Abstract:
Video processing image manipulation and trick play modes provide seamless normal play and trick play functioning. Specifically, a system processes video data including encoded digital packetized data representative of a sequence of individual images and ancillary data identifying characteristics of the individual images. The system involves a pre-processor for parsing the ancillary data to determine characteristics of the individual images prior to storing the encoded digital packetized data. The system also includes a multiplexer for filtering the encoded digital packetized data based on the determined characteristics to identify particular images of the image sequence for storage in a memory and for excluding other images of the sequence from storage.

Description:
This divisional application of original application Ser. No. 09/024,400 filed Feb. 17, 1998 by Mark A. Schultz et al which is a non-provisional application of provisional application Ser. No. 60/039,573 filed Feb. 18, 1997 by Mark A. Schultz et al. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the processing of a digitally encoded signal containing video and ancillary data and to the selection of picture type for decoding. 
     BACKGROUND OF THE INVENTION. 
     The introduction of disks recorded with digitally compressed audio and video signals, for example, utilizing MPEG compression protocols, offers the consumer sound and picture quality virtually indistinguishable from the original material. However, consumer users will expect such digital video disks or DVDs to offer features similar to those of their analog video cassette recorder or VCR. For example, a VCR may reproduce in either forward or reverse directions at speeds other than the recorded speed. Such non-standard speed playback features are also known as trick play modes. The provision of trick play features are less easily provided with MPEG encoded video signals due to the hierarchical nature of the compression which forms pictures into groups having varying degrees of compression. These groups are termed groups of pictures or GOPs, and require decoding in sequence. A detailed description of the MPEG 2 standard is published as ISO/IEC Standard  13818-2 . However, in simple terms, an MPEG 2 signal stream may comprise three types of pictures having varying degrees of content compression. An intra-coded frame or I frame has the least compression of the three types and may be decoded without reference to any other frame. A predicted frame or P frame is compressed with reference to a preceding I or P frame and achieves greater degree of compression than an intra-coded frame. The third type of MPEG frame, termed a bi-directionally coded or B frame, may be compressed based on predictions from preceding and / or succeeding frames. Bi-directionally coded frames have the greatest degree of compression. The three types of MPEG frames are arranged in groups of pictures or GOPs. The GOP may for example contain 12 frames arranged as illustrated in FIG.  1 A. Since only an intra-coded frame is decodable without reference to any other frame, each GOP may only be decoded following the decoding of the I frame. The first predicted frame or P frame, may be decoded and stored based on modification of the stored, preceding I frame. Subsequent P frames may be predicted from the stored preceding P frame. The prediction of P frames is indicated in FIG. 1A by the curved, solid arrow head lines. Finally, bi-directionally coded or B frames may be decoded by means of predictions from preceding and or succeeding frames, for example, stored I and P frames. Decoding of B frames by predictions from adjacent stored frames is depicted in FIG. IA by the curved, dotted arrow head lines. 
     The hierarchical nature of the coded frames comprising MPEG groups of pictures necessitates that the I and P frames of each GOP are decoded in the forward direction. Thus, reverse mode features may be provided by effectively jumping back to an earlier, or preceding I frame and then decoding in a forward direction through that GOP. The decoded frames being stored in frame buffer memories for subsequent read out in reverse to achieve the desired reverse program sequence. FIG. 1B illustrates play back in the forward direction at normal speed and at a time prior to time t0, a reverse three times speed mode trick play mode is selected. The trick play mode is initiated at time t0 where I-frame I( 25 ) is decoded and displayed. The next frame required for decoding is I-frame I( 13 ), thus the transducer is repositioned, as indicated by arrow J 1  to acquire frame I( 13 ). Having recovered and decoded I-frame I( 13 ), the transducer tracks, as indicated by arrow J 2  to acquire and decode frame P( 16 ). The process is repeated as indicated by arrows J 3 , J 4 . Following the acquisition and decoding of frame P ( 22 ) the transducer is moved as depicted by arrow Jn to recover frame I ( 1 ). To smoothly portray scene motion requires the decoding and display of I, P, and possibly B-frames. The jump and play process is repeated for preceding GOP, thereby progressing haltingly backwards through the records whilst smoothly portraying the program material in a reverse sequence at the video output. 
     The provision of visually smooth reproduction during trick mode reproduction requires timely disk retrieval and access to specific pictures from memory. Although each digital disk is encoded with navigation data which provides picture access points within each video object unit, these are limited in number, and may inherently contribute to temporally aliased image motion. To achieve temporally smooth trick mode reproduction, at multiple speeds in forward and reverse directions requires access to, and decoding of all encoded pictures. Although such performance is attainable at the cost of storage capacity, however trick play mode specific picture decoding provides opportunities for improved trick mode reproduction. 
     SUMMARY OF THE INVENTION 
     A system processes video data including encoded digital packetized data representative of a sequence of individual images and ancillary data identifying characteristics of the individual images. The system involves a pre-processor for parsing the ancillary data to determine characteristics of the individual images prior to storing the encoded digital packetized data. The system also includes a multiplexer for filtering the encoded digital packetized data based on the determined characteristics to identify particular images of the image sequence for storage in a memory and for excluding other images of the sequence from storage. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates an MPEG 2 group of pictures. 
     FIG. 1B illustrates recorded groups of pictures, during replay and reverse trick play at three times speed. 
     FIG. 2 is a block diagram of an exemplary digital disk player including inventive arrangements. 
     FIG. 3 shows in greater detail part of FIG.  2  and depicting various inventive arrangements. 
     FIG. 4 shows the digital disk player of FIG. 2 including other advantageous arrangements to those of FIG.  2 . 
     FIGS. 5A and 5B depict an exemplary bit stream before track buffering. 
     FIGS. 5C-5D depict exemplary data in buffer memory. 
     FIG. 6 is a flow chart illustrating an inventive arrangement for the recovery of start codes distributed across sector boundaries. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 depicts an exemplary block diagram of a digital video disk player. In block  10  a deck is shown which may accept a digitally recorded disk  14  for rotation by a motor  12 . A digital signal is recorded on disk  14  as a spiral track containing pits with respective pit lengths determined by an 8/16 modulation coding responsive to respective signal data bits. The record on disk  14  is read by pick up  15  which gathers reflected illumination from a laser. The reflected laser light is collected by a photo detector or opto pick-up device. An imaging device, for example a lens or mirror, which form part of transducer pick-up  15 , is servo controlled and driven by motor  11  to follow the recorded track. Different parts of the recording may be accessed by rapidly repositioning the imaging device. Servo controlled motors  11  and  12  are driven by integrated circuit drive amplifier  20 . Pick up  15  is coupled to an opto preamplifier, block  30 , which includes drive circuitry for the laser illuminator and a preamplifier which provides amplification and equalization for the reflected signal output from the opto pick-up device. The amplified and equalized replay signal from opto preamplifier  30  is connected to a channel processor block  40  where the replay signal is employed to synchronize a phase locked loop which is utilized to demodulate the 8:16 modulation employed for recording. 
     The MPEG encoded bitstream is encoded for error detection and correction by means of Reed Solomon product coding which is applied in blocks of 16 sectors, where each sector contains 2048 bytes of payload data. Thus following  8 : 16  demodulation the replay data stream is de-interleaved or unshuffled and error corrected by means of Reed Solomon product correction implemented in ECC buffer memories  45  and  46  of FIG.  4 . Each buffer stores 16 sectors of the replay data stream arranged as an array to facilitate de-interleaving and to enable the required row and column product processing. The cascaded ECC buffer memories introduce a delay to reproduced serial bit stream of approximately (2 * 16 * 1.4) milliseconds, where 2 represents the pair of ECC buffers, 16 represents the sectors over which the correction is applied and 1.4 milliseconds represents a sector period at 1X rotational speed. Thus the reproduced serial bit stream is delayed by a minimum of approximately 45 milliseconds. 
     The error corrected signal bitstream  41  is coupled via a link processor to a bit stream or mechanical / track buffer memory  60 A. The track buffer comprises a DRAM memory type and is used to store an amount of replayed data such that data losses during transducer or pickup  15  repositioning will not result in any visible deficiency when decoded. Thus the final output image stream will appear to be continuous or seamless to the viewer. Bitstream buffer memory  60 A is part of an exemplary 16 megabit DRAM memory. A further exemplary 16 megabit SDRAM memory block is partitioned to provide frame buffers  60 C and  60 D which provide storage for at least two decoded image frames, compressed video bit stream storage in buffer  60 B prior to decoding, an audio bit stream buffer  60 E and other storage in buffers  60 F, G and H. The channel processor  40  also includes timing control circuitry which control writing by link  505  to bitstream buffer  60 A. Data may be intermittently written to the bitstream buffer as a consequence of changes in replay track addresses, for example, resulting from user defined replay video content such as a “Directors cut”, parental guidance selection, or even user selectable alternative shot angles. To facilitate more rapid access and recovery of the recorded signal, disk  14  may be rotated at an increased speed resulting in the transduced bitstream having a higher bit rate, and possibly intermittent delivery. 
     As has been described, the recorded data stream is arranged in ECC blocks of 16 sectors. Each sector has a unique sector identification address which is protected with error correction bits that are processed by ECC block  47  of FIG.  4 . However, because the sector address is short and sector specific, any delay to sector address signal  42  resulting from error correction processing block  47  is insignificant. Sector address signal  42  is coupled to provide positional information to servo control integrated circuit  50 . Integrated circuit  50  provides drive and control signals for servo motors  11  and  12 . Motor  12  rotates disk  14  and provides servo controlled rotation at a plurality of speeds. The opto pickup or transducer  15  is positioned and servo controlled by motor  11  responsive to sector address signal  42 , and in addition, may be controlled to rapidly reposition or jump to another sector address, or location on the disk surface in response to a sector address request, transmitted by I 2 C control bus  514 , and coupled via element  54  of FIG.  4 . 
     The digital video disk player is controlled by a central processing unit or CPU, element  510  of block  500 , which accepts the reproduced bitstream and error flags from channel IC  40 , and provides control instructions to servo IC  50 . In addition CPU  510  accepts user control commands from user interface  90 , and MPEG decoder control functions from the MPEG decoder element  530  of block  500 . A system buffer memory  80  is addressed by and provides data to CPU  510 . For example, buffer  80  may comprise both RAM and PROM memory locations. The RAM may be used to store various data extracted from bitstream  41  by CPU  510 , for example such data may include descrambling or decryption information, bitstream and frame buffer memory management data, and navigation data. The PROM may, for example contain advantageous transducer jump algorithms which facilitate trick mode operation at a selection of speeds in forward or reverse directions. 
     The MPEG encoded bitstream is coupled to link processor  505  in FIG. 3, which may function as a hardware demultiplexor to separate MPEG encoded audio, video and control information from the DVD formatted bitstream. Alternatively, bitstream demultiplexing may be accomplished by software control of direct memory access or DMA of buffer  60 A, from CPU  510  of FIG.  3 . The encoded bitstream prior to or within track buffer  60 A is searched by microcontroller  510  to locate and read headers and to extract navigation data. Advantageous bit stream searching will be discussed with reference to FIG.  6 . 
     Microcontroller  510  is coupled the front end via I 2 C control bus signal  514  to control or request transducer repositioning to acquire the next sector required by a trick play sequence. The transducer positioning may be controlled by an advantageous stored sequence, or jump play pattern which is indexed with reference to replayed sector addresses and GOP sector addresses read from navigation pack data contained in each video object unit or VOBU. Exemplary sector addresses and VOBU navigation pack are depicted in FIG.  5 A. However, following transducer repositioning, the sectors initially retrieved from the front end may be identified by exemplary microcontroller  510  as not those requested by the jump instruction. Thus, microcontroller  510  advantageously overwrites this unwanted data in track buffer  60 A and ensures that only the requested data is present in the buffer. 
     Having identified sector addresses or headers, microcontroller  510  controls direct memory access of buffer  60 A which effectively separates MPEG data from other DVD formatted data stored in the buffer. Thus, video DMA  515  separates compressed video bits which are coupled for storage in exemplary video bit buffer  60 B. Similarly compressed audio bits are read from buffer  60 A and stored in audio buffer  60 E. Sub-picture data is also retrieved from track buffer  60 A by DMA and stored in buffer  60 F. 
     The compressed video bit stream in video bit buffer  60 B is searched to locate picture or higher level start codes by start code detector  520 . A detected start code signal  512  is coupled to microcontroller  510  which then communicates with MPEG decoder  530 , via signal  511 , to indicate the next picture type, the quantizer setting and to initiate decoding. A decoder status signal  513  is coupled back to microcontroller  510  to indicate the completion of decoding and that picture data available for display or storage. Compressed video bit buffer  60 B may be considered to function as a FIFO or circular buffer where the stored bitstream is sequentially accessed for MPEG decoding, however, trick mode operation may be advantageously facilitated by random access of buffer  60 B, as will be described. 
     Within MPEG decoder  530  the video bit stream is processed by a variable length decoder  531  which searches the bitstream to locate slice and macro-block start codes. Certain pictures decoded from each group of pictures are written to frame buffers  60 C and  60 D for subsequent use as predictors when deriving or constructing other pictures, for example P and B pictures, of the GOP. Frame buffers  60 C and  60 D have a storage capacity of at least two video frames. Separated audio packets are stored in audio bit buffer  60 E which is read out and coupled for audio decoding in block  110 . Following MPEG or AC 3  audio decoding a digitized audio signal results which is coupled to an audio post processor  130  for digital to analog conversion and generation of various base band audio signal outputs. A digital video output signal is transformed into raster scan format by display buffer  580  from decoded blocks read from reference frame buffer  60 C/D. However, during trick mode operation the output signal source may be a field memory advantageously reconfigured from memory unused during trick mode operation. Thus block to raster scan conversion within display buffer  580  may be advantageously controlled responsive to trick mode operation. The display buffer is coupled to encoder  590  which provides digital to analog signal conversion and generates baseband video components and encoded video signals. 
     Operation of the exemplary video player illustrated in FIG. 2 may be considered with reference to FIG. 1B which illustrates a forward play and reverse trick play sequence. As described previously, the coded relationship existing within each GOP necessitates that each group of pictures is decoded in a forward direction starting from an I-frame or picture. Thus, reverse mode features may be provided by effectively jumping back to transduce an earlier, or preceding I picture and then decoding in a forward direction through that GOP. The decoded pictures are stored in frame buffer memories for subsequent read out in reverse order. However, sequences that include B pictures may utilize further advantageous features which will be described. In FIG. 1B it will be assumed that at some time prior to time t0, for example at I-picture I( 1 ), the exemplary video player assumed a forward play condition in response to a user command. Each group of pictures is decoded in the forward direction as illustrated in FIG. 1A by the arrow headed lines linking I, B and P frames. At a time prior to time t0, a three times play speed reverse trick mode is selected, and initiated at time t0 where I-picture I( 25 ) is decoded and displayed. As previously described the next picture required for reverse trick play decoding is I-picture I( 13 ), thus the transducer is moved, as indicated by arrow J 1  to acquire picture I( 13 ). The signal recovery and decoding then follows a play sequence indicated in FIG. 1B by arrows J 1 , to acquire I( 13 ), J 2 , to acquire P( 16 ), J 3 , to P( 19 ), J 4  to P( 22 ). . . Jn. The intervening B pictures shown in FIG. 1B are transduced but may be discarded, for example, in the buffer by over writing or by decoder inhibit, as required specific to each trick play mode. To avoid the previously described requirement for additional reverse mode video buffering, various advantageous methods for MPEG decoder, buffer memory control and allocation are employed. 
     The determination of picture data may be performed in units of sectors referenced in bit stream  41  or track buffer  60 A. However, since an MPEG picture start code is buried within DVD data formatting and is not constrained to start coincident with a sector boundary the resulting location of picture start codes in units of sectors may inevitably include fragments of a preceding, possibly non-video sector. FIG. 5A shows part of exemplary bit stream  41  including a video object unit containing audio video and sub-picture data sectors. Each sector contains 2048 payload bytes with sector addresses shown shaded at the sector boundary. In FIG. 5B video picture A is shown ending in sector  54  and is immediately followed by the start code for video picture B. However, the remainder of video picture B start code occurs in sector  65 , with intervening sectors  55 - 64  containing sub-picture and audio data. Determination or location of picture data/video sectors in units of sectors is illustrated in FIG. 5C where a start code for exemplary picture A is shown in sector  2  with the start code of next picture B, occurring in sector  9 . Equation 1 shows picture data location by sector count, since picture A starts in sector  2  and ends in sector  9 , picture A has a duration of 8 sectors. Unwanted data fragments are illustrated FIG. 5C, where video data is depicted referenced to (video) sector numbers. However such video sector numbering may be directly related to the sector number or address in the reproduced bit stream. In FIG. 5C a video bit stream is shown with an exemplary picture A depicted with a picture start code initiated at byte  1000  of video sector  2 . Clearly the preceding 999 bytes of sector  2  correspond to data from a preceding picture. It is possible to employ more detailed processing where the picture data is located in units of bytes. Byte accurate processing may require more complexity of memory control than that required for sector level accuracy. However, if byte accurate processing is employed only complete picture data are stored in the video bit buffer, thus fragments are eliminated and hang up of MPEG decoder  530  is avoided. Byte accurate picture determination is shown in FIG. 5C for exemplary picture A, where a picture start code begins at byte  1000  of video sector  2  and picture B start code starts at byte  500  of sector  9 . Hence the size of picture A may be calculated, by use of equation 2, as 13,835 bytes. Thus byte accurate picture addressing allows microprocessor  510  to point to a specific byte in exemplary video bit buffer  60 B from which variable length decoder VLD  531 , of FIG. 3, is to start decoding. 
     If picture data is determined in units of sectors, the MPEG decoder reading pictures from the video bit buffer must be protected from hang up due to fragments of discarded pictures occurring before or after the wanted picture is decoded. Such picture fragments are depicted in exemplary video bit buffer of FIG. 5D which shows multiple sectors containing P and B pictures where unwanted data from a previous, or following picture are shown with diagonal shading. Each video object block unit or VOBU includes navigation data that identifies the end sector address of the first I picture and the last sector addresses of two following reference or P pictures of the first GOP of the VOBU. In addition the navigation data includes sector addresses of I-pictures in preceding and succeeding VOBUs, hence an I-picture only trick mode may be readily provided. However, problems resulting from picture fragments may be avoided if the end byte of the wanted picture can be identified. Microprocessor  510 /A, for example type ST20, is advantageously configured as a hardware search engine which searches through data stored in the track buffer to locate the end sector to find the last byte of the I-picture stored in buffer  60 A. Thus by precisely identifying an I-picture, it alone may be loaded into video bit buffer  60 B, hence avoiding the storage of partial pictures which may cause problems of decoder lockup. The exemplary microprocessor  510 /A may be employed to find start codes in an I-picture only mode since the ending sector is known from the navigation data. However, for P, B or multiple I-pictures, the exemplary microprocessor may not provide a practical solution since testing has to be performed on every byte of data in the bitstream, which represents an operationally intensive usage of microprocessor  510 . 
     The location and determination of start codes prior to decoding may be facilitated by an arrangement which utilizes the link interface block  505  of FIG. 3 to search for start codes in the bitstream prior to track buffer  60 A. Such use of link interface  505  advantageously provides early pre-processing or parsing for picture and or audio headers which may be signaled to microprocessor  510 . Thus, having identified headers in the incoming bitstream prior to the track buffer, pictures and audio required by a specific mode may be stored in exemplary truck buffer  60 A with unwanted pictures and or other data deleted in the buffer by overwriting. 
     In a first arrangement start codes are located by use of start code detector  520  which searches the bit stream in the video bit buffer  60 B. Although this method has an advantage in that MPEG start code detector design is known, the detector however requires contiguous data. Hence only data in the video bit buffer, stripped of DVD and transport data structure may be searched. Thus searching for MPEG data within the mechanical/track buffer is difficult to facilitate, and may not optimally use memory, and exemplary microprocessor  510  may be heavily loaded with interrupts, thus requiring the addition of a second microprocessor for example,  510 A specifically to implement start code detection. 
     In an advantageous arrangement, start code detection is facilitated by a start code detector which searches the bit stream exclusively for MPEG start codes prior to, or within track buffer  60 A. Thus by advantageously providing early parsing for MPEG video headers within the bit stream, trick play picture requirements may be anticipated and memory manipulation specific to trick play operation may be performed. The same advantageous parsing may be applied to the video packet stream prior to the video bit buffer during trick mode operation. For example, in a reverse replay mode, such pre-processing permits trick play specific selection between pictures to be buffered for decoding, and those unwanted pictures to be discarded before storage. Such picture selection, for example discarding B-frames, may approximately double the number of I and P pictures stored in exemplary video bit buffer  60 B during trick play operation. Thus the identification of wanted from unwanted data is a direct consequence of pre-processing or parsing prior to buffer storage which allows video bit buffer  60 B to store only wanted, or trick play specific pictures. Hence more trick play specific video object units or VOBUs may be stored facilitating smooth trick play motion rendition. 
     In an advantageous arrangement the storage capacity track buffer  60 A and video bit buffer  60 B are increased during a trick play mode by only selecting for storage data that is to be used subsequently. For example, in an exemplary trick play mode B frames may not be decoded, hence need not be stored in the track or video bit buffers. Thus only needed pictures are stored, and unwanted picture or other data is discarded. To facilitate this advantageous selection between wanted and unwanted pictures requires that the bit stream or video packet stream be pre-processed, parsed or searched to locate a sequence_header, group_of picture_header or picture_header prior to storage. Thus parsing or pre-processing of the compressed bit stream allows the determination of MPEG parameters such as, time_code, closed_gop, and broken_link data for each group of pictures or GOP. In addition, by pre-processing the packet stream the picture_start_code may be located thus permitting processing of the picture_header which in turn allows the determination of, for example, the temporal_reference, picture_coding_type (I, P and B). However, such advantageous MPEG parsing is difficult, as has been described, due to DVD partitioning MPEG like data into sectors of 2048 bytes. In addition, because the MPEG start codes (4 bytes) are not sector aligned, an exemplary picture start code may be distributed across a sector boundary. FIG. 5B illustrates a bitstream prior to track buffer  60 A where video picture A ends in sector  54  and is immediately followed by the start code for video picture B. However, the remainder of video picture B start code occurs in sector  65 , with intervening sectors  55 - 64  containing sub-picture and audio data. FIG. 5C illustrates a demultiplexed video sector bitstream prior to video bit buffer  60 B where a start code for exemplary picture A is shown in sector  2  with the start code of next picture B, occurring in sector  9 . A distributed start code occurs for picture C which is initiated at byte 2046 of sector  12  and continues in sector  13 . Hence, part of the start code is in one video sector with the remainder in the next video sector. 
     To enable a bitstream with distributed start codes to be parsed an advantageous method shown in FIG. 6 is employed. The method shown in FIG. 6 identifies and saves sector types and addresses, and in addition identifies and saves wanted start codes. Distributed or partial start codes are identified and saved by the use of an inventive partial start code flag which indicates the occurrence. remainder of the start code occurring in the next video sector is identified and recovered to complete the start code. The inventive method of FIG. 6, depicts searching and MPEG parsing applied to bit stream  41  prior to track buffering. The bit stream is searched for wanted sectors, for example a video sector, which is then searched for distributed start codes. A distributed start code may be separated by other non-video sectors containing for example audio, sub-pictures, navigation data etc. Thus the bit stream is searched and the subsequent video sector identified and processed, while the intervening non-video sectors, not currently required, for example during a specific trick mode, are not processed and may be discarded prior to storage or over written in exemplary track buffer  60 A. Thus having identified the next video sector, the packet data is searched to locate the next start code. However, because the partial start code flag is set, the remainder of the partial start code is searched for, and with its occurrence this remainder is combined with that of the preceding video sector to complete the start code. 
     The chart in FIG. 6 illustrates the inventive method employed for bit stream searching to identify wanted sector addresses, picture types and addresses, and for detecting and reassembling distributed start codes. The method starts at step  10  where an error corrected bitstream is searched to locate specific wanted sectors from a plurality of sectors including, navigation, audio video sector sub-picture data sectors. A video sector is detected at step  100 , where a NO result forms a loop which continues the bitstream search. Similarly, an audio sector may be detected at step  105  and its sector address stored accordingly. If step  100  tests YES, a video sector is detected and the sector address is stored at step  101 . A detected video sector initiates a further test at step  200  to detect a start code within the video sector. Step  200  depicts picture start code, however, various start codes may be present, for example sequence-headers, GOP headers or picture-headers all exist within the video sector hence any may suffer distribution across a sector boundary. A NO at step  200  forms a loop which continues to search for a start code within the video sector. A YES at step  200  indicates detection of a start code which initiates a further test to detect a partial start code at step  250 . A NO at step  250  forms a loop to wait for a partial start code occurrence. In addition, the NO at step  250  also indicates detection of a complete start code which is tested at step  255  to determine if it is a wanted type. Wanted start codes test YES at step  255  which results in the storage of the type and byte location within the sector address at step  260 . 
     Detection of a partial start results in a YES at step  250  which causes the sequence to restart searching the bitstream to locate the next video sector by looping back to step  100 . The YES at step  250  also initiates a test at step  300  to determine if a partial start code flag is set. The partial start code flag is not set until a first distributed or partial start code is detected. Thus NO at step  300  sets the partial start code flag at step  350 , and in addition stores the value of the partial start code at step  400 . A YES at test  300  indicates detection of the remainder or residue of the distributed start code and results in resetting the partial start code flag at step  500 . The YES at step  300  also results in storage of the detected start code remainder at step  450 . At step  550  the values of the partial start code from step  400  and its remainder from step  450  are combined to reform the distributed start code. Finally at step  575  the reformed start code type, byte and sector address are stored. Hence the inventive method described identifies and stores specific sector types and addresses, identifies and stores start code types and byte addresses within sectors, and identifies and reassembles distributed start code fragments. Thus a DVD format bit stream may be parsed to determine for example, specific MPEG coded picture types, prior to buffer storage. 
     During trick mode operation, and particularly during reverse play speed operation, maximized picture buffer capacity is required to store a group of pictures for read out in reverse order. During such trick modes certain player functions or features may not be required, may be non-useful or unavailable. Such functions or features include audio, multiple languages, sub-pictures and on screen displays and all utilize buffer memory capacity. Thus buffer memory capacity unused by these functions or features may be reallocated during trick mode operation to provide additional picture storage. However, during certain trick modes, for example a fast play mode, there may be a beneficial requirement for the accompanying audio to be reproduced at high speed, and pitch corrected to assist in scene location. In addition a limited on screen display may be required to indicate trick play speed and direction. Thus unused buffer memory capacity may be dynamically reconfigured to advantageously facilitate trick mode buffering for compressed pictures, decoded frame predictors, and video display fields. 
     The block diagram of FIG. 4 shows the same functions and element numbering as depicted in FIG. 2, however, FIG. 4 includes additional advantageous arrangements which enhance digital disk player operation. The exemplary digital video disk player shown in FIGS. 2,  3  and  4  may be considered to comprise two parts namely a front end and a back end. The front end controls the disk and transducer with the back end providing MPEG decoding and overall control. Such functional partitioning may represent an obvious solution for consistent, steady state, MPEG decoding. However, such partitioning of processing with control from the back end may result in microcontroller overload, for example, during trick mode operation and particularly in the reverse direction. 
     MPEG picture decoding order is determined, as is well known, by the encoded picture hierarchy, hence in forward mode operation the MPEG decoding sequence is followed. However, trick play operation may be advantageously facilitated by controlling MPEG picture decoding order based on picture sequences required by a predetermined trick play algorithm and knowledge of where the pictures start and stop in the video bit buffer. Thus knowledge of picture location in the video bit buffer  60 B, for example as calculated in FIG. 5C, or as determined by bitstream search of FIG. 6, permits memory start pointers in the start code detector  520  and variable length detector  531  to be advantageously directed to randomly access pictures as required, for example, during trick mode operation. An exemplary video bit buffer is shown FIG. 5D containing picture fragments as described previously. A start code detector memory pointer is depicted as arrow SCD which searches through the exemplary video bit buffer to locate MPEG start codes. However in the third sector of the first P picture the start code detector memory pointer SCD 1  indicates detection of a start code from the next, but unwanted picture. Hence by advantageously directing the start code memory pointer to known byte accurate memory locations unwanted pictures and undesirable decoder hang ups are avoided, as is indicated by arrow SCD 2  of FIG.  5 D. 
     In a further advantageous trick mode arrangement, unwanted data from previous pictures is cleared from input and output FIFOs, first in first out registers of start code detector (SCD)  520  and variable length decoder (VLD)  531 . Signals  521 / 532  depicted in FIG. 3 clear or reset respective FIFOs to purge data remaining from previous decode operations. Such clearance or flushing of the FIFOs ensures that the SCD and VLD start the next decode operation with new data from exemplary bit buffer  60 B thus eliminating a further source of decoder mis-operation resulting from residual prior data. 
     Operation in reverse at play speed requires the reproduction of B-frames and in a further trick mode optimization reverse mode operation is advantageously simplified in terms of buffer memory requirements by reversing the order in which adjacent B pictures are decoded. This advantageous reversal of decoding order is achieved by setting or controlling the memory start pointers to enable decoding of specific pictures required by the trick mode. In another trick mode optimization, buffer memory size and control may be simplified during trick play operation by advantageously skipping or not reading pictures in the video bit buffer by address manipulation as required by specific trick play algorithms. Memory size and control maybe further optimized during trick play by advantageously enabling multiple decoding of pictures either immediately or as specifically required by the trick play algorithm. The provision of these advantageous features requires careful control of read/write functions and the synchronization therebetween. 
     In yet a further trick mode optimization, a decoder control capability which facilitates audio video synchronization or lip-syncing by skipping picture decoding is advantageously increased in control range and utilized during trick mode operation to allow a number of pictures, selectable between 2 and at least 6 to be skipped or not decoded. Such picture manipulation advantageous facilitate trick play operation at six times play speed by skipping over B-pictures within each GOP.