Music – Instruments – Electrical musical tone generation
Reexamination Certificate
2000-05-04
2001-05-15
Donels, Jeffrey (Department: 2837)
Music
Instruments
Electrical musical tone generation
C084S652000, C434S30700R, C704S503000
Reexamination Certificate
active
06232540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to time-scale modification methods and apparatuses that perform time-scale modification on digital signals, which are modified without being changed in original pitches with respect to time scale in accordance with desired time-scale modification factors. Particularly, this invention relates to time-scale modification of rhythm source signals.
This application is based on Patent Application No. Hei 11-126349 filed in Japan.
2. Description of the Related Art
Normally, time-scale modification techniques are effected to perform compression and expansion on digital audio signals with respect to time, wherein the digital audio signals are not changed in pitches. Those techniques are used in a variety of fields such as in so-called “scale adjustment” in which an overall recording time of digital audio signals being recorded is adjusted to a prescribed time and “tempo modification” used by Karaoke apparatuses, for example. Conventionally, engineers and scientists propose various examples of time-scale modification techniques. For example, Japanese Unexamined Patent Publication No. Hei 10-282963 teaches a cut-and-splice method in time-scale modification processing. In addition, an example of a time-scale modification algorithm is taught by the paper entitled “Time-Scale Modification Algorithm for Speech by Use of Pointer Interval Control Overlap and Add (PICOLA) and Its Evaluation”, which is written by Morita and Itakura on pp. 149-150 of monographs 1-4-14 issued for the autumn meeting of Japan Acoustics Engineering Society in October of 1986.
In general, the cut-and-splice method is used for time-scale modification processing to perform compression or expansion on signal waveforms (or envelopes) in accordance with a designated time-scale modification factor (e.g., compression factor or expansion factor), as follows:
Waveforms are divided into and cut to segments, regardless of correlation therebetween. Then, the cut segments of the waveforms are spliced together to achieve the time-scale modification in accordance with the designated time-scale modification factor. Herein, discontinuity is caused to occur at joints by which the cut segments of the waveforms are spliced together. To reduce the discontinuity, a cross-fade process is effected on the joints to smoothly connect the joints of frames. Intervals of distance (referred to as “cut intervals”) by which the waveforms are cut to segments are set such that it is difficult for listeners to sense echoes or sound repetition given human auditory capabilities. For example, the cut intervals are set at 60 millisecond or so. The aforementioned publication teaches a splendid method in which cut lengths of waveforms are determined in synchronization with speech timing information. As compared with general methods, the aforementioned method is advantageous in that variations in sound quality are relatively small at joints of waveform segments being spliced together because the joints emerge by the same period of rhythm as that of the original waveforms.
According to the aforementioned PICOLA method, two segments are extracted from a waveform of an original audio signal. Herein, the two segments each having the same length are arranged to adjoin each other on the waveform with highest correlation therebetween. Signals of those segments are subjected to duplicate addition to produce a specific signal, which is substituted for the original two segments or which is inserted between them. Thus, it is possible to shorten or extend an overall time sustaining the waveform. This method is advantageous in that connection between waveform segments can be made smooth as compared with the cut-and-splice method. Particularly, this method enables high-quality time-scale modification on highly-pitch-dependent sound sources that produce speech signals, musical tone signals of monophonic musical instruments and the like.
In general, the conventional cut-and-splice method has merits in which appropriate sound qualities are expected with respect to many types of sound sources. In the case of rhythm sources, however, it suffers from noticeable deterioration of sound quality such as “double beat” and “disorder in rhythm”. The aforementioned publication teaches the cut-and-splice method which is effected in synchronization with the rhythm of the original waveform. In some cases, two attacks are included in each of the segments which are cut from original waveforms. When expanding the waveforms consisting of the cut segments being spliced together with respect to time, a double-beat phenomenon is caused to occur. In contrast, the PICOLA method does not cause such a double-beat phenomenon in principle thereof because time-scale modification is performed in connection with time correlation of waveforms. However, the PICOLA method does not at all compensate for attack positions on waveforms being reproduced by time-scale modification. This causes a rhythm deviation to occur with ease.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a time-scale modification method and apparatus that inhibits rhythm disorder and double beat from being caused to occur by compensating attack positions on waveforms being reproduced by effecting time-scale modification on rhythm source signals.
A time-scale modification method or apparatus of this invention is basically designed to effect a time-scale modification process (i.e., expansion or compression with respect to time) on rhythm source signals containing waves such that rhythm sounds are not substantially changed in pitches. Herein, attack positions are detected from the rhythm source signals by using thresholds which are determined in advance. Hence, the time-scale modification process is performed on intermediate signal portions of the rhythm source signals between the attacks in accordance with a desired time-scale modification factor. Then, the intermediate signal portions subjected to the time-scale modification process are smoothly connected with other signal portions such as the attacks and their proximal portions, which are not subjected to the time-scale modification process. Therefore, it is possible to secure the attacks and their proximal portions, which are left without being substantially changed, while accomplishing the time-scale modification on the rhythm source signals. Thus, it is possible to avoid occurrence of double beat and rhythm disorder in rhythm sounds, which are conventionally caused to occur by the time-scale modification.
Incidentally, the time-scale modification process is effected by a series of steps such as similarity calculation, determination of a basic period, partitioning of waves, windowed multiplication and addition. For example, a combined wave is produced from two waves which are partitioned from original waves of rhythm source signals by the basic period and which are subjected to windowed multiplication and addition. In the case of compression, the combined wave is substituted for the two waves in the original waves, so that the rhythm source signals are compressed as a whole. In the case of expansion, the combined wave is inserted between the two waves in the original waves, so that the rhythm source signals are expanded as a whole.
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Morita, Naotaka & Fumitada Itakura, School of Engineering, Nagoya University, “Time-Scale Modification Algorithm for Speech by Use of Pointer Interval Control Overlap and Add (PICOLA) and its Evaluation”, pp. 149-150.
Donels Jeffrey
Pillsbury & Winthrop LLP
Yamaha Corp.
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