METHOD AND APPARATUS FOR FRAME ACCURATE ACCESS OF DIGITAL AUDIO-VISUAL INFORMATION

Abstract: A method and apparatus for use in a digital video delivery system is provided. A digital representation of an audio-visual work, such as an MPEG file (104), is parsed to produce a tag file (106). The tag file (106) includes information about each of the frames in the audio-visual work. During the performance of the audio-visual work, data from the digital representation is sent from a video pump (130) to a decoder. Seek operations are performed by causing the video pump (130) to stop transmitting data from the current position in the digital representation, and to start transmitting data from a new position in the digital representation. The information in the tag file (106) is inspected to determine the new position from which to start transmitting data. To ensure that the data stream transmitted by the video pump (130) maintains compliance with the applicable video format, prefix data that includes appropriate header information is transmitted by said video pump (130) prior to transmitting data from the new position. Fast and slow forward and rewind operations are performed by selecting video frames based on the information contained in the tag file (106) and the desired presentation rate, and generating a data stream containing data that represents the selected video frames. A video editor is provided for generating a new video file from pre-existing video files. The video editor selects frames from the pre-existing video files based on editing commands and the information contained in the tag files (106) of the pre-existing video files. A presentation rate, start position, end position, and source file may be separately specified for each sequence to be created by the video editor.

Description: METHOD AND APPARATUS FOR FRAME ACCURATE ACCESS OF DIGITAL AUDIO- VISUAL INFORMATION FIELD OF THE INVENTION The present invention relates to a method and apparatus for processing audio¬ visual information, and more specifically, to a method and apparatus for providing non-sequential access to audio-visual information stored in a digital format. BACKGROUND OF THE INVENTION In recent years, the media industry has expanded its horizons beyond traditional analog technologies. Audio, photographs, and even feature films are now being recorded or converted into digital formats. To encourage compatibility between products, standard formats have been developed in many ofthe media categories. MPEG is a popular standard that has been developed for digitally storing audio-visual sequences and for supplying the digital data that represents the audio¬ visual sequences to a client. For the purposes of explanation, the MPEG-1 and MPEG-2 formats shall be used to explain problems associated with providing non¬ sequential access to audio-visual information. The techniques employed by the present invention to overcome these problems shall also be described in the context of MPEG. However, it should be understood that MPEG-1 and MPEG-2 are merely two contexts in which the invention may be applied. The invention is not limited to any particular digital format. In the MPEG format, video and audio information are stored in a binary file (an "MPEG file"). The video information within the MPEG file represents a sequence of video frames. This video information may be intermixed with audio information that represents one or more soundtracks. The amount of information used to represent a frame of video within the MPEG file varies greatly from frame to frame based both on the visual content ofthe frame and the technique used to digitally represent that content. In a typical MPEG file, the amount of digital data used to encode a single video frame varies from 2K bytes to 50K bytes. During playback, the audio-visual information represented in the MPEG file is sent to a client in a data stream (an "MPEG data stream"). An MPEG data stream must comply with certain criteria set forth in the MPEG standards. In MPEG-2, the MPEG data stream must consist of fixed-size packets. Specifically, each packet must be exactly 188 bytes. In MPEG-1, the size of each packet may vary, with a typical size being 2252 bytes. Each packet includes a header that contains data to describe the contents ofthe packet. Because the amount of data used to represent each frame varies and the size of packets does not vary, there is no correlation between the packet boundaries and the boundaries ofthe video frame information contained therein. MPEG employs three general techniques for encoding frames of video. The three techniques produce three types of frame data: Inter-frame ("I-frame") data, Predicted frame ("P-frame") data and Bi-directional ("B-frame") data. I-frame data contains all ofthe information required to completely recreate a frame. P-frame data contains information that represents the difference between a frame and the frame that corresponds to the previous I-frame data or P-frame data. B-frame data contains information that represents relative movement between preceding I or P-frame data and succeeding I or P-frame data. These digital frame formats are described in detail in the following international standards: ISO/IEC 13818-1, 2, 3 (MPEG-2) and ISO/IEC 11172-1, 2, 3 (MPEG-1). Documents that describe these standards (hereafter referred to as the "MPEG specifications") are available from ISO/IEC Copyright Office Case Postale 56, CH 1211, Geneve 20, Switzerland. As explained above, video frames cannot be created from P and B-frame data alone. To recreate video frames represented in P-frame data, the preceding I or P- frame data is required. Thus, a P-frame can be said to "depend on" the preceding I or P-frame. To recreate video frames represented in B-frame data, the preceding I or P-frame data and the succeeding I or P-frame data are required. Thus, B-frames can be said to depend on the preceding and succeeding I or P-frames. The dependencies described above are illustrated in Figure la. The arrows in Figure la indicate an "depends on" relationship. Specifically, if a given frame depends on another frame, then an arrow points from the given frame to the other frame. In the illustrated example, frame 20 represents an I-frame. I-frames do not depend on any other frames, therefore no arrows point from frame 20. Frames 26 and 34 represent P-frames. A P-frame depends on the preceding I or P frame. Consequently, an arrow 36 points from P-frame 26 to I-frame 20, and an arrow 38 points from P-frame 34 to P-frame 26 . Frames 22, 24, 28, 30 and 32 represent B-frames. B-frames depend on the preceding and succeeding I or P-frames. Consequently arrows 40 point from each of frames 22, 24, 28, 30 and 32 to the I or P-frame that precedes each ofthe B-frames, and to each I or P-frame that follows each ofthe B-frames. The characteristics ofthe MPEG format described above allow a large amount of audio- visual information to be stored in a relatively small amount of digital storage space. However, these same characteristics make it difficult to play the audio-visual content of an MPEG file in anything but a strict sequential manner. For example, it would be extremely difficult to randomly access a video frame because the data for the video frame may start in the middle of one MPEG packet and end in the middle of another MPEG packet. Further, if the frame is represented by P-frame data, the frame cannot be recreated without processing the I and P- frames immediately preceding the P-frame data. If the frame is represented by B- frame data, the frame cannot be recreated without processing the I and P-frames immediately preceding the B-frame data, and the P-frame or I-frame immediately following the B-frame data. As would be expected, the viewers of digital video desire the same functionality from the providers of digital video as they now enjoy while watching analog video tapes on video cassette recorders. For example, viewers want to be able to make the video jump ahead, jump back, fast forward, fast rewind, slow forward, slow rewind and freeze frame. However, due to the characteristics ofthe MPEG video format, MPEG video providers have only been able to offer partial implementations of some of these features. Some MPEG providers have implemented fast forward functionality by generating fast forward MPEG files. A fast forward MPEG file is made by recording in MPEG format the fast-forward performance of an analog version of an audio- visual sequence. Once a fast forward MPEG file has been created, an MPEG server can simulate fast forward during playback by transmitting an MPEG data stream to a user from data in both the normal-speed MPEG file and the fast forward MPEG file. Specifically, the MPEG server switches between reading from the normal MPEG file and reading from the fast forward MPEG file in response to fast forward and normal play commands generated by the user. This same technique can be used to implement fast rewind, forward slow motion and backward slow motion. The separate-MPEG file implementation of fast forward described above has numerous disadvantages. Specifically, the separate-MPEG file implementation requires the performance of a separate analog-to-MPEG conversion for each playback rate that will be supported. This drawback is significant because the analog-to-MPEG conversion process is complex and expensive. A second disadvantage is that the use of multiple MPEG files can more than double the digital storage space required for a particular audio-visual sequence. A 2x fast forward MPEG file will be approximately half the size ofthe normal speed MPEG file. A half-speed slow motion MPEG file will be approximately twice the size ofthe normal speed MPEG file. Since a typical movie takes 2 to 4 gigabytes of disk storage, these costs are significant. A third disadvantage with the separate-MPEG file approach is that only the playback rates that are specifically encoded will be available to the user. The technique does not support rates that are faster than, slower than, or between the specifically encoded rates. A fourth disadvantage is that the separate-MPEG file approach requires the existence of a complete analog version ofthe target audio¬ visual sequence. Consequently, the technique cannot be applied to live feeds, such as live sports events fed through an MPEG encoder and out to users in real-time. Based on the foregoing, it is clearly desirable to provide a method and apparatus for sequentially displaying non-sequential frames of a digital video. It is further desirable to provide such non-sequential access in a way that does not require the creation and use of multiple digital video files. It is further desirable to provide such access for real-time feeds as well as stored audio- visual content. SUMMARY OF THE INVENTION A method and apparatus for use in a digital video delivery system is provided. A digital representation of an audio-visual work, such as an MPEG file, is parsed to produce a tag file. The tag file includes information about each ofthe frames in the audio-visual work. Specifically, the tag file contains state information about the state of one or more state machines that are used to decode the digital representation. The state information will vary depending on the specific technique used to encode the audio-visual work. For MPEG-2 files, for example, the tag file includes information about the state ofthe program elementary stream state machine, the video state machine, and the transport layer state machine. During the performance ofthe audio-visual work, data from the digital representation is sent from a video pump to a decoder. According to one embodiment ofthe invention, the information in the tag file is used to perform seek, fast forward, fast rewind, slow forward and slow rewind operations during the performance ofthe audio-visual work. Seek operations are performed by causing the video pump to stop transmitting data from the current position in the digital representation, and to start transmitting data from a new position in the digital representation. The information in the tag file is inspected to determine the new position from which to start transmitting data. To ensure that the data stream transmitted by the video pump maintains compliance with the applicable video format, prefix data that includes appropriate header information is transmitted by said video pump prior to transmitting data from the new position. Fast forward, fast rewind, slow forward and slow rewind operations are performed by selecting video frames based on the information contained in the tag file and the desired presentation rate, and generating a data stream containing data that represents the selected video frames. The selection process takes into account a variety of factors, including the data transfer rate ofthe channel on which the data is to be sent, the frame type ofthe frames, a minimum padding rate, and the possibility of a buffer overflow on the decoder. Prefix and suffix data are inserted into the transmitted data stream before and after the data for each frame in order to maintain compliance with the data stream format expected by the decoder. A video editor is provided for generating a new video file from pre-existing video files. The video editor selects frames from the pre-existing video files based on editing commands and the information contained in the tag files ofthe pre¬ existing video files. A presentation rate, start position, end position, and source file may be separately specified for each sequence to be created by the video editor. The video editor adds prefix and suffix data between video data to ensure that the new video file conforms to the desired format. Significantly, the new video files created by this method are created without the need to perform additional analog-to-digital encoding. Further, since analog-to-digital encoding is not performed, the new file can be created even when one does not have access to the original analog recordings ofthe audio-visual works. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example, and not by way of limitation, in the figures ofthe accompanying drawings and in which like reference numerals refer to similar elements and in which: Figure la is a diagram illustrating the dependencies between different types of frames in an MPEG data stream; Figure lb is a block diagram of an audio-visual information delivery system according to an embodiment ofthe present invention; Figure 2a illustrates the various layers in an MPEG file; Figure 2b illustrates the contents of a tag file generated according to an embodiment ofthe invention; Figure 2c illustrates the tag information generated for each frame in an MPEG-1 file. Figure 3a illustrates the commands sent from the stream server to the video pump in response to a seek request according to an embodiment ofthe invention; Figure 3b illustrates the data generated by the video pump to a client in response to the commands illustrated in Figure 3a; Figure 4a illustrates the commands sent from the stream server to the video pump during a rate-specified playback operation according to one embodiment of the invention; Figure 4b illustrates the data generated by the video pump to a client in response to the commands illustrated in Figure 4a; Figure 5 illustrates an MPEG editor configured to perform non-interactive MPEG editing according to an embodiment ofthe invention; Figure 6 is a flow chart illustrating the operation ofthe MPEG editor of Figure 5 according to an embodiment ofthe invention; and Figure 7 is a block diagram illustrating a multi-disk MPEG playback system according to an embodiment ofthe invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description, the various features ofthe invention shall discussed under topic headings that appear in the following order: I. OVERVIEW II. TAG FILE GENERATION III. DIGITAL AUDIO/VIDEO FILE STRUCTURE IV. TAG FILE CONTENTS V. SEEK OPERATIONS VI. PREFIX DATA VII. PACKET DISCONTINUITIES VIII. BUFFER LIMITATIONS IX. SPECIFIED-RATE PLAYBACK OPERATIONS X. BIT BUDGETING XI. FRAME TYPE CONSTRAINTS XII. SUFFIX DATA XIII. SLOW MOTION OPERATIONS XIV. REWIND OPERATIONS XV. RUNTIME COMMUNICATION XVI. FRAME ACCURATE POSITIONING XVII. DISK ACCESS CONSTRAINTS XVIII. VARIABLE RATE PLAYBACK OPERAΗONS XIX. NON-INTERACTIVE DIGITAL AUDIO- VIDEO EDITING XX. DISTRIBUTED SYSTEM I. OVERVIEW Figure lb is a block diagram illustrating an audio- visual information delivery system 100 according to one embodiment ofthe present invention. Audio-visual infoπnation delivery system 100 contains a plurality of clients (1 - n) 160, 170 and 180. The clients (1 - n) 160, 170 and 180 generally represent devices configured to decode audio- visual information contained in a stream of digital audio- visual data. For example, the clients (1 - n) 160, 170, and 180 may be set top converter boxes coupled to an output display, such as television. As shown in Figure lb, the audio-visual information delivery system 100 also includes a stream server 110 coupled to a control network 120. Control network 120 may be any network that allows communication between two or more devices. For example, control network 120 may be a high bandwidth network, an X.25 circuit or an electronic industry association (EIA) 232 (RS - 232) serial line. The clients (1- n) 160, 170 and 180, also coupled to the control network 120, communicate with the stream server 110 via the control network 120. For example, clients 160, 170 and 180 may transmit requests to initiate the transmission of audio¬ visual data streams, transmit control information to affect the playback of ongoing digital audio-visual transmissions, or transmit queries for information. Such queries may include, for example, requests for information about which audio-visual data streams are currently available for service. The audio-visual information delivery system 100 further includes a video pump 130, a mass storage device 140, and a high bandwidth network 150. The video pump 130 is coupled to the stream server 110 and receives commands from the stream server 110. The video pump 130 is coupled to the mass storage device 140 such that the video pump 130 stores and retrieves data from the mass storage device 140. The mass storage device 140 may be any type of device or devices used to store large amounts of data. For example, the mass storage device 140 may be a magnetic storage device or an optical storage device. The mass storage device 140 is intended to represent a broad category of non- volatile storage devices used to store digital data, which are well known in the art and will not be described further. While networks 120 and 150 are illustrated as different networks for the purpose of explanation, networks 120 and 150 may be implemented on a single network. In addition to communicating with the stream server 110, the clients (1 - n) 160, 170 and 180 receive information from the video pump 130 through the high bandwidth network 150. The high bandwidth network 150 may be any of type of circuit-style network link capable of transferring large amounts of data. A circuit- style network link is configured such that the destination ofthe data is guaranteed by the underlying network, not by the transmission protocol. For example, the high bandwidth network 150 may be an asynchronous transfer mode (ATM) circuit or a physical type of line, such as a Tl or El line. In addition, the high bandwidth network 150 may utilize a fiber optic cable, twisted pair conductors, coaxial cable, or a wireless communication system, such as a microwave communication system. The audio-visual information delivery system 100 ofthe present invention permits a server, such as the video pump 130, to transfer large amounts of data from the mass storage device 140 over the high bandwidth network 150 to the clients (1 - n) 160, 170 and 180 with minimal overhead. In addition, the audio-visual information delivery system 100 permits the clients (1 - n) 160, 170, and 180 to transmit requests to the stream server 110 using a standard network protocol via the control network 120. In a prefeπed embodiment, the underlying protocol for the high bandwidth network 150 and the control network 120 is the same. The stream server 110 may consist of a single computer system, or may consist of a plurality of computing devices configured as servers. Similarly, the video pump 130 may consist of a single server device, or may include a plurality of such servers. To receive a digital audio-visual data stream from a particular digital audio¬ visual file, a client (1 - n) 160, 170 or 180 transmits a request to the stream server 110. In response to the request, the stream server 110 transmits commands to the video pump 130 to cause video pump 130 to transmit the requested digital audio¬ visual data stream to the client that requested the digital audio-visual data stream. The commands sent to the video pump 130 from the stream server 110 include control information specific to the client request. For example, the control information identifies the desired digital audio-visual file, the beginning offset ofthe desired data within the digital audio-visual file, and the address ofthe client. In order to create a valid digital audio-visual stream at the specified offset, the stream server 110 also sends "prefix data" to the video pump 130 and requests the video pump 130 to send the prefix data to the client. As shall be described in greater detail hereafter, prefix data is data that prepares the client to receive digital audio-visual data from the specified location in the digital audio-visual file. The video pump 130, after receiving the commands and control information from the stream server 110, begins to retrieve digital audio-visual data from the specified location in the specified digital audio-visual file on the mass storage device 140. For the purpose of explanation, it shall be assumed that system 100 delivers audio-visual information in accordance with one or more ofthe MPEG formats. Consequently, video pump 130 will retrieve the audio- visual data from an MPEG file 104 on the mass storage device 140. The video pump 130 transmits the prefix data to the client, and then seamlessly transmits MPEG data retrieved from the mass storage device 140 beginning at the specified location to the client. The prefix data includes a packet header which, when followed by the MPEG data located at the specified position, creates an MPEG compliant transition packet. The data that follows the first packet is retrieved sequentially from the MPEG file 104, and will therefore constitute a series of MPEG compliant packets. The video pump 130 transmits these packets to the requesting client via the high bandwidth network 150. The requesting client receives the MPEG data stream, beginning with the prefix data. The client decodes the MPEG data stream to reproduce the audio- visual sequence represented in the MPEG data stream. II. TAG FILE GENERATION System 100 includes a tag file generator 112. The tag file generator 112 generates a tag file 106 from the MPEG file 104. For stored MPEG content, the tag file generation operation is performed by tag file generator 112 "off-line" (i.e. prior to any client request for MPEG data from the MPEG file 104). However, in certain situations, such a real-time MPEG feeds, tag file generation is performed in real- time during receipt ofthe MPEG data stream. Consequently, in the prefeπed embodiment, tag file generator 112 generates tag file 106 in real-time or faster. Tag file generation rates may be increased by parallelization ofthe tag file operation. Tag file generator 112, stream server 110 and video pump 130 are illustrated as separate functional units for the purpose of explanation. However, the particular division of functionality between units may vary from implementation to implementation. The present invention is not limited to any particular division of functionality. For example, tag file generator 112 is illustrated as a stand-alone unit. However, in one embodiment, tag file generator 112 may be incorporated into an MPEG encoder. Such an MPEG encoder would generate the information contained in tag file 106 simultaneous with the generation ofthe information contained in MPEG file 104. An implementation that combines the MPEG encoding process with the tag file generation process may increase efficiency by eliminating the need to perform redundant operations. Such efficiency gains are particularly useful when processing audio-visual feeds in real-time. The tag file 106 contains control information that is used by stream server 110 to implement fast forward, fast rewind, slow forward, slow rewind and seek operations. The use ofthe tag file 106 to perform these operations shall be described in greater detail below. The tag file 106 contains general information about the MPEG file 104 and specific information about each ofthe video frames in the MPEG file 104. Prior to discussing in detail the contents ofthe tag file 106, the general structure of MPEG file 104 shall be described with reference to Figure 2a. III. MPEG FILE STRUCTURE Digital audio-visual storage formats, whether compressed or not, use state machines and packets of various structures. The techniques described herein apply to all such storage formats. While the present invention is not limited to any particular digital audio-visual format, the MPEG-2 transport file structure shall be described for the purposes of illustration. Referring to Figure 2a, it illustrates the structure of an MPEG-2 transport file 104 in greater detail. The data within MPEG file 104 is packaged into three layers: a program elementary stream ("PES") layer, a transport layer, and a video layer. These layers are described in detail in the MPEG-2 specifications. At the PES layer, MPEG file 104 consists of a sequence of PES packets. At the transport layer, the MPEG file 104 consists of a sequence of transport packets. At the video layer, MPEG file 104 consists of a sequence of picture packets. Each picture packet contains the data for one frame of video. Each PES packet has a header that identifies the length and contents ofthe PES packet. In the illustrated example, a PES packet 250 contains a header 248 followed by a sequence of transport packets 251-262. PES packet boundaries coincide with valid transport packet boundaries. Each transport packet contains exclusively one type of data. In the illustrated example, transport packets 251, 256, 258, 259, 260 and 262 contain video data. Transport packets 252, 257 and 261 contain audio data. Transport packet 253 contains control data. Transport packet 254 contains timing data. Transport packet 255 is a padding packet. Each transport packet has a header. The header includes a program ID ("PID") for the packet. Packets assigned PID 0 are control packets. For example, packet 253 may be assigned PID 0. Other packets, including other control packets, are referenced in the PID 0 packets. Specifically, PID 0 control packets include tables that indicate the packet types ofthe packets that immediately follow the PID 0 control packets. For all packets which are not PID 0 control packets, the headers contain PIDs which serve as a pointers into the table contained in the PID 0 control packet that most immediately preceded the packets. For example, the type of data contained in a packet with a PID 100 would be determined by inspecting the entry associated with PID 100 in the table ofthe PID 0 control packet that most recently preceded the packet. In the video layer, the MPEG file 104 is divided according to the boundaries of frame data. As mentioned above, there in no conelation between the boundaries ofthe data that represent video frames and the transport packet boundaries. In the illustrated example, the frame data for one video frame "F" is located as indicated by brackets 270. Specifically, the frame data for frame "F" is located from a point 280 within video packet 251 to the end of video packet 251, in video packet 256, and from the beginning of video packet 258 to a point 282 within video packet 258. Therefore, points 280 and 282 represent the boundaries for the picture packet for frame "F". The frame data for a second video frame "G" is located as indicated by brackets 272. The boundaries for the picture packet for frame "G" are indicated by bracket 276. Structures analogous to those described above for MPEG-2 transport streams also exist in other digital audio- visual storage formats, including MPEG-1, Quicktime, AVI, Indeo, Cinepak, Proshare, H.261 and fractal formats. In the prefeπed embodiment, indicators of video access points, time stamps, file locations, etc. are stored such that multiple digital audio-visual storage formats can be accessed by the same server to simultaneously serve different clients from a wide variety of storage formats. Preferably, all ofthe format specific information and techniques are incorporated in the tag generator and the stream server. All ofthe other elements ofthe server are format independent. IV. TAG FILE CONTENTS The contents of an exemplary tag file 106 shall now be described with reference to Figure 2b. In Figure 2b, the tag file 106 includes a file type identifier 202, a length indicator 204, a bit rate indicator 206, a play duration indicator 208, a frame number indicator 210, stream access information 212 and an initial MPEG time offset 213. File type identifier 202 indicates the physical wrapping on the MPEG file 104. For example, file type identifier 202 would indicate whether MPEG file 104 is a MPEG-2 or an MPEG-1 file. Length indicator 204 indicates the length ofthe MPEG file 104. Bit rate indicator 206 indicates the bit rate at which the contents ofthe MPEG file 104 should be sent to a client during playback. The play duration indicator 208 specifies, in milliseconds, the amount of time required to play back the entire contents of MPEG file 104 during a normal playback operation. Frame number indicator 210 indicates the total number of frames represented in MPEG file 104. Stream access information 212 is information required to access the video and audio streams stored within MPEG file 104. Stream access information 212 includes a video elementary stream ID and an audio elementary stream ID. For MPEG-2 files, stream access information 212 also includes a video PID and an audio PID. The tag file header may also contain other information that may be used to implement features other than those provided by the present invention. In addition to the general information described above, the tag file 106 contains an entry for each frame within the MPEG file 104. The entry for a video frame includes information about the state ofthe various MPEG layers relative to the position ofthe data that represents the frame. For an MPEG-2 file, each entry includes the state ofthe MPEG-2 transport state machine, the state ofthe program elementary stream state machine and the state ofthe video state machine. For an MPEG-1 file, each entry includes the cuπent state ofthe Pack system MPEG stream and the state ofthe video state machine. Tag file entry 214 illustrates in greater detail the tag information that is stored for an individual MPEG-2 video frame "F". With respect to the state ofthe program elementary stream state machine, the tag entry 214 includes the information indicated in Table 1. TABLE 1 DATA MEANING PES OFFSET AT THE START OF The offset, within the PES packet that PICTURE 217 contains the frame data for frame "F" of the first byte ofthe frame data for frame "F". PES OFFSET AT THE END OF The offset between the last byte in the PICTURE 219 frame data for frame "F" and the end of the PES packet in which the frame data for frame "F" resides. With respect to the state ofthe video state machine, tag entry 214 includes the information indicated in Table 2. TABLE 2 DATA MEANING PICTURE SIZE 220 The size ofthe picture packet for frame "F". START POSITION 226 The location within the MPEG file of the first byte ofthe data that coπesponds to frame "F" TIME VALUE 228 The time, relative to the beginning of the movie, when frame "F" would be displayed during a normal playback of MPEG file 104. FRAME TYPE 232 The technique used to encode the frame (e.g. I-frame, P-frame or B-frame). TIMING BUFFER INFORMATION Indicates how full the buffer ofthe 238 decoder is (sent to the decoder to determine when information should be moved out ofthe buffer in order to receive newly arriving infoπnation). With respect to the state ofthe transport layer state machine, tag entry 214 includes the information indicated in Table 3. TABLE 3 DATA MEANING START OFFSET 234 The distance between the ofthe first byte in the frame data and the start of the transport packet in which the first byte resides. # OF NON- VIDEO PACKETS 222 The number of non- video packets (i.e. audio packets, padding packets, control packets and timing packets) that are located within the picture packet for frame "F". # OF PADDING PACKETS 224 The number ofpadding packets that are located within the picture packet for frame "F". END OFFSET 236 The distance between the last byte in the frame data and the end ofthe packet in which the last byte resides. CURRENT CONTINUITY COUNTER The Continuity value associated with 215 frame "F". DISCONTINUITY FLAG 230 Indicates whether there is a discontinuity in time between frame "F" and the frame represented in the previous tag entry. Assume, for example, that entry 214 is for the frame "F" of Figure 2a. The size 220 associated with frame "F" would be the bits encompassed by bracket 274. The number 222 of non-video packets would be five (packets 252, 253, 254, 255 and 257. The number 224 ofpadding packets would be one (packet 255). The start position 226 would be the distance between the beginning of MPEG file 104 and point 280. The start offset 234 would be the distance between the start of packet 251 and point 280. The end offset 236 would be the distance between point 282 and the end of packet 258. Figure 2c illustrates the tag information generated for each frame in an MPEG-1 file. Referring to Figure 2c, entry 214 includes data indicating the state of three state machines: a system state machine, a pack state machine, and a video state machine. Specifically, tag entry 214 includes the information shown in Table 4. TABLE 4 DATA MEANING AMOUNT OF NON- VIDEO DATA The amount of non- video data (in bytes) 221 contained within the start and end boundaries ofthe frame data for frame "F". AMOUNT OF PADDING DATA 223 The amount ofpadding data (in bytes) contained within the start and end boundaries ofthe frame data for frame "F". PACK OFFSET AT START 225 The offset between the start boundary of the frame data for frame "F" in the beginning ofthe pack packet that contains the start boundary for frame "F". PACK REMAINING AT START 227 The distance between the start boundary for frame "F" and the end ofthe pack packet that contains the start boundary of frame "F". PACK OFFSET AT END 229 The offset between the end boundary for frame "F" in the beginning ofthe packet that contains the end boundary for frame npit PACK REMAINING AT END 231 The distance between the end boundary for frame "F" and the end ofthe pack packet that contains the end boundary of frame "F". PICTURE SIZE 233 The distance (in bytes) between the start boundary for frame "F" and the end boundary for frame "F". PICTURE START POS 235 The distance between the start ofthe MPEG-1 file and the start boundary for frame "F". PICTURE END POS 237 The position, relative to the beginning ofthe MPEG-1 file, ofthe end boundary for frame "F". FRAME TYPE 239 The technique used to encode the data that represents frame "F". TIME VALUE 241 The time, relative to the beginning of the movie, when frame "F" would be displayed during a normal playback of MPEG file 104. TIMING BUFFER INFO 243 Indicates how full the decoder is (sent to the decoder to determine when infoπnation should be moved out ofthe buffer in order to receive newly arriving information). As explained above with reference to MPEG-1 and MPEG-2 formats, the tag information includes data indicating the state ofthe relevant state machines at the beginning of video frames. However, the state machines employed by other digital audio- visual formats differ from those described above just as the state machines employed in the MPEG-1 format differ from those employed in MPEG-2. Consequently, the specific tag information stored for each frame of video will vary based on the digital audio-video format ofthe file to which it coπesponds. V. SEEK OPERATIONS Having explained the contents of tag file 106, the use of tag file 106 to perform seek operations shall now be described. When a client wishes to perform a seek operation, the client transmits a seek operation request to stream server 110. The seek operation request may specify, for example, to jump ahead in the MPEG sequence to a position five minutes ahead ofthe cuπent playing position. In response to the request, stream server 110 inspects the tag file 106 to determine the I-frame (the "target frame") that would be playing in five minutes if the playback operation proceeded at a normal rate. The target frame may be easily determined by inspecting the time value 228 and frame type 232 information stored in tag file 106. When the target frame is determined, stream server 110 determines the position within the MPEG file 104 ofthe frame data that coπesponds to the target frame (the "target position"). Stream server 110 performs this determination by reading the start position 226 stored in the entry in tag file 106 that coπesponds to the target position. Significantly, all ofthe operations performed by stream server 110 are performed without the need to access MPEG file 104. This allows for the stream server 110 and the video pump 130 to be distributed among the various servers in the server complex. For the purpose of explanation, various components of system 100 are said to read data from a particular storage medium. For example, tag file generator 112 and video pump 130 are described as reading data from MPEG file 104 located on mass storage device 140, and stream server 110 is described as reading data from tag file 106 stored on mass storage device 140. However, when data is to be frequently accessed, it is typically cached in a faster, temporary storage medium such as dynamic memory. Rather than read the data directly from the slower storage, the components read the data from the faster temporary storage. In the prefeπed embodiment, at least a portion ofthe tag file 106 is stored in a cache memory to reduce the number of disk accesses performed by stream server 110. Once the target position has been determined, the stream server 110 constructs prefix data for the transition. As mentioned above, prefix data is data that must be inserted into the MPEG data stream prior to a transition to ensure that the MPEG data stream remains MPEG compliant. Prefix data shall be described in greater detail below. Once stream server 110 constructs the prefix data, stream server 110 transmits commands to video pump 130 to instruct video pump 130 to transition from the current position in the MPEG file to the target position. For a seek operation, the commands generated by stream server 110 will typically include an insert command and a play command. The insert command instructs the video pump 130 to cease transmission of MPEG data from the current position, and to transmit the prefix data. This process effectively "inserts" the prefix data into the MPEG data stream. The play command instructs the video pump 130 to begin transmitting data starting at the target position within the MPEG file 104. The video pump 130 inserts this data in a byte-contiguous way such that the client does not see any boundary between the prefix data, the MPEG data, and the suffix data. Referring to Figure 3a, it illustrates the commands sent by the stream server 110 to the video pump 130 in response to a seek request from a client. In the illustrated example, the stream server 110 transmits two commands 302 to the video pump 130. The first command is an insert command instructing video pump 130 to insert "PREFIX_DATA" into the MPEG data stream that the video pump 130 is sending to a client. The second command is a play command. The play command instructs the video pump 130 to transmit data beginning at the position "START_POS". START_POS is the position within MPEG file 104 of the first byte of the target frame. In the prefeπed embodiment, the "play" instruction supports a "begin position" parameter and an "end position" parameter. In response to a play instruction, the video pump 130 transmits data from the MPEG file beginning at the begin position, and continues to transmit data from the MPEG file until me specified end position is reached. In a seek operation, it is assumed that the playback will continue from the target position to the end ofthe MPEG file. Therefore, only the begin position parameter ofthe play command is required for seek operations. Referring to Figure 3b, it illustrates the information sent from video pump 130 to a client (e.g. client 160) in response to the "insert" and "play" commands transmitted by stream server 110. At the time that the video pump 130 receives the insert command, the video pump 130 will be sending MPEG data from some position in the MPEG file (the "cuπent position"). Block 320 represents information transmitted by video pump 130 up to the cuπent position. Upon receiving the insert command, the video pump 130 finishes sending the cuπent transport packet, ceases to transmit data from the cuπent position and transmits the prefix data 322. After transmitting the prefix data 322 to the client, the video pump 130 responds to the play command. Specifically, the video pump 130 begins transmission to the client of data 324 beginning at the target location in the MPEG file. There is no interruption in the MPEG data stream transmitted by video pump 130 to the client during this process. In addition, the MPEG data stream received by the client fully complies to the MPEG standard. Consequently, the MPEG decoder within the client remains completely unaware that a seek operation was performed. Because seek operations performed by the technique discussed above produce an MPEG compliant data stream, custom MPEG decoders are not required. VI. PREFIX DATA As mentioned above, MPEG data is packaged in layers. Clients expect the data stream that they receive from video pump 130 to be packaged in those same layers. If video pump 130 simply jumps from one point in the MPEG file 104 to another point, packaging information will be lost and the clients will not be able to properly decode the data. For example, if video pump 130 simply starts transmitting data from point 280 in Figure 2a, the PES header 248 for PES packet 250 and the header for transport packet 251 will be skipped. These headers contain data which indicates how to decode the information which follows them. Consequently, without the information contained in these headers, the client will not know how to decode the subsequent data. Therefore, prefix data must be constructed and sent to smoothly transition between the current location in the MPEG file 104 and a new location. The prefix data contains packaging information which begins packages for the data at the new location. In the prefeπed embodiment, the prefix data includes the information described in Table 5. TABLE 5 DATA MEANING DISCARD INFORMATION For MPEG 2: This is a list of PIDs to keep. All other transport packets are discarded. For MPEG 1 : This is a list of elementary streams to keep. SYSTEM & PACK HEADER DATA Includes a valid system header and a (MPEG-1 ONLY) valid Pack Header. TRANSPORT PACKET HEADER Includes private data and MPEG video DATA (MPEG-2 ONLY) header data, described below. PRIVATE DATA Includes a private time stamp and other data described below. VIDEO INITIALIZATION DATA Includes an MPEG sequence header which indicates frames per second and horizontal and vertical resolutions. POSSIBLE EXTRA PADDING AND Explained below. SECOND TRANSPORT PACKET HEADER (MPEG-2 ONLY) MPEG VIDEO HEADER MPEG-2: Includes a valid PES header, a video presentation time and, under certain conditions, discontinuity data which causes the client's clock to be reset. MPEG-1 : Contains a valid picture header. With respect to the discard information, assume that the target video frame of a seek operation is the video frame located between points 280 and 282 in Figure 2a. The discard infoπnation contained in the insert command generated in response to the seek operation may instruct video pump 130 to discard all ofthe non- video packets located between points 280 and 282. According to one embodiment, the packets are identified by their PID numbers. With respect to private data, the mechanism used to convey this data differs between MPEG-1 and MPEG-2. For MPEG-1, private data is sent in a pack packet on the ISO/IEC private data-1 stream. (See section 2.4.4.2 of ISO 11172-1 for more information). For MPEG-2, private data is sent in a packet on the video PID, but in a section ofthe adaptation field titled private data. (See section 2.4.3.4 of ISO/IEC 13818-1 for more information). Since may clients may desire specific information about the operation in progress (seek, fast forward, rewind, frame advance or rewind) which cannot be encoded in the file's digital audio-visual storage format, private data is used. When the server knows that "client specific" information is needed, it places it into whatever private data mechanism is supported by the file's audio-visual storage format. Thus, the output to the network maintains its conformance to the required format. This is necessary in case the network is performing checks to be sure that data is not corrupted in fransmission. By virtue of being in private data, the "client specific" data will not be checked. With respect to the possible extra padding, since transport packets have a fixed size in MPEG-2, an extra padding packet is required when the prefix data is too large to fit into d e same packet as die first block of video data. For example, assume that point 280 is ten bytes from the beginning of video packet 251. If the prefix data required to transition to point 280 is greater than ten bytes, then the prefix data will not fit in the same packet as the first block of video data. Under such circumstances, the prefix data is sent in a transport packet that is completed with padding. A second transport packet is constructed to transmit the video data located between point 280 and the end of video packet 251. The first ten bytes in this second transport packet are filled with padding. Since MPEG-1 has variable size packets, this issue for MPEG-1 does not arise. Rather, a correct packet size for the prefix data is simply computed. VII. PACKET DISCONTINUITIES In the original MPEG file 104, each packet has an associated time stamp. Typically, the time stamps of packets sequentially located within MPEG file 104 will be sequential. During playback operations, d e client tracks the time stamps to determine the integrity ofthe MPEG data stream. If two sequentially-received packets do not have sequential time stamps, then the client determines diat a discontinuity has occuπed. If t-he difference between two sequentially-received time stamps is small, then the client can usually compensate for die discontinuity. However, if the difference between two sequentially-received time stamps is too great, t-he client may reset itself or initiate some other type of recovery operation. When a seek operation is performed, the client will sequentially receive packets diat are not sequentially located within the MPEG file 104. Because the packets are not sequentially located within MPEG file 104, the time stamps associated with die packets will not be sequential. If the jump specified by the seek operation is relatively large, then the discontinuity between the time stamps may be sufficient to cause the client to terminate normal playback. To avoid d is situation, data which causes the client to reset its clock is included in the prefix data. Upon receipt of such data, die client simply resets its clock based on the time stamp contained in the following packet. As noted above, die time stamps of packets sequentially located within an MPEG file will typically be sequential. However, it is possible to have sequentially stored packets that do not have sequential time stamps. If a large discontinuity occurs between packets in the original MPEG file, then die original MPEG file will itself contain data which causes die client's clock to reset. Stream server 110 inspects the discontinuity flags 230 in tag file 106 to determine whether a particular seek operation will skip any packets which contain data to reset the client's clock. If tiie seek operation skips over any discontinuous packets, then data that causes die client's clock to reset is added to the prefix data. Though in concept die same operation is performed in MPEG-1 and MPEG- 2, the mechanism by which the operation is performed differs because of d e different timing mechanisms used in MPEG-1 and 2. Specifically, in the MPEG-1 embodiment, the "System Clock Reference" (SCR) is the clock used (see Section 2.4.2 of ISO/IEC 11172-1). In the MPEG-2 embodiment, the "Program Clock Reference" (PCR) and "Presentation Time Stamp" (PTS) are both used. See sections 2.4.2.1 and 2.4.3.6 of ISO/IEC 13818-1 respectively for definitions ofthe PCR and PTS. VIII. BUFFER LIMITATIONS The MPEG decoder in each client has a buffer of a certain limited size. Typically the buffer must be large enough to hold information from two sequential frames of video. Consequently, the data for the later frame of video may be written into the buffer at the same time mat the data for the previous frame of video is being read out ofthe buffer by the decoder. In many clients, the size of d e buffer is selected based on die assumption that the incoming MPEG data stream will never contain two sequentially-ordered large I-frames of video data. During normal playback from an MPEG-compliant file, this assumption will hold true, since P and B-frames will occur between successive I-frames. However, seek operations may cause a jump from a large I- frame located at a first location in the MPEG file 104 to a second I-frame located at a second location in the MPEG file 104. If an attempt is made to write the second I- frame into the buffer before the first I-frame has been entirely read from me buffer, die decoder may lose synchronization or otherwise fail. Stream server 110 detects when a seek operation would cause such an overflow by inspecting the timing buffer information 238 stored in the tag file 106. To avoid such buffer overflow, the stream server 110 inserts data into the prefix data that will cause the arrival ofthe second large I-frame to the decoder buffer to be delayed. While the second I-frame is delayed, the client has time to complete the processing of me first I-frame. By the time the data for the second I- frame begins to arrive, the first I-frame has been completely processed so that die portion ofthe buffer used to hold d e previous I-frame is available to hold the second I-frame. According to one embodiment, the second I-frame is delayed by placing a delayed time stamp in transport packet header portion ofthe prefix data. The transport packet header portion ofthe prefix data serves as the header for the packet that contains the beginning ofthe second I-frame (the "transition packet"). The transition packet is received by a network buffer that feeds d e decoder buffer. The network buffer determines when to send d e video information contained in die transition packet to the decoder buffer based on d e time stamp in die transition packet. Because the time stamp indicates a delay between d e transition packet and die previous packet, the network buffer delays die transfer ofthe video information from the transition packet into the decoder buffer. According to an alternate embodiment, me second I-frame is delayed by adding padding packets to the prefix data prior to the data that serves as d e heading for the transition packet. Such padding packets will arrive at the client prior to the transition packet. As the client receives and discards die padding packets, d e first I- frame is being read from the decoder buffer. By the time all ofthe padding packets have been processed, die first I-frame has been completely read out ofthe decoder buffer and the decoder buffer is ready to receive the second I-frame. IX. SPECIFIED-RATE PLAYBACK OPERATIONS Most video cassette recorders allow viewers to watch analog-based audio¬ visual works at playback speeds other than normal lx forward playback. For example, some video cassette recorders provide multiple rates of fast forward, slow forward, slow rewind and fast rewind. The present invention provides similar functionality to the viewers of MPEG-encoded works. In the prefeπed embodiment, d e functionality of typical video cassette recorders is surpassed in d at any speed of forward and rewind playback is supported. For example, a viewer could select lOOOx fast forward or fast rewind, or .0001 slow forward or slow rewind. In d e prefeπed embodiment, the processes used to implement fast forward, slow forward, slow rewind and fast rewind operations include the same general steps. Therefore, for the purpose of explanation, these steps shall be described with reference to a fast forward operation. After the fast forward process is explained, it shall be described how and when slow motion and rewind operations differ from fast forward operations. To initiate a fast forward operation, a client transmits a fast forward request to the stream server 110. In embodiments diat support more tiian one fast forward rate, die fast forward request includes data designating a presentation rate. As used herein, "presentation rate" refers to the rate at which the audio- visual work is presented to a viewer. The stream server 110 receives the fast forward request from the client and, in response to the request, inspects the information contained in tag file 106. Specifically, stream server 110 determines from die information in tag file 106 which frames should be displayed to produce the specified presentation rate. The frame selection process performed by stream server 110 must take into account various constraints that will be described in greater detail below. X. BIT BUDGETING The simplest method for selecting frames during a fast forward operation would be to select every Nth frame, where N is the specified presentation rate relative to normal presentation rate. For example, assume that the client requests a 5x fast forward operation. In response to such a request, stream server 110 could select every fifth frame for display. Stream server 110 would tiien transmit a series of play commands to video pump 130 to cause video pump 130 to transmit an MPEG data stream that contains data for every fifth frame. Thus, the presentation rate would be 5x. The simple frame selection process described above could work if all ofthe frames in the MPEG file 104 were encoded in I-frame format and if either all I- frames were the same size or the bandwidtii of network 150 was unlimited. However, die bandwidth of network 150 is not unlimited, I-frames do not all have d e same size and, as explained above, MPEG files also include frames encoded in P-frame and B-frame formats which cannot be decoded independent of information from other frames. The bandwidth between video pump 130 and its clients is limited. For example, video pump 130 may be allocated a 1.5 or 2 Megabits per second channel for each MPEG data stream it transmits to a client. To determine whetiier selection of a particular frame (die "frame at issue") will exceed d e available bandwidtii, stream server 110 determines d e size of die time window diat will be available to send d e particular frame. The size of die time window is equal to (T2-T1)/PR, where Tl is the time value associated with die previously selected frame, T2 is the time value associated with the frame at issue, and PR is die cuπent presentation rate. For example, assume that the time associated with previously selected frame is one second away from d e time of die frame at issue. Assume also that d e presentation rate is lOx. Therefore, the time window for sending die frame at issue would be (1 second)/ 10 or .1 seconds. Once die stream server 110 determines d e time window available to send the data for the frame at issue, the stream server 110 determines the cuπent "bit budget" by multiplying the time window by d e data transfer rate ofthe channel dirough which the MPEG data stream is being sent to the client. For example, if the applicable data transfer rate is 2 Megabits per second and die time window is .1 seconds, tiien die cuπent bit budget is 200K bits. The stream server 110 then reads the frame size from the tag information to determine if die frame at issue falls wϊtiiin the cuπent bit budget. If die size ofthe frame at issue exceeds die current bit budget, then die frame at issue is not selected. This is the case, for example, if the size of die frame data for d e frame at issue is 50K bytes (400K bits) and the bit budget is 200K bits. Otherwise, if the frame at issue falls witiiin the bit budget, tiien the frame at issue is selected to be sent. If a particular frame is not sent, then it is more likely that a future frame will be sent, because ofthe unused timespace (and thus bits in the bit budget) of d e unused frames. XI. FRAME-TYPE CONSTRAINTS As explained above, a frame cannot be accurately recreated from P-frame data unless die preceding I-frame has been decoded. A frame cannot be accurate recreated from B-frame data unless die preceding and succeeding P or I-frame data is decoded. Consequently, stream server 110 is limited with respect to which frames it can select. Assuming that die bandwidth is available, any I-frame can be selected. According to one embodiment of die invention, only I-frames are even considered for selection. Stream server 110 accesses the tag information to determine the frame type ofthe frame at issue. If die frame at issue is not an I-frame, then it is automatically skipped, and stream server 110 moves on to evaluate die subsequent frame. At some playback rates, this technique may result in unused bandwidtii. That is, the transmission of every I-frame will require less bandwidth tiian is available. Therefore, stream server 110 transmits insert commands to cause video pump 130 to transmit MPEG padding between the transmission of I-frame information. In the prefeπed embodiment, die padding packets are sent as one component of suffix data, which shall be described in greater detail below. According to the prefeπed embodiment, P and B-frames are not automatically skipped in die frame selection process. Rather, P and B-frames are considered for selection unless information that tiiey require has already been skipped. Specifically, if any I-frame is not selected by stream server 110, men the frames at fall between the skipped I-frame and die subsequent I-frame are skipped. In addition, if any P-frame is not selected, then die B and P-frames that fall between the skipped P-frame and die subsequent I-frame are skipped. Based on diese rules, any additional bandwidth available between the transmission of I-frames may be filled witii P-frame and B-frame data. As a result, the resulting MPEG data stream will have more frames per second. According to yet another embodiment, stream server 110 is programmed to skip some I-frames even when the bandwidth is available to send them. For example, stream server 110 may skip every fifth I-frame that otherwise qualifies for selection. Because I-frames are significantly larger than P and B-frames, numerous P and B frames may be sent in the bandwidth made available by skipping a single I- frame. Consequently, the resulting MPEG data stream has more frames per second than it would odierwise have if all qualifying I-frames were selected. In the prefeπed embodiment, a client may specify parameters for the selection process performed by stream server 110. For example, the client may request more frames per second. In response, the stream server 110 transmits more P and B frames in the MPEG data stream by increasing the number of qualifying I- frames that it skips. On the other hand, die client may request a more continuous picture. In response, the stream server 110 transmits a higher percentage of qualifying I-frames, leaving less bandwidtii for transmitting P and B-frames. XII. SUFFIX DATA While the stream server 110 is selecting die frames to be displayed during a fast forward operation, the stream server 110 is simultaneously transmitting commands to the video pump 130 to cause the video pump 130 to send an MPEG video stream containing the frames that have already been selected. The portion of the MPEG data sfream used to convey data for a selected frame is refeπed to herein as a "segment". To maintain compliance with the MPEG standards, segments include prefix data that is sent prior to transmitting the frame data for the selected video frames. The process of generating prefix data was described above witii reference to seek operations. Performing a fast forward operation is similar to performing a series of seek operations in which each seek operation causes the video pump 130 to jump to the data for the next selected frame. Specifically, for each selected frame, die stream server 1 10 must generate prefix data, transmit an insert command to die video pump 130 to cause d e video pump 130 to insert the prefix data into die data stream, and transmit a play command to the video pump 130 to cause the video pump 130 to transmit data from the appropriate frame. Fast forward operations differ from seek operations in that die play command specifies an end position as well as a beginning position. The end position is d e location within the MPEG file 104 of die last byte ofthe frame data for the selected frame. For example, assume that the frame boundaries for a selected frame F are points 280 and 282 illustrated in Figure 2a. The stream server 110 would send video pump 130 an insert command to cause video pump 130 to send prefix data to the client, and a play command to cause video pump 130 to send die video data located between points 280 and 282 to d e client. Typically, the end position (e.g. point 282) specified in the play command will not coincide with a packet boundary. Therefore, to maintain MPEG compliance, additional information ("suffix data") must be inserted into die data stream after die transmission of die frame data. The suffix data includes padding which completes the transport packet that contains the end ofthe selected frame. For example, the suffix data that would be inserted into the data sfream after sending die frame F would contain a length ofpadding equal to die distance between point 282 and die end of video packet 258. Under certain conditions, die suffix data also includes padding packets. As shall be described hereafter, the number ofpadding packets sent in d e suffix data depends on die size ofthe frame data, die presentation rate, the minimum padding rate and die number ofpadding packets that were left inside die frame data. Thus, a segment consists of prefix data, die frame data of a selected frame, and suffix data. The stream server 1 10 generates the suffix data and transmits an insert command to the video pump 130 to cause die video pump to insert die suffix data into die MPEG data stream. Consequently, during a fast forward operation, die commands sent by die stream server 110 to the video pump 130 appear as illustrated in Figure 4a. Referring to Figure 4a, stream server 110 has thus far selected diree frames to be displayed: frame_l, frame_2 and frame_3. Upon selecting frame_l, stream server 110 transmits three commands 402 to die video pump 130. The three commands 402 include a first insert command 408, a play command 410 and a second insert command 412. The first insert command 408 instructs video pump 130 to transmit prefix data "PREFIX_DATA_1 " to a client. The play command 410 instructs video pump 130 to transmit the data located between die positions START_POS_l and END_POS_l to the client. In the illustrated example, START_POS_l would be the position ofthe first byte of frame_l, and END_POS_l would be d e position of die last byte of frame_l. The second insert command 412 instructs the video pump 130 to transmit suffix data "SUFFIX_DATA_1 " to die client. The data that is specified by these three commands constitutes a segment for frame_l . As explained above, many transport packets may be required to store the frame data for a single video frame (e.g. frame_l). Other packets that do not contain video infoπnation, such as padding packets, timing packets and audio packets, may be interspersed between die video packets for the video frame. In the prefeπed embodiment, stream server 110 not only transmits the boundaries of each frame to video pump 130, but stream server 110 also indicates what to do with the non-video packets within those boundaries. Typically, d e audio packets will be discarded. However, the other non-video packets may or may not be retained based on various factors. For example, to sustain the minimum padding rate stream server 110 may indicate tiiat the padding packets are to be maintained. The value of maintaining a minimum padding rate shall be discussed in greater detail below. Video pump 130 receives this information from stream server 110 and strips from the MPEG data stream those non-video packets indicated by die stream server 110. Consequently, the information sent by video pump 130 in response to play command 410 will typically include less tiian all ofthe data located between START_POS_l and START_POS_2. Referring again to Figure 4a, stream server 110 has transmitted diree commands 404 to cause video pump 130 to transmit a segment for frame_2, and diree commands 406 to cause video pump 130 to transmit a segment for frame_3. Stream server 110 will continue to transmit commands in this manner to cause video pump 130 to transmit segments for every frame that it selects to be displayed during the fast forward operation. Referring to Figure 4b, it illustrates the data transmitted by video pump 130 in response to d e commands described above. Specifically, in response to the first insert command 408, video pump 130 fransmits PREFIX_DATA_1 450 to the client 160. In response to play command 410, video pump 130 transmits the data located between START_POS_l and END_POS_l. This data, illustrated as DATA_1 452, contains die frame data of frame_l . In response to the second insert command 412, video pump 130 transmits SUFFIX_DATA_1 to the client 160. The segment consisting of PREFIX_DATA_1, DATA_1 and SUFFIX_DATA_1 conveys the frame data of frame_l to client 160 while maintaining compliance with die MPEG standards. In die prefeπed embodiment, diese commands between the stream server 110 and video pump 130 are sent over a very fast lightweight network or through shared memory. For a typical stream, supporting 15 frames-per second of fast forward, 45 commands per second shall be sent thus stressing communications inside the server. In die prefeπed embodiment, the commands are sent from d e stream server 110 to die video pump 130 in batches. XIII. SLOW MOTION OPERATIONS As explained above, frames are selectively skipped for playback operations tiiat exceed normal playback speed. For playback operations that are slower than normal playback speed, no frames are skipped. Ratiier, stream server 110 selects every frame. As in fast forward operations, the video pump 130 transmits segments for each ofthe selected frames in response to commands generated by stream server 110. The suffix data in the segments include padding packets which delay die arrival ofthe subsequent segments. Consequently, die frame data arrives and is decoded at a slower rate than during normal playback operations. Alternatively, the time delays may be imposed by causing the stream server 110 to insert delayed time stamps into the prefix data that it sends to die video pump 130. XIV. REWIND OPERATIONS Rewind operations are performed in die same manner as fast and slow forward operations with the exception that only I-frames are selected for rewind operations (regardless of whether the rewind operations are fast or slow). P and B frames are automatically skipped because they cannot be decoded unless frames that precede diem in die original MPEG file are processed before them. However, during rewind operations, die frames on which P and B frames depend will be processed after the P and B frames that depend on diem. The concept of "multisfream" fast forward or rewind has been mentioned above. Multisfream fast forward or rewind is accomplished by storing multiple copies ofthe movie, where the copies have been recorded at various rates. In the prefeπed embodiment, when a client requests a certain fast forward or rewind presentation rate, the sfream server 110 will determine whetiier it has a prerecorded file at tiiat rate. If so, it will play that file. This will give the user more frames per second and will also cause less computational and communication load on die stream server 110 and video pump 130. However, if die requested rate is not available, d e sfream server 110 will determine the best file from which to choose individual frames, and will process that file as described above. The best file will be the file which has the most I-frames to select from at the requested presentation rate. This integration of "multi-stream" and "single-stream" fast forward and rewind thus allows servers to choose between any level of quality, disk storage requirements, and server computational and communication load, providing significant advantage over the use of multi-stream operations alone. XV. RUNTIME COMMUNICATION In die prefeπed embodiment, stream server 110 is configured to receive and transmit responses to queries made by clients while video pump 130 is transmitting an MPEG data stream to the clients. The stream server 110 conveys the responses to die queries to the client by causing video pump 130 to insert the responses into the MPEG data stream that is being sent to die client. This process is complicated by die fact that die communication channel between video pump 130 and each client is completely filled by the MPEG data stream that die video pump 130 is sending. However, some packets in die MPEG data stream are merely padding, and do not contribute to die resulting audio-visual display. To take advantage of die bandwidth occupied by diese padding packets, d e stream server 110 causes video pump 130 to replace these padding packets widi data packets diat contain responses to the queries. When the data packets arrive at the client, the MPEG decoder in the client determines that the data packets do not contain audio-visual data and passes the data packets to a higher level application. The higher level application inspects die data packets and extracts from the data packets any information contained dierein. During fast forward and fast rewind operations, die ability of die stream server 110 to communicate with the client in diis manner would be lost if die frame selection process did not leave room for padding packets tiiat may be replaced with data packets. Therefore, in one embodiment of die invention, d e stream server 1 10 selects frames in such a way as to ensure some available minimum padding rate. If selection of a frame would cause die padding rate to fall below the specified minimum rate, then the frame is skipped. The stream server 110 also tells the video pump 130 where to put the requisite padding. According to one embodiment, die video pump 130 does not replace padding packets widi data packets, but actually generates die padding packets. The MPEG data stream transmitted by the video pump 130 passes through a downstream manager 131 prior to arriving at the client. The downstream manager replaces the padding packets witii data packets tiiat contain the responses generated by stream server 110. Because the MPEG data stream maintains a minimum level ofpadding, die downstream manager is guaranteed a minimum bandwidth for placing data packets into the MPEG data stream. XVI. FRAME ACCURATE POSITIONING For many uses, it is important to be able to determine exactly which frame is being displayed by die client at any given time. For example, a user may wish to pause the playback of an MPEG movie, select an item on the screen, and select a menu option that places an order for the item over d e network. If die cuπently displayed frame is not accurately identified, then the wrong item may be ordered. During normal movie play, frame accurate positioning is encoded as part of die normal MPEG data stream. Specifically, time stamps are interleaved with the frame data in die MPEG data stream. Hardware in die client extracts this timing information. Typically, numerous frames follow each time stamp. Therefore, the client uniquely identifies the cuπently displayed frame based on die last timing information and die number of frames tiiat have been processed since receipt of die last timing information. During fast forward and fast rewind, die identity of frames cannot be determined by die timing information contained in die MPEG data stream. For example, the third frame after a particular time stamp may be one of any number of frames depending on the current playback rate and frame selection technique. Consequently, to provide frame accurate positioning, the sfream server 110 is configured to insert a time stamp in front of every frame transmitted in die MPEG data stream. Video pump 130 receives the time stamp information from the stream server 110, which retrieves the time stamp from the tag file 106. Many clients are not able to decode more tiian a certain number of time stamps per second because die MPEG specification does not require them to decode more than a certain amount of time stamps per second. Therefore, in the prefeπed embodiment, the time stamp inserted before each frame is not an MPEG time stamp. Rather, the time stamps are placed in packets that are tagged as MPEG "private data packets". When a client receives a private data packet, it determines whether it recognizes the data in die packet. Clients diat do not support private data time stamps simply discard die private data packets containing die time stamps and tiius will not be able to do perfect frame accurate positioning. Such clients will still be able to perform approximate frame positioning based on the MPEG time stamps mat are coincidentally included in the MPEG data stream. Clients that support private data time stamps extract the time stamps from the private data packets and tiius can exactly determine the identity ofthe frames that follow the time stamps. XVII. DISK ACCESS CONSTRAINTS In some video playback systems, a single MPEG file may be stored across numerous disk drives to increase die fault tolerance of die system. Consider, for example, die multi-disk system 700 illustrated in Figure 7. System 700 includes N+l disk drives. An MPEG file is stored on N ofthe N+l disks. The MPEG file is divided into sections 750, 752, 754 and 756. Each section is divided into N blocks, where N is the number of disks tiiat will be used to store the MPEG file. Each disk stores one block from a given section. In the illustrated example, die first section 750 ofthe MPEG file includes blocks 710, 712 and 714 stored on disks 702, 704 and 706, respectively. The second section 752 includes blocks 716, 718 and 720 stored on disks 702, 704 and 706, respectively. The third section 754 includes blocks 722, 724 and 726 stored on disks 702, 704 and 706, respectively. The fourth section 756 includes blocks 728, 730 and 732 stored on disks 702, 704 and 706, respectively. The disk 708 which is not used to store the MPEG file is used to store check bits. Each set of check bits coπesponds to a section ofthe MPEG file and is constructed based on die various blocks that belong to the coπesponding section. For example, check bits 734 coπesponds to section 750 and is generated by performing an exclusive OR operation on all ofthe blocks in the first section 750. Similarly, check bits 736, 738 and 740 are the products of an exclusive OR performed on all of die blocks in die section 752, 754 and 756, respectively. System 700 has a higher fault tolerance than a single disk system in diat if any disk in die system ceases to operate coπectly, the contents ofthe bad disk can be reconstructed based on the contents ofthe remaining disks. For example, if disk 704 ceases to function, the contents of block 712 can be reconstructed based on die remaining blocks in section 750 and the check bits 734 associated witii section 750. Similarly, block 718 can be constructed based on die remaining blocks in section 752 and the check bits 736 associated with section 752. This enor detection and conection technique is generally known as "Redundant Array of Inexpensive Disks" or RAID. During real-time playback using RAID, a video pump reads and processes die MPEG file on a section by section basis so that all ofthe information is available to reconstruct any faulty data read from disk. During normal playback operations, there is sufficient time to perform the disk accesses required to read an entire section while die data from the previous section is being transmitted in die MPEG data sfream. However, during fast forward and fast rewind operations, less than all ofthe data in any section will be sent in the MPEG data stream. Because less data is sent, the transmission ofthe data will take less time. Consequently, less time will be available to read and process the subsequent section. For example, assume that only one frame X from section 750 was selected for display during a fast forward operation. During the time it takes to transmit the segment for frame X, the data for the next selected frame Y must be read and processed. Assume that the next frame Y is located in section 752. If the MPEG file is read and processed on a section by section basis (required for RAID), then all of the blocks in section 752 must be read and processed during die transmission of die single frame X. Even if it were possible to read and process all of die blocks in section 752 in die allotted time, it may still be undesirable to do so because of die resources that would be consumed in performing the requisite disk accesses. In light of die foregoing, video pump 130 does not use RAID during fast forward and fast rewind operations. Rather, video pump 130 reads, processes and transmits only the data indicated in die commands it receives from die stream server 110. Thus, in the example given above, only the frame data for frame Y would be read and processed during die transmission ofthe segment for frame X. By bypassing RAID during fast forward and fast rewind operations, disk bandwidtii remains at the same level or below that used during normal playback operations. Since RAID is not used during real-time fast forward and fast rewind operations, faulty data cannot be reconstructed during tiiese operations. Consequently, when the video pump 130 detects tiiat die data for a selected frame is corrupted or unavailable, the video pump 130 discards die entire segment associated witii the problem frame. Thus, if the data associated witii a frame cannot be sent, then the prefix and suffix data for die frame is not sent eitiier. However, any padding packets tiiat were to be sent along with the prefix or suffix data will still be sent. By sending data in entire "segments", conformance with d e digital audio¬ visual format is maintained. In one embodiment, die video pump 130 will send down padding packets to fill die line to maintain the coπect presentation rate. In the prefeπed embodiment, this behavior is selectable by the client. XVIII. VARIABLE RATE PLAYBACK OPERATIONS As mentioned above, a client may change the presentation rate ofthe audio¬ visual work by transmitting a rate change request to die stream server 110. Typically, clients issue change rate requests in response to input received from a user. For example, a user may press a fast forward button on a remote control. The remote control transmits a signal that identifies the button that was pressed. The client receives and decodes the signal transmitted by the remote control to determine tiiat die fast forward button was requested. The client then transmits a change rate request to the stream server 1 10 that specifies some presentation rate greater than lx. According to one embodiment of die invention, the client is configured to detect if the user continues to hold down the fast forward button. If the user holds down die fast forward button for more than a predetermined interval, then the client transmits a second change rate request that designates a faster presentation rate than the previously requested presentation rate. While the user continues to hold down die fast forward button, the presentation rate is continuously increased. Another button, such as the rewind button, may be pressed to incrementally decrease die presentation rate. The process described above appears to the user as a variable rate fast forward operation. However, to the stream server 110, the operation actually consists of a series of distinct fast forward operations. This incremental rate adjustment process has been described with reference to fast forward operations. However, it may equally be applied to slow forward, slow rewind and fast rewind operations. Further, rate changes may be performed in response to die how many times a particular button is pressed rather than or in addition to how long die button is pressed. In addition, a visual indication of die cuπent presentation rate, such as an aπow that has a length that reflects the presentation rate, may be displayed on die screen while the presentation rate does not equal lx. XIX. NON-INTERACTIVE DIGITAL AUDIO- VISUAL EDITING By initiating seek operations and rate-specified playback operations, a user is effectively performing interactive MPEG editing. That is, die MPEG data stream mat is produced in response to these operations is based on but differs from the content ofthe original MPEG file. In addition to such interactive presentation of content, the present invention provides a mechanism for non-interactive MPEG editing. During non-interactive MPEG editing, an MPEG file is produced which is based on but differs from one or more pre-existing MPEG files. The mechanism for non-interactive MPEG editing shall now be described with reference to Figures 5 and 6. Referring to Figure 5, an MPEG editor 502 is provided for generating new MPEG sequences based on pre-existing MPEG content. According to one embodiment, die MPEG editor 502 reads a command file 504 containing editing commands. The commands contained in the command file 504 include parameters for specifying "splices" from pre-existing MPEG files. For example, each ofthe commands in command file 504 may have die following format: "filename" [start_pos] [end_pos] [presentation_rate] In this exemplary command, d e "filename" parameter represents a pre¬ existing MPEG file. The remaining parameters specify a splice from the specified MPEG file. Specifically, the start_pos parameter represents the position within the specified MPEG file at which to begin the splice. If no start_pos is designated, it may be assumed tiiat the splice is to begin at the first frame ofthe specified MPEG file. The end_pos parameter represents the position at which to end d e splice. If no end_pos is designated, it may be assumed tiiat d e splice is to end at the end ofthe specified MPEG file. The presentation_rate represents the presentation rate of die splice relative to the original MPEG file. If no presentation rate is specified, tiien a normal (i.e. lx) presentation rate is assumed. In the prefeπed embodiment, the start_pos and end_pos parameters are specified in terms of time because timing information is typically more accessible to a user than file position information. For example, a user may want to specify a two minute splice that begins ten minutes into a particular MPEG movie and ends twelve minutes into the MPEG movie. The user typically will not know the file position of the first byte in the frame that is displayed ten minutes into the movie, or the last byte in the frame that is displayed twelve minutes into the movie. As shall be explained hereafter, the MPEG editor 502 determines file positions tiiat conespond to die specified times by inspecting the tag information for the specified MPEG file. The operation of MPEG editor 502 shall now be described with reference to Figure 6. At step 600, the MPEG editor 502 reads a command in the command file 504. Preferably die commands are read in die same sequence as they appear in the command file 504. Therefore, MPEG editor 502 will read die first command in command file 504 the first time that step 600 is performed. At step 602, the MPEG editor 502 determines whether die command specified a lx presentation rate. If a presentation rate other than lx was specified, tiien control passes to step 604. Steps 604 and 606 are analogous to the steps performed by stream server 110 and video pump 130 during a specified-rate playback operation. Specifically, at step 604 MPEG editor 502 selects frames in the specified MPEG file tiiat fall witiiin the specified time period (start_pos to end_pos). Frames are selected based on the specified presentation rate and the tag information according to the selection process described in detail above. Once die frames are selected, segments are generated (step 606) which package the frame data conesponding to die selected frames in MPEG-compliant packets. These segments are stored in sequence to produce a portion of an edited MPEG file 510. Control then passes to step 612, which either causes the next command to be processed or the editing operation to end if there are no more commands to be processed. If a lx presentation rate was specified, tiien control passes from step 602 to step 614. At steps 614 and 616, MPEG editor 502 performs an operation analogous to the seek operation described above. Specifically, MPEG editor 502 compares the specified starting position wid die time stamp information contained in die tag file 106 to deteπnine die position of a target frame. MPEG editor 502 then generates prefix data (step 614) to perform the transition to the specified frame. After generating the prefix data, MPEG editor 502 copies data from the specified MPEG file into the edited MPEG file 510 beginning at the start ofthe target frame (step 616). Once the data between start_pos and end_pos has been copied into edited MPEG file 510, MPEG editor 502 determines whether the splice terminated at die end ofthe specified MPEG file (step 610). If die splice terminated at die end of die specified MPEG file, tiien the splice ended on a packet boundary. Otiierwise, suffix data is generated (step 618) to complete the cunent packet (step 618). Control then passes to step 612, which either causes the next command to be processed or the editing operation to end if there are no more commands to be processed. When all of die commands in die command file 504 have been processed by MPEG editor 502, the edited MPEG file 510 will be an MPEG compliant file containing the splices specified by die commands in die command file 504. Significantly, the edited MPEG file 510 was generated witiiout having to perform additional analog-to-MPEG encoding. Further, editing may be performed even if one does not have access to any ofthe analog versions ofthe original works. By generating MPEG files in this manner, a user may quickly create unique and original movies based on preexisting MPEG content. Typically, non-interactive MPEG editing does not have to be performed in real-time. Therefore, some ofthe time constraints that apply to real-time operations do not apply to non-interactive MPEG editing. For example, it was explained above tiiat due to timing constraints RAID enor conection techniques are not used during fast forward and fast rewind operation. Since such timing constraints do not apply to non-interactive MPEG editing, RAID is used during die fast forward and fast rewind operations performed to produce edited MPEG file 510. For the purpose of explanation, the various data repositories used in die editing process are illustrated as files stored on storage device 140. However, tiiis form and location of this data may vary from implementation to implementation. For example, the various files may be stored on separate storage devices. Further, a user interface may be provided which allows a user to operate graphical controls to specify die parameters for a series of splices. XX. DISTRIBUTED SYSTEM As explained above, die tasks performed during die real-time transmission of MPEG data streams are distributed between die sfream server 110 and die video pump 130. The distributed nature of this architecture is enhanced by the fact that the video pump 130 does not require access to tag file 106, and stream server 110 does not require access to MPEG file 104. Consequently, stream server 110 and video pump 130 may operate in different parts ofthe network without adversely affecting the efficiency ofthe system 100. In the foregoing specification, the invention has been described witii reference to specific embodiments tiiereof. It will, however, be evident diat various modifications and changes may be made thereto without departing from the broader spirit and scope of die invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. PAGE INTENTIONALLY LEFT BLANK

Claims: CLAIMS What is claimed is: 1. A meihod for playing at a second presentation rate an audio-visual work that has been digitally encoded in a digital data stream for playback at a first presentation rate, wherein said digital data stream includes a sequence of video frame data, each video frame data in said sequence of video frame data coπesponding to a video frame in said audio-visual work, the metiiod comprising the computer-implemented steps of: selecting a selected set of video frames from said audio-visual work based on said second presentation rate; constructing a second digital data stream that includes die video frame data tiiat coπesponds to each video frame of said selected set of video frames; and transmitting said second digital data stream to a decoder. 2. The method of Claim 1 wherein said step of selecting said selected set of video frames includes repeatedly performing the steps of: determining a bit budget based on a first time value associated with a most recently selected video frame, a second time value associated witii a current frame, said second presentation rate and a data transfer rate; determining a size of die frame data mat conesponds to die cuπent frame; if the size of the frame data that conesponds to die cuπent frame exceeds said bit budget, tiien not selecting said cuπent frame as a video frame in said selected set of video frames, and selecting a new frame as a new cuπent frame; and if die size of die frame data that coπesponds to die cuπent frame does not exceed said bit budget, then selecting said cunent frame as a video frame in said selected set of video frames. 3. The metiiod of Claim 2 wherein said step of selecting said selected set of video frames includes the steps of: determining whetiier selection of said cunent frame would cause said second digital data stream to have a padding rate less than a predetermined padding rate; and if selection of said cuπent frame would cause said second digital data stream to have a padding rate less tiian said predetermined padding rate, then not selecting said cunent frame as a video frame in said selected set of video frames. 4. The method of Claim 3 further comprising the steps of: receiving a query from a user; determining a response to said query; and replacing padding data in said second digital data stream with said response to said query. 5. The method of Claim 1 wherein said sequence of video frame data includes video frame data from which said conesponding video frame can be constructed without reference to any other video frame data, and video frame data from which said coπesponding video frame cannot be constructed without reference to any other video frame data, said step of selecting a selected set of video frames including not selecting for said selected set any video frame mat conesponds to video frame data from which said conesponding video frame can be constructed without reference to any other video frame data. 6. The metiiod of Claim 1 wherein said sequence of video frame data includes video frame data from which said conesponding video frame can be constructed without reference to any other video frame data, and video frame data from which said conesponding video frame cannot be constructed witiiout reference to any other video frame data, said step of selecting a selected set of video frames including selecting for said selected set video frames that conespond to video frame data from which said conesponding video frame cannot be constructed witiiout reference to any other video frame data if and only if die video frames that conespond to said otiier video frame data have also been selected for said selected set of video frames. 7. The method of Claim 5 wherein said second presentation rate is negative relative to said first presentation rate. 8. The method of Claim 1 wherein said steps of selecting, constructing and transmitting are performed in parallel. 9. The method of Claim 1 wherein said step of constructing said second digital data stream includes causing said second digital data stream to conform to a predetermined format by inserting prefix data into said second digital data stream prior to each video frame data, and inserting suffix data into said second digital data stream after each video frame data. 10. The method of Claim 9 wherein said step of constructing said second digital data stream includes inserting delay data into said second digital data stream when said second presentation rate is slower than said first presentation rate, said delay data causing more time to elapse between decoding successive video frames represented in said second digital data stream than elapses between decoding successive video frames in said digital data stream. 11. The method of Claim 10 wherein said step of inserting delay data includes inserting padding data into said second digital data stream between frame data that coπesponds to successive video frames. 12. The method of Claim 10 wherein said step of inserting delay data includes inserting decode time stamps into said second digital data stream that indicate when coπesponding video frames represented in said second digital data stream are to be decoded. 13. The metiiod of Claim 1 wherein said steps of selecting is performed by a stream server running on a first computer and said steps of constructing and transmitting are performed by a video pump running on a second computer, the method further comprising the step of said stream server transmitting to said video pump a sequence of instructions that indicate how said video pump is to construct said second digital data stream. 14. The method of Claim 1 further comprising the steps of: parsing the digital data stream to generate tag information, said tag information including for each frame represented in said digital data stream: a location within said digital data stream ofthe frame data that represents said frame, a size ofthe frame data that represents said frame, and a time value that indicates when said frame is to be displayed during a performance of said audio-visual work at said first presentation rate; and selecting said selected set of video frames based on said tag information. 15. The method of Claim 14 wherein said digital data stream is divided into a plurality of packets, said step of parsing including die step of generating in said tag information, for each frame represented in said digital data stream, data indicating an offset between a starting boundary of the frame data for said frame and a start of die packet of said plurality of packets in which said starting boundary resides. 16. The method of Claim 9 wherein one or more non- video packets are contained witiiin a start boundary and an end boundary of frame data for a given frame in said selected set of video frames, the method further comprising the step of removing at least one of said one or more non- video packets from said frame data for said given frame prior to transmitting said frame data for said given frame in said second digital data sfream. 17. The method of Claim 1 further comprising the steps of: transmitting said digital data stream to said decoder prior to receiving a rate change request from a user; receiving said rate change request from said user; and in response to said rate change request from said user, ceasing to transmit said digital data stream to said decoder, and performing said steps of selecting said selected set of video frames, constructing said second digital data stream, and transmitting said second digital data stream to said decoder. 18. The method of Claim 17 wherein said rate change request includes data identifying said second presentation rate, the method including die step of determimng said second presentation rate by reading said data identifying said second presentation rate from said rate change request. 19. The method of Claim 17 wherein said step of transmitting said digital data stream includes reading said digital data stream from a plurality of storage devices using redundant anay of inexpensive disks (RAID) enor conection, and said step of transmitting said second digital data sfream includes reading frame data for said selected set of video frames from said plurality of storage devices witiiout using RAID enor conection. 20. The method of Claim 1 further comprising the steps of: selecting a selected data stream from a plurality of digital data streams, each of said plurality of digital data streams representing said audio-visual work at a different presentation rate; if said selected data sfream represents said audio- visual work at said presentation rate, then transmitting data from said selected data stream witiiout performing said steps of selecting a selected set of video frames and constructing said second digital data sfream; If said selected data stream does not represent said audio- visual work at said second presentation rate, then selecting said selected set of video frames from frames represented in said selected data stream and; constructing said second digital data stream to include frame data from said selected data stream. 21. The method of Claim 20 wherein: said plurality of digital data streams include a plurality of frame types including a first frame type that contains all ofthe information required to reconstruct a frame; and said step of selecting said selected data stream is performed based on how many of said video frames in said selected set of video frames would have said first frame type. 22. A metiiod of preprocessing an original digital data sfream that represents an audio-visual work to create tag information, the audio-visual work including a sequence of frames, the original digital data stream including a sequence of frame data, each frame data conesponding to a frame in said sequence of frames, the method comprising the steps of: for each frame in said sequence of frames, performing die steps of determining boundaries witiiin said original digital data stream for the frame data coπesponding to said frame; generating tag data that includes boundary data diat indicates said boundaries of said frame data; and storing said tag data separate from said original digital data stream. 23. The method of Claim 22 further comprising the steps of: generating a second digital data stream based on said original digital data sfream; when said second digital data stream reflects data at a first location in said original digital data sfream, performing the steps of receiving a skip confrol signal that indicates a second location in said original digital data stream; reading said tag data to determine a boundary of a frame that conesponds to said second location; causing said second digital data sfream to reflect data beginning at the boundary ofthe frame that conesponds to said second location. 24. The method of Claim 23 wherein said skip control signal indicates said second location by specifying an amount of time, said second location being the location of data tiiat would be reflected in said second digital data stream after said amount of time had elapsed during a normal speed, sequential playback operation. 25. The method of Claim 22 wherein said sequence of frame data includes multiple types of frame data, die metiiod further comprising, for each frame in said sequence of frames, performing the step of generating frame type data diat indicates a frame type for the frame data coπesponding to said frame. 26. The method of Claim 22 wherein: a decoder uses one or more state machines to decode said original digital data stream; said tag data includes, for each frame in said sequence of frames, state data tiiat represents a state of said one or more state machines, said state data for a given frame indicating a state tiiat said one or more state machines would be in when said decoder receives the frame data conesponding to said given frame during a normal-speed performance of said audio-visual work. 27. The method of Claim 26 wherein said original digital data sfream is an MPEG-2 data stream, said one or more state machines including a program elementary stream state machine, a video state machine and a fransport layer state machine. 28. The method of Claim 26 wherein said original digital data stream is an MPEG-1 data stream, said one or more state machines including a pack layer state machine and a system state machine. 29. A method for selecting frames for display during a performance at a specified presentation rate of a work represented in a digital video file, wherein the performance is produced by decoding a data stream generated from the digital video file over a channel having a predetermined data transfer rate, the method comprising: selecting for display a first frame represented by frame data in said digital video file, said first frame being associated widi a first time; inspecting a second frame that is represented by frame data in said digital video file, said second frame being associated widi a second time; determining a bit budget based on said presentation rate, a time difference between said first time and said second time and said predetermined data transfer rate; comparing the bit budget to die size of die frame data diat represents said second frame; if the size of the frame data exceeds the bit budget, tiien skipping the second frame; and if die size of die frame data is less tiian the bit budget, tiien selecting the second frame for display . 30. The method of Claim 29 further comprising the steps of: determining a frame type of said second frame; and skipping said second frame if said frame type does not conespond to an encoding technique that preserves all ofthe information required to reproduce said frame. 31. The method of Claim 29 wherein said digital video file is an MPEG- 1 or MPEG-2 file which includes I-frames, P-frames and B-frames, the method further comprising the steps of: determining whetiier said second frame is an I-frame, a P-frame or a B- frame; if said second frame is a P-frame, then skipping said second frame if any I- frames or P-frames located between said first frame and said second frame have been skipped; and if said second frame is a B-frame, then skipping said second frame if any I- frames or P-frames located between said first frame and said second frame have been skipped. 32. The method of Claim 29 further comprising the steps of: determimng whetiier selection of said second frame would cause said data stream to have a padding rate less than a predetermined padding rate; and skipping said second frame if selection of said second frame would cause said data sfream to have a padding rate less tiian said predetermined padding rate. 33. The method of Claim 29 further comprising the step of retrieving data diat indicates said first time, said second time and die size of die frame data diat represents said second frame from a tag file maintained separate from said digital video file. 34. The method of Claim 29 wherein said method is performed in real time during the performance at the specified presentation rate ofthe work represented in the digital video file. 35. The method of Claim 29 further comprising the steps of: receiving data representing a rate for skipping a particular type of reference frames; determining whether said second frame is said particular type of reference frame; and if die size of die frame data is less than the bit budget, tiien skipping said second frame if necessary to maintain said rate of skipping said particular type of reference frame. 36. The method of Claim 35 wherein said digital video file is an MPEG-1 or MPEG-2 file, wherein: the step of receiving data representing a rate for skipping said particular type of reference frames includes receiving data representing a rate for skipping I-frames; die step of determining whetiier said second frame is said particular type of reference frame includes determimng whether said second frame is an I-frame; and die step of skipping said second frame if necessary to maintain said rate of skipping said particular type of reference frames includes skipping said second frame if necessary to maintain said rate of skipping I- frames. 37. A method for creating a second digital video stream from one or more other digital video streams, the method comprising the computer-implemented steps of: receiving a series of editing commands, each editing command in said series of editing commands specifying a start position, an end position, and a presentation rate; for each editing command in said series of editing commands, performing the steps of selecting a selected set of video frames between said start position and said end position in said one or more other digital video streams based on said presentation rate; and storing frame data coπesponding to said selected set of video frames in said second digital video sfream. 38. The method of Claim 37 wherein: said one or more otiier digital video sfreams includes a plurality of otiier digital video streams; each editing command in said series of editing commands specifies one of said other digital video sfreams; said step of selecting said selected set of video frames includes selecting video frames that are represented by data tiiat is located between said start position and said end position in die digital video stream specified in said editing command. 39. The method of Claim 37 wherein: said start position indicates a first amount of elapsed time; and said end position indicates a second amount of elapsed time. 40. The method of Claim 37 wherein the step of selecting said selected set of video frames between said start position and said end position in said one or more other digital video streams based on said presentation rate comprises the steps of: determining a bit budget based on a first time value associated witii a most recently selected video frame, a second time value associated widi a cuπent frame, said presentation rate and a data transfer rate; determining a size of die frame data diat coπesponds to the cuπent frame; if the size ofthe frame data that coπesponds to die cuπent frame exceeds said bit budget, tiien not selecting said cunent frame as a video frame in said selected set of video frames, and selecting a new frame as a new cuπent frame; and if die size of d e frame data that coπesponds to die cunent frame does not exceed said bit budget, tiien selecting said cuπent frame as a video frame in said selected set of video frames. 41. The method of Claim 37 further comprising storing data between said frame data coπesponding to said selected set of video frames to cause said second digital video stream to conform to a predetermined format. 42. The method of Claim 41 wherein: said predetermined format is MPEG-2; and said step of storing data between said frame data includes storing data to serve as a valid transport packet header and PES packet header. 43. The method of Claim 41 wherein: said predetermined format is MPEG- 1 ; and said step of storing data between said frame data includes storing data to serve as a valid pack header and system header. 44. The method of Claim 41 further comprising the step of removing one or more non-video packets from frame data conesponding to a frame of said selected set of video frames prior to storing said frame data in said second digital video sfream. 45. A method for performing a seek operation during a performance of an audio- visual work, said performance being performed by decoding a digital data stream transmitted to a decoder, the metiiod comprising the steps of: receiving a seek instruction while transmitting to said decoder data from a first position in a digital representation of said audio-visual work; in response to said seek instruction, performing the steps of: ceasing to transmit data from said first position; transmitting data from a second position in said digital representation of said audio- visual work. 46. The method of Claim 45 further comprising the steps of: generating prefix data tiiat indicates a state of one or more state machines; transmitting said prefix data after ceasing to transmit data from said first position and prior to transmitting data from said second position. 47. The method of Claim 45 wherein said seek instruction indicates a predetermined period of time, the method further comprising the step of: determining said second position based on said first position and said predetermined period of time. 48. The method of Claim 47 wherein said step of determining said second position based comprises die steps of: inspecting a first time value associated widi a first frame tiiat is represented by data located at said first position in said digital representation of said audio-visual work; calculating a second time value based on said first time value and said predetermined period of time; determining a second frame associated witii said second time value; and determining said second position to be a position within said digital representation of said audio- visual work of frame data that represents said second frame. 49. The method of Claim 48 further comprising the steps of: parsing said digital representation of said audio- visual work to generate tag data, said tag data including, for each frame in said audio-visual work: a time value; and a start position; said step of inspecting a first time value including reading said first time value from said tag data; said step of determining said second frame associated with said second time value including inspecting said tag data to find a time value d at coπesponds to said second time value; and selecting die frame associated with die time value that coπesponds to said second time value as said second frame.

Inventors: PAWSON DAVE, PORTER MARK A, PAWSON, DAVE, PORTER, MARK, A.
Applicants: ORACLE CORP, ORACLE CORPORATION
Application year: 1996
Main classification: H04N7
IPC codes: G11B27, H04N7