could find application in portable and home organs.

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