Patent GB-433050-A (GB)
Title: Improvements in or relating to electrical musical instruments
IPC: G10H3/10
Filed: 1934-02-08
Abstract: 433,050. Electrical musical instruments ; sound - records. COMPTON ORGAN CO., Ltd., J., Chase Road, North Acton, Willesden, London, and BOURN, L. E. A., 22, Stanwell Road, Ashford, Middlesex. Feb. 8, 1934, No. 4171. [Classes 40 (ii) and 88 (ii)] In an electrical musical instrument wherein each note is produced by an undulating potential generated electrostatically by the relative rotation of two parts one of which carries a wave-form and the other a scanning form, in order to compensate for constructional inaccuracies producing irregularities in the output both the wave-form and the scanning form are repeated around the circle about the axis of rotation. Specification 403,444 is referred to, wherein a number of insulating discs carry twelve identical sets of concentric wave-form rings (each set corresponding to all the octaves of a note with the partials thereof), and stops and keys connect the required rings to the source of potential, the output being led to an amplifier and loud - speaker. The scanning element shown in Fig. 4 with the portion y omitted consists of a circular grid having sets of short radial scanning forms 2<a> extending between supporting rings 2<b>. The radial scanning forms 2<a> in each circle lie over, and are spaced apart by a distance equal to the wavelength of, the co-operating wave-form ring. Each wave-form ring may consist of a strip formed between two sine-waves of the same wave-length cut in a graphite coating on the insulating disc, the alternate strips between these being earthed. The scanning grid may be formed by graphite remaining in channels engraved in a disc subsequently coated and rubbed down. Less than a complete ring of radial scanning forms (or of wave-forms) may be used if a complete wave-form ring (or ring of scanning forms) is used, and provided the scanning forms are in phase with the waveforms. For instance, the scanning forms may be spaced apart twice the wave length, or may be in two diametrically opposite groups. Wave-form rings may be of other than sinewave form, for instance damped trains of oscillations. In order to obtain additive effects where the same sound is required as a partial of more than one note, the wave-form ring is divided into twice as many mutually insulated arcuate portions as the number of times the frequency is required, opposite pairs of arcuate portions being electrically connected, and a complete ring of radial scanning members 2<a> is used. Or a separate complete wave-form ring for each partial of each note may be employed. Where a key 8, Fig. 5, connects its wave-forms 1 to bus-bars 6 a preselection of which is energized by a stop 10 as in the prior Specification, each bus-bar may be permanently earthed through a resistance r<1> of 100,000 ohms, and its connection to the source of potential S made through a similar resistance r<2> to give additive effects as regards the stops. Also each contact 7 is connected to its wave-form 1 through an adjustable resistance r<3> and a leak l<1> of 20 megohms, a leak l<2> of 5 megohms and a condenser c being placed as shown. Adjustment of resistance r<3> corrects individual intensity. Condenser c causes potential to build up slowly, preventing generation of a sharp sound on engagement of the key contact. According to the Provisional Specification, a wave-form ring may be formed from the graphite-coated disc by dividing the ring formed between two concentric cuts, by a sinewave cut. One of the pair of rings thus produced is removed or earthed, or it may also be used for generating by connecting it to opposite potential.