Abstract:
A method and system respond to a user request for trick-mode play. A decoding device conveys the request to a device for storing a video stream. A current point location in the memory is frozen and the pointer is set back by a preselected back track distance. Then a search for a first target header is begun. The target header is either a sequence or GOP (group of pictures) header. When a target header is found, a data segment is selected which includes the target header. This is the start of the trick-mode stream. Further target headers are sought and further data accessed until a full trick-mode stream has been generated.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to devices for recording video stream data and more particularly to creating trick-mode streams.  
         BACKGROUND OF THE INVENTION  
         [0002]    Personal video recording is a new digital technology in which a video stream is recorded on a medium such as a hard drive. In its most common form, an MPEG2 video steam is decoded. The decoder may be included in a digital television, a cable/satellite set top box or personal video recorder. “Trick-mode” refers to such functions as pause, rewind and slow-motion. The trick is that these modes may be applied to a live broadcast. The illusion of pausing a live broadcast is given by continuing to record a broadcast after a pause button is activated and playing the recording back on command. Trick-mode operation is achieved by playback of a video stream called the “trick-mode stream” on the decoding device. A trick-mode stream can be created in different ways. The speed of response and picture quality in utilization of the trick-mode stream will be functions of the manner in which the stream is created. Two different ways of creating the trick mode stream are the most prevalent.  
           [0003]    One form of video stream creation is based on a look-up table. During recording of the stream, a table is created which contains pointers to various information in headers each associated with a video signal payload. A table is assembled which can point to headers for a Group of Pictures (GOP), sequence or picture. This table is used to assemble the trick-mode stream during trick-mode playback. Headers are selected in correspondence with the particular trick-mode that is commanded. Implementing this form of recording requires software that provides extensive CPU intervention during the recording process in order to parse the video stream and select header information to generate the pointer table. More recently, in the alternative, specialized hardware modules have been developed to implement the algorithms for generating the look-up tables.  
           [0004]    Another prevalent way of creating the trick-mode stream is concatenating segments of a full-recorded stream. Selection of segments is based on heuristic criteria. For example, at initiation of playback, hardware could select segments of the recorded stream at each new sector of the storage device. This method is fast and simple. However, the frames accessed while producing the trick-mode stream may be partial in some cases rather than complete. Including partial frames in the trick-mode stream causes presentation of undesirable video artifacts in the playback display.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides trick-mode playback which is efficient in terms of the degree of processing required to access desired streams for playback and reliable in terms of accessing and decoding I-frames (frames with information defining fold video data rather than frame-to-changes). Briefly stated, in accordance with the present invention, there are provided a method and system in which in response to a user request for trick-mode play, a decoding device conveys the request to a device for storing a video stream. A current point location in the memory is frozen and the pointer is set back by a preselected back track distance. Then a search for a first target header is begun. The target header is either a sequence or GOP (group of pictures) header. When a target header is found, a data segment is selected which includes the target header. This is the start of the trick-mode stream. Further target headers are sought and further data accessed until a full trick-mode stream has been generated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The invention may be further understood by the following description taken in connection with the following drawings.  
         [0007]    Of the drawings:  
         [0008]    [0008]FIG. 1 is a block diagram of a video stream decoder and recorder system, such as a personal video recorder;  
         [0009]    [0009]FIG. 2 is a diagram of a transmission stream packet including both a header and a payload;  
         [0010]    [0010]FIG. 3 is an illustration of a video stream including a number of successive target headers and illustrating a data segment; and  
         [0011]    [0011]FIG. 4 is a flowchart illustrating software structure and operation of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1 is a block diagram of a personal video recorder (PVR)  10  which receives video input from a video stream source  20 . The video stream source  20  could, for example, comprise a cable head-end, internet service provider or local video server. While video streaming is a significant application, as discussed further below, the invention is applicable to other forms of audio-video encoded signals. Video streaming is cited by way of example. A network adapter  25  couples the video source  20  to the PVR  10 . Preferably, the PVR  10  further comprises a remote control unit  15 . The PVR  10  provides an output to a monitor  30 , which may comprise an analog or digital television set. Within the PVR  10 , internal communication is done over a communication bus  40 .  
         [0013]    A transmission stream (TS) demultiplexer unit  44  receives the video stream from the network adapter  25  and provides an output to the digital bus  40 . The demultiplexer unit  44  may also include fixed and programmable filters. Video signals are provided to a decoder  50  including an audio decoder  52  and a video decoder  54 . The decoders  52  and  54  provide outputs to an interface circuit  56  providing an output for coupling to the monitor  30 . The video stream is also provided to the storage means  60  for recording. In the currently preferred form, the storage means  60  comprises a hard disk drive. The hard disk drive  60  is coupled to the communications bus  40  via a hard disk drive interface  64 . Operations are controlled by a microprocessor  70 , and programs are stored in a memory  74 . A buffer  80  is used in the creation of a trick-mode video stream.  
         [0014]    [0014]FIG. 2 is an illustration of transport stream structure. The transport stream is formed by multiplexing packetized elementary stream (PES) packets. During the formation of the transport stream, additional packets containing tables needed to de-multiplex the transport stream are inserted. The tables are referred to as PSI. It should be noted that the transport stream is a structure intended to be transported over lossy networks, such as every day transmission networks. An alternative form of stream, namely PS, is used in non-lossy contexts such as digital video disk (DVD) players. TS packets  200  comprise a header  202  and a payload  204 . Packets are of a fixed length of 188 bytes. The header is at least four bytes. The header  202  includes a sync byte  210 , nominally 5 bits long. A number of other well-known bits are included. For example, a transport error indicator bit  212  and a transport scrambling control bit  214  are provided. Other packets  216  perform other known functions. Null packets may be inserted to fill intervals between packets that bear information. Null packets contain dummy payloads. In accordance with the present invention, headers will be sought by processing methods for generating the trick-mode stream. These headers will be referred to as target headers  300  in a video stream  304 . In the illustration of FIG. 3, three different target headers  300 - 1 ,  300 - 2  and  300 - 3 , are illustrated.  
         [0015]    [0015]FIG. 4 is a flow diagram in connection with which operation of the present invention is explained. Hardware references are two components illustrated in FIG. 1, and references to communication structures are with reference to FIGS. 2 and 3.  
         [0016]    A trick-mode is generally initiated by a user operating the remote control  15 . The remote control receiver  42  issues commands to the microprocessor  70 , and the microprocessor  70  provides user information to the graphical user interface  46 . Operation is initiated at block  400  of FIG. 4 in which trick-mode command play is received by the CPU  70 . At block  402 , a current file pointer pointing to a location on the hard disk  60  is frozen and the pointer location is incremented. The current pointer is set to a location displaced by a back_track_distance. Different functions will call for different incrementing. For example, a pause command will hold the current pointer location until the PVR  10  is again commanded to play. For fast forward, or rewind, target headers will be beyond or prior to current locations respectively.  
         [0017]    At block  404 , an exhaustive search for the first target header is initiated. At block  406 , if the target header is not found, then operation stops at block  408 . However, when the target header is found, operation proceeds to block  410 . The data segment including the target header is the start of the trick-mode stream or trick-mode signal. Trick-mode signal is used in the present description to comprise an audio-video encoded signal that is not produced by streaming. Streaming is a subset of available types of encoding. The data segment is provided from the memory  60  to the buffer  80 . The next target header is found in accordance with operation at block  412 . The next target header is a function of the operation commanded. For example, where fast forward is commanded, only a fraction of the video frames are desired to be displayed. This is described as finding one in N headers. The Nth target header is searched for. In the case of pause, the second target header will equal the first target header. At decision block  414 , if a second target header is not found, operation is stopped at block  416 . However, it is contemplated that the second target header will be found and operation proceeds to block  418 .  
         [0018]    At block  418 , the target header distance, i.e. the distance between the two target headers found in accordance with the search is calculated. A next data segment is selected that includes the second target header. That data segment is appended to the trick-mode stream by delivering the segment to the buffer  80  from the memory  60 . Operation proceeds similarly. At decision block  420 , it is determined if the end of the stream has been reached. If so, operation is stopped at block  422 . Otherwise, operation proceeds to block  424 . The pointer jumps ahead. This may be described as file_PTR+=target_HDR_DIST-Delta (offset). At block  426 , the next target header is searched for. If it is not found, after decision block  428 , operation stops at block  430 . However, usually the target header will be found, and at block  432 , the target header distance is updated, a next data segment that includes the target header is selected and appended to the trick-mode stream. Operation returns to block  420 . Successive next target headers are selected until operation in the trick-mode is commanded to be stopped.  
         [0019]    The process of selecting a data segment that includes the target header is described with respect to FIG. 3. As described with respect to FIG. 2, each header includes a sync signal. Searching for the data segment comprises searching for two consecutive sync signals. For example, a first search is done for the form of the sync signals and then a next sync signal should follow 187 bytes later. The second target header  300 - 2  is selected for purposes of this explanation to correspond to the detected target header. Once the detected target header is selected, a stream segment from TH−α to TH+β is chosen. Typically, the α value is very much less than β. A typical value for α is 512 bytes. A typical value for β is dependent on the stream content as well as the target header. The target header may either comprise a sequence or GOP, i.e. a group of pictures. The value β is chosen such that an I-frame which follows the target header is included in the data segment. An I-frame is a frame which includes full data for the frame as opposed to merely including increments of an occurring frame with respect to a preceding frame. In one nominal application, β was set to 512 KB. A high value for β will produce complete I-frames most of the time.  
         [0020]    A further test may be performed to ensure that complete I-frame is included within the selected data segment. In addition to searching for a sync byte, the presence of a B or P picture start header may also be checked.  
         [0021]    The current technique is applicable to other forms of audio-video encoding besides MPEG2. These other forms include, for example, digital video (DV), JPEG 2000, MPEG4, an amorphous streaming format, or other audio-video encoding formats of which streaming is a subset. Where the MPEG2 TS stream is encapsulated with another form, such as IEEE 1394, ATM (Asynchronous Transmission Mode) or IP, the structure will include additional header bytes beyond those illustrated in FIG. 2. In the current MPEG2 example, a separation of 187 bytes is assumed between sync bits. In processing other forms, additional header bytes need to be accounted for in the separation between successive sync test bytes. It does not make a difference in operation if the transfer stream is scrambled or not.  
         [0022]    Many variations will occur to those skilled in the art to provide a trick-mode efficient operation in accordance with the present invention while departing from specific examples illustrated herein.