PATENT DOCUMENT

Publication Number: US-7425674-B2
Application Number: US-70671307-A
Country: US
Kind Code: B2

Title: Method and apparatus for time compression and expansion of audio data with dynamic tempo change during playback

Abstract:
A method and apparatus implement time compression and expansion of audio data, with dynamic tempo change during playback. Dynamic changes in tempo are implemented at specific points in the audio signal corresponding to local minimums in the fade-in and fade-out characteristics of the compression/expansion scheme. An audio signal is marked to define temporal slices of audio data. Mark positions may be selected to minimize significant transient activity midway between consecutive marks. Fade-in and fade-out functions are associated with the leading side and trailing side, respectively, of each mark, creating a series of cross-fading “mounds” with peaks at each mark. When a tempo change is requested (e.g., a user selects a new tempo value in a user interface), the tempo change is delayed until the start of the next “mound” (i.e., the next fade-in). Thus, despite the tempo change, each mound uses a contiguous set of audio data, preventing the clicks and pops associated with skips in the audio’ data. Cross-fading minimizes any effects of desynchronization caused by overlapping mounds of differing speeds.

Claims:
1. A computer program product comprising:
 a computer readable storage medium having computer program code embodied therein for adjusting tempo of audio data from a first tempo to a second tempo, said computer program code configured to cause a processor to perform a plurality of steps comprising: 
 performing a fade-out of a second slice of said source audio data contiguous with a fade-in from a first slice of said source audio data; 
 determining an offset into said second slice; and 
 performing a fade-in of said second slice of said source audio data beginning at said offset into said second slice based on said second tempo. 
 
     
     
       2. The computer program product of  claim 1 , wherein said second slice of said source audio is contiguously subsequent to said first slice of said source audio data. 
     
     
       3. The computer program product of  claim 1 , wherein said computer program code is further configured to cause a processor to perform:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo; and 
 wherein receiving comprises receiving said request while processing said first slice of said source audio data. 
 
     
     
       4. The computer program product of  claim 1 , wherein said computer program code is further configured to cause a processor to perform:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo; and 
 wherein receiving comprises receiving said request while processing a slice of said source audio data contiguously precedent to said first slice of said source audio data. 
 
     
     
       5. The computer program product of  claim 1 , wherein said fade-out of said second slice of said source audio data is based on a fade-out function and said fade-in from said first slice of said source audio data is based on a fade-in function, and wherein said fade-out function and said fade-in function, when combined, result in a substantially constant power level over a length of said audio data to which said fade-out and fade-in functions are applied. 
     
     
       6. The computer program product of  claim 1 , wherein said computer program code is further configured to cause a processor to perform:
 determining an output slice length for destination audio data corresponding to said second slice of said source audio data; and 
 wherein performing said fade-out comprises completing performing said fade-out of said second slice of said source audio data within said output slice length. 
 
     
     
       7. The computer program product of  claim 6 , wherein determining comprises determining, based at least in part on said second tempo, said output slice length. 
     
     
       8. The computer program product of  claim 1 , wherein said steps of performing adjust said tempo of said source audio data from said first tempo partially to said second tempo, and wherein said computer program code is further configured to cause a processor to perform:
 iteratively performing said steps of performing for one or more subsequent slices of said source audio data until said tempo of said source audio data is adjusted to said second tempo. 
 
     
     
       9. The computer program product of  claim 1 , wherein said computer program code is further configured to cause a processor to perform:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo; and 
 wherein said request for a tempo change represents an expansion of said source audio data, and wherein said computer program code is further configured to cause a processor to perform: 
 between performing said fade-out of said second slice of said source audio data and performing said fade-in of said second slice of said source audio data, performing a fill function on said second slice of said source audio data. 
 
     
     
       10. The computer program product of  claim 1 , wherein said computer program code is further configured to cause a processor to perform:
 parsing said source audio sequence into a plurality of source slices. 
 
     
     
       11. The computer program product of  claim 10 , wherein said parsing comprises:
 detecting a plurality of transients; and 
 selecting boundaries of said plurality of source slice based on respective locations of said plurality of transients. 
 
     
     
       12. The computer program product of  claim 10 , wherein said parsing comprises:
 obtaining information about musical characteristics of said source audio sequence; and 
 determining boundaries of said plurality of source slices based on said musical characteristics. 
 
     
     
       13. The computer program product of  claim 12 , wherein said plurality of source slices correspond temporally to musical units of time. 
     
     
       14. The computer program product of  claim 13 , wherein said musical units are sixteenth notes. 
     
     
       15. The computer program product of  claim 1 , wherein said plurality of source slices are of varying source slice lengths. 
     
     
       16. A method for adjusting tempo of an audio signal from a first tempo to a second tempo, the method comprising:
 performing a fade-out of a next slice of said source audio data contiguous with a fade-in from a current slice of said source audio data; 
 determining an offset into said next slice; and 
 performing a fade-in of said next slice of said audio data beginning at said offset into said next slice based on said second tempo. 
 
     
     
       17. The method of  claim 16 , wherein said second slice of said source audio is contiguously subsequent to said first slice of said source audio data. 
     
     
       18. The method of  claim 16 , further comprising:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo; and 
 wherein receiving comprises receiving said request while processing said first slice of said source audio data. 
 
     
     
       19. The method of  claim 16 , further comprising:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo; and 
 wherein receiving comprises receiving said request while processing a slice of said source audio data contiguously precedent to said first slice of said source audio data. 
 
     
     
       20. The method of  claim 16 , wherein said fade-out of said second slice of said source audio data is based on a fade-out function and said fade-in from said first slice of said source audio data is based on a fade-in function, and wherein said fade-out function and said fade-in function, when combined, result in a substantially constant power level over a length of said audio data to which said fade-out and fade-in functions are applied. 
     
     
       21. The method of  claim 16 , further comprising:
 determining an output slice length for destination audio data corresponding to said second slice of said source audio data; and 
 wherein performing said fade-out comprises completing performing said fade-out of said second slice of said source audio data within said output slice length for said destination audio data corresponding to said second slice of said source audio data. 
 
     
     
       22. The method of  claim 21 , wherein determining comprises determining, based at least in part on said second tempo, said output slice length for said destination audio data corresponding to said second slice of said source audio data. 
     
     
       23. The method of  claim 16 , wherein said steps of performing adjust said tempo of said source audio data from said first tempo partially to said second tempo, said method further comprising:
 iteratively performing said steps of performing for one or more subsequent slices of said source audio data until said tempo of said source audio data is adjusted to said second tempo. 
 
     
     
       24. The method of  claim 16 , further comprising:
 receiving a request for a tempo change to at least a portion of source audio data, wherein said tempo change is from said first tempo to said second tempo, wherein said request for a tempo change represents an expansion of said source audio data; and 
 between performing said fade-out of said second slice of said source audio data and performing said fade-in of said second slice of said source audio data, performing a fill function on said second slice of said source audio data. 
 
     
     
       25. The method of  claim 16 , further comprising:
 parsing said source audio sequence into a plurality of source slices. 
 
     
     
       26. The method of  claim 25 , wherein said parsing comprises:
 detecting a plurality of transients; and 
 selecting boundaries of said plurality of source slice based on respective locations of said plurality of transients. 
 
     
     
       27. The method of  claim 25 , wherein said parsing comprises:
 obtaining information about musical characteristics of said source audio sequence; and 
 determining boundaries of said plurality of source slices based on said musical characteristics. 
 
     
     
       28. The method of  claim 27 , wherein said plurality of source slices correspond temporally to musical units of time. 
     
     
       29. The method of  claim 28 , wherein said musical units are sixteenth notes. 
     
     
       30. The method of  claim 16 , wherein said plurality of source slices are of varying source slice lengths. 
     
     
       31. A system configured for adjusting tempo of an audio signal from a first tempo to a second tempo, the system comprising:
 one or more processors; 
 memory coupled to said one or more processors; 
 wherein said memory stores instructions which, when executed by said one or more processors, cause performance of: 
 performing a fade-out of a next slice of said source audio data contiguous with a fade-in from a current slice of said source audio data; 
 determining an offset into said next slice; and 
 performing a fade-in of said next slice of said audio data beginning at said offset into said next slice based on said second tempo. 
 
     
     
       32. The system of  claim 31 , wherein said second slice of said source audio is contiguously subsequent to said first slice of said source audio data. 
     
     
       33. The system of  claim 31 , wherein receiving comprises receiving said request while processing said first slice of said source audio data. 
     
     
       34. The system of  claim 31 , wherein receiving comprises receiving said request while processing a slice of said source audio data contiguously precedent to said first slice of said source audio data. 
     
     
       35. The system of  claim 31 , wherein said fade-out of said second slice of said source audio data is based on a fade-out function and said fade-in from said first slice of said source audio data is based on a fade-in function, and wherein said fade-out function and said fade-in function, when combined, result in a substantially constant power level over a length of said audio data to which said fade-out and fade-in functions are applied. 
     
     
       36. The system of  claim 31 , wherein said instructions further cause performance of:
 determining an output slice length for destination audio data corresponding to said second slice of said source audio data; and 
 wherein performing said fade-out comprises completing performing said fade-out of said second slice of said source audio data within said output slice length for said destination audio data corresponding to said second slice of said source audio data. 
 
     
     
       37. The system of  claim 36 , wherein determining comprises determining, based at least in part on said second tempo, said output slice length for said destination audio data corresponding to said second slice of said source audio data. 
     
     
       38. The system of  claim 31 , wherein said steps of performing adjust said tempo of said source audio data from said first tempo partially to said second tempo, and wherein said instructions further cause performance of:
 iteratively performing said steps of performing for one or more subsequent slices of said source audio data until said tempo of said source audio data is adjusted to said second tempo. 
 
     
     
       39. The system of  claim 31 , wherein said request for a tempo change represents an expansion of said source audio data, and wherein said instructions further cause performance of:
 between performing said fade-out of said second slice of said source audio data and performing said fade-in of said second slice of said source audio data, performing a fill function on said second slice of said source audio data. 
 
     
     
       40. The system of  claim 31 , wherein said instructions further cause performance of:
 parsing said source audio sequence into a plurality of source slices. 
 
     
     
       41. The system of  claim 40 , wherein said parsing comprises:
 detecting a plurality of transients; and 
 selecting boundaries of said plurality of source slice based on respective locations of said plurality of transients. 
 
     
     
       42. The system of  claim 40 , wherein said parsing comprises:
 obtaining information about musical characteristics of said source audio sequence; and 
 determining boundaries of said plurality of source slices based on said musical characteristics. 
 
     
     
       43. The system of  claim 42 , wherein said plurality of source slices correspond temporally to musical units of time. 
     
     
       44. The system of  claim 43 , wherein said musical units are sixteenth notes. 
     
     
       45. The system of  claim 31 , wherein said plurality of source slices are of varying source slice lengths. 
     
     
       46. An apparatus for adjusting tempo of an signal data from a first tempo to a second tempo comprising:
 means for performing a fade-out of a second slice of said source audio data contiguous with a fade-in from a first slice of said source audio data; 
 determining an offset into said second slice; and 
 means for performing a fade-in of said second slice of said source audio data beginning at said offset into said second slice based on said second tempo. 
 
     
     
       47. An apparatus for audio playback comprising:
 means for obtaining a source audio sequence containing source audio data and having a source tempo; 
 means for cross-fading a first source slice from said source audio sequence to determine destination audio data for a first output slice having a first output slice length corresponding to a first output tempo; 
 means for receiving a request for a second output tempo during said cross-fading of said first source slice; 
 means for performing a fade-out of a second source slice using source audio data contiguous with a fade-in from said cross-fading of said first source slice; and 
 means for performing a fade-in of said second source slice using an offset into said source audio data based on a second output slice length corresponding to said second output tempo.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of and claims priority to U.S. patent application Ser. No. 10/407,837 filed on Apr. 4, 2003, now U.S. Pat. No. 7,189,913, issued Mar. 13, 2007, the content of which is incorporated by reference in its entirety for all purposes as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to audio processing applications, and more particularly to a method and apparatus for adjusting the tempo of audio data. 
     BACKGROUND 
     With the proliferation of personal computers into the homes of consumers, media activities formerly reserved to professional studios have migrated into the household of the common computer user. One such media activity is the creation and/or modification of audio files (i.e., sound files). For example, sound recordings or synthesized sounds may be combined and altered as desired to create standalone audio performances, soundtracks for movies, voiceovers, special effects, etc. 
     To synchronize stored sounds, including music audio, with other sounds or with visual media, it is often necessary to alter the tempo. (i.e., playback speed) of one or more sounds. Changes in tempo may also need to be made dynamically, during playback, to achieve the desired listening experience. Unfortunately, straightforward approaches to implementing tempo changes, including merely playing the given sound at a faster or slower rate, result in undesired audible side effects such as pitch variation (e.g., the “chipmunk” effect of playing a sound faster) and clicks and pops caused by skips in data as the tempo is changed. These problems may be better understood in the context of an audio file example. 
     An audio file generally contains a sequence (herein referred to as an “audio sequence”) of digital audio data samples that represent measurements of amplitude at constant intervals (the sample rate). In a computer system, this audio sequence is often represented as an array of data like the following:
 
SourceAudioData[ ]={0.0, 0.2, 0.4, 0.3, 0.2, −0.04, −0.15, −0.2, −0.15, −0.05, 0.1, . . . }
 
       FIGS. 1A-1C  show a sound waveform example as might be stored in an audio file.  FIG. 1A  represents 2000 milliseconds of audio in waveform  100 .  FIG. 1B  represents 200 milliseconds of audio taken from the beginning of waveform  100  and shown in expanded view.  FIG. 1C  shows 10 milliseconds of audio in an even greater expanded view, showing individual samples associated with waveform  100 . 
     In  FIG. 1A , waveform  100  contains ten occurrences of sharp rises in signal value that taper over time. These occurrences are referred to herein as transients and represent distinct sound events, such as the beat of a drum, a note played on a piano, a footstep, or a syllable of a vocalized word.  FIG. 1C  illustrates how these sound events, or transients, are represented by the sequence of samples stored in an audio file. It should be clear that modifying the sample values or the time-spacing of the samples in  FIG. 1C  will result in a change in the transient behavior at the level of  FIG. 1A , and a corresponding change in the associated sound during playback of the audio sequence. 
     The resolution of  FIG. 1B  highlights the periodic nature of waveform  100  during the first transient. The frequency of this periodicity influences the pitch of the sound resulting from that transient. A faster oscillation provides a higher pitched sound, and a slower oscillation provides a lower pitched sound. Also clear from  FIG. 1B  is the continuous nature of waveform  100 . Discontinuities in waveform  100  would be audible on playback as clicks and pops in the audio. 
     Assuming that waveform  100  represents an adult speaking, if an audio enthusiast attempts to fit the audio sequence into a 1500 millisecond timeslot (e.g., to synchronize the audio sequence with another musical audio sequence) by simply playing back the samples at 4/3 speed, then the result will sound like a child&#39;s voice. This occurs because the frequency behavior of the transients speeds up with the playback rate, causing an increase in pitch. This same phenomenon occurs when the incorrect playback speed is selected on a dual-speed tape recorder. 
     Now assuming that the audio enthusiast only wishes to speed up a portion of the audio file, not only will the pitch change when the speed is changed, but the speed transition will be marked by a click as the continuity of the waveform is temporarily disrupted by the output waveform skipping forward. Neither the pitch change nor the audible clicking are desirable from a listening standpoint, particularly if the audio is to be of professional quality. Clearly, a mechanism is needed for providing tempo (i.e., speed) control without the undesired side effects of pitch variations and audible clicks or pops. 
     The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     A method and apparatus for performing time compression and expansion of audio data, with dynamic tempo change during playback, are described. Prior tempo adjustment schemes create undesired clicks and pops at tempo changes, caused by jumping and skipping in the audio playback signal where such changes occur. Embodiments of the invention avoid undesired pops and clicks by maintaining contiguous audio data for playback during significant audio transient activity. Dynamic changes in tempo are implemented at specific points in the audio signal corresponding to local minimums in the fade-in and fade-out characteristics of the compression/expansion scheme. In one or more embodiments, the compression/expansion scheme is substantially pitch-independent. 
     In accordance with one or more embodiments of the invention, an audio signal is marked to define temporal slices of audio data. In a preferred embodiment, marking may be performed to minimize significant transient activity midway between consecutive marks. A fade-in function is associated with the leading side of each mark, and, similarly, a fade-out function is associated with the trailing side of each mark, creating a series of cross-fading “mounds” with peaks at each mark. “Cross-fading” refers to the overlapping of the fade-out associated with each mound with the fade-in of a following mound to smooth the transition between respective transient activity associated with each mark. 
     In accordance with one or more embodiments, when a tempo change is requested (e.g., a user selects a new tempo value in a user interface), the embodiment delays implementing the tempo change until the start of the next “mound” (i.e., the next fade-in). Thus, despite the tempo change, each mound uses a contiguous set of audio data, preventing the clicks and pops associated with skips in the audio data. Cross-fading minimizes any effects of desynchronization caused by overlapping mounds of differing speeds. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIGS. 1A-1C  are waveform diagrams illustrating the behavior of a sample audio waveform over time. 
         FIG. 2A  is a waveform diagram illustrating a slicing method for parsing audio data at a constant rate, in accordance with one or more embodiments of the invention. 
         FIG. 2B  is a waveform diagram illustrating a slicing method for parsing audio data based on transient detection, in accordance with one or more embodiments of the invention. 
         FIG. 2C  is a waveform diagram illustrating a slicing method for parsing audio data based on musical characteristics, in accordance with one or more embodiments of the invention. 
         FIG. 3  is a process diagram illustrating a process for cross-fading within a slice of audio data, in accordance with one or more embodiments of the invention. 
         FIG. 4  is a flow diagram illustrating a method for processing audio data with dynamic tempo changes, in accordance with one or more embodiments of the invention. 
         FIG. 5  is a timing diagram illustrating time compression with a dynamic tempo change during playback of audio data, in accordance with one or more embodiments of the invention. 
         FIG. 6  is a timing diagram illustrating time expansion with a dynamic tempo change during playback of audio data, in accordance with one or more embodiments of the invention. 
         FIG. 7  is a flow diagram illustrating a method for processing audio data with dynamic tempo changes under compression and expansion conditions, in accordance with one or more embodiments of the invention. 
         FIG. 8  is a block diagram illustrating an embodiment of an audio processing system in which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The present invention is a method and apparatus for performing time compression and expansion of audio data, with dynamic tempo change during playback. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the invention. 
     Embodiments of the invention may include mechanisms or steps that provide substantial pitch independence in the process of altering the playback speed of audio data. For example, regions of audio data with greater influence on the listening experience (e.g., locations of greater transient activity and/or signal power) are identified, and, to the extent possible, the frequency characteristics of those audio regions are maintained regardless of the selected playback speed. Pitch variations can thus be avoided. 
     The original audio signal is processed as a sequence of transient events that may be pushed apart or compressed together as needed to meet the desired tempo. To avoid clicks and pops from instantaneous skips in the audio data, tempo changes are implemented only at the beginning of a new transient event. For example, when a tempo increase is signaled during a first transient event, the first transient is processed to completion without change. The leading edge of the following transient event, however, is moved closer to the first transient event (i.e., closer in time) to provide the increase in tempo. A cross-fading function provides smoothing of the transition between the trailing edge of the first transient event and the leading edge of its successor. 
     Parsing Audio Data into Slices 
     In one or more embodiments of the invention, audio data is processed in units of consecutive audio samples referred to herein as “slices.” The number of samples in each slice depends on the temporal length of the slice (e.g., the number of milliseconds in each slice), as well as the sample rate of the original audio data (e.g., 44 kHz=44,000 samples per second or 44 samples per millisecond). Embodiments of the present invention may be practiced with any slice length or sample rate. However, preferred criteria are that the length of each slice be sufficiently large to cause only minimal frequency distortion in the audible playback signal, yet sufficiently small to avoid any rhythmic distortion. This preferred criteria can be expressed as: f sound &gt;&gt; (slices per second) ≧f beat . For example, a typical slicing rate can be, but is not limited to, the range of 1-40 Hz (slices per second). 
     Embodiments of the invention implement a cross-fading scheme that maintains signal fidelity at the beginning and end of each slice, while sacrificing the fidelity of audio data in the middle of the slice, where necessary to modify playback tempo. Because fidelity of audio data in the middle of a slice may be reduced, it is preferable that the original audio data be parsed into slices that minimize the amount of significant transient activity near the middle of each slice. 
       FIGS. 2A-2C  illustrate three methods for parsing an audio data sequence into slices. In each of the parsing methods, the audio sequence is marked in some fashion to delineate slice boundaries. Each figure shows signal strength over time for an audio sequence  200 . Audio sequence  200  comprises transients (“transient events”)  201 - 210 , each transient representing, for example, a note played by an instrument. 
     In  FIG. 2A , audio sequence  200  is marked at an arbitrary constant rate. (e.g., 20 slices per second). The constant marking rate allows every slice to be treated similarly (e.g., no need to track the length of each slice in the original audio data). However, as shown in  FIG. 2A , the arbitrary selection of the marking rate (and phase) can result in the occurrence of significant transient activity in the center of some slices (e.g., transients  204 ,  207  and  208  begin in the middle of defined slices). Thus, as the tempo is changed, transients  204 ,  207  and  208  may experience some distortion due to cross-fading. 
     Marking schemes may also use detection schemes based on amplitude and/or frequency changes in the audio sequence.  FIG. 2B  illustrates marking of audio sequence  200  based upon the detection of transients. Transient detection uses power analysis to mark where the audio sequence has the largest changes in signal energy. Generally, the largest energy change corresponds to the beginning of a transient, also known as the “attack.” 
     As shown in  FIG. 2B , audio stream  200  is marked on or about the beginning of each of transients  201 - 210 . As opposed to the constant slice length used in  FIG. 2A , the transient detection of  FIG. 2B  results in varying slice lengths. In embodiments solely using transient detection to define slices, the length of each slice (or the marking positions) may be stored or tracked in memory to facilitate proper processing of each respective slice during playback. 
       FIG. 2C  illustrates marking audio sequence  200  into musical time slices. Because music typically has predictable rhythmic characteristics (apart from slight performance inflections), musical audio sequences are more amenable than random sound sequences to time-based parsing. For example, assuming that audio sequence  200  is one measure (a musical unit having a prescribed number of beats) of music in what is referred to as 4/4 time (i.e., four beats per measure, with a quarter note getting one beat), then slices may be defined by marks at intervals corresponding to the duration and phase of a small, music-based unit of time, such as a sixteenth note (one-sixteenth of a measure). A resolution corresponding to a sixteenth note is sufficient for most musical audio sequences, though it will be understood that other resolutions (e.g., thirty-second notes, etc.) may also be used in other embodiments of the invention. 
     Given an audio music sequence and an associated rhythm and time description (e.g., starting tempo of 120 beats per minute, 4/4 time, etc.), such as from meta data or user input, an audio processing program can approximate suitable marks in the audio sequence (e.g., the above example may be marked on the sixteenth note boundaries, with one slice every 125 milliseconds). In  FIG. 2C , the “attack” of each of transients  201 - 210  begins on or near the boundary of a slice (though the transients may or may not end near a slice boundary). Also, because the marks are based on constant slice lengths and not on actual transient occurrences, some slices contain no transients. 
     In addition to the individual parsing schemes shown in  FIGS. 2A-2C , a user&#39;s input may be used to specify slices, for example, by inputting or selecting, via a user interface in the audio processing system, a slice length in time or samples. Also, a graphic representation of the audio sequence, similar to that shown in  FIGS. 2A-2C ; may be displayed to a user, allowing a user to mark the sequence manually by, for example, clicking a mouse cursor on the sequence representation at a desired marking point along the time line. 
     Other embodiments of the invention may use parsing schemes beyond those previously described, or multiple parsing schemes may be combined. For example, transient detection may be used to insure that musical time slices are in proper phase, to extract an estimate of the initial tempo if one is not provided, or to combine empty slices with a preceding transient-filled slice to form a larger slice in a variable slice length implementation. 
     Cross Fading Within a Slice 
     As previously indicated, embodiments of the present invention use crossfading within each slice to seamlessly blend two transients together. The crossfading method uses a fade-in function, which begins at zero value and increases to a value of one, and a fade-out function, which begins at a value of one and decreases to zero value. In general terms, the fade-out function is used to scale the sample values of the trailing portion of the transient associated with the earlier marker. Similarly, the fade-in function is used to scale the sample values associated with the leading portion of the transient associated with the later marker. The scaled results of both functions are combined (e.g., using addition) to achieve the sample sequence for the output slice. 
     The actual fade-in and fade-out functions may vary for different embodiments. For example, the fade functions may be linear, exponential or non-linear. A preferred embodiment uses curves that approximate equal power over time when combined. The length of the fade-in and fade-out functions is generally equal to the output slice length. Some embodiments of the invention may use fade-in and fade-out lengths shorter than the output slice length, where some overlap of the fade-in and fade-out functions remains to provide the desired blending effect of the cross-fade. 
       FIG. 3  illustrates a sample application of a cross-fade to a slice of original sample data to create an output slice at four times the tempo (i.e., new slice length is one-fourth the slice length of original data). Elements  300  and  301  illustrate the fade-out and fade-in processes, respectively, whereas element  302  illustrates the process of combining the fade-in and fade-out results. 
     In fade-out process  300 , original data slice  303  (of length N samples) contains transient  311  associated with the left-most mark and transient  312  associated with the right-most mark. Transient  312  lies primarily in the following slice, but a small lead-in portion rests within slice  303 . The designated speed factor in this example is four (4.0). Thus, a new output slice region  304  is calculated as N/4 samples (i.e., original slice length/speed factor) in length. For the fade-out process, the fade-out function  305  is aligned with the beginning of the original slice  303 , with the fading completed within the new slice length of region  304  (i.e., completed N/4 samples from the beginning of slice  303  or within the first quadrant of original slice  303 ). Multiplying the data of the original slice  303  by the derived fade-out function  305  yields fade-out result  306 , which primarily contains a representation of the trailing portion of transient  311  forced to zero value within N/4 samples. Note that this process may change the duration of transient  311 , but it maintains the frequency characteristics of transient  311  that determine pitch. 
     In fade-in process  301 , a new output slice region  307  is calculated as N/4 samples, beginning N/4 samples before the right marker and completing on the right marker (i.e., the last quadrant of original slice  303 ). The fade-in function  308  is aligned with region  307 , with the fade-in completed by the end of slice  303 . Multiplying the data of the original slice  303  by the derived fade-in function  308  yields fade-in result  309  of length N/4 samples, which primarily contains a representation of the leading portion of transient  312 . 
     Combination process  302  obtains fade-out result  306  and fade-in result  309 , aligns them in time, and adds the fade-out and fade-in results together. The sum of the fade-out and fade-in results forms output slice  310 . Output slice  310  contains one-fourth the number of samples of original slice  303 , and thus provides playback at four times the speed of the original audio data, as desired in this example. Despite containing seventy-five percent less data than original slice  303 , output slice  310  retains the most significant transient activity of the original, with the associated frequency characteristics intact. 
     Dynamic Tempo Change During Audio Playback 
       FIG. 4  illustrates a general flow diagram of one embodiment of a process for playing back an audio sequence with dynamic tempo changes. The method shown assumes that parsing of the original audio sequence is completed before slice processing begins during playback. In other embodiments, the parsing may be performed one slice at a time and thus be embedded within a per-slice cross-fading loop (particularly if the parsing is performed at a constant rate that only requires incrementing a prior value by a constant value). Parsing may also be performed in a parallel computer application, process or thread that provides slice markers to the application, process or thread implementing cross-fades. 
     In step  400  of  FIG. 4 , the original audio data sequence or stream is parsed into time slices for processing, using, for example, one or more of the parsing schemes previously described. In step  401 , prior to beginning the crossfade processing loop, the value for the “end of first fade-in” sample location is initialized to the beginning of the first source slice. Also, an initial speed factor is determined (e.g., by program default or a preset user value). 
     Given the source slice length of the original audio data sequence and a current speed factor, the output slice length (e.g., in samples or time units) of the current slice is calculated in step  402 :
 
“output slice length”=“original slice length”/“speed factor” where
 
“speed factor”=“new tempo”/“original tempo”
 
     In step  403 , the fade-out of a current transient is calculated using the specified fade-out function and the output slice length as previously calculated. The original data read for the fade-out determination begins at the end of a fadein from the prior slice (i.e., at the left marker or slice boundary), so that there is no discontinuity in the sequence of data read. 
     In step  404 , the fade-in of the next transient is calculated using the specified fade-in function. The fade-in data read from the original audio sequence begins at the sample or time value corresponding to the right marker or slice boundary less the output slice length (i.e., the output slice length determines the read offset into the original data). The transition of initiating the fade in data is minimized by the fade-in function, making the initiation of a fade-in a suitable point in time to change speed or tempo of the playback. The revised read offset caused by the speed change is effectively hidden. 
     In step  405 , the fade-in and fade-out results of steps  403  and  404  are combined (via addition) to yield the destination audio data of the output slice. Steps  403 - 405  thus perform the desired cross-fade. For explanatory simplicity, this embodiment shows fade-in and fade-out calculations being performed to completion before combination occurs. Other embodiments may perform fadein, fade-out and combination calculations one sample at a time (as in the computer code example discussed below). 
     After the cross-fading of the current slice is complete, at step  406 , the playback process may query whether a new speed factor has been introduced by a speed change request during the processing of the current slice. If so, that new speed factor will take effect in the processing of the next slice. Alternatively, the speed change may be spaced over several slices (e.g., possibly, though not necessarily consecutive slices) for a smoother ramping up (or down) of tempo. 
     For example, a change from a speed factor of 1.2 to 4.8 may first transition from 1.2 to 2.4, then from 2.4 to 4.8 at a later slice. Any such one-step or multi-step speed transitions are within the scope of the present invention. 
     By checking for speed changes at the end of each slice, speed changes may be delayed up to one full slice length from when those changes are first requested. For most applications, this delay is of negligible consequence (e.g., delay on the order of 50 milliseconds). This delay insures that the speed change occurs at the beginning of a fade-in where a skip in read offsets is muted by the fade-in function. 
     After the speed factor query, if there are more slices to process, step  407  branches to step  408  where the next slice is designated as the new “current” slice, and the method flow returns to step  402  to begin processing the new slice. If, at step  407 , there are no further slices, then, at step  409 , if the audio playback is not set to create an audio loop, the method ends. However, if the audio playback is set to create an audio loop, then the first slice of audio data is again designated as the “current” slice, and processing continues at step  402 . 
     The following is a sample of computer pseudocode that implements steps  401 - 408  (i.e., slice processing for playback), in accordance with an embodiment of the invention. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 function float FadeInMultiplierFunction( position, length) 
               
               
                 { 
               
               
                  return sqrt( position/length); 
               
               
                 } 
               
               
                 function float FadeOutMultiplierFunction( position, length) 
               
               
                 { 
               
               
                  return sqrt( 1.0 − (position/length)); 
               
               
                 } 
               
               
                 function stretch( PositionMarkers[ ], SourceAudioData[ ], 
               
               
                 DestinationAudioData[ ]) 
               
               
                 { 
               
               
                  OutPosition = 0; 
               
               
                  EndOfLastFadeIn = 0; 
               
               
                  speed = getInitialSpeed( ); 
               
               
                  for n = 0 to number of PositionMarkers − 1 { 
               
               
                   OldSliceLength = PositionMarkers[n+1] − PositionMarkers[n]; 
               
               
                   NewSliceLength = OldSliceLength/speed; 
               
               
                   for i = 0 to NewSliceLength { 
               
               
                    AudioFadingOut = SourceAudioData[ EndOfLastFadeIn + i] * 
               
               
                    FadeOutMultiplierFunction( i, NewSliceLength); 
               
               
                    AudioFadingIn = SourceAudioData[ PositionMarkers[n+1] − 
               
               
                    NewSliceLength + i] * FadeInMultiplierFunction( i, 
               
               
                    NewSliceLength); 
               
               
                    DestinationAudioData[OutPosition] = AudioFadingOut + 
               
               
                    AudioFadingIn; 
               
               
                    OutPosition = OutPosition + 1; } 
               
               
                   EndOfLastFadeIn = PositionMarkers[n+1]; speed = 
               
               
                   GetNewSpeed( )} 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     In the above code segment, the functions “FadeInMultiplierFunction” and “FadeOutMultiplierFunction” represent the fade-in and fade-out functions, respectively, that are used to cross-fade the audio data. Those functions take a “position” value and a “length” value as inputs and generate a single floatingpoint value for multiplying with the audio data at the sample point designated by the integer “position.” The integer “length” specifies the length, in samples, of the entire fade function for the given slice. 
     The function “stretch” is the main loop for processing slices during playback. The function call for “stretch” has three arrays for parameters. The “PositionMarkers” array contains an array of sample numbers (integers) corresponding to parsing markers (i.e., slice boundary marks). For example, if PositionMarkers[ 0 - 2 ] contain the values “1”, “51” and “101”, then the first, second and third slices of audio data in the original audio sequence begin at sample 1, sample 51 and sample 101, respectively. A parsing function would fill this array with values prior to “stretch” being called. Some embodiments may not require that all marker values be stored in an array, e.g., because the marker values may be trivially determined using an incrementing mechanism. However, generalizing with the use of this array allows the code segment to handle parsing schemes with variable slice lengths. 
     The array “SourceAudioData” contains the original audio data sequence (e.g., floating-point sample values) indexed by sample number. Prior to calling “stretch”, “SourceAudioData” may be loaded with data from an audio file, or audio data created or captured in an audio application (possibly the same application containing “stretch”). 
     The array “DestinationAudioData” represents the processed audio data to be output during playback. The function “stretch” reads original audio data out of “SourceAudioData” and writes the cross-faded slice data into “DestinationAudioData”. The function “stretch” contains two nested loops. The outer loop steps through a new slice of “SourceAudioData” with each iteration, checking for a new “speed” value at the end of each cycle (may alternatively check at the beginning of each cycle). The inner loop steps through pairs of samples to be cross-faded, with the single sample result of each iteration written to “DestinationAudioData”. The data sample to be faded out is initially read from the current position marker location (i.e., beginning of the slice). Subsequent iterations of the inner loop cycle through consecutive samples in “SourceAudioData” for the length of the calculated output slice length, forming a contiguous sequence of read data from the fade-in data of the prior slice. The data sample to be faded in is initially offset in time from the right position. marker (i.e., the end of the slice) by the length of the new output slice. Further cycles read contiguous “SourceAudioData” samples for fade-in through the end of the slice. 
       FIG. 5  illustrates the application of a dynamic tempo change in accordance with one or more embodiments of the invention. In this example, as shown by speed control waveform  531 , the starting speed factor is 1.2, with a speed change input for a speed factor of 2.0 occurring during processing of slice  524 . (For example, control waveform  531  may be, but is not limited to, a realtime user input, a pre-programmed speed parameter, or an automated control parameter such as a synchronization system feedback signal.) Implementation of the speed change is withheld until processing of subsequent slice  525 . 
     In  FIG. 5 , waveform  500  represents a source audio data sequence parsed into four slices  523 - 526  having N samples each. Transients  505 - 508  are associated with slices  523 - 526 , respectively. Waveforms  501  and  502  illustrate cross-fade functions used to process audio sequence  500 . Waveform  503  illustrates output audio slices  527 - 530 , showing how the cross-fading functions correspond to those output slices. Waveform  504  represents the output audio waveform after processing. 
     Fade-out function  515  is applied to source audio data  500  from position marker number  1  to sample  510  (representing the length of one output slice given a speed factor of 1.2). Fade-in function  516  is applied to source audio data  500  from sample  509  through position marker number  2 . The results of the application of fade functions  515  and  516  are then combined within output slice  527 . 
     Similarly, in the processing of slice  524 , fade-out function  517  is applied to source audio data  500  from position marker number  2  to sample  512  (representing the length of one output slice given a speed factor of 1.2). Fade-in function  518  is applied to source audio data  500  from sample  511  through position marker number  3 . The results of the application of fade functions  517  and  518  are then combined within output slice  528 . During the processing of slice  524 , a request for a speed factor change (from 1.2 to 2.0) is recorded (see control waveform  531 ), but no speed adjustment action is taken during this slice. 
     In the processing of slice  525 , the new speed factor is taken into account. Fade-out function  519  is applied to source audio data  500  from position marker number  3  to sample  513  (representing the length of one output slice given the new speed factor of 2.0). Fade-in function  520  is applied to source audio data  500  from sample  513  through position marker number  4 . The results of the application of fade functions  519  and  520  are then combined within output slice  529 . 
     Likewise, fade-out function  521  is applied to source audio data  500  from position marker number  4  to sample  514  (representing the length of one output slice given a speed factor of 2.0). Fade-in function  522  is applied to source audio data  500  from sample  514  through position marker number  5 . The results of the application of fade functions  521  and  522  are then combined within output slice  530 . 
     As shown, the various fade-in and fade-out functions form arches or mounds approximately centered on each position marker and associated transient in the original audio sequence  500 . Conceptually, as the speed factor increases, the widths of the mounds become smaller, and the peaks of the mounds get closer together (as can be seen by the overlapping mounds within output slices  527 - 530 ). The opposite occurs when the speed factor is reduced. 
     In embodiments of the invention, speed changes are delayed so as to avoid changing speeds within any mound. Speed changes are recognized when mounds are at a minimum value (i.e., zero), to avoid audible skips. The instantaneous read offset that would normally cause a skip is instead implemented at the beginning of a fade-in, allowing the rest of the fade-in and fade-out of the mound to be completed with a contiguous sequence of samples from the source audio sequence. 
     In the example of  FIG. 5 , the speed change is requested during processing of slice  524 , but implementation of the speed change is delayed until the next fade-in ( 520 ) in slice  525 . The mound formed by fade functions  518  and  519  is asymmetrical because the output slice length changes with the speed change in slice  525 ; however, no read offset is incurred during fade-out  519 . This means that fade-out  519  and fade-in  520  use different speeds in calculating output slice  529 . This speed difference is imperceptible as it occurs only for a brief time (one slice) and it is cross-faded as usual. The following output slice ( 530 ) is fully synchronized. 
     Application to Time Expansion 
     The foregoing description of embodiments of the invention applies to speed changes wherein a single cross-fade per slice is sufficient to process the source audio sequence into destination slices. Audio compression (i.e., where the output slice length is smaller than the source audio slice length (speed factor &gt;1.0)) is satisfied by single cross-fades. However, where the speed factor is less than 1.0, the output slice length is larger than the source audio slice length. This means that the ‘source audio data must be expanded in time. While the previously described cross-fading schemes may be used for expansion (e.g., by permitting the fade-in and fade-outs to extend beyond the current slice boundaries), a variety of other expansion methods are also possible. 
     Expansion methods use a variety of schemes for filling the output slice with more data, such as repeating center portions of source audio slices or extending periods of near silence (where present). Examples of expansion schemes are disclosed in co-pending U.S. patent application Ser. No. 10/407,852, entitled “Method and Apparatus for Expanding Audio Data”, filed on Apr. 4, 2003, the disclosure of which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein. 
     In one or more embodiments of the invention, regardless of the means by which the source audio slice data is expanded, cross-fading is used to blend regions of the slice together. As with time compression, there is an initial fadeout at the beginning of the slice, which, consistent with the foregoing disclosure, is continued in a contiguous fashion from a fade-in at the end of the previous slice. A change in speed does not affect the contiguous nature of this cross-fading “mound” that overlaps slice boundaries. The change in speed is reflected, however, in determining the initial source data offset of each mound used to fill (i.e., expand) the middle portion of the new slice, as well as the source data offset of the fade-in performed at the end of the current slice. Consequently, as with the preceding compression examples, all mounds processed during playback expansion contain contiguous sequences of source data, minimizing clicks and pops associated with skips in the reading of data.  FIG. 6  illustrates the application of a dynamic tempo change, under time expansion, in accordance with one or more embodiments of the invention. In this example, as shown by speed control waveform  631 , the starting speed factor is 0.5, with a speed change input for a speed factor of 0.833 occurring during processing of slice  523 . Implementation of the speed change is withheld until processing of subsequent slice  524 . 
     In  FIG. 6 , waveform  500  represents a source audio data sequence parsed into four slices  523 - 526  having N samples each. Transients  505 - 508  are associated with slices  523 - 526 , respectively. Waveforms  600 ,  601  and  602  illustrate crossfade functions used to process audio sequence  500 . Waveform  603  illustrates output audio slices  627 - 628 , showing how the cross-fading functions correspond to those output slices. Waveform  604  represents the output audio waveform after processing. 
     Fade-out function  615  is applied to source audio data  500  from position marker number  1  to sample  611 , with the region from position marker number  1  to sample  610  at full gain and the region from sample  610  to sample  611  fading from 1.0 to 0.0. Fade-in function  616  is applied to source audio data  500  from sample  610  through position marker number  2 , with full fade-in achieved by sample  611 . Fill function  605 , comprising a fade-in from sample  609  to sample  610  and a fade-out from sample  610  to sample  611 , provides a mound of contiguous data from the relatively less significant portion of slice  523  for the purpose of expanding through replication. 
     The results of the application of functions  615 ,  616  and  605  are combined as needed to fill output slice  627 . In this example, the results corresponding to function  615  combine in a cross-fade with the results from fill function  605 . The results of fill function  605  are then repeated (two more times in this example) in a cross-fading manner. The fade-out of the last repetition of fill function  605  is then combined in a cross-fade with the results of function  616  to complete the output slice of the desired length. 
     Similarly, in the processing of slice  524 , fade-out function  617  is applied to source audio data  500  from position marker number  2  to sample  614 , with the region from position marker number  2  to sample  613  at full gain and the region from sample  613  to sample  614  fading from 1.0 to 0.0. Fade-in function  618  is applied to source audio data  500  from sample  613  through position marker number  3 , with full fade-in achieved by sample  614 . Fill function  606 , comprising a fade-in from sample  612  to sample  613  and a fade-out from sample  613  to sample  614 , provides a mound of contiguous data from the relatively less significant portion of slice  524 . 
     The results of the application of functions  617 ,  618  and  606  are combined as needed to fill the output slice  628 . In this example, the results corresponding to function  617  combine in a cross-fade with the results from fill function  606 . The fade-out of the results of fill function  606  is then combined in a cross-fade with the results of function  618  to complete the output slice of the desired length. The speed change that occurred during prior output slice  627  is processed in output slice  628 , shortening the output slice length so that only one copy of the results from function  606  are needed to complete the slice. 
     As with the single cross-fade processing scheme, the starting points for the final fade-in of a slice may vary with changes in the speed factor (i.e., changes in tempo). Further, the starting and ending points of the fill function (as well as the number of fill function replications required) can vary with changes in speed factor. Yet, because the speed change is delayed, and because the first fade-out of a new slice always begins where the final fade-in of the prior slice left off, all source-data read operations are made from contiguous sets of samples. Clicks and pops in the output are thus prevented. 
       FIG. 7  illustrates the flow of a method for time compression and expansion, in accordance with one or more embodiments of the invention. Steps  400 - 402 , as well as steps  406 - 410  are as described with respect to  FIG. 4 . However, after step  402  is completed, the present method inserts step  700 , wherein it is determined whether time compression or time expansion is appropriate for the current slice. For example, if the speed factor is greater than 1.0, then compression is in order, and steps  403 - 405  of  FIG. 4  are appropriate. If the speed factor is less than 1.0, then expansion begins with step  701 . 
     In step  701 , the leading portion of the source slice, starting from the end of the last fade-in, is copied to the output slice without fading. Referring to  FIG. 6 , the leading portion would be from position marker  1  to sample  610 . In step  702 , the number of replicated fill portions needed to fill the output slice length is determined. The replicated fill portion comprises the combination of the fade-in portion of function  605  (i.e., sample  609  to sample  610 ) overlapped with the fadeout portion of function  605  (i.e., sample  610  to sample  611 ). (Note that the fade-out portion of function  605  matches the fade-out portion of function  615 .) Various methods are possible for determining the size of the leading and replicating portions of the slice. One method, for example, uses a best fit analysis to fill an output slice with appropriately sized fill portions. 
     Steps  703  and  704  form a loop to continue performing cross-fades of the fill portions until the calculated number is reached. Then, in step  705 , the trailing portion of the source slice, from the last fade-in of the fill portion to the next position marker, is copied to the output slice. (This corresponds to combining the fade-out of function  605  with the fade-in of function  616 , when equal power fade functions are used.) With the slice completed, the flow returns to step  406  to continue as described above with respect to  FIG. 4 . 
     By delaying the implementation of a speed change until a following slice, the expected phase of the playback may be offset. Where phase is important, the compression and expansion implementations can be modified to overcompensate for the speed change during the first slice after the change. That is, where the speed factor changes from 1.2 to 2.0, a temporary speed factor of approximately 2.5 may be used in the first slice after the change to jump the phase forward. The speed and phase will thus be appropriate and consistent when the following slice “catches up.” One or more embodiments may track the time the change was requested to provide a closer estimate of the temporary speed factor needed. 
     Processing Environment Example 
     An embodiment of the invention can be implemented as computer software in the form of computer readable code executed on a general-purpose computer. Also, one or more elements of the invention may be embodied in hardware configured for such a purpose, e.g., as one or more functions of a dedicated audio processing system. 
     An example of a general-purpose computer  800  is illustrated in  FIG. 8 . A keyboard  810  and mouse  811  are coupled to a bi-directional system bus  818 . The keyboard and mouse are for introducing user input to the computer system and communicating that user input to processor  813 . Other suitable input devices may be used in addition to, or in place of, the mouse  811  and keyboard  810 . I/O (input/output) unit  819  coupled to bi-directional system bus  818  represents such I/O elements as a printer, A/V (audio/video) I/O, etc. Audio input may include a microphone, for example, and audio output may be, for example, a connection to speakers or external audio sound system (not shown). Audio I/O may also be carried out through a MIDI or other standard audio device interface. 
     Computer  800  includes video memory  814 , main memory  815  and mass storage  812 , all coupled to bi-directional system bus  818  along with keyboard  810 , mouse  811  and processor  813 . The mass storage  812  may include both fixed and removable media, such as magnetic, optical or magneto-optical storage systems or any other available mass storage technology that may be used for. example, to store audio files that represent input and/or output of an audio application executed by process  813 , as well as to store a persistent copy of the audio application itself. Bus  818  may contain, for example, thirty-two address lines for addressing video memory  814  or main memory  815 . The system bus  818  also includes, for example, a 64-bit data bus for transferring data between and among the components, such as processor  813 , main memory  815 , video memory  814  and mass storage  812 . 
     In one embodiment of the invention, the processor  813  is a microprocessor capable of executing computer readable program code such as an audio application. Main memory  815  may comprise, for example, dynamic random access memory (DRAM) that may be used to store data structures for computer program code executed by processor  813 . Video memory  814  may be, for example, a dual-ported video random access memory. One port of the video memory  814  is coupled to video amplifier  816 . The video amplifier  816  is used to drive the cathode ray tube (CRT) raster monitor  817 . Video amplifier  816  is well known in the art and may be implemented by any suitable apparatus. This circuitry converts pixel data stored in video memory  814  to a raster signal suitable for use by monitor  817 . Monitor  817  is a type of monitor suitable for displaying graphic images. Alternatively, the video memory could be used to drive a flat panel or liquid crystal display (LCD), or any other suitable data presentation device. 
     Computer  800  may also include a communication interface  820  coupled to bus  818 . Communication interface  820  provides a two-way data communication coupling via a network link  821  to a local network  822 . For example, if. communication interface  820  is an integrated services digital network (ISDN) card or a modem, communication interface  820  provides a data communication connection to the corresponding type of telephone line, which comprises part of network link  821 . If communication interface  820  is a local area network (LAN) card, communication interface  820  provides a data communication connection via network link  821  to a compatible LAN. Communication interface  820  could also be a cable modem or wireless interface. In any such implementation, communication interface  820  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  821  typically provides data communication through one or more networks to other data devices. For example, network link  821  may provide a connection through local network  822  to local server computer  823  or to data equipment operated by an Internet Service Provider (ISP)  824 . ISP  824  in turn provides data communication services through the data communication network now commonly referred to as the “Internet”  825 . Local network  822  and Internet  825  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  821  and through communication interface  820 , which carry the digital data to and from computer  800 , are exemplary forms of carrier waves transporting the information. 
     Computer  800  can send messages and receive data, including program code or audio data files, through the network(s), network link  821 , and communication interface  820 . In the Internet example, remote server computer  826  might transmit a requested code for an application program through Internet  825 , ISP  824 , local network  822  and communication interface  820 . 
     The received code may be executed by processor  813  as it is received, and/or stored in mass storage  812 , or other non-volatile storage for later execution. In this manner, computer  800  may obtain application code (or data) in the form of a carrier wave. 
     Application code may be embodied in any form of computer program product. A computer program product comprises a medium configured to store or transport computer readable code or data, or in which computer readable code or data may be embedded. Some examples of computer program products are CD-ROM disks, ROM cards, floppy disks, magnetic tapes, computer hard drives, servers on a network, and carrier waves. 
     The computer systems described above are for purposes of example only. An embodiment of the invention may be implemented in any type of audio processing system or audio playback environment. 
     Thus, a method and apparatus for performing time compression and expansion of audio data, with dynamic tempo change during playback, have been described in conjunction with one or more specific embodiments. The invention is defined by the claims and their full scope of equivalents. 
     Extensions and Alternatives 
     Alternative embodiments of the invention are described throughout the foregoing description, and in locations that best facilitate understanding the context of the embodiments. Furthermore, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of embodiments of the invention. Therefore, the specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 
     In addition, in this description certain process steps are set forth in a particular order, and alphabetic and alphanumeric labels may be used to identify certain steps. Unless specifically stated in the description, embodiments of the invention are not necessarily limited to any particular order of carrying out such steps. In particular, the labels are used merely for convenient identification of steps, and are not intended to specify or require a particular order of carrying out such steps.

Metadata:
Filing Date: 20070213
Publication Date: 20080916
Grant Date: 20080916
Priority Date: 20030404
Inventors: MOULIOS CHRISTOPHER
FRIEDMAN SOL
Assignee: APPLE INC
CPC Classifications: [{"code": "G10H2210/385", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10H2250/035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10H1/0091", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10H1/0091", "inventive": true, "first": true, "tree": "[]"}, {"code": "G10H2250/035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G10H2210/385", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 33097639