Patent GB-1396148-A (GB)
Title: Chorus and ensemble animation rate compensation system
IPC: G10H1/10
Filed: 1972-11-14
Abstract: 1396148 Electrophonic musical instrument A B FREEMAN 14 Nov 1972 [18 Nov 1971] 52485/72 Heading G5J In an electronic musical instrument, a chorous effect is produced by a plurality of tone sources, the number minimized for economy, providing for different octaves notes relatively slightly detuned by an amount related to either the highest or lowest pitch present in the output so that the animation rate (beat frequency), is a desired function of pitch. Master tone generators 18, 19 supply tones in the top octave in two separately detunable sets feeding notes C# to F# and G to C; tones for all notes below F# in the octave below top are available from both sets, either via a chain of divide-bytwo frequency dividers from the same note in the top octave or through a separate chain of divide-by-two dividers headed by a divide-bythree stage from the note in the top octave a fifth above the required note. Alternatively, the two master tone generator sets may be replaced by two detunable high frequency oscillators, with the two halves of the top octave produced from them by non-octave frequency division. Key switches 12 cause the required tones to be connected to output via the tone keying unit 13, notes in successive octaves being selected from different master tone generator sets 18 or 19, via the appropriate type of divider chain in 22 or 23. Voltage to energize the key switches is supplied from the rate control unit 15 via buses 14a-14e, each covering half an octave, current drain through each bus being detected by a transistor feeding the bus: sustaining circuits on the key switches in keying unit 13 maintain the tone and the drain on the bus after release of the key if required. In Fig. 2, switches 49, 50, 51, closed mechanically when 16, 8 and 4 foot stops are in use, saturate transistor 41a when both switches 49 and 50 are closed, and saturate transistor 41b when both 50 and 51 are closed, so modifying the level of voltage supplied to the base of transistor 36, and thus compelling the maximum percentage detuning to be higher with 16 and 8 foot stops in use than with 8 and 4 foot. The level of detuning, adjustable pro rata by variable resistor 39, is further modified by the highest pitch bus 14a-14e which passes current, determining the first point in the resistor chain 35a-35f at which a saturated transistor is encountered, effectively short circuiting the remainder of the chain. The detuning voltages, positive-going above ground for line 17 and negative-going below positive supply potential for line 16, detune master tone generator sets (or HF oscillators), 86 and 93 in opposite directions. Detuning voltage on 16 causes diodes 80, 81 to conduct for a proportionately increased part of their oscillator cycle, during which extra capacitances 83 are inserted in the resonant circuit, detuning it downwards; detuning voltage on 17 causes diodes 88, 89 to conduct for a proportionately decreased part of the cycle, removing capacitances 90 from the resonant circuit and detuning upwards. In the circuit for note G shown, the tone signals from the detuned oscillators, subdivided by the appropriate frequency dividers, appear at G1 and G2 whence one or other is connected to output line 26 via diodes 64-67, held conducting by virtue of the charge on electrolytic capacitor 61 if sustaining is required, or by the voltage in the bus if not. When note G is sounded in two octaves, one will be supplied from G1 and the other from G2. The effect of the complete circuit is to reduce the percentage detune for higher pitches, giving an animation rate that increases less rapidly than frequency. In a second embodiment (Fig. 3, not shown), current through the buses 14a-14e inserts series chains of capacitors in parallel with the tuning capacitors of two high frequency oscillators, the one chain being shortened as the pitch of the active bus rises, while the other is lengthened; the effect is to detune one oscillator above datum by an amount depending on the highest active bus, and the other oscillator below datum by an amount depending on the lowest active bus. In a third embodiment (Fig. 4, not shown), the tone output 26 is filtered by a series of halfoctave pass-band wave filters normally used as flute filters for picking sinusoidal flute tones from complex tone signals. Transistors brought to saturation by the presence of a signal in the filter outputs short circuit at intermediate points, a chain of resistors controlling the detuning voltage, corresponding to the resistors 35a-35f in the first embodiment.