Data processing: speech signal processing – linguistics – language – Speech signal processing – Synthesis
Reexamination Certificate
1999-03-16
2001-10-30
Korzuch, William (Department: 2641)
Data processing: speech signal processing, linguistics, language
Speech signal processing
Synthesis
C704S268000
Reexamination Certificate
active
06311158
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to signal processing, and more particularly to techniques for synthesizing time-domain signals by use of non-overlapping inverse Fourier transforms.
Sinusoids are fundamental building blocks used in the synthesis of waveforms for speech, audio, music, and other applications. It is known that a particular time domain signal can be decomposed into a sum of sinusoids, with each sinusoid having a particular amplitude, frequency, and phase. In fact, a time-domain signal can be fully represented by its corresponding frequency-domain spectrum.
In sinusoidal modeling or additive synthesis of speech, audio, or music signal, it is often necessary to synthesize and sum a large number of sinusoids with time-varying amplitude, frequency, and phase parameters. For example, an accurate representation of a low piano note can require over 100 sinusoids. Several techniques currently exist for the synthesis of sinusoids, including) wavetable synthesis and synthesis using overlapping Fourier transforms.
Wavetable synthesis is a popular technique for synthesizing waveforms. A wavetable synthesizer typically stores samples of a limited number of representative waveforms in a read-only memory (ROM) that are later retrieved and manipulated to generate the desired waveform. For example, a music wavetable synthesizer implementing a piano may store a set of representative notes (i.e., eight notes out of eighty-plus possible notes the piano is capable of playing). To synthesize a desired note, one of the representative notes is retrieved from memory, shifted in pitch to match that of the desired note, and converted to a desired output format (e.g., an analog signal). As can be seen, the cost to implement a wavetable synthesizer can be very high when large numbers of sinusoids need to be synthesized. Further, the need to determine and store representative waveforms can limit the use of the wavetable synthesizer to specific applications. Wavetable synthesizer is further described in U.S. Pat. No. 5,809,342.
Synthesis using overlapping inverse Fourier transforms is another technique for synthesizing waveforms. In this technique, the signal to be synthesized is partitioned into overlapping frames, with each frame including a number of samples from preceding and succeeding frames. The overlapping attempts to minimize the amount of discontinuity at the frame boundary. The signal is then synthesized frame by frame. Each frame typically includes a number of sinusoids, with each sinusoid corresponding to a “peak” in the frequency domain. For each frame, a peak is synthesized in the frequency domain for each of the sinusoids. The peaks in the frame are added together and an inverse Fourier transform is calculated to generate a time-domain frame. Consecutive time-domain frames are synthesized in the above-described manner, overlapped with adjacent frames, and added together with these frames. This technique is further described in U.S. Pat. No. 5,401,897.
The use of inverse Fourier transforms that overlap results in additional cost and can generate artifacts that degrade performance. For example, for implementations having fifty percent overlapping, half of the samples in any particular frame is from the preceding frame and the remaining half of the samples is from the succeeding frame. Overlapping the frames thus results in more frames being calculated per second of output signal. Moreover, it has been noted that artifacts can occur in the overlapping regions whenever the frequency of the sinusoids changes from one frame to the next, which commonly occurs. The artifacts include undesirable amplitude modulation that arises from summing sinusoids from adjacent frames having similar, but different frequencies. To counter this undesirable modulation, sweeping sinusoids can be generated such that the frequency of these sinusoids varies linearly (i.e., instead of being constant) within a particular frame or exhibits two sweep rates within one frame. The generation of sweeping sinusoids can significantly complicate the synthesis process and typically requires additional computations.
Thus, techniques that efficiently synthesize time-domain signals with reduced complexity and minimal amounts of artifacts are highly desirable.
SUMMARY OF THE INVENTION
The invention provides techniques for synthesizing time-domain signals using less computations and having improved signal quality. The synthesis is achieved using non-overlapping Fourier transforms. The time-domain signal is decomposed to a series of waveforms, with each waveform being generated by a sum of sinusoids. Each sinusoid is synthesized by a spectral pattern in the frequency domain that corresponds to a selected (e.g., Hanning) window function. Discontinuities in the amplitude and phase of adjacent waveforms are minimized by matching the amplitude and phase of pairs of corresponding sinusoids in adjacent frames. Matching of amplitude and phase can be achieved by synthesizing sinusoids with linearly varying amplitude and phase.
An embodiment of the invention provides a method for synthesizing a time-domain signal. In accordance with the method, the time-domain signal is partitioned into a number of time-domain frames and a waveform is then generated for each time-domain frame. Each waveform includes one or more sinusoids. The waveform is generated by first selecting a sinusoid for synthesis. A set of parameter values (e.g., the start and end amplitude, frequency, and phase values) is computed for the selected sinusoid. A template is then determined for the selected sinusoid and added to a frequency-domain frame. The template is based on the computed parameter values and a selected window function. The process can be repeated for each sinusoid in the waveform. After all sinusoids have been processed, the frequency-domain frame is transformed to a time-domain frame. In an implementation, the time-domain frame is re-normalized with a re-normalization function that is generated based on (i.e., the inverse of) the selected window function. A predetermined number of samples from each end of the time-domain frame can be discarded. The waveform is defined by the non-discarded samples in the time-domain frame. The waveforms from the time-domain frames are concatenated to generate the time-domain signal.
Various additional features can be provided. For example, the selected window function can be oversampled to provide higher frequency resolution. The template typically includes a component corresponding to a sinusoid having constant amplitude and a component corresponding to a sinusoid having amplitude that varies linearly across the frame.
Another embodiment of the invention provides for a computer program product that implements the method described above.
Yet another embodiment of the invention provides for a signal synthesizer that includes an electronic storage unit and a processor. The electronic storage unit is configured to store values of a spectral pattern corresponding to a sinusoid. The processor couples to the electronic storage unit and is configured to generate a sequence of non-overlapping waveforms. Each waveform corresponds to a time-domain frame and includes one or more sinusoids. Each sinusoid is synthesized by placement of a template at a particular amplitude value and frequency corresponding to the sinusoid being synthesized.
The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.
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Griffin, Daniel W. and Jae S. Lim, “Signal Estimation from Modified Short-Time Fourier Transform,” IEEE trans. Acoust., Speech, and Sig. Proc. vol. ASSP-32, No. 2, Apr. 1984, pp. 236-243.
Creative Technology Ltd.
Korzuch William
Storm Donald L.
Townsend and Townsend / and Crew LLP
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