Apparatus and method for modifying a musical tone to produce celeste and other effects

#This invention relates generally to methods and apparatus for modifying musical tones to achieve desired musical effects, and is more particularly concerned with modifying electrical tone signals in electrical musical instruments such as those of the organ type to produce celeste and other musical effects.

#Celeste is the effect produced by playing two or more closely tuned tones together. In a pipe organ, celeste may be produced by sounding together sets of pipes which are naturally or purposely tuned slightly flat and slightly sharp, so that their sounds randomly move into and out of phase, with the true frequency of the note represented by the key played by the organist. Celeste therefore has slow "beating" associated with it, depending on the frequency spread between the tones involved; that is, the extent to which they are out of tune as compared to the reference note.

#Various attempts have been made to utilize electrical phasing circuits to modify tone signals to produce chorus or celeste effects. A system described in Leslie, U.S. Pat. No. 3,372,225 utilizes rotary electrostatic devices for producing celeste or chorus effects. However, in this system the beat rates between the signals in two auxiliary electric-acoustic channels are constantly varying, and the same beat rate is applied to the whole musical instrument. The resultant sound is thus an undulating, varying sound, whereas in true celeste the beat frequency should remain constant throughout the period of time the note is played.

#Other objects and features of the invention will become apparent, and its construction and operation better understood from the following detailed description taken in conjunction with the accompanying drawings in which:

#FIG. 1 is a block diagram illustrating a single channel device embodying the principles of the invention;

#FIG. 2 is a circuit diagram of a voltage-to-period clock pulse generator, and FIGS. 2A an 2B are waveform diagrams used to illustrate its operation;

#Referring now to FIG. 1 for a description of the general character of the invention, the apparatus there shown comprises a musical tone-modifying device which includes an input terminal 10 which receives an analog signal representative of a musical tone from a suitable source, such as an electronic organ, a shift register delay device 12 for controllably detuning the input signal, and an output terminal 14. The tone modifier is normally used in an audio signal processing channel which further includes a suitable power amplifier 16 and a sound-reproducing device, for example, a loudspeaker 18. The desired modulation of the tone signal is achieved with a shift register through which sampled electrical representations of the amplitude of the input signal are shifted progressively from the input to the output, thus delaying the signal. The timing of the shifting function of the shift register 12 is controlled by trigger pulses from a clock oscillator 20, which are frequency-modulated under control of a signal generated by a control voltage generator 22, so as to vary the time delay imposed and thereby to cause frequency modulation of the delayed input tone signal appearing at the output terminal 14. The shift register 12 may be of the type described in the aforementioned Doughty U.S. Pat. No. 3,749,837, a "bucket-brigade" form of which is commercially available and the construction and operation of which are described in the IEEE Journal of Solid State Circuits June, 1969, in an article by F. L. J. Sangster and K. Teer, entitled "Bucket-Brigade Electronics-New Possibilities for Delay-Time Conversion and Scanning".

#Alternatively, the shift register 12 may be a digital shift register provided with analog-to-digital and digital-to-analog converters of known form, a parallel implementation of which is also described in the Doughty patent. A system utilizing parallel-to-serial conversion, and one long delay line, may also be used. The memory in either of these embodiments can be magnetic bubble memories, this from of memory being particularly suitable for achieving the long delay in a parallel-to-serial system. It is intended that the term "shift register" as used in the specification and claims shall encompass all such possible implementations.

#Although superficially similar to FIG. 1 of the Doughty patent, the system illustrated in FIG. 1 has the important difference that the period of the trigger pulses which control the timing of the shifting function of the shift register is proportional to a control voltage applied to the clock oscillator. Stated another way, instead of using a voltage-controlled oscillator of a kind disclosed by Doughty and conventionally used as clock generators, whose frequency is proportional to an applied control voltage, the clock generator 20 generates a train of trigger pulses the period of which is proportional to an applied control voltage. Consequently, the frequency of the trigger pulses is inversely proportional to the control voltage. The use in the arrangement of FIG. 1 of a voltage-to-period clock generator, instead of the conventionally used voltage-to-frequency clock generator, provides the unexpected important advantage that the percentage change of the frequency of the output signal as compared to the frequency of the input signal is directly related to the rate of change, or slope, of the control voltage applied to the clock generator. For example, a ramp voltage of fixed slope applied to the clock generator provides an output signal that is detuned by a fixed percentage from the input frequency which does not change during the period of the ramp. On the other hand, if a signal having a triangular waveform, the slope of which reverses as its goes through each cycle, is used as the control voltage for the clock generator, the frequency of the output signal is successively and uniformly shifted in opposite directions by equal amounts from the frequency of the input signal on a cyclical basis determined by the frequency of the triangular waveform signal, with an abrupt reversal of the direction of frequency shift at successive half cycles. Thus, the output signal is alternately slightly sharp and slightly flat for substantially identical periods relative to the input tone signal. Knowing that the percentage change of the output frequency as compared to the input frequency is directly related to the rate of change of the control voltage, it is possible accurately to predict the frequency modulation of the output signal that will occur when various wave shapes are used for the control voltage; this predictability, in turn, enables the design of systems for producing a variety of musical effects, the detuning characteristics necessary for the production of which are generally known by ones skilled in the electrical musical instruments art.