Music – Instruments – Electrical musical tone generation
Reexamination Certificate
2000-04-10
2001-05-29
Donels, Jeffrey (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084S607000
Reexamination Certificate
active
06239345
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of signal processing, and more particularly to a method and apparatus for looping signals from a wavetable to form synthesized signals.
BACKGROUND OF THE INVENTION
Wavetable sound synthesis refers to a process for synthesizing sounds from stored information. A wavetable is a table of data representing samples of sound information. Sounds may be synthesized by reading the samples from the wavetable into a digital to analog converter, thereby producing an analog sound signal representative of the desired sound. All types of sounds may be synthesized using this technique, however, large amounts of data storage are needed to store the individual sound samples. In a process known as looping, sounds are synthesized from a wavetable using fewer data samples. During the looping process, the desired sound signal can be created by repeatedly reading out a portion of a wavetable. This technique forms a continuous data stream that, when converted to analog form, will produce the desired sound signal. As a result of using the looping process, a wide range of sounds can be synthesized from a much smaller wavetable, thereby providing a more efficient way to synthesize sound signals.
Although the looping process improves wavetable efficiency, it can result in side effects detrimental to the quality of the synthesized sound. These side effects occur because the looping process produces artifacts as a result of mismatches, or discontinuities, in amplitude, phase or timbre between the beginning and end of the loop.
Techniques have been used in the past to overcome the problems associated with discontinuities in looped signals For example, one technique is called cross-fading. In a typical looping process, the end of one loop flows directly into the beginning of the next loop. At the meeting point there may exist severe discontinuities causing noticeable artifacts sometimes referred to as “clicks.” In cross-fading, the loop end and loop beginning are overlapped. To compensate for the increased signal level at the overlapped portion, the end of the loop is faded out while the beginning of the loop is faded in. Although cross-fading can reduce the effects of discontinuities it does not eliminate them. These reduced, but still noticeable, artifacts are sometimes referred to as “blips.”
Another technique used to compensate for the effects of looping is described in “Method and apparatus for producing an electronic representation of a musical sound using extended coerced harmonics” (U.S. Pat. No. 5,466,882). The method disclosed in this patent uses a sinusoidal model to represent the synthesized sound signal. According to the disclosed method, synthesized harmonics are forced into strict harmonic relationship with the fundamental frequency to eliminate phase discontinuities at the loop start and end points. However, forcing the signal to be strictly harmonic is a drastic measure, which can seriously alter the quality of the resulting sound. For example, piano sounds synthesized using this technique can have very poor sound quality.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus are provided wherein loop discontinuities in a looped signal are eliminated. In the case of amplitude discontinuities, the harmonic amplitudes contained in the loop are progressively scaled over the length of the loop, so that for each harmonic the loop end amplitude matches the loop beginning amplitude. In the case of phase discontinuities, the harmonic phases are progressively shifted over the length of the loop, so that for each harmonic the loop end phase matches the loop beginning phase. Shifting the phase is accomplished by slightly altering the frequency of the harmonics to produce the desired amount of phase-shift at the loop end. Modification of one or more of the harmonics or frequency components is accomplished independently of the other frequency components. In accordance with the present invention, the method also provides a technique to select loop begin and end points to minimize the amount of phase adjustment.
In an embodiment of the invention a method is provided for looping a signal to form a synthesized signal. The method begins by transforming the signal into a sinusoidal representation comprising at least one frequency component having at least one signal characteristic. The frequency component is adjusted to have a beginning value and an ending value that match thereby forming an adjusted sinusoidal representation. The adjusted sinusoidal representation is then converted to a time domain signal and looped for a plurality of loops to form the synthesized signal.
In another embodiment of the invention, apparatus is provided for forming a loop signal used to create a synthesized signal. The apparatus comprises a memory having a stored signal and a controller coupled to the memory and having logic to access the memory to output the stored signal. A transformer has logic to transform the sinusoidal signal to a sinusoidal representation having a plurality of frequency components. A signal splitter is coupled to the memory and the transformer and has logic to form a residual signal. A harmonic adjuster is coupled to the transformer and has logic to adjust the plurality of harmonic signals to form an adjusted sinusoidal representation. An inverse transformer is coupled to the harmonic adjuster and has logic to inverse transform the adjusted sinusoidal representation to form an adjusted signal. Finally, a combiner is coupled to the signal splitter and the inverse transformer and has logic to combine the residual signal and the adjusted signal to form the loop signal.
REFERENCES:
patent: 5194681 (1993-03-01), Kudo
patent: 5446882 (1995-08-01), Lee
patent: 5637821 (1997-06-01), Izumisawa et al.
Creative Technology Ltd.
Donels Jeffrey
Townsend and Townsend / and Crew LLP
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