Patent Publication Number: US-6661430-B1

Title: Method and apparatus for copying an audiovisual segment

Description:
This application claims the benefit of U.S. provisional Application No. 60/046,822 filed on Nov. 15, 1996, the disclosure of which is incorporated herein by reference. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following U.S. patent applications: (1) U.S. patent application Ser. No. 08/947,771 (attorney docket No. FUT1P001) filed on the same day as the instant application, naming Eric T. Brewer, Andrew Palfreyman and Thomas S. Gilley as inventors, and entitled “Method and Apparutus For Editing Video Files”; (2) U.S. patent application Ser. No. 08/948,380 (attorney docket No. FUT1P002) filed on the same day as the instant application, naming Eric T. Brewer, Andrew Palfreyman, and Greg C. Ames as inventors, and entitled “Method and Apparatus For Synchronizing Edited Audiovisual Files”; (3) U.S. patent application Ser. No. 08/947,646 (attorney docket No. FUT1P003) filed on the same day as the instant application, naming Eric T. Brewer, and Andrew Palfreyman as inventors, and entitled “Method and Apparatus For Seeking within Audiovisual Files”; (4) U.S. patent application Ser. No. 08/948,352 (attorney docket No. FUT1P004) filed on the same day as the instant application, naming Eric T. Brewer, Andrew Palfreyman, and Thomas S. Gilley as inventors, and entitled “Method and Apparatus For Clipping Video Segments From Audiovisual File” and (5) U.S. patent application Ser. No. 08/940,350 (attorney docket No. FUT1P005) filed on the same day as the instant application, naming Eric T. Brewer, Andrew Palfreyman, and Thomas S. Gilley as inventors, and entitled “Method and Apparatus For Stitching Edited Video Segments.” All above identified applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to editing audiovisual files. More particularly, the invention relates to various methods and apparatuses for maintaining the audio component of a bit stream substantially synchronized with the video component after performing editing operations are discussed. 
     2. Description of the Related Art 
     MPEG (motion pictures experts group) is a standard promulgated by the International Standards Organization (ISO) to provide a syntax for compactly representing digital video and audio signals. The syntax generally requires that a minimum number of rules be followed when bit streams are encoded so that a receiver of the encoded bit stream may unambiguously decode the received bit stream. As is well known to those skilled in the art, a bit stream will also include a “system” component in addition to the video and audio components. Generally speaking, the system component contains information required for combining and synchronizing each of the video and audio components into a single bit stream. 
     Since the initial unveiling of the first MPEG standard entitled MPEG-1, a second MPEG standard known as MPEG-2 was introduced. In general, MPEG-2 provided an improved syntax to enable a more efficient representation of broadcast video. By way of background, MPEG-1 was optimized to handle data at a rate of 1.5 Mbits/second and reconstruct about 30 video frames per second, with each frame having a resolution of 352 pixels by 240 lines (NTSC), or about 25 video frames per second, each frame having a resolution of 352 pixels by 288 lines (PAL). Therefore, decoded MPEG-1 video generally approximates the perceptual quality of consumer video tapes (VHS). In comparison, MPEG-2 is designed to represent CCIR 601-resolution video at data rates of 4.0 to 8.0 Mbits/second and provide a frame resolution of 720 pixels by 480 lines (NTSC), or 720 pixels by 576 lines (PAL). For simplicity, except where distinctions between the two versions of the MPEG standard exist, the term “MPEG,” will be used to reference video and audio encoding and decoding algorithms promulgated in current as well as future versions. 
     Typically, a decoding process begins when, an MPEG bit stream containing video, audio and system information is demultiplexed by a system decoder that is responsible for producing separate encoded video and audio bit streams that may subsequently be decoded by a video decoder and an audio decoder. Attention is now directed at the structure of an encoded video bit stream. Generally, an encoded MPEG video bit stream is organized in a distinguishable data structure hierarchy. At the highest level in the hierarchy is a “video sequence” which may include a sequence header, one or more groups of pictures (GOPs) and an end-of sequence code. GOPs are subsets of video sequences, and each GOP may include one or more pictures. As will be described below, GOPs are of particular importance because they allow access to a defined segment of a video sequence, although in certain cases, a GOP may be quite large. 
     Each picture within a GOP is then partitioned into several horizontal “slices” defined from left to right and top to bottom. The individual slices are in turn composed of one or more macroblocks which identify a square area of 16-by-16 pixels. As described in the MPEG standard, a macroblock includes four 8-by-8 pixel “luminance” components, and two 8-by-8 “chrominance” components (i.e., chroma red and chroma blue). 
     Because a large degree of pixel information is similar or identical between pictures within a GOP, the MPEG standard takes particular advantage of this temporal redundancy and represents selected pictures in terms of their differences from a particular reference picture. The MPEG standard defines three general types of encoded picture frames. The first type of frame is an intra-frame (I-frame). An I-frame is encoded using information contained in the frame itself and is not dependent on information contained in previous or future frames. As a result, an I-frame generally defines the starting point of a particular GOP in a sequence of frames. 
     A second type of frame is a predicted-frame (P-frame). P-frames are generally encoded using information contained in a previous I or P frame. As is well known in the art, P frames are known as forward predicted frames. The third type of frame is a bi-directional-frame (B-frame). B-frames are encoded based on information contained in both past and future frames, and are therefore known as bi-directionally predicted frames. Therefore, B-frames provide more compression that both I-frames and P-frames, and P-frames provide more compression than I-frames. Although the MPEG standard does not require that a particular number of B-frames be arranged between any I or P frames, most encoders select two B-frames between I and P frames. This design choice is based on factors such as amount of memory in the encoder and the characteristics and definition needed for the material being coded. 
     Although the MPEG standard defines a convenient syntax for compactly encoding video and audio bit steams. Audio synchronization difficulties arise when a copied audiovisual bit stream segment is joined with another copied audiovisual bit stream segment. The synchronization problem is partially due to the fact that audio frames and video frames rarely have a one-to-one correlation. Therefore, when a segment of video frames is identified for copying from a file, the identified video frames will not have a pre-determined number of audio frames that correspond to the identified video frames. 
     Consequently, when a segment of video is copied from a file and then subsequently joined to another copied segment, the audio component of the copied segment may not be synchronized with the proper video frame. Once the video and audio frames are no longer synchronized, an “error” representing the number or percentage of an audio frame for which the video and audio frames fail to be synchronized is introduced into the resulting bit stream. By way of example, the synchronization error introduced from two bit stream segments being joined may be as little as a fraction of an audio frame, to as large as a few audio frames. 
     Although the error associated with joining only two bit stream segments may in certain cases only be a few audio frames, when a multiplicity of bit stream segments are joined in a more sophisticated editing task, the errors for each joined segment are summed. Therefore, the resulting error may be quite large, and the resulting audio frames may be severely un-synchronized and fail to make sense upon playback. Further, un-synchronized audio and video bit streams typically produce audio discontinuities at the bit stream locations where segments are joined. This problem is commonly described as a “popping” sound. Thus, as discontinuities are introduced to joined bit stream segments, discomforting popping sounds are introduced causing the resulting audio stream to not only be un-synchronized, but also intolerable. 
     In view of the foregoing, what is needed are methods and apparatuses for editing audio and video bit streams while ensuring that the audio component remains substantially synchronized with the video component. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing in accordance with the purpose of the present invention, methods and apparatuses for maintaining edited audiovisual files substantially synchronized during editing operations performed through the use of an editing engine are disclosed. Preferably, the editing engine performs editing operations in two passes through an edit list. In one embodiment, the edit list may contain a number of copying requests instructing the editing engine to create a copy operator for copying segments of audio and video from certain files. To initiate copy operations, the editing engine preferably performs a first pass where the copied segments of an audio and video have an audio component that is preferably longer in time than the video component. 
     In another embodiment, a predetermined number of audio frames at each end of the copied audio segment may be decoded and re-encoded to generate glue frames which may provide, e.g., sound fading and blending effects. Once the copied segments of audio are processed in the first pass, the editing engine will initiate a second pass through the editing list to stitch together (i.e., join) the processed audio and video segments into a single file. Advantageously, during the stitching operation, frames at the ends of each copied audio segment (i.e., tab-in and tab-out audio frames) may be dropped or retained in order to maintain the audio component in the newly created audiovisual file substantially synchronized with the video component. Therefore, the newly created file is advantageously made up of one or more audiovisual segments that preferably has an audio component that is no more than about half an audio frame in error. 
     In yet another embodiment, a method for copying a segment from an audiovisual file having a multiplicity of audio frames and a multiplicity of video frames is disclosed. In a first step, a mark-in location in a video file is selected to correspond to a first video frame in the segment such that the first video frame has an associated start time. Next, a mark-out location in the video file is selected to correspond to a last video frame in the segment, and the last video frame having an associated end time. Once the mark-in video frame is selected, a first audio frame having a first audio frame start time that is at least as early as the first video frame start time is designated as an initial audio frame. A second audio frame having a second audio frame start time that is at least as late as the last video frame end time is designated as the last audio frame. The audiovisual file is copied to include a video portion extending from the first video frame to the last video frame and an audio portion extending from the initial audio frame to the last audio frame. In this manner, the audio portion of the segment may preferably be longer than the video portion of the copied segment. 
     In still another embodiment, a method of stitching a first and second audiovisual segment together is disclosed. In this embodiment, each audiovisual segment has a multiplicity of audio frames including a first audio frame, a second audio frame that sequentially follows the first audio frame and a last audio frame. The audiovisual segment further includes a multiplicity of video frames having a first video frame and a last video frame. The method includes the step of aligning an initial audio frame in the first audiovisual segment with the first video frame in the first audiovisual segment. The first audio frame from the first audiovisual segment is designated as the initial audio frame when a tab error associated with the first audio frame from the first audiovisual segment is less than about half a frame. On the other hand, the second audio frame from the first audiovisual segment is designated as the initial audio frame when a tab error associated with the first audio frame from the first audiovisual segment is greater than half a frame. Stitching the first and second audiovisual segments together. 
     In another embodiment, a method of joining a first and a second audiovisual segment together while maintaining substantial audio to video synchronization is disclosed. Each audiovisual segment having a multiplicity of audio frames including a first audio frame, a second audio frame that sequentially follows the first audio frame and a last audio frame. A multiplicity of video frames including a first video frame and a last video frame are also disclosed. In this embodiment, the method includes a step of aligning an initial audio frame in the first audiovisual segment with the first video frame in the first audiovisual segment. Preferably, the first audio frame from the first audiovisual segment is designated as the initial audio frame when a tab error associated with the first audio frame from the first audiovisual segment is less than about half an audio frame. Further, the second audio frame from the first audiovisual segment is designated as the initial audio frame when a tab error associated with the first audio frame from the first audiovisual segment is greater than about half an audio frame. On the other hand, the first audio frame from the first audiovisual segment is dropped when the second audio frame from the first audiovisual segment is designated as the initial audio frame. The method further includes determining whether a cumulative error associated with the last audio frame in the first segment exceeds half a frame, and dropping the last audio frame in the first segment when it is determined that the cumulative error associated with the last audio frame exceeds half a frame. The method then determines whether a cumulative error associated with the first audio frame in the second segment exceeds about half a frame, and dropping the first audio frame in the second segment when it is determined that the cumulative error associated with the first audio frame exceeds about half a frame. 
     Although the advantages are numerous, a particular advantage of this invention is that the stream error is prevented from exceeding about half an audio frame, and the video frames are substantially synchronized with the audio frames without regard to the number of segments being stitched together after successive copy operations. It should also be appreciated that if corrections were not made by dropping or retaining audio frames in the second pass as described above, the cumulative stream error would grow and propagate as additional audiovisual segments are stitched together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1A shows a number of exemplary audiovisual frame sequences used to describe the processing steps associated with generating an audio component that is substantially synchronized with the video component in accordance with one embodiment of this invention. 
     FIG. 1B is an exemplary audiovisual segment copied from the display order stream of FIG. 1A in accordance with one embodiment of the present invention. 
     FIG. 2 is a data flow architecture used for editing audiovisual files in accordance with one embodiment of this invention. 
     FIG. 3 is an overview flowchart identifying the preferred steps of editing audiovisual files in accordance with one embodiment of the present invention. 
     FIG. 4 is a flowchart illustrating a method of optionally generating glue segments for any suitable operator in accordance with one embodiment of the present invention. 
     FIG. 5 is a flowchart illustrating the method steps associated with executing a copy operator in accordance with one embodiment of the present invention. 
     FIG. 6 is a flowchart illustrating the method steps associated with outputting a middle-glue of FIG. 5 in accordance with one embodiment of the present invention. 
     FIG. 7 is a flowchart illustrating the method steps associated with outputting an in-glue of FIG. 5 in accordance with one embodiment of the present invention. 
     FIG. 8 is a flowchart illustrating the method steps associated with outputting an out-glue of FIG. 5 in accordance with one embodiment of the present invention. 
     FIG. 9 is an overview flowchart of the method steps associated with creating the requested output stream during a second pass of an editing operation in accordance with one embodiment of the present invention. 
     FIG. 10 is a more detailed description of the method steps associated with multiplexing data pulled from input sources in accordance with one embodiment of the present invention. 
     FIG. 11 is a general description of the method steps performed by stitcher objects in accordance with one embodiment of the present invention. 
     FIG. 12 is a more detailed description of the method steps performed by stitcher objects in accordance with one embodiment of the present invention. 
     FIG. 13 is a flowchart illustrating the method steps associated with processing tabs in accordance with one embodiment of the present invention. 
     FIG. 14 is a diagrammatic illustration of a plurality of audiovisual segments being stitched together in accordance with one embodiment of the present invention. 
     FIG. 15 shows a table illustrating a plurality of tab processing calculations in accordance with one embodiment of the present invention. 
     FIG. 16 is a diagram illustrating the audio frame errors after tabs are processed in accordance with one embodiment of the present invention. 
     FIG. 17 is a block diagram of an exemplary computer system for carrying out the editing steps in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Broadly speaking, the present invention discloses methods and apparatuses for maintaining edited audiovisual files synchronized during editing operations performed through the use of an inventive editing engine. Preferably, the editing engine performs editing operations in two passes through an edit list. Generally, an edit list may be provided by an application requesting that certain operations be performed on a number of files containing MPEG audio and video data. In one embodiment, the edit list may contain a number of copying requests instructing the editing engine to create a copy operator for copying audiovisual segments from certain files. 
     To initiate copy operations, the editing engine preferably performs a first pass where the audiovisual segment will have an audio component that is preferably longer in time than the video component. In another embodiment, a predetermined number of audio frames at each end of the copied audio segment may be decoded and re-encoded to generate glue audio frames which may provide, e.g., sound fading and blending effects. Once the copied segments of audio are processed in the first pass, the editing engine may initiate a second pass through the editing list to stitch together (i.e., join) the processed audio and video frames into one file. Advantageously, during the stitching operation, frames at the ends of each copied audio segment may be dropped or retained in order to maintain the audio component in the newly created file substantially synchronized with the video component. Therefore, the newly created file is preferably made up of one or more copied audiovisual segments. In one embodiment, the new file preferably has an audio component that is no more than about half an audio frame off from being exactly synchronized with the video component. 
     FIG. 1A shows a number of exemplary audio and video frame sequences used to describe the processing steps associated with generating an audio component that is substantially synchronized with the video component. An exemplary encode order stream  50  of video frames are presented to illustrate the order in which video frames are encoded after being processed in accordance the MPEG standard format. By way of example, in encode order stream  50 , the first frame is an I-frame which is followed by a P-frame, a B-frame, a B-frame, a P-frame, a B-frame, a B-frame, a B-frame, etc. Although the editing algorithm of this invention may process a sequence of frames in any suitable arrangement, the editing algorithm of this invention preferably processes frame sequences in a display order. Therefore, before processing operations are performed, the encode order stream  50  is converted into a display order stream. 
     Thus, a frame stream arranged in temporal order from frame  0  to  36  identifies the order in which frames are processed in a display order stream  52 . For comparison, the corresponding temporal order of the frames in encode order stream  50  are illustrated under the corresponding frames. Of course, it should be understood that display order stream  52  is merely exemplary, and other suitable display order streams may also be suitably processed in accordance with the teachings of this invention. 
     When a segment of video frames are copied from display order stream  52 , a mark-in location and a mark-out location is selected to identify the number of video frames being copied. By way of example, a mark-in location is selected at frame  9 , which is a P-frame, and a mark-out location is identified as frame  28 , which is a B-frame. Accordingly, the segment of frames copied from display order stream  52  will include frames  9  through  28 . As shown, the identified segment will also include associated audio frames. 
     As is well known in the art, each audio frame may vary in size depending on the type of MPEG audio layer being copied. The MPEG audio standard specifically identifies three layers, each layer having an associated frame rate and a variety of identifying characteristics. By way of example, MPEG layer  2  audio may have frame rates between about 28 and 38 frames per second. Other exemplary characteristics may include an audio mode (e.g., stereo, mono, surround sound, etc.), and a sampling frequency (e.g., 32 kHz, 44.1 kHz and 48 kHz). As described in the MPEG audio documents, each audio frame preferably include an associated header which identify the particular characteristics of the audio samples that follow each header. However, for ease of illustration, the audio frames will be described as pure pulse code modulation (PCM) audio samples. 
     As illustrated in display order stream  52 , exemplary audio frames are shown lying under their associated video frames. The pictorial audio and video frame representation is used to identify the “time” positioning of audio frames with respect to the associated video frames in a representative MPEG bit stream. 
     FIG. 1B shows an audiovisual segment  60  after it has been copied from display order stream  52  of FIG. 1A in accordance with one embodiment of the present invention. As shown, video frames  9  through  28  and an initial audio frame  56  and an end audio frame  62  were copied from display order stream  52 . During the initial copying step, the copied audio segment preferably occupies a longer length of time than the copied video segment. 
     As will be described in greater detail below, once frame  9  is identified as the mark-in video frame, a determination is made to copy audio frames such that the beginning time of the initial audio frame  56  is the same as a start time  54  of the mark-in frame  9  or earlier. Similarly, once frame  28  has been identified as the mark-out video frame, a determination is made to copy audio frames such that the beginning time of the end audio frame  62  is the same time as a end time  53  of the mark-out frame  28  or earlier. 
     In simple terms, if an audio frame does not perfectly align with the start time  54  of the mark-in video frame  9  or the end time  53  of the mark-out video frame  28 , then the initial audio frame  56  will have an earlier start time than the start time  54  of the mark-in video frame, and the end audio frame  62  will have an earlier start time than the end time  53  of the mark-out video frame  28 . In this example, audio frame  56  will be selected as the initial audio frame and audio frame  62  will be selected as the end audio frame. It is of particular importance to appreciate that audio frame  64  has a start time that is later than the end time  53  of mark-out video frame  28 , and is therefore not copied. Accordingly, only audio frames up to audio frame  62  are copied during the first pass. 
     FIG. 2 is a data flow architecture  100  used for editing audiovisual files in accordance with one embodiment of this invention. As shown, a similar architecture (e.g., shown as shadowed objects) is used for editing the video component of a file. As described in a co-pending related application, video files may be edited in parallel using the shadowed architecture described herein. For a detailed description of the methods and apparatuses used for editing video files, reference may be made to the above incorporated by reference related U.S. patent applications: (1) Ser. No. 08/947,771; (2) Ser No. 08/948,352; and Ser. No. 08/948,350. 
     The data flow architecture  100  is preferably driven by an editing engine referred to as MEDIT engine  102  which is capable of performing a number of editing tasks. By way of example, such tasks may include copy operations requesting that a segment from a source or input stream file be copied for use in another file. Other suitable editing tasks may include a fade operation, a blend operation, a morphing operation, a titling operation, a text annotation operation, etc. In general, MEDIT engine  102  is a dynamic engine that is capable of managing numerous editing tasks which may vary depending on the types of operators provided by an application requesting an editing task. It should therefore be understood that, MEDIT engine  102  may manage any number of operator types, including operator “plug-ins” provided by future applications requesting sophisticated editing tasks. 
     As an overview, the following discussion will provide a general description of the processing steps taken by MEDIT engine  102  in performing editing tasks such as copying an audiovisual segment from a source file. Generally, a copy operation is initiated when an application  106  requests that a copy operation be performed. 
     Initially, application  106  will provide MEDIT engine  102  with a suitable edit list  108  that includes a number of “channel operators”  110  identifying the number of channels requiring some type of editing, “function operators”  112  identifying the type of editing functions requested by application  106 , and an “end operator”  114  identifying the end of an editing request. In the example shown, the function operators  112  identify “copy” requests. By way of example, the first copy request identified in function operators  112  is a request for copying frames  9  through  28  in a file called A.MPEG for channel  1 . As shown, there may be numerous other copy requests in function operators  112  leading up to a request for copying frames  10  through  25  in a file called B.MPEG for channel N. Of course, once the video frames are identified for copying, the associated audio frames are preferably selected for copying as described above. 
     Once MEDIT engine  102  receives edit list  108 , the copy requests are processed in two identifiable passes through edit list  108 . In a first pass, MEDIT engine  102  walks through edit list  108  selecting the correct number of audio frames such that the audio component is longer in time than the video component. Preferably, the initial audio frame is selected to begin at or before the start time of the mark-in video frame, and the end audio frame is selected to begin at or before the end time of the mark-out video frame. For ease of description, the initial audio frame will be referred to as a “tab-in” audio frame and the end audio frame will be referred to as the “tab-out” audio frame. 
     Once the appropriate tab-in and tab-out frames are selected, and the number of audio frames in a copied segment may be ascertained, and the copy operator may process (i.e., decode and re-encode) a predetermined number of audio frames beginning with the tab-in frame to generate in-glue segments, and may also process a predetermined number of audio frames up to the tab-out frame to generate out-glue segments. Once any glue segments are generated for the copied audio segment, the glue segments are stored in an appropriate storage medium  140 . It should be understood that storage medium  140  may be any suitable storage medium such as a cache memory, a computer hard drive, floppy disk, or a remotely located storage medium connected by a suitable network. 
     In the second pass, the MEDIT engine  102  may make use of the previously generated glue segments by joining the glue segments and un-processed audio frame segments (i.e., middle glue) with the aid of a plurality of stitcher objects  147  and  148  that are created by MEDIT engine  102 . As will be described in greater detail below, a stitcher object will be created for each channel in edit list  108 , and each created stitcher object associated with a particular channel is responsible for walking through edit list  108  and joining glue segments for its own channel (e.g., ignoring information associated with other channels). 
     In this manner, multiple stitcher objects may be created such that each channel identified in edit list  108  has its own stitcher object. In a preferred embodiment, each stitcher will be responsible for joining the particular glue segments in a proper time ordered manner, such that each generated segment is time stamped to generate an appropriate audio sequence. Further, each created stitcher object uses a glue object such as glue objects  130  and  131  to pull the glue segments from the previously generated in-glue or out-glue files, or retrieve the middle glue from the original file by using pointers which identify the location of the middle-glue segment. With reference to audiovisual segment  60  of FIG. 1B, if audio frames  56 ,  58 ,  59 , and  61  were decoded and re-encoded to generate an in-glue segment, and audio frames  66 ,  65 ,  63  and  62  were decoded and re-encoded to generate an out-glue segment, the remaining frames lying between audio frames  61  and  66  will represent an exemplary middle-glue segment. Once the stitched frame data is output as a program elementary stream (PES) to a multiplexer  150 , the multiplexer  150  will pull PES data from all of the created stitchers (i.e., input sources) and output the copied segments to application  106  through MEDIT engine  102 . 
     To illustrate the overall data flow of FIG. 2, assume application  106  requests a copy operation of video frames  9  through  28  from A.MPEG file  124  (i.e., display order stream  52  of FIG. 1A) from channel  1 . As MEDIT engine  102  walks through edit list  108  during a first pass, MEDIT engine  102  determines whether audio glue segments have already been generated and stored in a glue file  126  during a previous editing request. Assuming that no audio glue segments already exist for a copy operation of video frames  9  through  28  from A.MPEG file  124 , MEDIT engine  102  will create copy operator  104  which creates a control object  111  (e.g., control object). 
     In this embodiment, control object  111  uses a seek engine  118  to locate the appropriate video frames identified for copying data from A.MPEG file  124 . For a more detailed description on suitable seeking engines, reference may be made to a related U.S. patent application Ser. No. 08/947,646, which is hereby incorporated by reference. 
     Once the appropriate frames are located and the appropriate number of audio frames including the tab-in and tab-out frames have been selected, a decoder  120  may decode a predetermined number of audio frames beginning with the tab-in or ending with the tab-out audio frame. Generally speaking, the audio glue frames represent audio frames that are processed to introduce audio effects such as, e.g., fading “to or from” zero. Further, it should be understood that generating “in and out” glue segments is an optional process step that may be implicitly set by the parameters in the copy operator  102  or may be expressly requested by the parameters sent by the application  106 . Therefore, if glue generation is required, a predetermined number of audio frames may be decoded one frame at a time by decoder  120 . It should be understood that a decoder buffer used should be managed in order to satisfy decoding requirements defined in the MPEG standard. 
     Once an audio frame is decoded, the decoded data is sent to copy operator  104 . Copy operator  104  then sends the decoded data to another control object  113  (e.g., control object) created by copy operator  104  and having an encoder  114 . At this point, encoder  114  re-encodes the audio frame data into an appropriate format and calls a glue object  116  that stores re-encoded audio frames into a glue file. As shown, the glue file is preferably stored in storage medium  140  which may be cache memory. Once all of the predetermined number of audio glue frames are optionally decoded and re-coded for each of the in-glue and the out-glue segments, the segments are stored in an appropriate glue file such as A.MPEG glue file  126 . 
     It should be appreciated, that MEDIT engine  102  will generally create separate copy operators for each copy request in edit list  108 . Therefore, the second copy operation request in the edit list (i.e., video frames  10  through  50  from B.MPEG file, channel N) is preferably processed by a separate copy operator  104  which will in turn create a new control object  111  for its own seeking and decoding functions, and a new control object  113  for re-encoding and transferring the generated glue frames to its corresponding glue file that may be stored within storage medium  140 . 
     In one embodiment, execution of each copy operator may be processed by multiple processing units in a parallel format which advantageously expedites any editing requests identified in edit list  108 . Further, parallel processing is facilitated since there is no set evaluation order in the edit list, and each editing operation may be performed independently of each other. In a further embodiment, multiple processing may be accomplished through the use of internet video servers. As is well known in the art, internet video servers may be used to simultaneously process editing requests in edit list  108 . 
     Referring still to FIG. 2, once appropriate glue files are generated for each copy request in edit list  108 , MEDIT engine  102  will walk through edit list  108  in a second pass to create stitcher objects such as stitcher objects  147  and  148  for each channel identified in edit list  108 . Although only two stitcher objects are shown created for channel  1  and cannel N, it should be understood that there may be any number of stitcher objects created depending on the number of channels identified in edit list  108 . By way of example, in some embodiments, edit list  108  may contain stitcher objects for multiple channels up to about 8,000 audio channels and about 4,000 video channels under an MPEG-2 platform. 
     Once a stitcher object is created for each channel, each stitcher object  147  and  148  will preferably create glue objects  130  and  131 . In this embodiment, each stitcher object will walk through the edit list searching for editing requests for its associated channel. By way of example, stitcher  147  will walk through edit list  108  to identify editing requests for channel  1 , and likewise, stitcher  148  will walk through edit list  108  to identify editing requests for channel N, and so on. Once glue objects  130  and  131  are created, glue objects  130  will to provide each stitcher  147  and  148  with glue data that may have been generated during the first pass. 
     In this example, glue object  130  is charged with retrieving the various glue segments for the copied segment. By way of example, glue object  130  may retrieve glue data stored in A.MPEG glue file  126  and provide it to stitcher  147 . Further, if any middle-glue data (i.e., un-processed portion of the copied segment) is required, glue object  130  will use pointers  134  to a streamer  122  controlled by control object  111 . In this manner, glue object  130  will be able to retrieve the correct number of audio frames from the A.MPEG file  124 . In this embodiment, middle-glue may be associated with audio frames lying between audio frame  61  and audio frame  66  in copied segment  60  of FIG.  1 B. Of course, if glue segments are not generated for the audio frames, all of the audio frames beginning with the tab-in audio frame and ending with the tab-out audio frame will be identified as middle-glue. 
     Therefore, as each stitcher  147  and  148  requests glue data, glue objects  130  and  131  will retrieve the data from the appropriate location. As each stitcher receives requested data in a time ordered manner, each stitcher will transfer PES data streams to a MUX unit  150  that multiplexes the received PES data streams and sends a single audiovisual multiplexed stream to application  106  through MEDIT  102 . 
     FIG. 3 is an overview flowchart identifying the preferred method steps for editing video files in accordance with one embodiment of the present invention. The method begins at a step  300  where MEDIT engine receives an edit list. As described above, an edit list will generally contain a number of channel operators that identify the number and type of channels required for a particular editing request. For example, there are typically separate channels for both audio as well as video. There may also be a number of separate video channels and a number of separate audio channels. 
     Referring to FIG. 2, once application  106  sends an edit list  108  to MEDIT engine  102 , the method will proceed to a step  302  where the audio frames including the tab-in and tab-out frames are identified, and glue segments may be generated for each copy request in edit list  108  if requested. If glue segments are requested, there may be any number of glue segments for a particular copy operation in edit list  108 . Thus, the audio glue segments may include in-glue, middle-glue (i.e., “un-processed” audio frames ) and an out-glue. If glue segments are generated for a predetermined number of audio frames, then the generated glue segments are preferably stored as “in or out” glue files for use in the second pass, and in future editing operations. 
     Thus, if the same range of frames is copied in a future editing operation, the previously generated glue segments may be re-used. Advantageously, this avoids having to inefficiently re-generate the same glue files from scratch. In fact, the glue segment files may be distributed throughout a network and be retrieved upon a requested editing operation. 
     Once the appropriate glue segments have been generated and stored to an appropriate memory location (e.g., cache memory), the method proceeds to a step  304  where the requested output stream is created during the second pass by the MEDIT engine  102  shown in FIG.  2 . As shown, multiple stitcher objects are created such that each channel identified in the edit list will have its own stitcher object, and each stitcher object may walk through the edit list requesting data for each function operator in the edit list. Thus, each stitcher object will be responsible for pulling data from various audio glue files with the aid of a glue manager (i.e., glue objects  130  and  131 ). In this manner, each stitcher will receive data from the glue objects, and then a multiplexing unit  150  will request PES stream data from each stitcher. 
     As the multiplexer pulls data from the associated stitcher objects, the multiplexer also sends the multiplexed data to the application via the MEDIT engine  102 . It should be understood that the stream output by the multiplexer may be audio, video or a multiplexed combination of video and audio data. Once the requested output stream has been sent to the application in step  304 , the method is complete. FIGS. 4 through 17 will now be used to provide a more detailed description of the method steps associated with advantageously generating an edited output stream that maintains the audio component substantially synchronized with the video component. 
     FIG. 4 is a more detailed illustration of the method steps associated with generating glue for any suitable operator in accordance with one embodiment of the present invention. Initially, the MEDIT engine will walk through an edit list that may be provided by an application. Generally, the method begins at a step  310  where the MEDIT engine obtains the next entry in the edit list. Once the MEDIT engine has the current entry in the edit list, the method proceeds to a decision step  312 . At decision step  312 , it is determined whether the current entry in the edit list is an “END” operator. If the current entry is an END operator, the method of FIG. 4 will be done. 
     If the current entry is not an END operator, the method will proceed to a second decision step  314  where a determination is made as to whether glue exists, if glue is required for a current entry in the edit list. If required glue already exists for the current entry in the edit list, the method will proceed back to step  310  where the next entry in the edit list is processed as described above. On the other hand, if in step  314 , it is determined that glue does not exist or is not required for the current entry, the method will proceed to a step  316  where an operator is created by MEDIT for the current entry. Of course, the type of operator created will depend on the type of entry in the edit list. By way of example, if the entry is a copy request, then a “copy operator” will be created as described in FIG.  2 . 
     It should therefore be appreciated that any suitable operator may be created by MEDIT depending on the type of editing request provided in an edit list. By way of example, suitable editing operators may include blend operators, fade operators, morphing operators, titling operators, and text annotation operators. Further, new operators may be created by MEDIT in the future depending on the type of “plug-in” operators installed by applications making use of the MEDIT editing engine of this invention. 
     Once the appropriate operator is created in step  316 , the method will proceed to a step  318  where the operator is executed to generate appropriate audio segments and generate any requested glue segments for the particular type of function in the edit list. A more detailed description of the method steps associated with executing an operator is described with reference to FIG.  5 . Once the appropriate audio segments and any glue segments are generated for the editing operation of step  318 , the method proceeds to a step  320  where the operator is destroyed. Once the current operator is destroyed, the method will revert back to step  310 , where the next entry in the edit list is received and again processed through to step  320  as described above. 
     FIG. 5 is a more detailed description of the process steps associated with executing a copy operator in accordance with one embodiment of the present invention. The method begins at a step  402  where a mark-in video frame is identified. For ease of description, reference will be made to the exemplary display order stream  52  of FIG. 1A where the “mark-in” video frame is frame  9 . Once the mark-in frame is identified, a mark-out frame is identified in a step  404 . In this example, the mark-out frame is frame  28  as shown in FIG.  1 A. 
     The method then proceeds to a step  406  where an audio frame is selected to be associated with the mark-in frame  9 . In the example given, the selected audio frame will preferably be the tab-in audio frame. As described above, the tab-in audio frame will preferably have a start time that is before or at the start time of the mark-in video frame  9 . The tab-in audio frame is therefore selected by performing an “audio-to-video” seeking operation that uses a known video start time (i.e., mark-in frame  9  start time) to perform a seek on the audio component. The audio seeker is therefore able to identify the presentation time stamps and the decode time stamps of the audio frames closest to mark-in frame  9 . With this information, the seeker determines which audio frame has the closest start time to a start time  54  of the mark-in video frame  9 . 
     As shown in FIG. 1B, audio frame  58  has a start time that is closer to start time  54 . In this case, audio frame  58  is identified as the “mark-in audio frame” which has its own associated start time. At this point, the seeker will determine whether the start time of the mark-in audio frame  58  is at least as early (i.e., in time) as start time  54  of the mark-in video frame  9 . In this example, the start time of the mark-in audio frame  58  is not at least as early as start time  54 . Therefore, the seeker will back-up one audio frame to audio frame  56 , which is now identified as the tab-in audio frame. 
     Once the audio frame associated with the mark-in video frame is selected, the method will proceed to a step  408  where an audio frame is selected to be associated with the mark-out video frame  28 . As described above, an audio-to-video seek operation is again performed to identify a “mark-out audio frame”. The mark-out audio frame will preferably be the audio frame that has a start time that is closest in time to the end time  53  of mark-out video frame  28 . In this example, audio frame  64  has a start time that is closest to the end time  53  of mark-out video frame  28 . Once the mark-out audio frame  64  is identified, the seeker will determine whether the mark-out audio frame  64  has a start time that is no later than the end time  53  of mark-out video frame  28 . Since the exemplary mark-out audio frame  64  has a start time that is later than the end time  53  of the mark-out video frame  28 , the seeker will back-up one frame to audio frame  62  which is now identified as the “tab-out” audio frame. Once the tab-in and tab-out audio frames have been selected in steps  406  and  408 , the method proceeds to a decision step  410 . 
     In step  410 , the method determines whether an “in-glue” is required for the copied segment. As described above, glue segments are generally identified as decoded and re-encoded audio frames. In this embodiment, a predetermined number of audio frame may be decoded and re-encoded at the beginning of the copied segment. By way of example, although any number of audio frame may be decoded and re-encoded to introduce sound blending effects, fading effects, etc., audio frames  56 ,  58 ,  59  and  61  may be decoded and re-encoded based on an implicit requirement of the copy operator  104  of FIG.  2 . On the other hand, the number of glue audio frames and the type of sound effects may be requested explicitly through application  106  of FIG.  2 . 
     If in-glue is required in step  410 , the method will proceed to a step  412  where in-glue is output for a predetermined number of audio frames beginning with the tab-in audio frame. On the other hand, if in-glue is not required, the method will proceed to a step  414  where it is determined whether “out-glue” is required. If out-glue is required, the method will proceed to a step  416  where out-glue (i.e., decoded and re-encoded audio frames) is output for a predetermined number of frames. As in the case of in-glue, an exemplary predetermined number of frames may be audio frames  66 ,  65 ,  63 , and  62  of FIG.  1 B. On the other hand, if out-glue is not required in step  414 , the method will proceed to a step  418  where it is determined whether middle-glue is required for the copy operation. If middle glue is required for the copy operation in step  418 , the method will proceed to a step  420  where a middle-glue segment of audio frames is output. 
     In one embodiment, a middle-glue audio segment may include: (a) audio frames beginning with the tab-in frame and ending at the tab-out frame; (b) audio frames between the tab-in frame and extending to one frame before the first out-glue audio frame; (c) audio frames beginning with an audio frame after the last in-glue frame and extending to the tab-out frame; or (d) audio frames beginning with an audio frame after the last in-glue frame and extending to one frame before the first out-glue audio frame. Once the appropriate optional glue segments are output, the method of executing the copy operator is done. 
     FIG. 6 is a flowchart diagram illustrating the method steps associated with outputting a middle-glue as described in FIG.  5 . The method begins at a step  451  where the glue range is identified. As described above, the middle-glue range may vary depending on whether in-glue and out-glue is required. For exemplary purposes, assuming that in-glue for audio frames  56 ,  58 ,  59 , and  61 , and out-glue for audio frames  66 ,  65 ,  63 , and  62  are required, the middle-glue segment may be identified as extending from an audio frame  69  to an audio frame  68 . Of course, if no “in or out” glue is required, the middle-glue segment may extend from tab-in frame  56  to tab-out frame  62 . 
     Once the middle-glue range is identified in step  451 , the method will proceed to a step  452  where a middle-glue file is output that includes a number of identifiers. By way of example, the output file will preferably have a file name, the number of audio frames associated with the middle-glue segment, the initial audio frame number (middle-glue-in), the final audio frame number (middle-glue-out), the audio frame rate of the middle-glue, pointers to an input stream identifying the “middle-glue-in” frame, and pointers to the input stream identifying the “middle-glue-out” frame. In one embodiment, the middle-glue audio frames maybe un-processed audio frames  69  through  68  (e.g., not decoded and re-encoded) that are “copied” from an input file when the stitcher calls for the middle-glue segment in the second pass. Once the middle-glue output file has been generated in step  452 , the method of generating middle-glue will be done. 
     FIG. 7 is a flowchart diagram describing the method steps associated with outputting an in-glue as described in FIG.  5 . The method begins at a step  461  where the glue range is identified for the segment of in-glue which includes an “in-glue-in” 0  frame and extends to an “in-glue-out” frame. By way of example, with reference to audiovisual segment  60  of FIG. 1B, the in-glue segment will preferably include audio frames  56  through  61 . 
     Once the glue range for the in-glue has been identified in step  461 , the method proceeds to a step  462  where the first in-glue frame is decoded. By way of example, the first frame that will be decoded is preferably tab-in audio frame  56 . Referring to the data flow architecture of FIG. 2, once frame  56  has been selected from A.MPEG file  124  by an appropriate seek engine  118  of control object  111 , the identified data (i.e., tab-in audio frame  56 ) is retrieved and demultiplexed by a DEMUX unit  121  which isolates the audio bit stream from the video bit stream. Thereafter, tab-in audio frame  56  is sent to decoder  120  where the audio sample data is decoded. The decoded sample data is then sent to copy operator  104  which then sends the data to encoder  114 . At this point, the method proceeds to a step  464  where tab-in audio frame  56  is encoded by encoder  114  lying within control object  113 . 
     In this embodiment, frame  56  is may be re-encoded to smooth in a transition between audio segments being stitched together (e.g., to substantially remove popping effects). As described above, the re-encoded audio frames may be encoded to include, e.g., a fade to zero or from zero for half a second, adding a  60  Hz “humm”, etc. Once frame  56  has been encoded, the method proceeds to a step  466  where the encoded frame is appended to the output in-glue file (i.e., A.MPEG GLUE  126 ) by glue object  116 . 
     The method now proceeds to a decision step  468  where it is determined whether there are anymore audio frames in the in-glue range of frames identified in step  461 . If there are more audio frames, the method will again proceed to a step  462  where the next frame in the in-glue segment is decoded as described above. Once the next frame is decoded in step  462 , the method will again proceed to step  464  where the frame may be encoded with any number of continuity producing sound effects. Therefore, once the frame has been encoded, the method will proceed to a step  466  where it is again appended to the output glue file. The method then proceeds to step  468  where it is again determined whether there are anymore audio frames in the in-glue range of frames identified in step  461 . 
     If there are no more frames in the in-glue range of frames identified in step  461 , the method will proceed to a step  469  where an in-glue file that includes the appended frames is output (e.g., A.MPEG glue file  126 ). By way of example, the glue file may include a file name, the number of frames in the in-glue segment, the initial frame number for the (“in-glue-in”) frame, the final frame number for the (“in-glue-out”) frame, and the audio frame rate of the in-glue segment. Once the output glue file is complete, the method is done for the method steps associated with outputting an in-glue as described in FIG.  5 . 
     FIG. 8 is a flowchart diagram illustrating the method steps associated with outputting an out-glue as described in FIG.  5 . The method begins at a step  471  where the glue range is calculated for the out-glue segment. By way of example, in audiovisual stream  60  of FIG. 1B, the out-glue segment may begin at audio frame  66  and extend to tab-out audio frame  62 . Once the out-glue range has been calculated in step  471 , the method will proceed to a step  472  where audio frame  66  in the out-glue segment is decoded. 
     With reference to the data flow architecture of FIG. 2, once seek engine  118  has located and retrieved audio frame  66  in a file such as A.MPEG file  124 , the audio frame data is demultiplexed in DEMUX  121  to isolate the audio component. Frame  66  is then decoded in DEC  120  which generates decoded audio sample data from within control object  111 . The decoded sample data is then sent to copy operator  104  which sends the data to encoder  114  of control object  113 . Once the data has been re-encoded by encoder  114  in a step  474 , glue object  116  will append the re-encoded audio frame to a glue file (e.g., A.MPEG GLUE file  126 ) in a step  476 . 
     The method will then proceed to a step  478  where it is determined whether there are anymore frames in the glue-out range. Since there are more frames in the glue-out range, the method will proceed back again to step  472  where the next frame is processed. By way of example, the next frame may be audio frame  65  which is decoded and then re-encoded in step  474 . As described above, frame  66  is then re-encoded into a suitable encoding format to produce a desired sound effect. Once the frame is encoded in step  474 , the method will proceed to step  476  where the re-encoded frame is appended to the out-glue file as described above. 
     The method will then continue to loop back to step  472  until all audio frames in the predetermined out-glue segment are processed in accordance with one embodiment of the present invention. When it is determined that there are no more frames for processing into out-glue in step  478 , the method will proceed to a step  479  where an output glue file is generated. By way of example, the glue file may include a file name, the number of frames in the out-glue segment, the initial frame number for the (“out-glue-in”) file, the final frame number for the (“out-glue-out”) file, and the frame rate of the out-glue segment. Once the output glue file is complete, the method is done for the process steps associated with optionally outputting an out-glue as described in FIG.  5 . 
     FIG. 9 is an overview flowchart of the method steps associated with creating the requested output stream during the second pass performed by MEDIT engine  102  as described in step  304  of FIG.  3 . The method begins at a step  502  where MEDIT walks through an edit list and creates stitcher objects for each channel in the edit list. By way of example, an edit list may have numerous channels for displaying different video files. As shown in FIG. 2, exemplary channel operators  110  are identified for a channel  1  and extending to a channel N. Thus, associated stitcher objects are created for channel  1  and channel N, and are shown as stitcher object  147  and stitcher object  148 , respectively. 
     Once the stitcher objects have been created for each channel identified in the edit list in step  502 , the method will proceed to a step  504  where MEDIT calls a multiplexer  150  and gives the multiplexer a list of input sources. In this embodiment, multiplexor  150  is configured to pull data from input sources such as stitcher object  147  and stitcher object  148 . However, it should be understood that multiplexor  150  may pull data from any number of suitable input sources other than stitcher objects  147  and  148 . By way of example, the input sources may be embodied in any suitable form such as a file containing appropriate MPEG data. 
     The method then proceeds to a step  506  where the stitcher objects created for each channel are deleted once the stitcher objects have provided multiplexor  150  with appropriate input data from the un-processed input stream and the various glue files that may have been generated during the first pass as described above. After multiplexer  150  generates the requested copied segment, the copied segment is sent to the application through MEDIT engine  102 . Once the copied segments are output, the stitcher objects are deleted in step  506 , and the second pass is done. 
     FIG. 10 is a more detailed description of the method steps associated with multiplexing data pulled from input sources as described in step  504  of FIG.  9 . At a first step  530 , the method determines whether data is available on any input sources provided to the multiplexor. If the multiplexer is not provided with any input sources, the multiplexer will be done. On the other hand, if there are input sources provided to the multiplexer, the method will proceed to a step  532  where data provided by the input sources is read by the multiplexer. 
     Once any available data has been read from the input sources in step  532 , the method will proceed to a step  534  where the read data is multiplexed by a suitable multiplexing engine. By way of example, a suitable public domain multiplexing engine may be a one or two pass MPEG multiplexing engine, file name MPEG-1: Multi-Stream System Layer Encoder (multiplexer), developed by Z. Yaar, J. Boucher, J. Palmer, and E. Rubin (public domain, 1994). These multiplexing engines are available from Boston University, of Boston, Mass. 
     Once the data has been multiplexed in step  534 , the method will proceed to a step  536  where the multiplexed data is read to MEDIT engine  102  and then sent to the application requesting the editing operation as described in FIG.  2 . Once the multiplexed data is written to MEDIT, the process again proceed to decision step  530  where it is determined whether there are anymore available input sources. If there are available sources, the method will again loop through steps  532 ,  534 , and  536  until there are no more input sources. Once there are no more input sources, the method will be done. 
     FIG. 11 is a more detailed description of the method steps performed by the stitcher objects when reading data from input sources as described in step  532 . Initially, the method begins at a step  540  where the stitcher objects are called by the MEDIT engine  102 . As described above, a stitcher is preferably created for each channel (i.e., for all audio and video channels) provided in an edit list. Once the appropriate number of stitcher objects have been created, the method will proceed to a step  542  where each stitcher implements a finite state machine in order to generate an appropriate program elementary stream (PES) for the multiplexer. 
     In general, the finite state machine is charged with opening the input sources, reading the input sources, and closing the input sources in a time ordered manner. Thus, each stitcher will preferably walk through the state machine attempting to open the various input sources and attempting read the appropriate audio data. Once the data is read, the files are closed. If no data is found in the input sources (i.e., no “in, middle or out” glue was generated or needed), the state machine will proceed to the next file and proceed performing open, read, and close operations. 
     As described above, each of the stitchers use a glue object such as glue objects  130  and  131  to retrieve the glue files when requested. Therefore, each glue object is charged with retrieving the various pieces of glue files that may have been generated during the first pass as described in step  302  of FIG.  3 . Advantageously, by implementing glue objects, it is irrelevant to each stitcher object where the glue file is actually stored since the glue objects will retrieve the glue files from the appropriate location when requested by each stitcher. In this manner, each stitcher will loop through asking its associated glue object for glue files until there are no more glue files available for a particular copy operation. 
     FIG. 12 is a more detailed description of the method steps performed by each stitcher when implementing the finite state machine as described in step  542  of FIG.  11 . The method begins at a step  550  where it is first determined whether an “open” is required for an in-glue or an out-glue. If an open is required for an in-glue or an out-glue, the method will proceed to a step  552  where the appropriate glue file is opened and the file header is processed as is well known in the art. 
     Once the, file headers are processed, the method will proceed to a step  558  where a time re-stamping is performed for the opened glue file for the whole duration of a read operation. Also performed in step  558  is a tab processing operation. In general, during a read operation, the data is read into a buffer where the data is temporally held. Once in the buffer, the read contents are processed from beginning to end to determine appropriate time re-stamping, and to determine whether to drop or retain the tab-in and tab-out audio frames for the copied audio frame segments. Once processed, the entire contents of the buffer are output to the multiplexer (e.g., MUX  150  of FIG.  2 ). 
     As will be described in greater detail with reference to FIG. 13, tab processing is generally performed to assure that no more than about half an audio frame error is produced once two or more audio and video segments are joined. Broadly speaking, tab processing is performed at each of the tab-in and tab-out audio frames, and if certain conditions are met, the tab-in and tab-out audio frames may be dropped or retained. 
     Once re-stamping and tab processing has been performed in step  558 , the method will proceed to a step  560  where the state machine closes the open files. On the other hand, if an open was not required for an in-glue or an out-glue, the method will proceed to a step  554  where an open-middle-glue file and process file header step is performed. In this step, the headers of the middle-glue file are processed to assure that MPEG stream standards are met. Next, the method will proceed to a step  556  where the middle-glue file is open with reference to pointers indicating the location of the middle-glue. By way of example, as shown in FIG. 2, pointers  134  and  136  will identify the location of the beginning and ending audio frames in the input stream from which reading will be performed. Once the input stream has been opened in  556 , the method will again proceed to step  558  where time re-stamping and tab processing is performed as described above. Once time re-stamping and tab processing is performed, the method will proceed to step  560  where the open files are closed. 
     FIG. 13 is a flowchart diagram illustrating the method steps associated with performing tab processing in accordance with one embodiment of the present invention. For ease of illustration, concurrent reference will be made to FIG. 14 which illustrates a plurality of audiovisual segments that will be stitched together, and FIG. 15 which shows a tabulation table for an exemplary tab processing operation. 
     The method in FIG. 13 beings at a step  602  where a current tab  706  is processed and the existing stream error is determined. As shown in FIG. 14, the first segment is SEGMENT A, and the existing stream error is zero. For example, since there are no prior tabs carrying forward an existing stream error, and SEGMENT A is the first segment, the existing stream error is zero. Once the existing stream error is determined to be zero in step  602 , the method proceeds to a step  604  where the tab error is determined for tab  706 . In this example, the tab error is 0.2 as shown in the table of FIG.  15 . As used herein, reference to an “error” means the percentage an of an audio frame for which an audio frame is un-synchronized with the associated video frames. By way of example, a 0.2 error shall mean 20% of an audio frame. Further, although round numbers are used for ease of description, an associated error may have any suitable precision. 
     Once the error for tab  706  is determined in step  604 , the method proceeds to a step  606  where it is determined whether the sum of the existing stream error and the tab error (i.e., cumulative error) is greater than half a frame (i.e., &gt;0.5 error). In this example, the sum of the existing error (0.0 error) and tab  706  error (0.2) is not greater than half a frame. When the error is not greater than half a frame, the method proceeds to a step  608  where tab  706  is retained as shown in the table of FIG.  15 . The method now proceeds to a decision step  612  where it is determined whether there are any more tabs in the stitching operation illustrated in FIG.  14 . Since there are, the method will return to step  602  where the existing stream error is determined for the current tab. As shown in FIG. 14, the current tab is now tab  708 . At this stage, the existing stream error is the error carried from a previous tab processing operation. 
     As shown in the table of FIG. 15, the existing stream error is now 0.2. Once the current stream error is determined in step  602 , the method will proceed to a step  604  where the tab error for tab  708  is determined. As shown in the table of FIG. 15, the tab error for tab  708  is 0.5. The method now proceeds to the decision step  606  where it is determined whether the sum of the existing stream error (0.2) and the tab error for tab  708  (0.5) is greater than half a frame. Since the sum of errors is 0.7 (i.e., &gt;0.5), the method will proceed to a step  610  where tab  708 is dropped. After tab  708  is dropped, the new stream error will be −0.3 as shown in FIG.  15 . 
     Once tab  708  which represents the tab-out for SEGMENT A is processed, the method will again proceed to step  612  where it is determined whether there are any more tabs to process. Since there are, the method will return to step  602  where the current tab is tab-in  710  of SEGMENT B. Since the new stream error was −0.3 after the last tab was processed, the existing stream error will be −0.3 when tab  710  is processed. The method will now proceed to step  604  where the tab error for tab-in  710  is determined to be 0.4 as shown in the table of FIG.  15 . 
     The method then proceeds to decision step  606  where it is determined whether the sum of the existing stream error and the tab error for tab  710  is greater than half an audio frame. In this example, the sum is (−0.3+0.4) 0.1, which is less than half an audio frame (i.e., &lt;0.5). Therefore, tab  710  will be retained as illustrated in FIG.  15 . The method again continues to decision step  612  where it is again determined whether there are any more tabs. As shown in FIG. 14, tabs  712 ,  714 ,  716  and  720  will also be process through the method steps of FIG. 13 as described above. Once each tab is processed, a determination will be made to either drop or retain each tab. For completeness, reference may be made to FIG. 15 where exemplary calculations are shown for each tab associated with SEGMENT A through SEGMENT D illustrated in FIG.  14 . 
     FIG. 16 is a diagrammatic illustration of the existing frame error for each tab processed through the method steps of FIG.  13 . As shown, the existing stream error after the first tab  706  is processed is zero although the tab error was 0.2. This is possible since the entire audio component is shifted forward in time to aligned the first audio frame start time with the start time of the first video frame. However, when the second tab  708  is processed, the existing stream error will be 0.2 which is a result of shifting the entire audio component “20% of an audio frame” forward in time (i.e., the audio is 20% of an audio frame ahead of the video component). After the third tab  710  is processed, the existing error will be −0.3, which means that the audio component as a whole is shifted back “30% of an audio frame.” 
     For completeness, the following illustrates how the error is substantially maintained to be not more than “half a frame error” after a particular tab is processed. By way of example: after the fourth tab  712  is processed, the audio component will be 10% of an audio frame ahead of the video component; after the fifth tab  714  is processed, the audio component will be 50% of an audio frame behind the video component; after the sixth tab  716  is processed, the audio component will be 40% of an audio frame ahead of the video component; after the seventh tab  718  is processed, the audio component will be 50% of an audio frame ahead of the video component; and after the eight exemplary tab is processed, the audio component will be 40% of an audio frame ahead of the video component. 
     Since the existing stream error is prevented from exceeding a half an audio frame, the video frames will be substantially synchronized with the audio frames without regard to the number of segments being stitched together after successive copy operations. It should be appreciated that if corrections were not made by dropping or retaining audio frames as described above, the cumulative stream error would grow and propagate as additional audio and video segments were stitched together. Consequently, when the error grows to multiple audio frames, the audio component will no longer be synchronized with the video component and therefore be incomprehensible. That is, the audio content of a copied segment will not match the content of its associated video frame. 
     The invention employs various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Further, the manipulations performed are often referred to in terms, such as producing, identifying, determining, or comparing. 
     Any of the operations described herein that form part of the invention are useful machine operations. The invention also relates to a device or an apparatus for performing these operations. The apparatus may be specially constructed for the required purposes, or it may be a general purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general purpose machines may be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. An exemplary structure for the invention is described below. 
     FIG. 17 is a block diagram of an exemplary computer system  800  for carrying out the processing according to the invention. The computer system  800  includes a digital computer  802 , a display screen (or monitor)  804 , a printer  806 , a floppy disk drive  808 , a hard disk drive  810 , a network interface  812 , and a keyboard  814 . The digital computer  802  includes a microprocessor  816 , a memory bus  818 , random access memory (RAM)  820 , read only memory (ROM)  822 , a peripheral bus  824 , and a keyboard controller  826 . The digital computer  800  can be a personal computer (such as an IBM compatible personal computer), a workstation computer (such as a Sun Microsystems or Hewlett-Packard workstation), or some other type of computer. 
     The microprocessor  816  is a general purpose digital processor which controls the operation of the computer system  800 . The microprocessor  816  can be a single-chip processor or can be implemented with multiple components. Using instructions retrieved from memory, the microprocessor  816  controls the reception and manipulation of input data and the output and display of data on output devices. According to the invention, a particular function of microprocessor  816  is to assist in the processing of audio and video MPEG editing tasks as described above. 
     The memory bus  818  is used by the microprocessor  816  to access the RAM  820  and the ROM  822 . The RAM  820  is used by the microprocessor  816  as a general storage area and as scratch-pad memory, and can also be used to store input data and processed data. The ROM  822  can be used to store instructions or program code followed by the microprocessor  816  as well as other data. 
     The peripheral bus  824  is used to access the input, output, and storage devices used by the digital computer  802 . In the described embodiment, these devices include the display screen  804 , the printer device  806 , the floppy disk drive  808 , the hard disk drive  810 , and the network interface  812 . The keyboard controller  826  is used to receive input from keyboard  814  and send decoded symbols for each pressed key to microprocessor  816  over bus  828 . 
     The display screen  804  is an output device that displays images of data provided by the microprocessor  816  via the peripheral bus  824  or provided by other components in the computer system  800 . The printer device  806  when operating as a printer provides an image on a sheet of paper or a similar surface. Other output devices such as a plotter, typesetter, etc. can be used in place of, or in addition to, the printer device  806 . 
     The floppy disk drive  808  and the hard disk drive  810  can be used to store various types of data. The floppy disk drive  808  facilitates transporting such data to other computer systems, and hard disk drive  810  permits fast access to large amounts of stored data. 
     The microprocessor  816  together with an operating system operate to execute computer code and produce and use data. The computer code and data may reside on the RAM  820 , the ROM  822 , or the hard disk drive  820 . The computer code and data could also reside on a removable program medium and loaded or installed onto the computer system  800  when needed. Removable program mediums include, for example, CD-ROM, PC-CARD, floppy disk and magnetic tape. 
     The network interface  812  is used to send and receive data over a network connected to other computer systems. An interface card or similar device and appropriate software implemented by the microprocessor  816  can be used to connect the computer system  800  to an existing network and transfer data according to standard protocols. 
     The keyboard  814  is used by a user to input commands and other instructions to the computer system  800 . Other types of user input devices can also be used in conjunction with the present invention. For example, pointing devices such as a computer mouse, a track ball, a stylus, or a tablet can be used to manipulate a pointer on a screen of a general-purpose computer. 
     The invention can also be embodied as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data which can be thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, magnetic tape, optical data storage devices. The computer readable medium can also be distributed over a network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     The following MPEG audio and video standards described above are hereby incorporated by reference: (1) a document entitled “Generic Coding of Moving Pictures and Associated Audio Information: Video,” ISO/IEC 13818-2; (2) a document entitled “Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 MBit/s” (Part 1 System, Part 2 Video, Part 3 Audio) 11171/11172 (1995/1996); and (3) a document entitled “Generic Coding of Moving Pictures and Associated Audio Information” ISO/IEC 13818-3. All above-referenced MPEG standard documents and future MPEG standard documents may be obtained form ISO/IEC Case Postale 56, CH-1211, Geneva 20, Switzerland. 
     Although the preferred embodiments of the present invention have been described in detail, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. In the above embodiments a distributed architecture has been described. Such an architecture has a number of advantages particularly in terms of modularity and ease of introducing new functionalities. 
     By way of example, new functionalities may be created merely by providing an additional “plug-in” operator object which may utilize many of the same component objects, such as the seeker, the decoder, the encoder, etc. While such a distributed architecture is believed to work particularly well, it should be appreciated that similar functionalities may be accomplished using other architectures as well. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.