Patent GB-561729-A (GB)
Title: Improvements in and relating to electrical musical instruments
IPC: G10H5/00
Filed: 1943-02-17
Abstract: 561,729. Electrical musical instruments. HOBSON, P. T., and BOOSEY & HAWKES, Ltd. Feb. 17, 1943, No. 2631-. [Class 88 (ii)] A keyboard electrical musical instrument comprises means for producing a complete series of primary electrical oscillations having frequencies corresponding to all the fundamental frequencies of a plurality of octaves of musical notes, the wave-form of these primary oscillations being, for each frequency, a mirrorsymmetrical isosceles saw-tooth, means for deriving from these primary oscillations a corresponding series of secondary oscillations of similar wave-form, means for deriving from. the primary oscillations, a corresponding series of tertiary oscillations having an unsymmetrical isosceles saw-tooth wave-form, means responsive to actuation of the playing key, for each note, to feed the output system of the instrument from said secondary and tertiary oscillations having a fundamental frequency corresponding to the note, and further means controlled by the player to determine whether symmetrical oscillations (corresponding to stopped pipe tones) or unsymmetrical oscillations (corresponding to open pipe tones) or both, are transmitted when playing keys are depressed. The fundamental frequencies are produced by a set of twelve oscillators of known type giving a symmetrical saw-tooth wave-form and whose fundamental frequencies correspond to those of the notes comprising the lowest octave of the instrument. Each oscillator feeds a series of frequency doublers comprising full wave rectifiers coupled in cascade by transformers, so that from each transformer can be taken oscillations whose frequency is double that fed into the preceding rectifier. In this way, from the twelve oscillators, are produced the fundamental frequencies corresponding to all the notes on the keyboard. Fig. 2 shows one transformer T13 corresponding to a particular note coupling full-wave rectifiers R12 and R13. A secondary coil A feeds a series of full-wave rectifiers SR to produce symmetrical secondary oscillations, while a secondary coil B feeds a set of half-wave rectifiers OR to produce unsymmetrical tertiary oscillations. The rectifiers are coupled by condensers C2 to the output 7 for further amplification and translation of the oscillations into sound by loud-speakers. Since . the full-wave rectifiers SR effect a doubling of the frequency, the set fed from transformer T13 supply the output for the note one octave higher corresponding to the transformer (not shown) fed from rectifiers R13. The note corresponding to the fundamental frequency supplied by transformer T13 is supplied by half-wave rectifiers OR and from a a set of full-wave rectifiers (not shown) corresponding to SR, but fed from a transformer one octave lower than T13. When a note key NK is in its raised position, a prohibitive bias is fed through resistance ER to a point P and hence by lead Q to the half-wave rectifiers OR and by lead X1 to the corresponding set of fullwave rectifiers. Removal of this bias by depression of the key leaves both sets of rectifiers under the control of a prohibitive bias applied from lead Z through the stop switches S1 ... S6, so that a note is sounded only if one or more stops is operated. Operation of a stop substitutes for bias Z a lesser bias from one of the tappings t1 ... t6 on potentiometer HR. Operation of. any of stops s1 . . . s3 allows energization of the full-wave rectifiers to give odd harmonics corresponding to stopped pipe tones, while operation of stops s4 ... s6 energizes the half-wave rectifiers OR which give the open pipe tones. The choice of stop within these two groups, selects the magnitude of the working bias and this varies the proportion of higher harmonics included in the transmitted wave. The attack and decay of the notes is determined by the values of the resistances ER and AR in combination with condenser c1 and variations of these by the player may be used for echo effects. Vibrato effects may be obtained by cyclic variation in the gain of the amplifiers by a motor-operated gain control. A celeste or side-band effect may be obtained by a slow frequency modulation of the basic generators. Clicks occasioned by changing stops are avoided by arranging for the pre-amplifiers to be biassed to cut-off until after the stop has been operated. The loud-speakers are individually chosen to deal with the frequencies they each handle, and further tone control may be effected by means of filters. The instrument may be controlled from a console, in which case electrical control is preferably avoided so as to reduce interference effects. To this end also the fundamental frequency generating stages are all mutually screened and are earthed.