Data processing: speech signal processing – linguistics – language – Audio signal bandwidth compression or expansion
Reexamination Certificate
2000-06-21
2002-11-26
McFadden, Susan (Department: 2654)
Data processing: speech signal processing, linguistics, language
Audio signal bandwidth compression or expansion
C704S503000, C084S612000
Reexamination Certificate
active
06487536
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a time-axis compression/expansion method and apparatus for performing time-axis compression/expansion on original digital signals at a desired compression/expansion rate without changing the pitch of the original digital signals, and more particularly to a time-axis compression/expansion method and apparatus of this kind which is suitable for processing multichannel signals.
2. Prior Art
The time-axis compression/expansion technique for time-axis compressing or time axis-expanding a digital audio signal without changing the pitch of the same is utilized e.g. for so-called “time length adjustment” for adjusting a total recording time period over which the digital audio signal is to be recorded to a predetermined time period, tempo conversion in a karaoke apparatus or the like, and so forth. Conventionally, this kind of time-axis compression/expansion technique includes a cut-and-splice method (as disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 10-282963), an overlap-add method based on pointer shift amount control (Morita & Itakura, “Expansion/Compression of Sound in Time Product by Using Overlap-Add Method Based on Point Shift Amount Control and Its Evaluation”, Lectures at the Autumn Conference of the Acoustical Society of Japan Vol. 1-4-14, p. 149, October, 1986), etc.
Time-axis compression/expansion processing by a general cut-and-splice method is performed such that waveform segments are cut out without considering correlation between the waveform segments and then the cut-out waveform segments are spliced together to thereby effect compression/expansion based on a specified compression/expansion rate. According to this method, discontinuities can occur in spliced portions of the cut-out waveform segments, and therefore cross-fading is carried out to smooth the spliced portions of the cut-out waveform segments. The time interval of the waveform cutout is set to such a time period that the human ears cannot sense an echo or doubling of sounds, e.g. approximately 60 msec. Particularly, according to the method disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 10-282963, the cutout length or length of the cutout waveform segment is determined in synchronism with sound timing information. This method is distinguished from other conventional methods in that spliced portions appear at the same repetition period as that of the rhythm of the original waveform, so that tone changes at the spliced portions cannot be easily perceived. Cross-fading between waveform segments which are largely different in phase from each other markedly degrades the tone quality. Therefore, the present assignee has proposed a phase-matching type cut-and-splice method in which cut-out waveform segments which are closest in phase to each other are detected and are then subjected to cross-fading.
On the other hand, the overlap-add method based on pointer shift amount control is performed such that two adjacent segments of the original audio signal closely correlated in waveform and equal in length to each other are extracted, and the two signal segments are overlapped or added together. Then, the two original signal segments are replaced by a new signal segment obtained by the overlapping/addition, or the new signal segment is inserted between the two original signal segments, whereby the total time of the original audio signal is reduced or increased. This method enables smoother splicing of waveforms than the cut-and-splice method. Particularly, this method can achieve higher-quality time-axis compression/expansion of pitch-based sound source signals, such as voice signals and sound signals generated by monophonous musical instruments.
However, the conventional phase-matching type cut-and-splice method and overlap-add method based on pointer shift amount control only deal with monophonic signals. If these methods, which select signal segments identical in phase or signal segments closely correlated in waveform to each other for cross-fading, are directly applied to processing of stereo signals, it may provide an odd auditory localization for the listener, which forms a serious problem. This results from the fact that left-channel and right-channel signals are processed as separate monophonic signals independent from each other so that a disagreement occurs between the cross-faded portions of the signals of the respective channels, causing a difference in phase between tones sensed by the two ears that determines the auditory localization of the stereo signal.
Aside from the time-axis compression/expansion apparatus, there have been proposed pitch conversion devices that perform processing for changing the readout ratio by using the cut-and-splice method (Japanese Laid-Open Patent Publication (Kokai) No. 5-297891). According to one of the devices, pitch conversion of left-channel and right-channel signals of a stereo signal is performed such that portions of the left-channel signal most closely correlated to each other are cut out and spliced together by cross-fading, and then portions of the right-channel signal close to the edited point of the left-channel signal and most closely correlated to each other are cut out and spliced together by cross-fading. According to another device, the pitch conversion is performed such that the editing method is switched, as required, according to the correlation between the left-channel signal and the right-channel signal in such a manner that if the correlation between the two channel signals is not high, portions of each channel signal which are most closely correlated to each other are edited on a channel-by-channel basis, while if the correlation between the two channel signals is high, portions of the left-channel signal (or right-channel signal) which are most closely correlated to each other and portions of the other channel signal corresponding to the portions of the left-channel signal (or right-channel signal) are both edited.
However, these proposed devices had the disadvantage that cross-fading is not fully synchronized between the left and right channel signals, which may cause a difference in phase between tones sensed by the two ears and hence provide an odd auditory localization for the listener. Such a transient odd auditory localization that is sensed is generally more conspicuous to the ears than improper splicing of waveform segments by cross-fading, which forms a problem to be solved.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a time-axis compression/expansion method and apparatus for multichannel signals, which is capable of performing time-axis compression/expansion on a multichannel signal without causing a disagreement between cross-fading points of the channels of the multichannel signal, to thereby ensure that a normal auditory localization is provided for the listener.
To attain the above object, according to a first aspect of the present invention, there is provided a time-axis compression/expansion method for time-axis compressing/expanding a multichannel signal comprising a plurality of channel signals at a specified compression/expansion rate, which comprises the steps of sequentially cutting out waveform segments from each of the channel signals, determining a cutting starting point of a leading end portion of a waveform segment of the cut out waveform segments following each preceding waveform segement of the cut out waveform segments, commonly between the channel signals, based on two portions of a waveform of a synthesized signal formed by synthesizing the channel signals within a range of a predetermined search starting point to a predetermined search ending point of the waveform of the synthesized signal, the two portions corresponding to a time period over which cross-fading is to be carried out and being most similar to each other, and splicing together the preceding waveform segment and the following waveform segment cut from each of the channel signals based on the determined cuttin
Koezuka Shinji
Kondo Kazunobu
McFadden Susan
Yamaha Corporation
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