Patent GB-1376093-A (GB)
Title: Sampling modulation system for an electronic musical instrument
IPC: H03K7/04
Filed: 1973-02-09
Abstract: 1376093 Electronic musical instrument MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 9 Feb 1973 [10 Feb 1972 7 July 1972 10 July 1972 20 July 1972 19 Oct 1972 1 Nov 1972] 6523/73 Heading G5J To modulate an alternating signal in phase or frequency, typically to provide vibrato on a musical tone, without limit of Œ# radians imposed on phase deviation by known methods, the signal 10, a is sampled 12 at a controlled rate 16, b; the samples c are stored 13 and after a delay d copied at a second controlled rate 17, e and finally filtered to give modulated signal output 19, f. At least one of the sampling and reading rates b, e must be frequency modulated 18. Typically the modulation rate is infrasonic, signal frequency is sonic, and sampling/reading rates ultrasonic. Phase deviation of components of the signal should be proportional to their frequency, giving vibrato through fixed musical interval: this corresponds to constant swing of time delay. The input must be filtered 11 to remove frequencies above half the minimum sampling/read rate, automatically accomplished by the "hold" circuit which stores signal level on a capacitor between samples. In all embodiments the instantaneous rate of reading delayed samples must differ from the instantaneous rate at which the same samples are stored, though the averages must be the same. In the first often embodiments (Figs. 3, 4, not shown) the same modulated square pulse generator controls both store and read operations. At the start of each pulse a capacitor stores the value of the signal while at the end of each pulse the potential on the capacitor is copied to a second capacitor in a memory chain, via a unity gain high impedance amplifier and pulse operated switch. Successive samples move a further stage up the chain at each pulse. The memory chain may be a charge-transfer device such as a "bucket-chain" or charge coupled device; it must be long enough to ensure that the time delay between storage and reading of any sample is always positive. Samples arriving at the end of the chain are read at the modulated rate which is the same as the simultaneous sampling rate but different from the rate at which emergent samples were stored. In embodiment 2 (Fig. 5, not shown) the memory chain of embodiment 1 is split into a plurality of parallel chains, entered and read in sequence. The starter chains introduce less noise and distortion. In embodiment 3 (Fig. 6, not shown) the sampling and sequential distribution means of embodiment 2 are combined. In embodiment 4 (Fig. 7, not shown), to reduce noise and distortion still further, the number of parallel chains is increased to the limit in which the length of each chain is only one stage. Distribution and reading may be under the control of the same modulated pulses, but the number of paths required is reduced by providing equal and oppositely phased modulation for two outputs from a pulse train generator. Delay between sample and read must be maintained positive at all times. Embodiment 5 (Fig. 8, not shown) is the same as embodiment 2 except only the sampling rate is frequency modulated, while the stepping along the memory chain and reading are governed by an unmodulated pulse train. In embodiment 6 (Fig. 9, not shown) separate buffer stages are provided to copy at intervals each stage of the memory. A clearing pulse initiates the storage of a series of samples which step along the memory chain at a modulated rate, and at the same instant records the number of pulses stored in the last cycle; this number of stored samples is then read out from the buffer in order of entry, under control of a pulse train modulated oppositely to the sampling train. When the reading operation reaches the last sample entered, the current series of samples in memory is copied to the buffer, and a new clearing pulse initiates a new cycle. In embodiment 7 (Fig. 10, not shown) the parallel store is first, leading to a sequential output store, as opposed to embodiment 6 in which the sequential store leads. At the beginning of a cycle, a clearing pulse causes the stages of the memory chain to take up the states of the parallel sample stores; the number of samples stored is recorded. While the sample stores are being loaded in succession from the input signal, under the control of a fixed frequency pulse train, the memory chain feeds samples to the output under the control of a modulated train. When the recorded number of samples has emerged a new clearing pulse restarts the cycle. In embodiment 8 (Fig. 11, not shown) a musical instrument is shown in which two loudspeakers are supplied from the input via identical modulating systems, which may be any of the embodiments 1-7, supplied with equal and opposite modulation feeds, giving spacial and temporal vibrato effect. In embodiment 9 (Fig. 12, not shown), only one modulating system supplies a loud-speaker, again via any embodiment 1-7, while the second loud-speaker carries unmodulated signal. In embodiment 10 (Fig. 13, not shown) the signal from a tone generator, supplying one loudspeaker, is frequency modulated by conventional means, and one of the embodiments 1-7 applied to this signal adds modulation in opposite phase, giving a resultant output to the second loud-speaker with opposite modulation to the first. Since the conventional modulation will have constant phase deviation with all signal components, it is possible to use one of the embodiments 1-7 to feed a loud-speaker cancelling the vibrato on signal components of one frequency, e.g. bass notes where it is undesirable, while maintaining it at other frequencies.