Patent GB-495271-A (GB)
Title: Improvements in and relating to keyboard instruments for the production of musical tones
IPC: G10H5/08
Filed: 1937-08-10
Abstract: 495,271. Electrical musical instruments; photomechanical printing - surfaces; modulating systems. SCOTT, E. Aug. 10, 1937, No. 22047. Convention date, Aug. 10, 1936. [Classes 40 (ii) and 88 (ii)] [Also in Groups XX and XL] In a keyboard musical instrument, highfrequency current is modulated at the audiofrequency of the note to be sounded, by passing the high-frequency current through an impedance varied at the said audio-frequency as a direct result of continuous relative mechanical movement between two parts such as condenserplates, such movement being continuously maintained at a constant speed dependent on the frequency of the respective note. The modulated current is subsequently demodulated in a detector and converted into musical notes. The use of high-frequency gives advantages pointed out below. The highfrequency carrier-waves, generated as by valves 1, 12, 26 are modulated electrostatically as to amplitude, frequency or phase, by stationary " capacity-islands " 63 &c., of various wave-forms, which are scanned by rotating arms 62 forming a condenser with the islands. Twelve scanning units are employed and are driven by a common 60-cycle synchronous motor, or by separate motors, at speeds differing in the geometric ratio of the twelfth root of two, to yield each of the twelve notes of the tempered scale. Six octaves are obtained by including multiple wave-forms and arms in each unit. The system includes a clickless method of note-keying, described below, by upsetting the balance of special centre-tapped resistances. Tremolo, more pronounced in the higher frequencies, is obtainable at will and celeste effects may be provided by an additional set of scanners, rotating at slightly different speeds. The wave-form records are produced by photo-etching from oscillograms of musical instruments, voices and synthetic waves, as described below. High-frequency carrier-waves are generated by valves 1, 12, 26, tuned to different frequencies and each supplying one manual 69, 86, 88 of the instrument. The carrier-waves are controlled by crystals 3 and are amplified by valves 4, the tuned output-circuits of which are loosely coupled to centre-tapped coils 6, 15, 29, feeding the respective manuals. For unison-coupling of manuals I (69) and II (86), switches 20 and 21 connect coils 5 and 7, loosely coupled to coil 6, in series with coil 15. For noiseless keying, coils 6, 15, 29 are connected across a series of centre-tapped impedance-branches 34, 38, 42, one for each note in each manual, the centre-tappings 46 leading off through the modulating scanners 63 &c., and wave-form selecting stop-switches, to the detecting and audio side of the instrument. The impedance-branches 34 &c. have impedances which decrease with increase in the applied voltage, and are constituted by lamps or by a ceramic material known under the Registered Trade Mark "Thyrite." In addition to being connected to the scanning- rings 67, their centre tappings are earthed through a high-frequency choke 83 and condenser 84. The centre-tappings 8 of the coils 6, 15, 29 are also earthed, so that initially a state of balance exists, and no current flows to the scanning-units. When a note-key 69 is depressed, direct-current flows from a battery 77 through a resistance 82, choke 83, centretapping 46, left-hand half of the impedancebranch 34, and upper half of the coil 6. Such current-flow, which builds up in accordance with the predetermined time-constant of the condenser 84, upsets the balance in the impedance-branch, and causes the carrier-wave to pass through the impedance-tapping 46 to the selected scanning-unit. In the scanning units, all capacity-islands of the same waveform are connected together and provided with leads 90, 119, 137, 144, feeding tuned circuits 95, 121, 140, 150 respectively, in the stop register. The circuits 95 and 121 are each triple-tuned to the frequencies of each of the main oscillating-valves 1, 12, 26. Such tuning is effected by the provision of additional tuned coils 92, 93, 122 and 123, and permits the timbre which it controls to be played in any desired one of the manuals. Three banks of stop switches 98, 103, 111 and 125, 129, 132 are provided for each such circuit, each switch serving to connect a loose-coupled coil such as 96 &c. to a grid 99, 108, 113 respectively, of valves 97, 109, 114, the outputs of which are tuned at 152 &c. to the respective oscillators 1, 12, 26. In the off-position of the stops, as shown, the grids 99, 108, 113 are all earthed. Thus, if it is desired to play in manual I (69). the timbre (e.g. cello) given by the capacityislands 64, the stop-switch 98 is drawn out and the grid 99 of the valve 97, tuned to the oscillator 1, is energized. The same timbre is played in manual II (86) by drawing the stop 103, energizing the grid of the valve 109, and in manual III (88) by drawing the stop 111, connected to the valve 114. Similarly, the timbre (e.g. diapason) provided by the capacityislands 63 can be played in the respective manuals by drawing the stops 125, 129, 132. The leads 137, 144 from the capacity-islands 66, 65 are connected to circuits 140, 150 which are singly-tuned to the oscillators 1 and 12 respectively. Thus, the timbre (e.g. harmonic flute) given by the capacity-islands 66 is played in manual 1 by drawing the stop 142, and that (e.g. violin) of the capacity-islands 65 in manual II by drawing the stop 147. The mixing of the carrier-waves in the triple-tuned circuits and their subsequent separation by the tuned valves 97, 109, 114 is rendered possible by the use of high-frequency carrier-waves. The output of the valves 97, 109, 114 passes to full-wave detector-valves 166, 173, 174 respectively, which feed a common audioamplifier and loud-speaker, Fig. 3 (not shown), through a direct channel, or through one which imposes a tremolo. With switches 167, 185, 187 in their lower position, the detector-output passes directly through an amplifier valve 176 to the input-lead 178 of the audio-amplifier. The switches 167 &c. of one or more of the manuals can be moved to an upper position to break the direct connection, and pass the detector-output to the main control-grids of valves 194, 195, which have additional controlgrids 204, connected to a low-frequency generator 205 so as to impose a tremolo on the output of the valves. The valves 194, 195 are connected to the audio-input lead 178 through filters 199, 202 respectively. In the filter 199, the attenuation varies directly with the frequency whilst, in the filter 202, it varies inversely with the frequency. By these means, the tremolo is more pronounced on the higher audio-frequencies. The speed of tremolo can be controlled by varying the frequency of the generator 205, and its degree can be controlled by a potentiometer 206. Independent volumecontrol for each manual is provided by swellpedals 170, 171, 172, operating potentiometers for biassing one of the grids 161 of each of the valves 97, 109, 114. These valves each include a " suppressor-grid " 162, and two additional biassing-grids 160. In a modified arrangement, Fig. 10 (not shown), employing frequencyinstead of amplitude-modulation of the carrierwave, the scanning units are directly keyed in parallel with a fixed condenser in the output of the carrier-wave oscillator. In the modification shown in Fig. 11, phase-modulation is employed and is obtained by directly keying the scanners 517 across condensers 511 which normally serve to maintain balance in equal and opposite coils 503, 504, loosely coupled to the carrier-wave oscillator 500, and connected across a detector-valve 509. When a notekey 508 is depressed, the voltage-vector in the transformer 504 is displaced, so that it is no longer in direct balancing opposition to that in the transformer 503. A resultant voltagecomponent is thus produced and is impressed on the detector-valve. The electrostatic scanning-units, shown in detail in Figs. 4-6, each comprise a stator-disc forming one half of a two part casing 255, and carrying on its face, concentric rings of capacity-islands 264, representing various instruments, voices and synthetic tones, which are scanned by radial arms 311, Fig. 5, on the opposed face of a rotor-disc 250. The distance between the arms is exactly equal to the length of the capacity islands in the ring which they scan. The number of arms in the ring 308 is twice that in the ring 309, so that the ring 308 produces the next octave above the ring 309. Two octaves are similarly obtained from the ring 310, by lengthening alternate arms 311 to provide in effect two rings like 309, 308. Two additional octaves, making six in all, are provided by providing double wave-forms of unit length, to be scanned by arms in the same row. Thus, the capacity-island 275 (cello, 4 feet stop), will yield the octave of the island 263 (cello, 8 feet stop). The islands are all individually connected as by leads 90, 119, 137 and 144, Figs. 1, 2, 4, to the stop register, as described above. The rings 308, 309, 310, &c. of the scanning arms receive their charge through a direct connection with rings 327, 328, 329 on the opposite face of the rotor-discs, such rings having a capacity coupling with rings 67, 89, 87, provided on the opposed face of the stator 255. The rings 67, 89, 87 are those shown in Fig. 1, being connected to the note-keys by leads 68 &c. Scanning-rings may be provided on both faces of the rotor 250. The stator with its capacity-islands 264, is produced in the following manner. Wave-forms of the instruments to be reproduced are obtained by a cathode-ray oscillograph or a string-galvanometer. The wave-forms are then enlarged on rectangular-co-ordinate paper, and are transposed to polar co-ordinates, from which they are drawn out at increased size by hand, checking the line-thickness by a microscope. The drawings are photographed on a glass-plate, reduced to the