Patent Publication Number: US-6704813-B2

Title: System for storing streaming information in a circular buffer by using padding block containing non-streaming information to fill a partition of the buffer

Description:
REFERENCE TO CO-PENDING APPLICATIONS 
     The present application is a divisional of and claims priority of U.S. patent application Ser. No. 09/286,747, filed Apr. 6, 1999. 
     Reference is hereby made to U.S. patent applications all filed on Apr. 6, 1999, Ser. No. 09/286,789, now abandoned, entitled “STREAMING INFORMATION APPLIANCE WITH CIRCULAR BUFFER”; Ser. No. 09/287,075, now U.S. Pat. No. 6,378,035, entitled “STREAMING INFORMATION APPLIANCE WITH BUFFER READ AND WRITE SYNCHRONIZATION”; Ser. No. 09/286,808, still pending, entitled “STREAMING INFORMATION APPLIANCE WITH BUFFER FOR TIME SHIFTING”; Ser. No. 09/286,746, now U.S. Pat. No. 6,463,486, entitled “AN INFORMATION APPLIANCE FOR RECEIVING STREAMING INFORMATION AND READING THE INFORMATION WITH A PLURALITY OF READER MODULES”; and Ser. No. 09/286,865, now U.S. Pat. No. 6,535,920 entitled “ANALYZING, INDEXING AND SEEKING OF STREAMING INFORMATION”, which are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to streaming information. More particularly, the present invention relates to recording streaming information and retrieving the stored information for selective playback. 
     With advances in technology including faster processors, improved graphics, and so forth, a desktop computer can easily receive and present streaming information to the user. Common examples of streaming information include streaming video and audio delivered over a wide area network, such as the Internet. For instance, television broadcast signals that would otherwise be transmitted wirelessly using satellites, television transmitters, etc., are encoded and made available for transmission to remote computer users via the Internet. Upon request by the desktop computer user, the encoded data packets containing audio and video data are sent to the desktop computer user sequentially. Upon receipt, the data packets are decoded and processed by the desktop computer in order to render the streaming information to the user in as close to real time as possible. After rendering or presentation, the data packets are discarded. 
     Although processing streaming information in the manner described above is useful, there exist a number of shortcomings. Currently, streaming information is provided at the request of each desktop computer. Thus, each user must form a separate connection with the source of streaming information in order to receive the desired streaming information. Once initiated, the user is unable to control the manner in which streaming information is rendered. For instance, the user cannot temporarily “pause” the incoming streaming information in order to perform another task and then resume viewing when desired. Likewise, the user is unable to repeat a previously rendered portion since the data packets are discarded, or skip ahead since the data packets have not been received. 
     There thus is an ongoing need to improve the manner in which streaming information is rendered. Although described above with respect to a desktop computer and streaming information received from the Internet, the improved process should be applicable to other information appliances or computing devices and other forms of streaming information. 
     SUMMARY OF THE INVENTION 
     A data block format for streaming information includes a first data block size field and a second data block size field, each of the fields indicating the size of the data block. A payload field is bounded by the first data block sized field and the second data size in the data block format. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first exemplary environment for the present invention. 
     FIG. 2 is a block diagram of a second exemplary environment for the present invention. 
     FIG. 3 is a perspective view of a mobile device. 
     FIG. 4 is a block diagram of a third exemplary environment for the present invention. 
     FIG. 5 is a system for processing streaming information. 
     FIG. 6 is a block diagram of a delay filter. 
     FIG. 7 is a block diagram of delay filter  112  showing an interface to a circular buffer in the filter. 
     FIG. 8 is a diagram illustrating the circular buffer  124  along a linear time line. 
     FIGS. 9A and 9B together form a flow chart for a write portion of a synchronization algorithm according to one embodiment of the present invention. 
     FIG. 10 is a diagram illustrating advancement of a Tail Pointer variable within the circular buffer. 
     FIGS. 11A and 11B together illustrate a flow chart for a read portion of the synchronization algorithm according to one embodiment of the present invention. 
     FIG. 12 is a diagram illustrating an example of the circular buffer when a reader module has overtaken a writer module. 
     FIG. 13 is a diagram illustrating a status register which is maintained for each data channel for specifying which user operations are permitted or prohibited in a present state of the channel. 
     FIG. 14 is a table listing examples of user operations corresponding to user operation fields UOP 0 -UOP 10  in FIG.  13 . 
     FIG. 15 is a flow chart of a software object that is called by an application in response to a user operation request. 
     FIG. 16 is a flow chart illustrating steps performed by the delay filter in maintaining the user operation bits of the status register shown in FIG.  13 . 
     FIG. 17 is a pictorial representation of a data block. 
     FIG. 18 is a pictorial representation of a first sequence of data blocks. 
     FIG. 19 is a pictorial representation of a second sequence of data blocks. 
     FIG. 20 is a block diagram of another embodiment of a system in accordance with the present invention. 
     FIG. 20A is a block diagram of an example of a filter graph. 
     FIGS. 21A-21D are block diagrams illustrating stream analysis. 
     FIGS. 22-23C are flow diagrams illustrating stream analysis. 
     FIG. 24 is a flow diagram illustrating indexing. 
     FIG. 25 is an illustration of a portion of a data buffer. 
     FIG. 26 is a flow diagram illustrating a seeking operation. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     Overview 
     FIG.  1  and the related discussion are intended to provide a brief, general description of a first exemplary computing environment in which the invention may be implemented. Although not required, the invention will be described, at least in part, in the general context of processor executable instructions, such as program modules being executed by a controller, processor, a personal computer or other computing device. Generally, program modules include routine programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Tasks performed by the program modules are described below and with the aid of block diagrams and flowcharts. Those skilled in the art can implement the description, block diagrams and flowcharts to processor executable instructions, which can be written on computer readable mediums. In addition, those skilled in the art will appreciate that the invention may be practiced with other information appliances, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention is also applicable in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     With reference to FIG. 1, the first exemplary environment for the invention includes a general purpose computing device in the form of a conventional personal computer  20 , including processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to processing unit  21 . System bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system  26  (BIOS), containing the basic routine that helps to transfer information between elements within personal computer  20 , such as during start-up, is stored in ROM  24 . Personal computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk (not shown), a magnetic disk drive  28  for reading from or writing to removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical medium. Hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to system bus  23  by a hard disk drive interface  32 , magnetic disk drive interface  33 , and an optical drive interface  34 , respectively. The drives and the associated computer readable medium provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for personal computer  20 . 
     Although the exemplary environment described herein employs a hard disk, a removable magnetic disk  29  and a removable optical disk  31 , it should be appreciated by those skilled in the art that other types of computer readable medium which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memory (ROM), and the like, may also be used in the exemplary operating environment. 
     A number of program modules may be stored on hard disk, magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more application programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into personal computer  20  through input devices such as a keyboard  40  and pointing device (mouse)  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to processing unit  21  through a serial port interface  46  that is coupled to system bus  23 , but may be connected by other interfaces, such as a sound card, a parallel port, a game port or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to system bus  23  via an interface, such as a video adapter  48 . In addition to monitor  47 , personal computers may typically include other peripheral output devices such as a speaker  49  connected to a sound card  57  and printers (not shown). 
     Personal computer  20  may operate in a networked environment using logic connections to one or more remote computers, such as a remote computer  49 . Remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device or other network node, and typically includes many or all of the elements described above relative to personal computer  20 , although only a memory storage device  50  has been illustrated in FIG.  1 . The logic connections depicted in FIG. 1 include a local are network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer network Intranets and the Internet. 
     When used in a LAN networking environment, personal computer  20  is connected to local area network  51  through a network interface or adapter  53 . When used in a WAN networking environment, personal computer  20  typically includes a modem  54  or other means for establishing communications over wide area network  52 , such as the Internet. Modem  54 , which may be internal or external, is connected to system bus  23  via serial port interface  46 . In a network environment, program modules depicted relative to personal computer  20 , or portions thereof, may be stored in the remote memory storage devices. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
     Streaming information can be received by computer  20  using a number of known methods and technologies. Commonly, a source of streaming information is a remote computer wherein computer  21  is connected to the remote computer using a wired or wireless modem. This technique is used often when streaming information is provided through an intranet or the Internet, typically in digital form. Digital streaming information can further comprise satellite signals received by a satellite receiver, dish or the like. 
     However, streaming information can also arrive as analog signals. For instance, the streaming information can also comprise broadcast radio or television signals. In such cases, computer  20  includes a radio tuner  60  and a television tuner  62  to receive the broadcast signals and convert the analog signals to digital form for transmission over system bus  23 . 
     It should be understood that the present invention can be used in other computing devices besides computer  20 , discussed above. FIG. 2 is a block diagram of a mobile device  68 , which is another exemplary computing environment. Mobile device  68  includes a microprocessor  70 , memory  72 , input/output (I/O) components  74 , a communication interface  76  for communicating with, for example, computer  20 . In a one embodiment, the afore-mentioned components are coupled for communication with one another over a suitable bus  78 . 
     Memory  72  is implemented as non-volatile electronic memory such as random access memory (RAM) with a battery back-up module (not shown) such that information stored in memory  72  is not lost when the general power to mobile device  68  is shut down. A portion of memory  72  is preferably allocated as addressable memory for program execution, while another portion of memory  72  is preferably used for storage, such as to simulate storage on a disc drive. 
     Memory  72  includes an operating system  80 , an application program  82  as well as an object store  84 . During operation, operating system  80  is preferably executed by processor  70  from memory  72 . Operating system  80 , in one preferred embodiment, is a “WINDOWS CE” brand operating system commercially available from Microsoft Corporation. Operating system  80  is preferably designed for mobile devices, and implements database features which can be utilized by application  82  through a set of exposed application programming interfaces and methods. The objects in object store  84  are maintained by application  82  and operating system  80 , at least partially in response to calls to the exposed application programming interfaces and methods. 
     Communication interface  76  represents numerous devices and technologies that allow mobile device  68  to receive streaming information. The devices are similar to those discussed above with respect to computer  20  and include wired and wireless modems, satellite receivers and broadcast tuners to name a few. Mobile device  68  can also be directly connected to computer  20  to exchange data therewith. In such cases, communication interface  76  can be an infrared transceiver or a serial or parallel communication connection, all of which are capable of transmitting streaming information. 
     FIG. 3 is a simplified pictorial illustration of mobile device  68 . Mobile device  68  can be a desktop assistant sold under the designation H/PC having software provided by the Microsoft Corporation. In one embodiment, mobile device  18  includes a miniaturized keyboard  83 , display  85  and stylus  86 . In the embodiment shown in FIG. 3, display  85  is a liquid crystal display (LCD) which uses a contact sensitive display screen in conjunction with stylus  86 . Stylus  86  is used to press or contact display  85  at designated coordinates to accomplish certain user input functions. Miniaturized keyboard  83  is preferably implemented as a miniaturized alpha-numeric keyboard, with any suitable and desired function keys, which are provided for accomplishing certain user input functions. In other known embodiments, keyboard  83  is omitted and a “soft” keyboard is provided through the contact sensitive display screen. In yet other embodiments, a character recognition module is employed to recognize characters written on the contact sensitive display screen using stylus  86 . 
     FIG. 4 illustrates yet another exemplary environment in which the present invention can operate. In FIG. 4, an entertainment appliance  90  is illustrated and includes processing unit  21 , system memory  22  and system bus  23 . Hard disk drive  27 , or any of the other storage devices described above, are further coupled to system bus  23  and used for temporary and permanent storage of program applications, data, etc. Unlike typical desktop computers such as computer  20  described above, entertainment appliance  90  may use a limited input device such as a handheld remote  92  operable with a receiver  94 , which can be an infrared receiver, wireless receiver, or the like. In entertainment appliance  90 , information is rendered to the user using monitor  47  or other display device that is coupled to system bus  23  with video adapter  48 . Audio information is also rendered and illustrated herein with speaker  49 . Speaker  49  is coupled to system bus  23  with sound card  57 , which can be combined with video adapter  48  to form a signal device, if desired. It should be further understood that audio and video information could be provided to external components, such as amplifiers or the like, which in turn, are connected to monitor  47  and speakers  49 . 
     Streaming information is provided to entertainment appliance  90  through a communications interface  98 . Communications interface  98  can be any of the devices and technologies described above with respect to the two previous environments. 
     Delay Filter 
     FIG. 5 is a general block diagram illustrating a system  110  for processing streaming information. System  110  includes a delay filter  112  for temporarily storing streaming information received from a streaming information source  114 . Delay filter  112  is further coupled to a rendering device or devices  116  to render streaming information upon request by the user. Also illustrated in FIG. 5 is an encoder  118  and a decoder  120 . Although not required, encoder  118  and decoder  120  can improve system performance wherein encoder  118  receives streaming information source  114  and compresses the streaming information prior to transfer and storage in delay filter  112 . Decoder  120  receives the streaming information temporarily stored in delay filter  112  in the compressed format and uncompresses the streaming information prior to transferring the streaming information to rendering device  116 . 
     At this point, it should be noted that system  110  can be operated in any of the computing environments described above, or similar computing environments. Those skilled in the art will appreciate that delay filter  112 , rendering device  116 , encoder  118  and decoder  120  can be implemented in hardware, software, or combinations thereof. In one embodiment by way of example, delay filter  112  is embodied in the operating system. Higher level application programs or other portions of the operating systems can access functions of delay filter  112  using application program interfaces (APIs) as is well known in the art. 
     In operation, streaming information source  114  provides an information stream to delay filter  112  (optionally through encoder  118 ). Generally, the streaming information comprises digital data representing one or more channels of content information. For instance, streaming information source  114  can comprise an Intranet or the Internet available through the communication interfaces described above. Likewise, streaming information source  114  can comprise an analog or digital television tuner wherein separate audio, video and data (e.g. closed captioning) information streams comprise a single channel. Other sources of streaming information include, but are not limited to, audio tuners, satellite receivers and the like. 
     In the embodiment illustrated, encoder  118  receives the streaming information and encodes or compresses the streaming information into a known format such as “MPEG”, “AVI”, “MOV” (Apple®QuickTime®) and “WAV”, although, if used, the present invention is not limited to any one particular encoding format. 
     Generally, as discussed below, delay filter  112  includes a writer module  122 , a circular buffer  124  and one or more reader modules  126 . Writer module  122  receives the streaming information provided by streaming information source  114  and writes the streaming information into circular buffer  124 . Circular buffer  124  can comprise any of the storage devices described above, for example, hard disk  27  or RAM memory. Reader module  126  accesses circular buffer  124  to retrieve the streaming information when the streaming information is to be rendered. If the streaming information stored in circular buffer  124  is encoded or compressed, decoder  120  decodes or uncompresses the streaming information, which is then provided to rendering device  116 . 
     FIG. 6 is a more detailed pictorial representation of delay filter  112 . In this illustrated embodiment, the streaming information comprises a television signal or channel that includes audio, video and data (closed captioning) streams. The streaming information is first provided to a stream analyzer  130 , which analyzes the incoming streams and provides such information as synchronization points that may be present in each of the streams. Synchronization points are used in rendering some types of streaming information and are discussed in detail below. 
     Streaming information and synchronization point information is provided to writer module  122 . In one embodiment, as illustrated, writer module  122  comprises a mux writer that receives multi-stream streaming information for storage in circular buffer  124 . As indicated above, one or more reader modules  126  (herein labeled as  126   1 ,  126   2  and  126   3 ) are provided to read the streaming information from circular buffer  124  for rendering. Writer module  122  stores synchronization information in an index  132 . Reader modules  126  may access index  132  in order to locate a particular portion of the streaming information and properly render the streaming information. Operation of writer module  122 , circular buffer  124 , reader modules  126  and index  132  are discussed in detail below. 
     In the embodiment illustrated, two separate playback reader modules  126   1 , and  126   2  are illustrated having outputs that provide streaming information to separate video, audio and data decoders  120  and rendering devices  116 . In general, this illustrates that separate reader modules  126  can be reading streaming information from circular buffer  124  at different points in circular buffer  124 , and thus, represents separate individuals accessing the data stored therein. In addition, other reader modules  126 , such as indicated at  136 , can be implemented to archive and store the streaming information in circular buffer  124  for later viewing. Generally, in archiving system  136 , reader module  126   3  provides streaming information to a mux formatter  138  that, in turn, provides the information to a writer module  140  for storage in any of the storage devices indicated above such as hard disk  27 . 
     Circular Buffer 
     Referring back to circular buffer  124 , circular buffer  124  has “floating” beginning and ending points, which are referred to as a logical “head”  150  and “tail”  152 . Head  150  corresponds to the logical head of valid data in circular buffer  124 , and tail  152  corresponds to the logical tail of valid data in circular buffer  124 . Writer module  122  always writes to the head of buffer  124 , which moves circularly through the buffer in the direction of arrow  154 . Buffer  124  therefore always has a fixed maximum time quantum of data available for reading. For example, when time shifting multimedia (e.g., audio and video) content, writer module  122  receives the streaming multimedia information and stores the information in circular buffer  124 . The user views the stored multimedia content through one of the readers modules  126 . The circular structure of buffer  124  allows some portion of the streaming information to be available for the user to “instant replay” or “pause” on demand, for example, without allowing the buffer to “fill-up” with “time-shifted” data. Circular buffer  124  can be implemented in volatile or non-volatile memory, such as random access memory (RAM), a hard disk, a floppy disk or an optical disk. In one embodiment, circular buffer  124  is implemented in hard disk drive  27 . 
     FIG. 7 is a block diagram of delay filter  112  showing an interface to circular buffer  124 . Buffer IO layer  200  interfaces between circular buffer  124  and the clients of the buffer, which include writer module  122  and reader modules  126  (labeled  126   1 - 126   N , where N is an integer variable greater than or equal to 1) Buffer IO layer  200  implements the circularity of buffer  124  and synchronizes writer module  122  with reader modules  126   1 - 126   N  Buffer IO layer  200  implements circularity by translating between logical addresses, used at upper interfaces  204 - 207  to identify logical positions within buffer  124 , and physical (wrap around) addresses, used at lower interface  208  for identifying particular physical addresses within buffer  124 . The logical addresses always increase (or decrease) with time and never wrap around. The logical addresses can be periodically reset as desired. The physical addresses wrap around (i.e. wrap from the highest address of the buffer to the lowest address of the buffer) at a frequency determined by the circular buffer size. As each successive block of the streaming information is received by writer module  122 , the block is associated with a respective logical address or a range of logical addresses, which increases with each successive block. Buffer IO layer  200  translates the logical addresses into corresponding physical addresses which are used to access circular buffer  124 . In one embodiment, buffer IO layer  200  translates the logical addresses into physical addresses as a function of the logical address modulo the buffer size (i.e. the number of storage locations in circular buffer  124 ). Buffer layer IO  200  can be implemented in an operating system, for example. 
     Write/Read Synchronization 
     Since writer module  122  and reader modules  126   1 - 126   N  can operate independently of one another and at different data rates, buffer IO layer synchronizes writer module  122  and reader modules  126   1 - 126   N  to maintain a predetermined temporal order between writing and reading. In one embodiment, buffer IO layer  200  prevents any reader  126   1 - 126   N  from reading data that is not yet logically available and prevents writer module  122  from overwriting data that is in the process of being read by one or more of the reader modules  126   1 - 126   N . In circular buffer  124 , a given physical position corresponds to multiple logical positions. Without synchronization, a reader module that is trailing writer module  122  by a distance that is close to the buffer size may be reading from the same physical area to which writer module  122  is writing. Buffer IO layer  200  also allows reader modules  126   1 - 126   N  to follow writer module  122  as close as possible to minimize latency. 
     Buffer IO layer  200  implements a synchronization algorithm for writer module  122  and reader modules  126   1 - 126   N . Each time writer module  122  wants to pass data to buffer IO layer  200 , its corresponding application calls the synchronization algorithm. Similarly, each time one of the reader modules  126   1 - 126   N  wants to read data from buffer IO layer  200 , its corresponding application calls the synchronization algorithm. The synchronization algorithm can be implemented in hardware, software, or a combination of both, as desired. 
     The synchronization algorithm uses “blocking” to (1) block a reader module  126   1 - 126   N  that is trying to read data which has not yet been written and/or (2) block writer module  122  if it is trying to write to an area of circular buffer  124  from which one of the reader modules  126   1 - 126   N  is currently reading. In both cases, one component is blocked until another component has completed the operation necessary to remove the offending condition. For example, if writer module  122  is blocked, it remains blocked until all of the reader modules  126   1 - 126   N , which who are reading from the area to be written, have completed their reads. When a reader module is blocked, it remains blocked until writer module  122  has written all of the data requested by that reader module. 
     The synchronization algorithm uses a plurality of shared variables. Each client has its own set of variables which are shared with the other clients. In FIG. 8, circular buffer  124  is arranged along a linear time line from zero to infinity. The synchronization algorithm maintains a “Tail Pointer”  230  which is an integer variable indicating the logical tail of valid data in circular buffer  124 . A “Head Pointer”  232  is an integer variable which indicates the logical head of valid data in circular buffer  124 . For a read operation to succeed, the logical read position within circular buffer  124  must be greater than or equal to Tail Pointer  230  and less than or equal to Head Pointer  232 . Since buffer  124  is circular, the logical position of Tail pointer  230  is also logically “ahead” of the logical position of Head Pointer  232 . 
     When writer module  122  issues a write command, it specifies an area in buffer  124  to be written, starting at a present write position  234 . A “Writer Blocked On” integer variable  236  is used to identify the logical position corresponding to the end of the data to be written when all or part of the area to be written is blocked by one or more of the reader modules  126   1 - 126   N . The Writer Blocked On variable is set when one of the reader modules  126   1 - 126   N  is currently reading from that area. For example, a reader module may be presently reading from logical position  235 , which is in the area between positions  234  and  236 . A “zero” value for the “Writer Blocked On” variable indicates that writer module  122  is not currently blocked by any of the reader modules  126   1 - 126   N . 
     A “Currently Reading From” integer variable is maintained for each reader module  126   1 - 126   N . The Currently Reading From variable is used to indicate that the reader module is currently performing a read that starts at this logical position in circular buffer  124 . For example, in FIG. 8, a reader module is currently reading from logical position  235  in circular buffer  124 . The Currently Reading From variable is used to prevent writer module  122  from overwriting the data in logical position  235  while the reader module is reading from logical position  235 . When a particular reader module  126   1 - 126   N  is not currently reading from circular buffer  124 , its corresponding Currently Reading From variable is set to infinity. 
     A “Writer Unblocked Event” variable is used to “wake-up” writer module  122  when it can proceed with its desired write command. For example, the Writer Unblock Event variable is set to an active state when writer module  122  is not blocked, and is reset to an inactive state when writer module  122  is blocked. The Writer Unblock Event can be implemented as a Windows® event (a “Win32” event) or any other similar synchronization mechanism familiar to those skilled in the art. 
     A “Reader Unblock Event” variable is used for each reader module  126   1 - 126   N  for “waking-up” the reader module when the data it is requesting is available. For example, the Reader Unblock Event variable is set to an active state when the corresponding reader module  126   1 - 126   N  is not blocked and is reset to an inactive state when the corresponding reader module  126   1 - 126   N  is blocked. 
     A “Critical Section” variable is used to protect access to each of the above shared variables. For example, a “Win32” Critical Section may be used or any other similar synchronization mechanism such as a mutual exclusion “mutex” object, as is known to those skilled in the art. 
     FIGS. 9A,  9 B,  11 A and  11 B together form a flow chart for the synchronization algorithm according to one example of the present invention. The steps performed during a write are shown in FIG.  9 A and are labeled  300 - 317 . At step  300 , when the application driving writer module  122  wishes to pass data to Buffer IO layer  200 , the application calls algorithm  290 . At step  301 , algorithm  290  locks the Critical Section to protect access to the variables used in steps&#39;  302 - 305 . At step  302 , algorithm  290  advances the “Tail Pointer” to the logical position in circular buffer  124  that corresponds to the end of the write command. This logical position will depend upon the amount of data being written by writer module  122 . The length of data can vary from one write command or data block to the next, and the data can have arbitrary data formats, which can also vary from one write command or data block to the next. 
     FIG. 10 is a diagram illustrating advancement of the Tail Pointer in circular buffer  124 . Tail Pointer  250  is advanced from logical position  260  to logical position  262 . Advancing Tail Pointer  250  immediately invalidates the area to be written (behind the advanced Tail Pointer  250 ) for future reads, even if writer module  122  must wait before it can actually start writing the data. As a result, once writer module  122  notifies buffer IO layer  200  that it wants to write to an area in circular buffer  124  by calling the writer algorithm, no new reader modules can start reading from that area. This minimizes the time during which writer module  122  must wait for reader modules  126   1 - 126   N  and prevents the writer from being perpetually blocked. 
     Referring back to FIG. 9A, synchronization algorithm  290  determines whether any of the “Currently Reading From” variables of readers  126   1 - 126   N  is less than (i.e. behind) the “Tail Pointer” variable, at step  303 . For example, in FIG. 10, a reader module may have a Currently Reading From variable  270  which is pointing to a logical position  272  in circular buffer  124  that is less than the logical position  262  of the advanced Tail Pointer  250 . If this is the case, synchronization algorithm  290  blocks or delays writer module  122  so that the conflicting reader module can “get out of the way”. If none of the reader modules&#39; “Currently reading From variable is less than the advanced Tail Pointer variable, then synchronization algorithm  290  proceeds directly to step  311  (shown in FIG. 9B) to write the data to circular buffer  124 . 
     At step  304 , algorithm  290  sets the “Writer Blocked On” variable to the value of the “Tail Pointer” variable  250 . This indicates the largest logical position in circular buffer  124  at which the reader modules  126   1 - 126   N  must be clear of before writer module  122  can write the data. Algorithm  290  then blocks writer module  122  by resetting the “Writer Unblock” event to the inactive state, at step  305 . Since writer module  122  is blocked, the Critical Sections are unlocked at step  306 . At step  307 , algorithm  290  waits for the “Writer Unblock” event variable to be activated by the synchronization algorithm for the conflicting reader that is accessing the area to be written. When the conflicting reader module activates the “Writer Unblock” event, algorithm  290  locks the Critical Section at step  308  and then resets the “Writer Blocked On” variable to zero, at step  309 . Algorithm  290  again unlocks the Critical Section at step  310  and proceeds to step  311  (shown in FIG.  9 B). 
     At step  311 , buffer IO layer  200  converts the starting logical address to a circular or physical address and writes the data in circular buffer  124 , beginning at that physical address. Once the data has been written, algorithm  290  locks the Critical Section for the variables used in steps  313 - 315  and advances the “Head Pointer” variable to the logical position in circular buffer  124  that corresponds to the end of the data written by writer module  122 . In the example shown in FIG. 10, Head Pointer  274  is advanced from position  276  to position  278  (which can also be viewed as being “behind” the advanced Tail Pointer  250 ). Advancing Head Pointer  274  validates the newly written data between Head Pointer  274  and Tail Pointer  250 . 
     At step  314 , algorithm  290  determines whether any of the “Reader Blocked On” variables for reader modules  126   1 - 126   N  is less than the advanced “Head Pointer” variable  274 . If not, none of the reader modules  126   1 - 126   N  were waiting for the data written by writer module  122 , and algorithm  290  proceeds to step  316 . If so, one or more of the reader modules  126   1 - 126   N  were waiting for writer module  122 , which has now validated the desired logical positions. Algorithm  290  sets any such “Reader Unblock” event variables at step  315  to unblock the corresponding reader module. 
     In the example shown in FIG. 10, a reader module may be waiting to read data at logical position  280  which was not yet available when Head Pointer  274  was pointing to logical position  276 . That reader module would have had a “Reader Blocked On” variable pointing to logical position  280 , as shown by arrow  282 . Now that “Head Pointer” variable  274  is pointing to logical position  278 , which is ahead of logical position  280 , the data in position  280  is available for reading, and algorithm  290  sets the “Reader Unblock” event variable for that reader module to the active state, thereby unblocking the reader module. 
     Referring back to FIG. 9B, the corresponding Critical Sections are unlocked at step  316 , and the algorithm completes at step  317 . 
     FIGS. 11A and 11B together illustrate a read portion of synchronization algorithm  290 , which includes steps  351 - 372 . When one of the reader modules  126   1 - 126   N  desires to read data from circular buffer  124 , that reader module calls algorithm  290  in buffer IO layer  200 , at step  351 . At step  352 , algorithm  290  locks the Critical Section to protect the variables used in steps  353 - 355 . At step  353 , algorithm  290  determines whether the amount of data to be read extends to a logical position in circular buffer  124  which is beyond the logical position of the “Head Pointer” such that a portion of the desired data is not yet valid. If not, algorithm  290  proceeds directly to step  360 . 
     FIG. 12 is a diagram illustrating an example of circular buffer  124  when the requested data extends beyond the Head Pointer. Circular buffer  124  has a Tail Pointer  330  at a logical position  332  and a Head Pointer  334  at a logical position  336 . If the reader module requests data beginning at logical position  337  and extending up to logical position  338 , which is beyond the logical position  336  of Head Pointer  334 , then algorithm  350  sets the “Reader Is Blocked On” variable for that reader module to the logical position corresponding to the end of the requested read data (e.g. logical position  338 ), as shown by arrow  340 . Referring back to FIG. 11A, algorithm  350  resets the “Reader Unblock” event variable to block the corresponding reader module at step  355 . Since the reader module is blocked, algorithm  350  unlocks the corresponding Critical Section, at step  356 . At step  357 , algorithm  350  waits for the “Reader Unblock” variable to be set for this reader (at step  315  in FIG.  9 B). Once the “Reader Unblock” variable is set, the corresponding reader module is unblocked and the Critical Section is locked at step  358 . The “Reader Blocked On” variable for that reader is then reset to infinity, at step  359 , and algorithm  350  proceeds to step  360 . 
     At step  360 , algorithm  350  determines whether the logical position corresponding to the beginning of the requested data is before the logical position of the “Tail Pointer” variable. If so, the requested data is invalid since it has already been overwritten by writer module  122 . In this case, algorithm  350  unlocks the Critical Section at step  361  and fails the requested read operation at step  362 . The failure can be passed to the corresponding reader module through a variety of mechanisms, such as a status variable. 
     If the requested data starts at a logical position that is not before the “Tail Pointer” variable, algorithm  350  proceeds to step  363  (shown in FIG.  11 B). At step  363 , algorithm  350  sets the “Currently Reading From” variable of the reader to the logical position at the start of the requested data. The Critical Section is unlocked at step  364 , and the starting logical address is converted to a circular or physical address for circular buffer  124  (shown in FIG.  7 ). At step  365 , buffer IO layer  200  reads the requested amount of data, beginning at the converted logical address and provides the data to the corresponding reader module, at step  365 . At step  366 , algorithm  350  locks the Critical Section. At step  367 , the “Currently Reading From” variable for the reader is reset to infinity since the read operation has completed. 
     At step  368 , algorithm  350  determines whether the “Writer Blocked On” variable points to a logical position in circular buffer  124  that is greater than the read position. The read position is the logical position in circular buffer  124  corresponding to the beginning of the data to be read. If the “Writer Blocked On” variable is not greater than this logical position, then this reader module was not blocking writing module  122 . Algorithm  350  then unlocks the Critical Section at step  369  and completes its function at step  370 . 
     If the “Writer Blocked On” variable is greater than this position, then this reader module was blocking writing module  122 , and algorithm  350  proceeds to step  371  to determine whether any other reader module is blocking writer module  122 . If not, algorithm  350  sets the “Writer Unblock” event variable to unblock writer module  122 , at step  372 . If there are other reader modules that are blocking writer module  122 , then algorithm  350  proceeds to step  369  without setting the “Writer Unblock” event variable. 
     The algorithm shown in FIGS. 9 and 11 can be modified in a variety of ways in alternative embodiments. For example, it may not be desirable to block writer module  122 . If the streaming information being provided to writer module  122  may be lost if writer module  122  is delayed by any significant period of time, such as when writer module  122  is coupled to a television tuner, then it would not be desirable to delay writer module  122 . In these embodiments, algorithm  290  is modified in FIGS. 9A and 9B to remove steps  303 - 310  which serve to delay writer module  122 . Algorithm  290  proceeds from step  302  directly to step  311 . Similarly, algorithm  290  is modified in FIGS. 11A and 11B to remove steps  366 - 369 ,  371  and  372  which serve to selectively unblock writer module  122 . 
     In addition, algorithm  290  can be modified to perform steps  360 - 362  (validating the read operation) either before or after the read is performed at step  365 , or both. Validating the read operation before the actual read operation avoids a wasteful read. However, steps  360 - 362  can be performed after step  366  if desired. Steps  360 - 362  can also be performed both before and after read step  365 . 
     The synchronization algorithm shown in FIGS. 9 and 11 does not require a circular buffer such as that shown in FIG.  6 . The synchronization algorithm can also be used with minor modification in a linear buffer having both logical and physical addresses that wrap around at a frequency based on the size of the buffer. However, a circular buffer allows a reader module to be time-shifted with respect to the writer module such that a portion of the streaming information is available to the user for “instant replay” on demand. A circular buffer allows the user to pause a program, but does not allow the user to inadvertently fill up the buffer with time shifted data. The writer module is always writing to the “beginning” of the buffer, which is point that moves circularly through the buffer. 
     The synchronization algorithm shown in FIGS. 9 and 11 also allows the user to fast forward through the data stored in the buffer. If the user has paused a reader module so that the writer module is considerably ahead of that reader, the user or the application has the option of fast-forwarding the reader module to catch up to the writer module. However, the synchronization algorithm stops fast-forwarding when the reader module catches up with the writer module. For example, the reader module may initiate a “DirectShow” event (or other appropriate event mechanism available to the software environment) whenever the reader module is blocked by the writer module. The application detects this event, and switches the state of the reader module from fast-forward to normal play mode. If the writer module catches up to the reader module, corruption by the writer module can be avoided by (1) blocking the reader until the data is available, (2) blocking the writer until the reader is out of the way, or (3) moving the reader ahead by unpausing or fast forwarding the reader. 
     User Operation Permission Checking 
     In one embodiment, delay filter  112  (shown in FIG. 6) further includes a user operation permission checking and reporting mechanism which maintains consistent operating states in the filter. FIG. 13 is a diagram illustrating a status register  420  which is maintained by delay filter  112  for each data stream, or channel, through the filter. Delay filter  112  can maintain register  420  in RAM, for example, or any other storage medium. Status register  420  includes a plurality of fields, with each field having one or more bits. In the example shown in FIG. 13, bits  0 - 10  of status register  420  correspond to user operation fields UOP 1 -UOP 10 , respectively. Bits  11 - 27  correspond to reserved fields. Bits  28 - 31  correspond to a channel streamer ID field which identifies the particular data stream through writer module  122  and reader modules  126   1 - 126   N  to which status register  420  is associated. Each user operation field UOP 0 -UOP 10  has a binary value which indicates whether the corresponding user operation is permitted or prohibited. For example, a binary “one” in the corresponding field would indicate that that user operation is permitted. A binary “zero” in a user operation field would indicate that that user operation is prohibited. FIG. 14 is a table listing examples of user operations that correspond to user operation fields UOP 0 -UOP 10 . Delay filter  112  maintains the UOP fields for each stream or channel based on the state of its reader module. Delay filter  112  updates the user operation bits whenever the state of the reader module changes. 
     Certain user operations might be permitted only in certain states of the delay filter. Use of a prohibited operation might lead to inconsistent states in the delay filter and user interface. For example, assume that a reader module that is performing “time-shifting” of multimedia content is paused at point X in the circular buffer  124 . In the meantime, writer module  122  is still writing data into circular buffer  124 . Depending on the size of the circular buffer  124  and the time for which the reader module has been paused, there is a possibility that writer module  122  can catch up to the point X where the reader module is paused and try to overwrite the data. In this case, delay filter  112  can forcibly “unpause” the reader module and send a notification to the application that the paused reader module has been unpaused. There is a small time window between the time at which the reader is “unpaused” and the time at which the application receives the notification. During this time window, the application might issue a prohibited user command, such as a forward scan. The reader module will now be performing the forward scan at the time the application receives the notification that the reader module has been unpaused and is playing at normal speed. The application and the delay filter  112  are therefore in inconsistent states. 
     To avoid this problem, delay filter  112  allows the application to check the user operation bits whenever a user operation is issued to check for validity of the operation. Also, whenever the user operation bits change, delay filter  112  reports this change to the application. The application can therefore update its user interface to ensure that invalid user operations are not issued to the delay filter  112 . 
     FIG. 15 is a flow chart of a software module or object that can be called by the application through an application program interface (“API”), for example, whenever the user requests an operation or the user operation status bits change. At step  430 , the application waits for the user to request an operation. Once a user operation has been requested, the application checks the status of the corresponding user operation bit, at step  431 . If the operation is permitted at step  432 , the application issues the requested operation to delay filter  112 , at step  433 . If the requested operation is prohibited, the operation is rejected at step  434 . 
     FIG. 16 is a flow chart illustrating steps performed by delay filter  112  in maintaining the user operation bits for each status register. At step  440 , delay filter detects a change in the state of one of the reader modules. At step  441 , the user operation bits of the status register for the corresponding stream or channel are updated as a function of the change in state. Delay filter  112  then notifies the application through an API, for example, for that stream or channel of the change in state, at step  442 . The application can then update the user interface to disable or enable selected subsets of the user operations listed in FIG. 14. A user interface update can include setting or resetting light indicators, issuance of an audible alert or changing screen displays, for example. The application can determine whether the user interface should be updated by querying the delay filter  112  for the user operation bits for a specific stream or channel. 
     Time Shifting 
     Another broad inventive aspect illustrated by the exemplary embodiment includes the concept of “time shifting” when streaming information is rendered. Generally, an information appliance for receiving streaming information implements time shifting by including a buffer (in a one embodiment circular buffer  124 ), writer module  122  which receives blocks of streaming information and writes blocks to the buffer, and at least one reader module  126  which selectively reads the blocks from the buffer. 
     Although all signal processing equipment inherently includes signal propagation delay, “time shifting,” as used herein, is distinguishable for the reasons discussed below separately or in any combination. In the first instance, the amount of time shifting (i.e., the relative position of any one reader module  126  with respect to the writer module  122  in circular buffer  124 ) is selective and adjustable. In a first mode of operation, the user can “rewind”, “pause”, “fast forward” and “play” in any desired order, thereby changing the relative reading position of an associated reader module  126  with respect to a position of writer module  122  in circular buffer  124 . Hence, rendered streaming information (which begins with a reader module  126 ) is delayed in time from the streaming information that is written by writer module  122 . 
     In another mode of operation, logic is provided to adjust the amount of “time shifting” based upon operation of the system and without user intervention. For instance, if a reader module has been “paused” by a user, thus maintaining the current position of reader module  126  in circular buffer  124 , at some time it may be necessary to begin advancing the reader position in order that writer module  122  can store new streaming information in circular buffer  124 . This situation may require reader module  126  to begin moving forward within circular buffer  124  at a rate greater than or at least equal to the rate at which streaming information is being written to circular buffer  124 . 
     Automatic adjustment of the position of a reader module  126  in circular buffer  124  can also be initiated by user commands not directly associated with movement of a position of reader module  122 , such as “play”, “pause”, “rewind” and “fast forward”. As probably best illustrated by example, the source of streaming information  114  to delay filter  112  can be a multiple broadcast channel device such as a television tuner, or the like. If the user begins rendering information pursuant to a selected broadcast channel, pauses, and then again resumes rendering information on the same broadcast channel, the associated position of reader module  126  will change position accordingly, thereby increasing the relative distance of the position of reader module  126  and the position of writer module  122  in circular buffer  124 . If the user then selects a different broadcast channel to be rendered, for example, a different television channel, writer module  122  will then record the streaming information associated with the new broadcast channel in circular buffer  124 . However, since the position of reader module  126  is “time shifted” from the position of writer module  122  by a delay in proportion to the amount of time the user had paused, the user will not notice the new broadcast channel selection until reader module  126  reaches the new streaming information in circular buffer  124  provided by the change in broadcast channel. In order to obviate the problems presented by the foregoing, the position of reader module  126  is automatically advanced to a position in circular buffer  124  adjacent the position of writer module  122 . This technique reduces the amount of delay so that rendered streaming information substantially follows broadcast channel selection in a multiple broadcast channel environment. 
     Another distinguishing feature between “time shifting” as used herein and typical signal propagation delay is the medium used to record streaming information. In particular, as discussed above, circular buffer  124  can be embodied using any of the storage devices described above such as RAM memory, hard disk  27 , or the like. In addition, the amount of memory present in circular buffer  124  is sufficient to provide a perceptible delay to the user, if desired. In one embodiment, the extent of circular buffer  124  provides at least five minutes of delay between writing and rendering of streaming information. In a further embodiment, the extent of circular buffer  124  is sufficient to provide at least 30 minutes of delay between writing and rendering streaming information. In yet a further embodiment, the extent of circular buffer  124  is sufficient to provide at least one hour of delay. 
     A further distinguishing feature of “time shifting” as used herein includes the presence of a single writer module  122  and a plurality of independent reader modules  126   1 - 126   N    
     In one embodiment, any information received from streaming source  114  is first recorded in circular buffer  124  prior to rendering the streaming information to the user. In other words, all information to be rendered to the user is read with an associated reader module  126  from circular buffer  124 , wherein there exists no direct connection from streaming source  114  to rendering devices  116 . Although recording and reading from circular buffer  124  prior to rendering streaming information, may slightly increase propagation delay when the position of reader module  126  in circular buffer  124  is substantially adjacent to the position of writer module  122 , advantages of always writing to and reading from circular buffer  124  include simplification in system design and operation, which enhances system stability. Although a switching mechanism could be provided in the system to switch from substantially “live” streaming information and time shifted streaming information as provided by delay filter  112 , the switching mechanism whether software, hardware or a combination thereof, nevertheless necessitates another component in this system that must be designed and tested to operate with the remaining components of the system. Furthermore, by eliminating a direct connection of the source of streaming information  114  and the rendering devices  116  and always reading from circular buffer  124  prior to rendering, the quality of rendered information is consistent. In other words, there is no change in quality (e.g., video quality or audio quality) of rendered streaming information since all information must pass through the same system components. Furthermore, by always writing to and reading from circular buffer  124 , the user need not remember to execute a particular command so that streaming information is recorded, for example, in order to provide “instant replay.” In this manner, the user is also able to retrieve and save an entire segment of streaming information (such as a television show) even when the user has already rendered some of the streaming information. Thus, if the user is viewing a show and then later decides to save the show, he can invoke archive system  136  to begin at the start of the show in circular buffer  124  and transfer a copy of the streaming information to a more permanent file. The latter can be performed while still viewing the show with a separate reader module  126 . However, the user must initiate saving or archiving prior to writer module  122  writing over any of the desired streaming information, but in a preferred embodiment, circular buffer  124  is of sufficient length to provide many minutes if not hours of stored information. 
     Data Block Structure 
     FIG. 17 is a pictorial representation of a block  500  of data stored in circular buffer  124  for one stream of a multi-stream source of streaming information. Generally, data block  500  includes a header portion  502 , a data or payload portion  504  and a tail or end portion  506 . Header portion  502  contains relevant information as to the size of the data block and the channel to which it pertains. In particular, header portion  502  includes a field  508  in which the size of the entire block, including this field, is stored. Field  509  contains information used to identify the streaming information to which the data block pertains. Field  510  provides a distance (typically bytes) from the beginning of the data block to where the actual streaming information or payload is stored in data block  500 . Field  511  provides a value indicating the size of the actual data or payload  504 . In one embodiment, each of fields  508 - 511  are of fixed length in order that their location in data block  500  is known. 
     In one embodiment, a field  512  is also provided in header portion  502 . Field  512  provides other information relevant to the streaming information. As used herein, field  512  is referred to as “prefix field” in view of its relation to the actual data or payload  502 . In a one embodiment, prefix field  512  is of variable length, thus the information need not be length restricted. If desired, a field  513  is also provided, indicating the size of the prefix field  512 . Typically, field  513  is also of fixed length, like fields  508 - 511 . 
     Prefix field  512  can be used to store information such as synchronization indicators, discussed below, a pointer to the next data block pertaining to the same stream, or other information as may be desired. As appreciate by those skilled in the art, a format is specified for information to be contained in prefix field  512  in order that relevant information can be decoded easily. However, since prefix field  512  can be of any length, additional information can be stored as necessary, thereby making data block  500  extensible. 
     It should also be noted that prefix field  512  can also include a selected amount of “fill” data. The fill data is used to adjust or increase the size of data block  500  to a predetermined length, which may be desirable for processing or storage in circular buffer  124 . 
     Data or payload field  504  stores digital data received from the source of streaming information  114 , which will be rendered. 
     In the embodiment illustrated, tail or end portion  506  includes two fields  520  and  521 . Field  520 , herein also referred to as “suffix field” is similar to prefix field  512  in that it can be used to store information about data blocks that are successive or preceding in the same stream. In one embodiment, suffix field  520  is at a known location from the end of data block  500  and stores a pointer indicating the location of the preceding data block corresponding to the same channel. This information is particularly useful when it may be necessary to perform a “rewind” operation upon the streaming information contained in circular buffer  124 . Suffix field  520  can also include other information present in the data block  500  such as data associated with field  511  and field  513 . In a manner similar to prefix field  512 , a format for the information contained in suffix field  520  is predetermined and known to the reader module in order that relevant information can be obtained therefrom. Suffix field  520  can be of fixed length, or can be of variable length, wherein it may be desired to provide an additional field similar to field  513  that indicates the length of suffix field  520 . Suffix field  520  can also be used to store “fill data”, if desired. 
     Field  521  provides the size of data block  500  and, as such, is identical to field  508  in header portion  502 . Field  521  is useful when a reader module progresses backward through the circular buffer  124 . In this manner, the reader module can ascertain the size of a data block  500  by reading the value contained in field  521 , at which point, the reader module has determined where the end of the preceding block is located. Without the presence of field  521 , the reader module would otherwise have to scan the data block  500  in order to determine its size or beginning location, which can consume processing time. 
     FIG. 18 illustrates a sequence of data blocks  530  from a multi-stream streaming information source. In particular, data blocks  532 A,  532 B and  532 A comprise a portion of a first stream; data blocks  534 A and  534 B comprise a portion of a second stream; and data blocks  536 A and  536 B comprise a portion of a third stream. The data blocks  532 A- 532 C,  534 A- 534 B and  536 A- 536 B together comprise a single channel and (e.g., audio, video and data) are generally interposed such as illustrated in FIG. 18; however, there is generally no requirement that a specific order be followed. 
     As indicated above, prefix fields  512  and suffix fields  520  can be used to store pointers indicating preceding and succeeding data blocks in each of the stream streams. In the illustration of FIG. 18, arrows  534  represent that prefix fields  512  of data blocks  530  store pointers or addresses of the immediate succeeding data block in each of the stream, respectively. Similarly, arrows  536  represent that suffix fields  320  of each of data blocks stores pointers or addresses of the immediate preceding data block in each stream, respectively. In one embodiment, each of the prefix fields  512  stores the address of the prefix field  512  in the immediate succeeding data block, while each of the suffix fields  520  stores the address of the suffix field  520  in the immediate preceding data block. In this manner, reader module  126  can quickly locate and process data blocks for a particular stream of streaming information. 
     FIG. 19 illustrates a second sequence of data blocks  550 . In sequence  550 , data blocks  552 ,  554  and  556  pertain to streaming information received from streaming information source  114 , which will be selectively rendered by rendering devices  116  in the manner discussed above and further below. However, sequence  550  further includes data blocks  558  and  560  that “pad” the sequence  550  and do not contain any streaming information to be rendered. Rather, data blocks  558  and  560  are used, if desired, to organize or structure the sequence  550  stored in circular buffer  124 , or other storage device, in a desired manner. For instance, it may be desirable that data blocks stored in circular buffer  124  coincide with defined medium boundaries partition such as sector or cluster boundaries. In FIG. 19, desired boundaries are indicated at  562 A and  562 B. Data blocks  552 ,  554  and  554  are organized along with a padding data block  560  of suitable size or length such that a data block  564  begins at the boundary  562 B. If desired, a plurality of padding data blocks can be used, and the order of streaming information data blocks and padding data blocks can be changed to meet any desired criteria. The sequence of FIG. 19 is but one exemplary sequence structure. 
     It should be noted that field  509  of each data block can be used to identify padding data blocks from other streaming information data blocks wherein writer module  122  can generate padding data blocks as necessary during processing and formation of streaming information data blocks. Furthermore, padding data blocks can be used in combination with “fill” data provided in prefix field  512  or suffix field  520 , if desired. Mux formatter  138  and writer module  140  can also implement the use of padding data blocks, and/or fill data contained in prefix field  512  or suffix field  520 . 
     Multiple Readers Per Buffer 
     It may be desirable, in some instances, to allow a plurality of users to access the streaming information at any one time. This can be accomplished in any number of different ways. For example, the streaming information could be duplicated and one reader module can be provided for each copy of the streaming information. However, this requires a great deal of storage, and may also require increased processing power for making multiple copies of the streaming information and for accessing the duplicate copies. Also, once the predetermined number of copies of the streaming information are in use, it can become very difficult to add new users. 
     FIG. 20 is similar to FIG. 6 described above. However, FIG. 20 illustrates portions of a system  610  in more detail and eliminates other portions for clarity. For example, FIG. 20 illustrates that system  610  includes circular buffer  124 , a plurality of reader modules  614 ,  616 ,  618  and  620  (which can be the same as or similar to reader modules  126  mentioned above) associated with a plurality of users (users  630 - 636 , respectively). Similarly, FIG. 20 illustrates that each reader module includes an interface  622 ,  624 ,  626  and  628 , respectively. Interfaces  622 - 628  are described in greater detail below. 
     In the illustrative embodiment shown in FIG. 20, a plurality of users  630 ,  632 ,  634  and  636  are depicted coupled to interfaces  622 - 628 , respectively. In the embodiment illustrated, users  630  and  632  are viewers, or rendering applications, for viewing or rendering streaming information stored in circular buffer  124 , while users  634  and  636  are data storage sites which are used for archiving the streaming information stored in circular buffer  124 . FIG. 20 also illustrates a separate application program  638 , separately coupled to interfaces  622 - 628 . 
     Again, it should be noted that FIG. 20 is illustrative only, and that any number of users or application programs can be coupled to any number of reader modules. Similarly, the users can be any suitable type of users desirous of accessing the streaming information stored in circular buffer  124 , and may be accessing the information for reasons other than viewing, or archival purposes, as is depicted in FIG.  20 . Similarly, in accordance with this aspect of the present invention, the buffered streaming information need not necessarily be stored in circular buffer  124 , but can be stored in a linear buffer, or any other type of buffer. However, circular buffer  124  is illustrated for purposes of simplicity only. 
     In one illustrative embodiment, reader modules  614 - 620  are implemented as objects which conform to the COM object-oriented programming model. Each reader module or “reader object” has independent access to the buffered streaming information. Each reader module  614 - 620  is depicted as accessing the streaming information at a different location on circular buffer  124 . This illustrates that reader modules  614 - 620  can access the buffered information at different times in the buffered information stream. This allows the multiple users  630 - 636  to independently employ all the features of time shifting mentioned above, without interfering with the other users accessing the buffered information stream. One user, for instance, can archive the streaming information for later viewing, while at the same time another user can view the information. 
     Similarly, the archival user can store the buffered streaming information from a time or location in the data buffer which is different than the time or location in the data buffer which the viewer is accessing. For instance, the archival user may simply be reading the information, as it is recorded in circular buffer  124 , and storing it in archival storage. By contrast, the viewer may be viewing the data, but intermittently pausing to take breaks or perform other tasks. Therefore, the reader module associated with the viewer may well be at a temporally displaced location in buffer  124  than the reader module associated with the archival process. 
     Thus, FIG. 20 illustrates a system by which multiple reader modules can access the buffered streaming information. The system depicted requires only one writer module for buffering the streaming information, and only a single copy of the streaming information. This greatly reduces the amount of storage required, and also reduces the processing power required. Similarly, since the reader modules, in one illustrative implementation, are simply objects configured to access the information in buffer  124 , virtually any number of objects can be added to the system, limited only by the computational resources of the system. This can be accomplished by simply instantiating another reader object. 
     Since the streaming information written in circular buffer  124  can be formed of one or more channels each having a plurality of streams (e.g., audio, video, closed captioning, etc.), the output from each of the reader modules  614 - 620  will likely have a plurality of effective output pins, each pin carrying one of the streams of a given channel in the streaming information. Since system  610  illustrates that a plurality of different reader modules can be used to access the same buffered streaming information, this can present some obstacles which must be overcome. 
     For example, in some streaming architectures, the need may arise to group the pins associated with each reader module  614 - 620  so that the user, application, or whatever other component is receiving the data from the reader module, knows which pins are associated with that reader module. An example of a streaming architecture which can be used in accordance with the present invention is an architecture known as DirectShow services. 
     DirectShow services is an architecture which is commercially available and well known. However, for a better understanding of certain aspects of the present invention, a brief description of the DirectShow services system may be beneficial. 
     DirectShow services refers to a modular system of pluggable components known as filters, arranged in a configuration known as a filter graph. A component referred to as a filter graph manager oversees the connection of these filters and controls the flow of the streaming information therethrough. An exemplary filter graph is composed of a collection of filters of different types. 
     Most filters can be categorized into one of three types: 
     1. Source filters take data from a source, such as a disk file, camcorders, satellite feed, internet server, or VCR, and introduce that data into the filter graph; 
     2. Transform filters process data and pass it along to other portions of the filter graph, or out of the filter graph; and 
     3. Rendering filters render data to a hardware device or to any location that accepts media input (such as memory or a disk file). 
     In addition to these three types of filters, there are other kinds of filters also. Examples of other filters include effect filters, which add effects without changing the data type, and parser filters, which are configured to understand the format of the source data and know how to read correct bytes, create time stamps, and perform seek operations. 
     Further, it is quite possible for some filters to represent a combination of filter types, or functions. In the DirectShow architecture, a filter is said to pass streaming information “downstream” to a next subsequent filter. An “upstream filter” refers to the filter which passes data to the downstream filter, and a “downstream filter” refers to the next filter in line to receive the data. In one illustrative embodiment, the filters are program modules written in any language which can generate objects adhering to component object model (COM) programming. Of course, COM programming refers to the object-oriented programming model which defines how objects interact with a single application or between applications. In COM, client software accesses an object through a pointer to an interface (e.g. API) which has a related set of functions, called methods, on the object. 
     By way of example only, a filter graph  640 , the purpose of which is to play back MPEG-compressed video information from a file may take the form set out in FIG.  20 A. Filter graph  640  includes source filter  642 , MPEG parser  644 , video decompression transform filter  646 , audio decompression transform filter  648 , video render filter  650  and audio render filter  652 . Source filter  642  reads data from a disk and provides it as streaming information to MPEG parser  644 . MPEG parser  644  parses the streaming information into its audio and video streams. Transform filters  646  and  648  decompress the video and audio data in the corresponding streams. Render filters  650  and  652  act to display the video data on a screen and send the audio information to a sound card, respectively. 
     It can thus be seen that, when using certain streaming architectures, such as the DirectShow architecture, it can be important that applications or other programming modules which are to receive streaming information from any of reader modules  614 - 620  know which output pins correspond to which of the reader modules. For example, since the streaming information being read by each of the reader modules  614 - 620  can include a channel having a plurality of streams, it is important that a user or application program receiving information from any given reader receive all streams associated with that reader, and no other reader. In other words, it would be undesirable for an application program (or a rendering filter, for example) to receive an audio stream from reader module  614  but a video stream and closed caption stream from reader module  616 . Reader module  616  may well be reading the stream information from a different location in buffer  124  than reader module  614 . Therefore, the audio and closed captioning streams would not correspond to the video stream. 
     In order to address this obstacle, one feature of the present invention includes a new COM interface which allows an object outside of reader modules  614 - 620  to enumerate the output pins which belong to that particular reader module. In other words, in the embodiment in which reader modules  614 - 620  are implemented as objects, those objects are configured to expose methods through associated interfaces  622 - 628  to the application (or other external components which have contact with the reader modules) which allow manipulation of the object. One exemplary method exposed by interfaces  622 - 628  is referred to as IenumChannelStreamPin, which, when invoked by an outside object, yields a value which represents an enumeration of the particular output pins belonging to the particular reader module which was queried. In this way, an outside object can quickly and easily obtain an enumeration of the particular pins belonging to any of the given reader modules  614 - 620 , through its associated interface  622 - 628 . 
     Interfaces  622 - 628  also expose another method which allows any specific pin output by any of reader modules  614 - 620  to be queried for its group&#39;s enumerator object. In other words, an outside object can query any pin with which it has contact to obtain the identity of the particular enumerator corresponding to the group which includes that pin. In this way, a user or application program which has only recently gained access to an output pin, can query that output pin to find the enumerator associated with that output pin. The external component can then invoke the method exposed by the enumerator to obtain a complete enumeration of the pins corresponding to that channel. The external component can thus quickly and accurately obtain the identity of the pins associated with any given reader module, or channel, being output by system  610 . 
     Indexing And Seeking 
     As discussed above, indexer  132  generates an index which contains index entries that are used for seeking. In other words, each of reader modules  614 - 620  can seek to any point in buffer  124  which has a corresponding entry in the index. In some instances, depending on the nature of the streaming information, substantially any point in buffer  124  can be indexed. However, for other types of streaming information, it may be desirable to index only certain points within the streaming information. Such points are referred to herein as sync points. In other words, the nature of the streaming information may render it unreasonable or undesirable to seek to certain points within the streaming information and begin rendering the data at that point. 
     For example, some video streaming information works on the well known MPEG2 video format. Such a format includes a number of different types of frames which are referred to as I frames, B frames, and P frames. I frames need no other information in order to be rendered, while B and P frames are dependent frames which require information in a preceding I frame in order to be rendered. In such an example, it may be undesirable to seek to a point within buffer  124  which begins with a B or P frame. In fact, many conventional video decoders may not even be able to decode data which is provided from a B or P frame, without the necessary information contained in the preceding I frame. Therefore, it may be highly desirable to index only I frames in such a video stream stored on buffer  124 . 
     Similarly, where the streaming information contains a stream, such as written text for closed captioning, it may be desirable to allow a user to seek to points which correspond to the beginning or ending of words, the beginning or ending of sentences, or the beginning or ending of paragraphs. The same is true for an audio stream. In other words, it may be desirable to allow a user to only seek to certain spots in the audio stream. Such spots may correspond to sentence or word boundaries, etc. 
     Therefore, one illustrative feature of the present invention includes a stream analyzer  654 , such as that illustrated in FIG.  21 A. Stream analyzer  654  is shown coupled to a source  656  of streaming information and a sink  658  of streaming information. Stream analyzer  654  is configured to receive the streaming information from source  656  (which can be implemented as a software object). 
     In one illustrative embodiment, stream analyzer  654  embodies knowledge of the organization of the streaming information provided by source  656 . Analyzer  654  also illustratively embodies knowledge regarding the detection of logical boundaries in the information in order to obtain the location of those boundaries (sync points). The location of the sync points can then be presented to any other software or hardware component which may desire the information in order to enable skipping forward or backward through the streaming information more practicable. 
     Stream analyzer  654  illustratively provides an indication of the sync points in one of two ways. In a first embodiment, stream analyzer  654  embeds the location of the sync points within the streaming information itself, as it is provided to information sink  658 . This is referred to as in-band communication of derived sync point information. 
     In an alternative embodiment, stream analyzer  654  can provide the sync point information as indicated by dashed arrow  660 , separately from the streaming information provided to sink  658 . This is referred to as out-of-band communication of the derived sync point information. 
     In one illustrative embodiment, stream analyzer  654  is implemented in the C++ programming language using classes and communicating with other software objects using COM interfaces. However, it should be noted that such an implementation is illustrative only and the present invention is not limited to this particular method of implementation. 
     The information sink  658  can, of course, be a downstream filter (downstream of stream analyzer  654 ) an application program, a rendering filter or other program component, etc. Sink  658  can use the derived sync point information itself, or simply pass it on to other components which may wish to use the derived sync point information. In one illustrative embodiment, the derived sync point information is provided to delay filter  112  which uses it in generating an index, as is described in greater detail below. 
     In another illustrative embodiment, not only does stream analyzer  654  analyze the incoming streaming information for logical boundaries which can be used as sync points, but stream analyzer  654  also analyzes the incoming streaming information for points of interest which may be desirable sync points. Such points of interest can be substantially any points which are deemed to possibly be of interest to the user. Therefore, in such an embodiment, stream analyzer  654  embodies knowledge of the type and format of information in the incoming stream, as well as knowledge of the types of events of interest to the user. Similarly, stream analyzer  654  is configured to contain knowledge of how to detect these points of interest in the incoming streaming information, and is provided with a mechanism to report the location in the data stream which corresponds to these points of interest. 
     FIGS. 21B and 21C are more detailed block diagrams illustrating stream analyzer  654  in accordance with illustrative embodiments of the present invention. FIG. 21B illustrates an embodiment of stream analyzer  654  in which the derived sync point information is provided out-of-band, while FIG. 21C illustrates an embodiment in which the derived sync point information is provided in-band. 
     In FIG. 21B, stream analyzer  654  includes point of interest (POI) analysis component  662 , point of interest interface component  664  and streaming information interface component  666 . The streaming information is provided from source  656  to point of interest analysis component  662  which calls on its base of knowledge regarding the type of information and the organization of information contained in the streaming information, as well as its knowledge regarding points of interest to the user. POI analysis component  662  then identifies points of interest in the streaming information and provides an output  668  which is indicative of the location of the points of interest (i.e., it is indicative of the sync points). Output  668  is provided to POI interface  664  which makes the information available to sink  658 . Similarly, POI analysis component  662  passes the incoming streaming information on to streaming information interface  666  such that the streaming information is separately available to sink  658 . 
     FIG. 21C is similar to FIG.  21 B and similar items are correspondingly numbered. However, rather than having two separate interfaces  664  and  666  for the sync point information and for the streaming information, stream analyzer  654  illustrated in FIG. 21C has a single combined interface  669  which integrates the point of interest (or sync point) information into the streaming information and provides it as an in-band output to sink  658 . It should, of course, be noted that the integration of the sync point information can also be accomplished in POI analysis component  662 . In any case, a single output of streaming information (which contains the sync point information in-band) is provided to sink  658 . 
     FIG. 21D is a more detailed block diagram of one embodiment of point of interest analysis component  662 , in which in the integration of the sync point information into the streaming information provided at its output, is performed within point of interest analysis component  62 . POI component  662  includes, in the illustrative embodiment shown in FIG. 21D, parsing component  670 , written language analysis component  672 , video analysis component  674  and audio analysis component  676 . While any type of information analysis components can be used, components  672 ,  674  and  676  are illustrated for exemplary purposes only. POI analysis component  662  also includes integration component  678 . 
     Parsing component  670  receives the streaming information at its input. In the embodiment illustrated, the streaming information may include a plurality of streams (although only a single stream may be processed as well), such as a written textual stream (e.g., closed caption information), a video stream and an audio stream. Parsing component  670  parses the incoming streaming information into its respective stream components and provides those components to appropriate analysis blocks  672 ,  674  and  676 . 
     Analysis components  672 ,  674  and  676  analyze the incoming streams, identify potential points of interest and generate information indicative of the location of the those points of interest. The streaming information, as well as the sync point information, is then provided from each analysis component  672 - 676 , to integration component  678 . In the embodiment illustrated, integration component  678  re-integrates the streams into the original streaming information, and also embeds the sync point information at appropriate locations within the streaming information. Thus, the in-band sync point information is provided in the data stream at the output of integration component  678 . 
     It should be noted that the analysis components  672 - 676  can look for substantially any desired points of interest. For example, written language component  672  can be configured to look for sentence boundaries or word boundaries. In looking for sentence boundaries, the analysis component can simply be configured to look for periods in the incoming stream. To look for word boundaries, component  672  can be configured to simply look for spaces in the incoming stream. It should also be noted that language analysis component  672  can be a more sophisticated analysis component, such as a natural language processing or natural language analysis component, in which case component  672  can be configured to identify certain types of clauses, surnames, parts of speech, etc. In any case, information indicative of the written language points of interest to be identified by component  672  is provided to component  672  such that the desired points of interest can be identified in the incoming stream. 
     Similarly, video analysis component  674  can be used to identify any number of different points of interest. For example, component  674  can be configured to identify commercial breaks, as well as to identify the I, B and P frames mentioned above. In identifying commercial breaks in the incoming video stream, video analysis component  674  can simply be configured to look for a black screen which exists for a certain predetermined amount of time. Similarly, in the MPEG2 video format, the I, B and P frames include markers identifying them as such, or are preceded by a header identifying the frames. Therefore, video analysis component  674  can be configured to look for the identifying information in the stream. 
     Audio analysis component  676  can also be configured to look for a wide range of points of interest. By way of example, if the incoming stream is from a horror genre film, a war genre film, or an action genre film or television program, audio analysis component can be configured to look for loud noises, such as screams, gun shots, or car chase noises. In that instance, component  676  can simply be configured to look for amplitude information which exceeds a predetermined threshold. Similarly, component  676  can be a more advanced component, such as a speech recognition or voice recognition component. In the instance in which component  676  includes a speech recognition component, it can be configured to look for sentence or word boundaries, or it can be used to look for particular words or phrases. For example, component  676  can be used to identify words of profanity which can be marked for later deletion. Similarly, where component  676  includes a voice recognition component, it can be configured to identify the voice of certain actors or actresses, or any other entity which can be so identified. 
     It should again be mentioned that the point of interest analysis component  662  can be configured to identify the location of substantially any point of interest which can be identified. Component  662  simply needs to be provided with the information indicative of points of interest to be analyzed, as well as information indicative of the type of information and format of information which will be received in the streams provided thereto. The specific embodiments mentioned above are simply provided as examples, and are not to be viewed as limiting the application of this feature of the present invention. 
     FIG. 22 is a flow diagram which illustrates the operation of POI analysis component  662  in more detail. First, POI analysis component  662  receives the streaming information. This is indicated by block  680 . Parsing component  670  then parses the streaming information into its respective streams for a point of interest analysis. This is indicated at block  682 . At some point, prior to point of interest analysis, point of interest analysis information indicative of the points of interest to be identified must be provided to the various analysis components in POI analysis component  662 . This is indicated by block  684 . The streams are then analyzed based upon the received point of interest information as indicated by block  686 . 
     Once the point of interest information is derived from the streams, the streams are then reintegrated and the point of interest information is provided either in-band or out-out-band, along with the integrated streams. This is indicated by block  688 . The data stream and the point of interest information is then transmitted to a downstream component (such as delay filter  112 ) for further processing. Again, it should be noted that the point of interest information can either be provided in-band or out-out-band. This is indicated by block  690  in FIG.  22 . 
     FIGS. 23A,  23 B and  23 C are more detailed flow diagrams illustrating the operation of written language analysis component  672 , video analysis component  674  and audio analysis component  676 . FIG. 23A illustrates that written language analysis component  672  first receives the written language stream information (such as closed caption information). This is indicated by block  692 . Component  672  then analyzes the stream information to locate word or sentence boundaries, certain clauses, surnames or other parts of speech, etc. Again, the point of interest identified can be substantially any points of interest for which component  672  is properly configured. This is indicated by block  694 . Component  672  then provides an indication of the locations in the streaming information where the points of interest reside. This is indicated by block  696 . 
     FIG. 23B is similar to FIG. 23A, and similar items are correspondingly numbered. However, rather than locating textual points of interest, video analysis component  674  analyzes the video stream information for a black screen, image changes (which may correspond to desired scene changes), I, B and P frame markers, etc. This is indicated by block  698 . 
     FIG. 23C is similar to FIGS. 23A and 23B, and similar items are similarly numbered. However, rather than analyzing written language or video information, component  676  analyzes audio stream information. In the embodiment illustrated in FIG. 23C, component  676  analyzes the audio stream information for amplitude changes, sentence or word boundaries, certain content words (e.g., profanity), a certain persons voice, etc. This is indicated by block  700 . 
     Index Generation 
     As described above, reader modules  614 - 620  in system  610  are configured such that they can seek to any indexed point, in buffer  124 . For information streams which do not have sync points, it is reasonable for the reader module to seek to substantially any sample within buffer  124 . Therefore, in such information streams, any sample can be indexed. In such an embodiment, indexer  132  generates index entries which simply correspond to the desired granularity of the indexer. For instance, it may be desirable in certain streams to index points which are temporally spaced by approximately 0.25 seconds. With other information, it may be desirable to index points which are spaced by two seconds or more. In information streams which have no sync points, indexer  132  simply generates index entries identifying locations in buffer  124  which are separated by the desired granularity of the indexer. 
     However, as discussed in the previous section, many data streams will be provided with sync points. In those cases, it may be desirable for indexer  132  to generate index entries which correspond only to sync points. It will, of course, be noted that if sync points occur more frequently than the desired granularity of the index generator, every single sync point may not be indexed. However, if sync points do not occur more frequently than the desired granularity of the index generator, substantially every sync point may be indexed. 
     Under such an arrangement, a problem can arise. For example, in a given streaming architecture, stream analyzer  654  described above may not exist, or it may not yet know whether the streaming information it is receiving contains any sync points. That being the case, indexer  132  may not know, at the time it begins receiving streaming information, whether it must simply begin indexing samples according to its granularity, or whether it is to wait to index only sync points. FIG. 24 is a flow diagram illustrating the operation of indexer  132  in addressing this problem. 
     First, indexer  132  simply executes a query against stream analyzer  654  to determine whether stream analyzer  654  can provide information as to whether the incoming streaming information contains sync points. This is indicated by block  702 . Stream analyzer  654  may not even provide a response, or it may provide a response indicating that it does not yet know whether the streaming information contains sync points. If analyzer  654  provides a response indicating that information indicative of whether sync points exist is available, a flag is set. The flag is referred to in FIG. 24 as the “Sync Point Info Is Authoritative” flag. This is indicated by blocks  703  and  704 . If there is no information available that is indicative of whether sync points exist, this flag is reset, as indicated by blocks  703  and  705 . 
     Next, if information as to the presence or absence of sync points is available, it is determined whether any sync points are present. This is indicated by block  706 . If stream analyzer  654  has already identified sync points in the incoming streaming information, it provides indexer  132  with a response indicating that sync points do exist. In that case, indexer  132  sets a sync point flag to a value which indicates that the incoming streaming data does, in fact, contain sync points. This is indicated by block  707 . However, if stream analyzer  654  does not respond, or it has not yet located sync points in the incoming streaming information, and provides an output indicative of that to indexer  132 , indexer  132  assumes, for the moment, that there are no sync points in the incoming streaming information, and resets the sync point flag. This is indicated by block  708 . Indexer  132  then receives a sample of the streaming information as indicated by block  710 . 
     Upon receipt of the sample, indexer  132  analyzes the sample to see if the sample has been marked as a sync point. This is indicated by blocks  712  and  714 . If the sample has not been marked as a sync point, indexer  132  examines the sync point flag to see whether the sync point flag is set. This is indicated by block  716 . If, at block  716 , it is determined that the sync point flag is indeed set, that indicates that the streaming information being processed does contain sync points, and the present sample is not marked as a sync point. Therefore, indexer  132  does not index the sample under analysis but simply returns to processing at block  710 . 
     If, however, at block  716 , it is determined that the sync point flag is not set, indexer  132  is still assuming that no sync points exist in the incoming streaming information. Therefore, the indexer simply determines whether it should index the present sample under analysis, based on the desired granularity of indexer  132 . In other words, if indexer  132  is to index points no more often than every one half second (for example), indexer  132  determines whether the present sample is temporally removed from the previously indexed sample by at least one half second. If so, the present sample is indexed. If not, processing simply returns to block  710 . This is indicated by block  718 . 
     If, at block  714 , indexer  132  determines that the present sample is marked as a sync point, indexer  132  then determines whether the sync point flag is currently set. This is indicated at block  720 . If the sync point flag is currently set, indexer  132  simply continues processing at block  718  and determines whether it has reached sufficient temporal displacement from the previous index entry to index the present sync point. 
     However, if, at block  720  it is determined that, even though the present sample is marked as a sync point, the sync point flag is not set, then indexer  132  determines whether the “Sync Point Info Is Authoritative” flag is set, at block  721 . If not, then indexer  132  realizes that it has been assuming that no points exist in the streaming information under analysis, but it also realizes that sync points do actually exist. Therefore, indexer  132  discards all the previous index entries (since they corresponded to non-sync point samples) as indicated at block  722  and sets the sync point flag as indicated by block  724 . Processing then again continues with respect to block  718 . Similarly, if at block  721  it is determined that the “Sync Point Info Is Authoritative” flag is set, processing continues at block  718 . 
     Therefore, it can be seen that by using the algorithm illustrated in FIG. 24, this feature of the present invention can be used to accurately index points in the streaming information, regardless of whether the indexer currently knows whether the sync points are present in the streaming information. Where sync points are not present, samples will be indexed according to the desired granularity of the indexer. When sync points are present, only sync points will be indexed. 
     Time Shift Seeking With Multiple Streams 
     As described above, the streaming information stored in buffer  124  can include multiple streams. As is also describe above, it may be desirable to allow reader modules  614 - 620  (illustrated in FIG. 20) to seek to various points in the streaming information indexed by indexer  132 . This presents some difficulty. It is difficult to decide at which particular file offset within buffer  124  the particular reader module should begin reading, after a seek is requested. This decision is made difficult by a number of factors. 
     For example, the streams can be out of sync within the file located in buffer  124 . In other words, audio information can be stored before or after video information, and in different size storage blocks, such that the two do not directly coincide. Also, the reader module may request to seek to a position in buffer  12  (to a sample with a particular time stamp) for which no sync point has been indexed. In other words, as set out above, indexer  132  does not index every single sample. Instead, it only indexes based on its own granularity, or based on sync points. Therefore, a reader module may request to seek to a time stamp for which no index entry is available. Further, some streams may contain sync points while others may not. In addition, not all sync points may be indexed, since they may occur more frequently than the desired granularity of the index. One illustrative feature of the present invention is directed to addressing these obstacles in seeking through a channel having multiple streams. 
     FIG. 25 is one exemplary illustration of a portion of buffer  124  which is divided into a plurality of buffered samples. Samples  730 , having cross-hatching in a first direction, represent samples of a first stream. Samples  732 , with cross-hatching in the opposite direction, correspond to samples of a second stream. Samples  734 A,  734 B,  734 C and  734 D, with no cross-hatching, correspond to samples of a third stream. 
     In order to address the obstacles identified above, one illustrative embodiment of the present invention is a seek algorithm which is described with respect to FIGS. 25 and 26. In the algorithm, the particular reader module conducting a seek seeks to a highest file offset within buffer  124  which is suitable to satisfy each of the stream&#39;s individual seeking requirements. The reader module then begins streaming information, but drops information for each of the streams being read which is read before the first sample that should actually be played for that stream. In addition, where no sync points with the exact time stamp have been indexed, the nearest sync point before the desired seek position is provided as a potential starting point, unless the nearest sync point before the desired seeking position is too far into the past. Since some streams contain no sync points, the present invention assumes that, in a stream with no sync points, every sample is a sync point. Finally, since not all sync points may be indexed, the seeking algorithm first finds a nearest indexed sync point before and after the desired seek position, and then determines the difference between the two indexed seek points to see whether there may be more sync points, which are not indexed, between the two. If so, the data between the two indexed sync points is read to determine whether additional sync points (and ones closer to the desired seek point) exist. 
     More specifically, FIG. 26 is a flow diagram illustrating a seeking operation in accordance with one illustrative feature of the present invention. In order to seek to a position within buffer  124  (illustrated in FIG. 25) the designated reader module first receives a seek request to seek to a position in buffer  124  having a time stamp indicating time T. This is indicated by block  740 . In FIG. 25, the time stamp T corresponds to sample  734 C. In the illustrative embodiment, sample  734 C is not indexed. 
     Next, a variable FILEOFFSET is set to infinity (or a very large number). This is indicated by block  742 . 
     The first of the plurality of streams is then chosen for analysis. This is indicated by block  744 . 
     For the first stream, a time variable T 1  is set to the index entry with the highest time stamp before the desired seek time T. In FIG. 25, it can be seen that sample  734 A for the designated stream corresponds to the index entry T 1  which has the highest time stamp before the desired seek time T. If no such index entry exists, then T 1  is set to T-Z. In one illustrative embodiment, Z=100. This basically causes T 1  to be discarded as will be illustrated in later processing. This is indicated by block  746 . 
     Next, a time period T 2  is set to an index entry for the selected stream with the lowest time stamp after the desired seek time T. It can be seen that sample  734 D corresponds to a sample in the selected stream which is indexed, and which has the lowest time stamp to the right of time T (e.g., after time T). If no such index entry exists, the presently selected stream is ignored for purposes of computing the file offset, and a new stream is selected. This is indicated by block  748 . 
     Thus, after processing at block  748 , it can be seen that the two index entries which lie on either side of time T, and which are closest to time T, have been identified. Those index entries correspond to times T 1  and T 2 . The temporal displacement between the desired time T and the previously indexed time T 1  is represented by X=T−T 1 . The temporal displacement between the desired seek time T and the subsequent index entry T 2  is represented by Y=T 2 −T. 
     If both X and Y are greater than a predetermined time interval (e.g., five seconds) that indicates that both of the indexed samples T 1  and T 2  are too far from the desired seek position T to be helpful. This determination is indicated by block  750 . If it is the case that X and Y are greater than the predetermined time interval, then the present stream is simply ignored in computing FILEOFFSET. This is indicated by block  752 . Processing then continues at block  754  where it is determined whether additional streams exist. If so, a next stream is selected at block  756  and processing continues with respect to block  746 . If no additional streams exist, a seek is performed at block  758 , which is described in greater detail below. 
     If, at block  750 , it is determined that both values X and Y are not greater than the predetermined time interval, then it is determined whether either X or Y are greater than the predetermined time interval. For example, at block  760 , it is determined whether value X (which corresponds to T-T 1 ) is greater than five seconds. If so, then the value Y must not be greater than five seconds, and a variable FILE_OFFSET_THIS_STREAM is set to index value T 2 . This is indicated by block  762 . 
     If, at block  760 , it is determined that the value of X is not greater than five seconds, then it is determined whether the value Y is greater than five seconds. If so, then the value T 2  can be ignored and the variable FILE OFFSET THIS STREAM is set to value T 1 . This is indicated by blocks  764  and  766 . 
     Basically, blocks  760 - 766  determine whether either index values T 1  or T 2  are too far separated from the desired seek time T to be of help. If they are, they are eliminated from further computing. 
     If, however, at block  764 , it is determined that the value Y is not greater than five seconds, that means that both samples corresponding to values T 1  and T 2  are close enough to desired seek time T to be of interest. That being the case, it is determined whether other sync points between T 1  and T 2  may exist. In other words, if a distance between T 1  and T 2  is so small that no other sync points could have been indexed between them (based on the desired granularity of the indexer) then additional sync points may exist between samples T 1  and T 2  which are closer to the desired seek position T, but which have simply been omitted from the index because the granularity of the indexer was not sufficient to accommodate an additional index entry. 
     Therefore, a search algorithm is conducted in which the samples of the selected stream which reside between samples T and T 1  are searched to determine whether they are in fact sync points. If so, that would mean that those sync points are closer in time to the desired seek position T, and are prior in time to that seek position. If that is true, then the variable FILE_OFFSET_THIS_STREAM is set to the value corresponding to that identified sync point. This is indicated by blocks  768  and  770 . 
     Therefore, at this point in the processing, the desired file offset for the stream under analysis has either been chosen to be T 1 , T 2 , or a value between T 1  and T. Then, for the stream under analysis, the seek algorithm requests subsequent filters in the streaming process to ignore any data samples which are read out of buffer  12  and have time stamps prior to that identified by the variable FILE_OFFSET_THIS_STREAM. This is indicated by block  772 . 
     Once the variable FILE_OFFSET_THIS_STREAM has been derived, it is determined whether that value is less than the current value of the variable FILEOFFSET. If this is the first selected stream, the value will be less than the current value of FILEOFFSET, which has been set to infinity. However, if this was a second, third, or later selected stream, then the file offset for the present stream under analysis will be used as the value for the variable FILEOFFSET only if it is further to the left (or earlier in time) than the current value for the variable FILEOFFSET. This has the effect of moving the seek point which will actually be used in seeking in buffer  124  to a point far enough left in the buffer to satisfy every stream&#39;s individual seeking requirements. This is indicated by block  774 . 
     Next, processing continues at block  754  where it is determined whether additional streams need to be analyzed. If so, processing continues at block  756 . If not, processing continues at block  758 . In block  758 , the selected reader module is commanded to seek to the sample identified by the variable FILEOFFSET and start streaming data from buffer  124  at that point. Since in block  772 , a value has been set for each stream indicating that streamed data prior to the desired sync point for that stream is to be ignored, the data streamed out of buffer  124  will not be rendered for a given stream unless that data is coincident with, or after, the earliest sync point identified for that given stream (and identified by the variable FILE_OFFSET_THIS_STREAM). 
     One embodiment of pseudocode corresponding to the flow diagram of FIG. 26 is set out below: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 FileOffset = INFINITY 
               
               
                   
                 for (every connected stream) { 
               
               
                   
                 T1 = Find the index entry with the highest 
               
            
           
           
               
               
            
               
                   
                 timestamp before T; 
               
            
           
           
               
               
            
               
                   
                 if (T1 does not exist) 
               
            
           
           
               
               
            
               
                   
                 T1 = T − 100;//so that we ignore T1 
               
            
           
           
               
               
            
               
                   
                 T2 = Find the index entry with the lowest 
               
            
           
           
               
               
            
               
                   
                 timestamp after T; 
               
            
           
           
               
               
            
               
                   
                 If (T2 does not exist) 
               
            
           
           
               
               
            
               
                   
                 Continue;//no index - ignore this stream in 
               
            
           
           
               
               
            
               
                   
                 computing file offset 
               
            
           
           
               
               
            
               
                   
                 If ((T − T1 &gt; 5sec) AND (T2 − T &gt; 5sec)) 
               
            
           
           
               
               
            
               
                   
                 Break; 
               
            
           
           
               
               
            
               
                   
                 else if (T − T1&gt;5sec)//“too far” 
               
            
           
           
               
               
            
               
                   
                 FileOffsetForThisStream = FileOffset (T2); 
               
            
           
           
               
               
            
               
                   
                 Else if (T2 − T &gt; 5sec)//“too far” 
               
            
           
           
               
               
            
               
                   
                 FileOffsetForThisStream = FileOffset (T1); 
               
            
           
           
               
               
            
               
                   
                 Else {//neither one is “too far” 
               
            
           
           
               
               
            
               
                   
                 Search through the portion of the file 
               
            
           
           
               
               
            
               
                   
                 between FileOffset (T1) and FileOffset (T2) 
               
            
           
           
               
               
            
               
                   
                 To find the sync point with the highest time 
               
            
           
           
               
               
            
               
                   
                 stamp that is still below T. 
               
            
           
           
               
               
            
               
                   
                 The position of that sync point becomes 
               
            
           
           
               
               
            
               
                   
                 FileOffsetForThisStream. 
               
            
           
           
               
               
            
               
                   
                 Then tell the splitter to discard any 
               
            
           
           
               
               
            
               
                   
                 samples for this stream whose timestamps 
               
            
           
           
               
               
            
               
                   
                 are smaller than the one we just found. 
               
               
                   
                 } 
               
               
                   
                 If (FileOffsetForThisStream &lt; FileOffset)\ 
               
            
           
           
               
               
            
               
                   
                 FileOffset = FileOffsetForThisStream; 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                 Seek to FileOffset and start streaming. 
               
               
                   
                   
               
            
           
         
       
     
     Thus, it can be seen that the algorithm illustrated in FIG. 26 solves a number of problems. With respect to streams that can be out of sync in a file, the algorithm seeks to the highest file offset suitable to satisfy every stream&#39;s individual seeking requirements. The reader module then begins streaming at that point, but drops everything prior to the first sample that should actually be played for each given stream. Similarly, where no sync point exists with the exact desired time stamp (the desired seek position), the seek position is set to the nearest sync point before the desired seek position, unless the nearest sync point is too far to the left (into the past). Also, since some streams may contain seek points while others may not, every sample in a stream without seek points is deemed to be a suitable seek point. Finally, since not all seek points may be indexed, the seeking algorithm first finds the nearest indexed seek points before and after the desired seek position and then looks at the difference between those indexed points to determine whether any sync points may reside between them. If so, the samples to the left of the desired seek position are read to look for additional sync points. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.