Patent GB-766180-A (GB)
Title: Music synthesizer
IPC: G10H1/06
Filed: 1952-10-31
Abstract: 766,180. Electrophonic musical instruments.  RADIO CORPORATION OF AMERICA. Oct. 31, 1952 [Dec. 26, 1951], No. 27468/52. Classes 88 (1) and 88 (2). [Also in Groups XXXIX and XL (c)] An electrophonic musical instrument is controlled by a perforated tune sheet to provide electrical oscillations corresponding to a single instrument playing one note at a time. The oscillations may be recorded or may be reproduced by a loud-speaker. When the oscillations are recorded, separate record tracks are successively made on the same blank of six different instruments playing their respective parts in the same musical arrangement. The six records are read and re-recorded simultaneously in a single track on a separate blank from which may be reproduced the sounds of a small orchestra. The process may be repeated to produce a record of a large orchestra. The musical instrument (Fig. 1 and Figs. 2a, 2b) comprises twelve tuning forks, controlled sinusoidal oscillators 50 generating the semitones of a single octave, a hiss generator and a mixer, providing in all, fourteen sources of oscillation connected to terminals 1-14, Fig. 2b. Tone wheels may be used as oscillators. One of the sources is connected over a tree 71 of relay contacts to an octaver 72 including frequency multipliers and dividers which produce sawtooth waves with the frequencies of the same note in eight different octaves. These eight frequencies are available on terminals 0-7 in a relay tree 97 which connects the desired frequency to an envelope shaper 121. Attack is determined by the envelope shaper which includes an amplifier 127 normally blocked by the potential on a condenser 128 charged by a battery 129. The amplifier is unblocked by discharging the condenser through one of a number of circuits 132, 134, 136, 138 with different time constants. The discharge circuit, and hence the attack, is selected by a relay tree 126. Volume is controlled by a device 173 which includes either a potential divider 176 or a variable gain amplifier. The tapping on the potential divider or the gain of the amplifier is selected by a relay tree 174. The output on lead 177, still in sawtooth waveform, is fed to a spectrum controller 191 which comprises a number of parallel connected filters which produce waveforms characteristic of different musical instruments on terminals 0 to 15 of a volume equalizer unit 194. The equalizer unit comprises a number of individual valve amplifiers which can be adjusted to bring the output on all the terminals to the same level. The output characteristic of one instrument is selected by means of a relay tree 196 and the final output is fed either to an amplifier 198 or a loud-speaker 199 or to a recording head. The relay trees are controlled by a perforated tune sheet 73, as shown, by a magnetic tape, or by a photographic code record. Brushes 1-18 are normally insulated from a metallic driving roller 74 by the sheet and make contact with it only through perforations. Each brush controls a relay (which may be thermionic as in Fig. 4a, not shown) which operates when the brush finds a perforation. Brushes 1 to 6 control relays la to 4a in the tree 71. Brushes 5, 6, 7 control the octaver, brushes 8, 9, 10, 11 the timbre, brushes 12, 13, 14 the attack and brushes 15, 16, 17, 18 the volume. The octaver, Figs. 2a, 5, includes a clipper 78 which converts the sinusoidal wave into a square wave, a cascade of multivibrator circuits 81, 82, 83, 84, 86 and frequency multipliers 87, 88. The clipper 78 and the multivibrators are connected to shapers 89, 91 to 94, 96, of which 89 is typical, to convert the square wave into sawtooth form. Capacitor 101 is charged slowly through an anode resistor 102 and is discharged through valve 103 when a positive pulse is applied to the grid by differentiation of the square wave from the clipper 78. Valve 103 feeds a cathode follower 107. In the multiplier 87, positive and negative pulses are produced by differentiation of the square wave from 78 and are fed to valve 114 to produce positive pulses alternately in tubes 108 and 109. The tubes 108, 109 discharge capacitor 116 at double the original frequency and cathode follower 117 provides a saw-tooth output. A clipper 85 follows multiplier 87 to provide a square-wave input for 88. Attack. In the envelope shaper, Fig. 6, one plate of the capacitor 128 is connected to the grids of valves 141, 142 in a push-pull stage of the amplifier. The other plate is maintained at 20 v. by a potential divider connected between +250 v. and - 105 v. Diodes 147, 148 restrict the amplifier control voltage to the range from - 30 v. to ground potential. The shape of the unblocking potential applied to the amplifier through one of the circuits 132, 134, 136, 158 is shown at 166, Fig. 6a. With resistor 167 adjusted almost to zero, circuit 138 provides a potential shaped as shown at 171 and a sound like a xylophone. With resistor 167 at a maximum, the potential is as shown at 172 and the sound is like a piano. One or both of the capacitors 161, 162 may be added to 128. Volume control. A variable gain amplifier may be used instead of the potential divider shown in Fig. 2b. The amplifier may be the amplifier stage 127 of the envelope shaper, as shown in Fig. 7a. A tapping on a potential divider 182 is selected by a relay tree 183 controlled by brushes 15 to 18. The selected potential is applied through a resistance capacity circuit to the second control grids of the amplifier pentodes 141, 142. Timbre is determined by the spectrum controller 191 comprising tuned resonator filters and compensator filters which attenuate high or low frequencies. A resonator filter may include a two-stage amplifier with negative feed-lock. Tune sheet. When two channels are combined, the second shannel is controlled by a second set of brushes 19 to 36, Fig. 1, and the codings for the two channels are perforated in line on the tune sheet, Fig. 22. The two channels preferably play alternate notes so that whilst one note is played by one channel, relays are set in the other channel for the next note. As shown, the frequency volume and other codings for a note overlap the envelope coding. The envelope codings of the two channels do not overlap. A portamento effect may be produced by including a frequency glider 122, Fig..1, in the channel. A frequency meter 226, Fig. 16 (see Group XXXVI) provides a direct current proportional to the frequency. This is applied to a beat frequency oscillator 228 through a filter 227 which retards any change in the direct current. A reactance valve 231 fed by the direct current controls the frequency of a variable frequency oscillator 229 which together with a fixed frequency oscillator 232 and a detector 233 produces a beat frequency which after time becomes the same frequency as that originally measured. The output is clipped, differentiated and shaped in units 234, 236, 237 to give a sawtooth waveform. With the filter 227 as shown, the control potential applied to the oscillator 228 includes a wobble as it approaches its final value as shown at 258, Fig. 16d. Resistance capacity filters produce a non-oscillatory change at 262, 263. A vibrato effect is obtained by applying a sawtooth wave control voltage to the first control grids of two pentodes in a push-pull amplifier 192, Fig. 1. The signal is applied to the second control grids. The vibrato unit 192 may be switched into or out of a channel by a switch 193.  A machine for punching, driving and reading the coded paper record includes the driving roller 74, Figs. 19, 20 with sprocket teeth, a supply roller 296 and a take-up roller 297. The paper runs over a supporting plate 298 and is punched by means of keys 293 (not all shown). The brushes are mounted on strip 334 which may be raised by a screw 336 to disengage the brushes from the record. Each brush comprises' a number of spring fingers 342, Fig. 21c, so that there is no break in the circuit as it passes from one perforation to the next. The driving roller is driven by gears from a motor shaft 299. For punching the record, the gear train is broken by sliding the gear 301 on its shaft and the paper is then advanced step-bystep by means of a pusher rod 307. The supply roller 296 is driven by a friction drive for rewinding. The take-up roller 297 is also driven by a friction drive. The friction drives are controlled by a lever 318, and a screw adjustment 333. When recording the disc recorder is coupled to the drive roller 74 by means of a flexible shaft. Six separate three-minute recordings are made on a 16-inch disc running at 33<SP>1</SP>/ 3  r.p.m. When re-recording, six pick-ups 358, Fig. 24, are used simultaneously and are connected to the record cutter 363 through six adjustable gain amplifiers 359, a pre-amplifier 361, an equalizer network 360 to compensate for the record characteristic and a power amplifier 362.