Abstract:
Methods and apparatus for encoding and decoding video subframes (e.g., lower-resolution video) with a DVC video coder are disclosed. The disclosed embodiments allow a DVC video coder to efficiently code a subframe. The disclosed encoder embodiments redistribute blocks of data from a subframe to correspond with the staggered locations used for video segment creation. This separates video segments at the DVC coder output into two group—those largely or completely composed of subframe data, and those containing discardable data. The present invention allows a DVC coder to be used efficiently for several different video resolutions, or in a low-resolution system.

Description:
This application is continuation of prior U.S. patent application Ser. No. 09/429,229 filed Oct. 28, 1999 now U.S. Pat. No. 6,683,986. 

   FIELD OF THE INVENTION 
   This present invention relates to digital image coding, and more particularly to a method and apparatus for encoding digital video into a compressed digital video stream, and a corresponding method and apparatus for decoding a compressed digital video stream. 
   BACKGROUND OF THE INVENTION 
   DVC is a common acronym for a digital video coding standard presently in widespread use for digital handheld camcorders, digital video recorders, digital video playback devices, etc. See  Recording—Helical - scan digital video cassette recording system using  6.35  mm magnetic tape for consumer use  (525-60, 625-50, 1125-60  and  1250-50  systems ), International Electrotechnical Commission Standard, IEC 61834 (1998). This standard describes the content, formatting, and recording method for the audio, video, and system data blocks forming the helical records on a DVC tape. It also specifies the DVC video frame format for compatibility with different television signal formats, including the 525-horizontal-line, 60 Hz frame rate broadcast format common in the United States (the 525-60 format), and the 625-horizontal-line, 50 Hz frame rate broadcast format common in many other countries (the 625-50 format). 
   Examining the 525-60 DVC video frame format in particular,  FIG. 1  illustrates the digital sample structure for the luminance component of a 525-60 format video frame. A video frame  30  is divided into a tiling of superblocks S 0 , 0  to S 9 , 4 . Each superblock takes one of three possible superblock shapes  32 ,  34 ,  36 , depending on the superblock&#39;s position in frame  30 . Also, each superblock is divided into 27 macroblocks. Most of these macroblocks are of the format shown for macroblock  38  (four blocks arranged horizontally), although for superblock shape  36 , three macroblocks have the format shown for macroblock  40  (four blocks arranged 2×2). 
   Macroblocks  38  and  40  each contain four luminance blocks  42 . Each luminance block  42  contains 64 digital samples  44 , arranged in a regular 8×8 grid. Each macroblock also contains one 64-sample Cr and one 64-sample Cb block (not shown), for a total of six blocks of samples per macroblock. The total frame size is 720 digital samples (90 luminance blocks) wide by 480 digital samples (60 luminance blocks) high. 
   DVC encoder chips are commercially available. These chips generally have two modes of operation: an encoding mode that converts video frames into an encoded stream of video segments, and a decoding mode that converts an encoded stream of video segments back into video frames. The basic operation of the encoding mode of a DVC encoder chip is shown in  FIG. 2  as two concurrent processes, Process A and Process B. 
   Process A operates on an incoming pixel stream representing a raster-sampled video frame. Block  50  performs a horizontal lowpass filter to smooth the data. The smoothed pixels are gathered at block  52  until eight lines are present, representing 90 blocks of luminance data (45 blocks of chrominance data are also processed concurrently, not shown). An 8×8 Discrete Cosine Transform (DCT) is performed on each of the 90 pixel blocks at  54 , and the blocks are stored to frame buffer A at block  56 . This process loops until an entire frame of DCT data has been stored to frame buffer A, and then repeats for the next frame using a frame buffer B. 
   At the same time that Process A is performing DCTs and storing data to frame buffer A, Process B is reading stored DCT data (from the previous frame) from frame buffer B. Thus at block  60 , process B selects DCT data corresponding to a video segment that is to be created next. At block  62 , it reads five macroblocks, corresponding to this DCT data, from frame buffer B. At block  64 , these five macroblocks are encoded together into a fixed-length video segment by a complex quantization and coding process that can allow less-detailed macroblocks to “share” unused portions of their bit allotment to more-detailed macroblocks. In general, block  64  results in some loss of data in order to fit the five macroblocks into the allowable space, although the data discarded is selected to (hopefully) have a low impact on perceived picture quality. Finally, at block  66 , the encoded video segment is output from the DVC chip and Process B loops back up to produce the next video segment. 
   The five macroblocks encoded in a DVC video segment are selected from scattered regions of the digital video frame in order to distribute the effects of physical data recording errors.  FIGS. 3 ,  4   a,    4   b,  and  4   c  illustrate how the five macroblocks corresponding to a particular video segment are selected. Generally, five superblocks S 0 , 0 , S 1 , 6 , S 2 , 2 , S 3 , 8 , and S 4 , 4  are coded into the first twenty-seven video segments, each video segment representing one macroblock from each of the five superblocks shown. Scan paths  72  ( FIG. 4   a ),  74  ( FIG. 4   b ), and  76  ( FIG. 4   c ) illustrate the order of macroblock selection for each particular superblock shape. Thus the first video segment will combine the first macroblock in scan path  72  for each of S 0 , 0  and S 2 , 2  with the first macroblock in scan path  74  for each of S 1 , 6  and S 3 , 8  and the first macroblock in scan path  76  from S 4 , 4 . The second video segment will combine the second macroblocks in these scan paths, etc. 
   When the five superblocks shown have been converted into twenty-seven video segments, encoding for those superblocks is complete. The process then performs a second encoding pass using the five superblocks immediately below the first five superblocks to generate 27 more video segments, and repeats. After the bottom superblock in any superblock column has been encoded, the process “wraps” to the head of that column on the next pass and continues until ten passes have been made. 
   SUMMARY OF THE INVENTION 
   The DVC process provides efficient digital compression for its designed frame formats, and low-cost DVC chips are available. Unfortunately, the staggered five-macroblock-shared video segment design hinders efficient use of the DVC chip with any frame format other than those of its design. For instance, a quarter-VGA (QVGA) frame is 320 pixels wide by 240 pixels high, less than one-fourth the size of a DVC 525-60 frame (720×480 pixels). If a QVGA subframe were inserted in the top left corner of an otherwise blank DVC 525-60 frame, 3.5 out of every 4.5 pixels in the frame (77.8%) would be blank. But because at least one macroblock of pixels from the QVGA subframe would appear in all but 21 of the 270 video segments created for this frame, over 92% of the fixed-sized video segments must be kept intact in order to preserve the QVGA subframe information. The net result is that the DVC-coded subframe requires more bits to represent a lossy-coded version of the QVGA subframe than the original QVGA subframe required. 
   The embodiments illustrated herein show an alternative approach that allows standard DVC chips to be used to efficiently code a QVGA subframe, or any other subframe data. Generally, this approach redistributes blocks, from a desired subframe, throughout a DVC frame to correspond with selected DVC video segments, ensuring that video segments of interest will generally be filled with subframe data. By judicious selection of a redistribution mapping, buffer space requirements can be decreased and full DVC compression efficiency can be realized on a subframe. For instance, with a proper redistribution mapping, a QVGA subframe can be represented while discarding 77.8% of the DVC video segments. 
   In one aspect of the invention, a method for encoding a digital image is disclosed. The method used a digital video coder, such as a DVC coder, that encodes a digital video frame using video segments. A digital image to be encoded is segmented into a set of blocks, and the blocks are presented to the digital video coder as part of a larger, synthesized digital video frame. The blocks are inserted into the digital video frame so as to substantially occupy frame locations corresponding to selected video segments in the video segment encoding order. The synthesized digital video frame is encoded with the digital video coder to produce a coded output stream comprising multiple video segments. From the coded output stream, those video segments corresponding to the digital image are selected. 
   In another aspect of the invention, a method for transmitting a digital video sequence is disclosed. The method used a digital video coder, such as a DVC coder, that encodes a digital video frame using video segments. An original frame of the digital video sequence is segmented into a set of blocks, and the blocks are presented to the digital video coder as part of a larger, synthesized digital video frame. The blocks are inserted into the digital video frame so as to substantially occupy frame locations corresponding to selected video segments in the video segment encoding order. The synthesized digital video frame is encoded with the digital video coder to produce a coded output stream comprising multiple video segments. From the coded output stream, those video segments corresponding to the digital image are selected and transmitted to a receiver. The selected video segments are inserted into a coded input stream, which is supplied to a digital video decoder for decoding into a second synthesized digital video frame. From the second synthesized digital video frame, reconstructed blocks corresponding to the set of blocks of the original frame of the digital video sequence are selected and combined to form an output digital video frame corresponding to the original frame. 
   In yet another aspect of the invention, a digital video encoding system is disclosed. The digital video encoding system uses a digital video coder that encodes input digital video frames into output video segments, each video segment representing data from multiple scattered regions of a digital video frame input to the digital video coder. The system also has an input frame buffer, and a mapper to map blocks of data from the input frame buffer to a synthesized digital video frame for input to the digital video coder. The blocks of data are mapped such that they substantially occupy frame locations of the digital video frame corresponding to selected video segments in the video segment encoding order of the digital video coder. The system also has a data selector to select video segments from the digital video coder output corresponding to the blocks of data mapped from the input frame buffer. 
   In a further aspect of the invention a digital video decoding system is disclosed. The digital video decoding system uses a digital video decoder that decodes input digital video segments into output video frames, each video segment representing data from multiple scattered regions of an output digital video frame. The system also has an input data buffer to buffer video segments, and a data padder to concatenate video segments from the input data buffer with dummy video segments for input to the digital video decoder. The system also has a subframe extractor to map the digital video frame regions corresponding to the video segments supplied from the input data buffer into a reconstructed digital video frame. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention may be best understood by reading the disclosure with reference to the drawing, wherein: 
       FIG. 1  illustrates the 525-60 frame format for DVC coding; 
       FIG. 2  illustrates the basic operation of a DVC coder; 
       FIGS. 3 ,  4   a,    4   b,  and  4   c  illustrate macroblock and superblock coding order for DVC video segments; 
       FIGS. 5 ,  6   a,    6   b,  and  7  illustrate a block tiling for a QVGA image useful with an embodiment of the invention; 
       FIGS. 8 and 9  illustrate a DVC image mapping, and corresponding video segment output timeline, for the block tiling of  FIG. 7 ; 
       FIGS. 10 and 11  show block diagrams for a QVGA encoding system and a QVGA decoding system according to one embodiment of the invention; 
       FIGS. 12 and 13  illustrate a DVC image mapping, and corresponding video segment output timeline, for a second embodiment of the invention; 
       FIG. 14  depicts a block diagram for a QVGA encoding system useful with the mapping of  FIG. 12 ; 
       FIG. 15  shows processing timing for the encoding system of  FIG. 14 ; 
       FIGS. 16 and 17  illustrate a DVC image mapping, and corresponding video segment output timeline, for a third embodiment of the invention; 
       FIG. 18  shows a QVGA superblock tiling useful with the mapping of  FIG. 16 ; 
       FIG. 19  shows a block diagram for a QVGA decoding system useful with the mapping of  FIG. 16 ; 
       FIG. 20  illustrates data padding mapping for an embodiment of the invention; 
       FIGS. 21   a  and  21   b  show a QVGA superblock tiling according to an embodiment of the invention; 
       FIG. 22  shows a DVC image mapping for the tiling of  FIGS. 21   a  and  21   b;    
       FIG. 23  shows a superblock tiling for a reduced horizontal resolution image; 
       FIG. 24  shows a DVC image mapping for the tiling of  FIG. 23 ; 
       FIG. 25  shows a video transmit/display system using an embodiment of the invention; and 
       FIG. 26  illustrates a multi-mode DVC coding system according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments disclosed below were selected by way of illustration and not by way of limitation. For instance, although QVGA (320×240) and reduced horizontal resolution (192×240) examples are shown, the principles taught by these examples may be used with virtually any image size. And although the specific examples use a DVC encoder operating in 525-60 mode, the principles taught by these examples are applicable to other DVC modes, and indeed, other encoders that operate similarly to produce video segments. 
   To introduce the first example, a 320×240 QVGA luminance frame  80  is shown in  FIG. 5 . Frame  80  has been divided for purposes of the example into thirty rows by ten columns of macroblocks  82 , each macroblock  82  constructed from four horizontally adjacent 8×8 sample blocks  84 . Typically, each macroblock will also contain a 4×16 Cr and a 4×16 Cb block (not shown). This macroblock configuration matches the configuration described for macroblock  38  of  FIG. 1 . 
     FIGS. 6   a  and  6   b  show, respectively, two superblock patterns  86  and  88 . Each of superblock patterns  86  and  88  contains fifteen macroblocks  82 , arranged as two-and-one-half columns of six rows of macroblocks. The selection of the particular shapes is related to the video segment macroblock scan order of the DVC encoder, and the reasons for these selected shapes will become more apparent as the example progresses. Note that as frame  80  contains 300 macroblocks, twenty of these fifteen-macroblock superblocks will completely tile frame  80  if a suitable tiling can be found. 
     FIG. 7  shows such a tiling  90 . A first column of five superblocks of the superblock  86  pattern are interlocked with a second column of five superblocks of the superblock  88  pattern. A third column of five superblocks of the superblock  86  pattern are located adjacent the second column, and interlocked with a fourth column of five superblocks of the superblock  88  pattern. This tiling  90  regularly tiles QVGA frame  80  with no samples or macroblocks either excluded or left over. 
   The tiling is now mapped onto an otherwise blank DVC 525-60 frame  92 , as shown in  FIG. 8  (the first five superblocks to be encoded are highlighted for reference, but contain blank data also). The particular mapping shown accomplishes several design goals. First, the Ay,x QVGA superblocks have been staggered to correspond to the staggered sampling order used for video segment construction. This ensures that an Ay,x superblock will be considered along with four other Ay,x superblocks (rather than with blank superblocks) at the time of video segment construction. Second, the Ay,x superblocks of  FIG. 7  have been aligned with the Sy′,x′ superblocks of  FIG. 8 , such that although the Ay,x superblocks are different in size than the Sy′,x′ superblocks, each video segment produced will either contain Ay,x data in all five of its macroblocks or blank data in all five macroblocks. 
     FIG. 9  illustrates the advantageous effects of this superblock mapping.  FIG. 9  shows, on a time line, the temporal video segment output  94  of a DVC chip. Each group of 27 video segments (e.g., group  96 ) represents the encoding time for five staggered DVC superblocks (e.g., S 0 , 0 , S 1 , 6 , S 2 , 2 , S 3 , 8 , and S 4 , 4  for group  96 ). The first four of these groups are blank, since the mapping of the QVGA frame placed no data in the corresponding superblocks. Beginning with subgroup  98   a,  QVGA frame data A. 1  is represented in 15 consecutive video segments. A. 1  corresponds to QVGA superblocks A 0 , 0 , A 0 , 1 , A 0 , 2 , A 0 , 3 , and A 4 , 0 . Subgroup  98   b,  consisting of twelve video segments, is again blank, since the QVGA superblocks do not completely fill the underlying DVC superblocks. This fifteen QVGA data/twelve blank data format is repeated three more times, followed by two final groups of 27 blank video segments each. 
   The tiling/mapping of this example has effectively placed the QVGA frame in the minimum possible number of DVC video segments ( 60 ), instead of spreading the QVGA frame information amongst virtually all of the 270 DVC video segments. The 210 blank DVC video segments can be discarded at the encoder output, and the 60 valid segments efficiently stored or transmitted. 
     FIG. 10  illustrates a block diagram for one possible hardware configuration useful with the preceding example. Encoding system  100  accepts an input video stream  102 . Video stream  102  is digitized in a QVGA frame format by video buffer writer  104 , which stores successive frames alternately to A buffer  106  and B buffer  108 . Mapper  110  provides synthesized, rasterized 525-60 formatted frame data to DVC encoder  114  according to the mapping shown in  FIG. 8 . Thus mapper  110  supplies 480 rows of 720 luminance sample data, by either selecting a constant pixel value for “blank” pixels, or by reading QVGA data for mapped QVGA superblock pixels from either A buffer  106  or B buffer  108  (whichever is not currently being overwritten by video buffer writer  104 ). 
   DVC encoder  114  uses frame buffers  112  to encode the synthesized frame data supplied by mapper  110  into DVC-formatted video segments. Finally, data selector  116  filters the video segment data to produce output data stream  118 , e.g., consisting of the video segment groups A. 1 , A. 2 , A. 3 , and A. 4  of  FIG. 9 . 
     FIG. 11  illustrates a block diagram for one possible hardware configuration useful in decoding a data stream produced by encoding system  100 . Decoding system  120  accepts an input data stream  122 , after system  120  has been configured to recognize the frame format represented in the data stream. For instance, if the frame format is DVC-coded QVGA, decoding system  120  will expect 60 DVC video segments per QVGA frame and will know where these 60 segments map to in the DVC frame format. An appropriate synchronization signal can be supplied to indicate the beginning of a new frame. 
   Input data stream  122  is read into data buffer  124 . Data mixer/padder  126  uses the data from buffer  124  to create a coded input stream, e.g., like that of  FIG. 9  (although the video segments from input data stream  122  could be placed anywhere in the coded stream, as long as they are treated consistently). Note that DVC decoder  130  accepts segments in a particular format specified for DVC, even if blank data is represented. This may include blank VAUX (video auxiliary) data segments. For blank video and VAUX segments, padding data such as the following hex data sequences can be used for padding: 
   VAUX 
   0×29, 0×7d, 0×50, 0×b7, 0×9c, 0×ac, 0×c1, 0×b5, 
   0×d1, 0×91, 0×02, 0×4d, 0×3d, 0×c3, 0×f8, 0×ec, 
   0×52, 0×fa, 0×a1, 0×6f, 0×39, 0×59, 0×83, 0×6b, 
   0×a3, 0×22, 0×04, 0×9a, 0×7b, 0×87, 0×f1, 0×d8, 
   0×a5, 0×f5, 0×42, 0×de, 0×72, 0×b3, 0×06, 0×d7, 
   0×46, 0×44, 0×09, 0×34, 0×f7, 0×0f, 0×e3, 0×b1, 
   0×4b, 0×ea, 0×85, 0×bc, 0×e5, 0×66, 0×0d, 0×ae, 
   0×8c, 0×88, 0×12, 0×69, 0×ee, 0×1f, 0×c7, 0×62, 
   0×97, 0×d5, 0×0b, 0×79, 0×ca, 0×cc, 0×1b, 0×5d, 
   0×19, 0×10, 0×24, 0×d3, 0×dc, 0×3f, 0×8e, 0×c5, 
   0×2f, 0×aa, 0×16, 0×f3, 0×95, 0×98, 0×36, 0×ba 
   Blank Video 
   0×29, 0×7d, 0×50, 0×b8, 0×9c, 0×aa, 0×c1, 0×b5, 
   0×d1, 0×91, 0×02, 0×4d, 0×3d, 0×c3, 0×f8, 0×ec, 
   0×52, 0×fa, 0×a1, 0×69, 0×39, 0×59, 0×83, 0×6b, 
   0×a3, 0×22, 0×04, 0×9a, 0×7b, 0×87, 0×f1, 0×d8, 
   0×a5, 0×f3, 0×42, 0×de, 0×72, 0×b3, 0×06, 0×d7, 
   0×46, 0×44, 0×09, 0×34, 0×f7, 0×0f, 0×e3, 0×b7, 
   0×4b, 0×ea, 0×85, 0×bc, 0×e5, 0×66, 0×0d, 0×ae, 
   0×8c, 0×88, 0×12, 0×69, 0×ee, 0×09, 0×c7, 0×62, 
   0×97, 0×d5, 0×0b, 0×79, 0×ca, 0×cc, 0×1b, 0×7b, 
   0×19, 0×10, 0×24, 0×d3, 0×dc, 0×3f, 0×8e, 0×c5, 
   0×2f, 0×aa, 0×16, 0×f3, 0×95, 0×98, 0×36, 0×ba 
   DVC decoder  130  accepts the coded input stream from data mixer/padder  126 . Using frame buffers  128 , decoder  130  produces a synthesized digital video frame. This synthesized frame is passed to subframe extractor  132 . Subframe extractor  132  gathers blocks from the synthesized frame that correspond to the input data stream  122 , i.e., those that represent reconstructed blocks corresponding to the original QVGA video frame input at the encoder. These blocks are written into either A buffer  134  or B buffer  136  in appropriate locations to reconstruct a QVGA image. Finally, video output selector  138  produces an output video stream  140  by reading raster data from either A buffer  134  or B buffer  136  (i.e., the buffer that is not currently being written to by subframe extractor  132 ). 
   Many other possibilities exist for mapping schemes and hardware according to embodiments of the invention. For example,  FIGS. 12–16  illustrate concepts in a second embodiment that reduces buffer requirements. 
   Referring to  FIG. 12 , a mapping of two QVGA frames (frames A and B) onto a DVC 525-60 frame is shown. This mapping illustrates that two frames can be mapped in a manner that avoids both appearing in the same video segment, and also in a manner that avoids placing data from frame A horizontally adjacent with data from frame B (this is a consideration, e.g., if the DVC encoder runs a horizontal smoothing filter). 
   The two frame mapping is desirable because it enables a reduction in the number of 720×480 DVC frames buffers required from two to one. This is achieved by presenting each QVGA frame to the DVC encoder for two successive DVC frame times. The A and B presentation times are staggered, i.e., the A frame can be changed at odd frame times and the B frame changed at even frame times. The frame that is not being changed represents the valid output data for that frame time. 
     FIG. 13  shows the valid video segments for an even and an odd DVC frame according to this mapping. Four A-groups of video segments A. 1 , A. 2 , A. 3 , A. 4  appear in the first four 27-video segment groups of an even DVC frame. Four B-groups of video segments B. 1 , B. 2 , B. 3 , B. 4  appear in the second four 27-video segment groups of an odd DVC frame. Note that although other video segments may possibly contain some valid data, this generally cannot be relied upon. 
     FIG. 14  depicts a block diagram for an encoding system  160  useful with the two frame mapping of  FIGS. 12 and 13 . Encoding system  160  has a C buffer  168  that is not present in encoding system  100  of  FIG. 10 . In exchange for this addition, encoding system  160  requires only one DVC frame buffer  172 , instead of the two DVC frame buffers  112  of encoding system  100 . As a DVC frame buffer is 4.5 times the size of a QVGA frame buffer, this results in substantial memory savings. 
   Video buffer writer  162  stores frames alternately to one of A buffer  164 , B buffer  166 , and C buffer  168 . Video buffer writer  162  also creates a frame sync signal  176 , to indicate to mapper  170  and data selector  174  where in the repeating storage sequence the encoding system is operating. Mapper  170  utilizes frame sync signal  176  to determine which two of the buffers  164 ,  166 ,  168  are not being written to by buffer writer  176 , and reads data from those two buffers to create an input stream for DVC encoder  114 . 
   In operation, DVC encoder  114  performs its first-pass operations (DCT calculation and storage) and second-pass operations (video segment creation) using the single frame buffer  172 . Although this means that the DCT values used to create video segments will be changing as video segments are created, because the input QVGA values are repeated for two frames, DCT values corresponding to the repeated areas can be relied on for one frame time. This concept is further illustrated in  FIG. 15 , as explained below. 
     FIG. 15  indicates the values present at various points in encoding system  160  during six consecutive frame times, T 0  through T 5 . During each frame time Tn, a corresponding QVGA frame F(Tn) is input to encoding system  160 . Thus at T 0 , the system is initialized and a first QVGA frame F( 0 ) is stored in Buffer A. At T 1 , frame F( 1 ) is stored in buffer B, while frame F( 0 ) is read from buffer A and a null image is read from Buffer C. Frame F( 0 ) and the null image are mixed at mapper  170 , F( 0 ) forming the “A” blocks of  FIG. 12 , the null frame forming the “B” blocks of  FIG. 12 . During T 1 , DCTs are performed on this mixed image and stored in frame buffer  172 , such that at the beginning of T 2 , frame buffer  172  contains DCTs for this mixed image. 
   During T 2 , F( 0 ) is again read from buffer A and used to form the “A” blocks of the mixed image. F( 1 ) is read from buffer B and used to form the “B” blocks of the mixed image. As DCTs are performed on the blocks and written to frame buffer  172 , the “A” blocks will be overwritten with the same data—these blocks are thus stable during time T 2 . But the null image DCT data stored in the B blocks will gradually be overwritten with DCT data from F( 1 ) during T 2 . Thus the “encoded image” represented in the sequence of video segments output by DVC encoder  114  during T 2  will consist of valid video segments corresponding to F( 0 ) (see timeline  152  of  FIG. 13 ), and generally unreliable video segments corresponding to either the null image or F( 1 ). Note that during T 2 , F( 2 ) is being stored to buffer C. Data selector  174  selects the F( 0 ) video segments for output from the system, two frame times after frame F( 0 ) was input to the system. 
   During T 3 , F( 1 ) is again read from buffer B and used to form the “B” blocks of the mixed image. F( 2 ) is read from buffer C and used to form the “A” blocks of the mixed image. Thus during T 3 , the B blocks are stable and the A blocks are changing. During this time period, data selector  174  selects its output according to timeline  154  of  FIG. 13 , extracting the video segments corresponding to F( 1 ) from the DVC encoder output. 
   A similar frame progression can be observed for the remaining time periods of  FIG. 15 , and is not detailed further in this description. 
   When output data must be buffered for writing to storage media, or transmission to a receiver, a data buffer can be added at output data stream  178 . The size of this buffer, as well as its latency, can be affected by the block mapping scheme chosen for the invention. For instance, in order to prevent buffer underflow in a system generating the output of  FIG. 13 , video segments for a frame must be buffered until after the first four groups of twenty-seven video segments are output from DVC encoder  114 . This means that the buffer must be capable of storing an entire collection of “A” video segments, and that the buffer must delay for almost a half-frame before beginning transmission. 
     FIG. 16  shows an alternate mapping  210  that decreases output buffer size and latency. In this mapping, the first fifteen video segments of an even DVC frame are “A” video segments (see timeline  212  of  FIG. 17 ), followed by 39 video segments to be discarded. In an odd DVC frame, the first twenty-seven video segments are discarded, and the next fifteen video segments are “B” video segments (see timeline  214  of  FIG. 17 ). Groups of “A” and “B” video segments are interspersed throughout the frame, such that the encoder  114  output of valid video segments is less bursty. Timelines  216  and  218  show the timing of buffered output can be arranged to avoid buffer underflow with a latency of just over one-tenth of a frame. Note that the buffer size required for these timelines is roughly three-eighths of a one-frame collection of valid video segments. 
   Mapping  210  shows an additional feature that can be used to reduce buffer size in both an encoding system and in a decoding system according to the invention. The block arrangement has been modified such that the order in which QVGA superblocks are used in synthesizing a 525-60 frame correlates roughly with the raster order of the synthesized 525-60 frame. This is best visualized by viewing  FIG. 18  in conjunction with  FIG. 16 . 
     FIG. 18  shows a tiling diagram  220 , divided into five sets of four QVGA superblocks each (sets  222 ,  224 ,  226 ,  228 , and  230 ). These superblocks are grouped according to a “superblock raster order”, i.e., superblocks are grouped in rows according to a left-to-right, top-to-bottom ordering. Superblock set  222  is mapped such that it occupies the first four valid QVGA superblock mapping positions in mapping  210 , i.e., either the four “A” positions or the four “B” positions shown in the first two superblock rows of  FIG. 16 , depending on whether the frame is even or odd. With this mapping, two effects are achieved. All superblocks in the top fifth of a QVGA frame are used within the first fifth of a frame time as input to the encoder, thus latency at the input buffer can be reduced from one frame to one-fifth of a frame, and buffer size can be reduced accordingly (i.e., for an embodiment like  FIG. 10 , A buffer  106  and B buffer  108  can be replaced by a single buffer two-fifths of a QVGA frame in length; for an embodiment like  FIG. 14 , A buffer  164 , B buffer  166 , and C buffer  168  can be replaced by a single buffer 1.4 times a QVGA frame in length). At the decoder, a single output buffer four-fifths of a QVGA frame in length can be utilized (see buffer  192  of  FIG. 19 ), and latency can be reduced from one frame to one-fifth of a frame. 
   Using each of the improvements shown in  FIGS. 16 and 17 , the total latency of a encode-transmit-receive-decode system can be reduced substantially. One-fifth of a frame latency is required at the input to the encoder. One frame latency is required in the DVC encoder. One-tenth of a frame latency is required at the transmit buffer. At the receive buffer, an additional two-tenths of a frame latency is required, followed by a one frame latency in the DVC decoder. Finally, an additional one-fifth of a frame latency is required at the output buffer of the decoding system. This is a total end-to-end delay of about 2.7 frames, or less than a tenth of a second. 
     FIG. 19  shows a decoding system that uses the above improvements in order to function with minimal buffer size. Data buffer  182  is approximately 85 video segments in length, allowing video segments to be used in two consecutive synthesized video segment frame inputs to DVC decoder  186 . One 525-60 frame buffer  188  is required for DVC decoder  186 . And a four-fifths of a QVGA frame buffer  192  is required at output video stream  196 . 
   Some DVC encoders perform a horizontal filtering operation on their input samples in order to reduce horizontal frequency prior to coding. With such an encoder, the boundaries of a QVGA superblock (where the superblock meets the blank background of a synthesized 525-60 image) are seen as high frequency edges by the filter and blurred with the blank background, resulting in visible artifacts in a reconstructed image. This effect can be avoided by appropriate padding of the QVGA input with surrounding QVGA pixels during synthesis of a 525-60 input image to the DVC encoder. 
     FIG. 20  illustrates a mapping  232  similar to mapping  210  of  FIG. 16 . Mapping  232 , however has been padded, i.e., additional pixels have been copied from the QVGA data in locations adjacent to the QVGA superblocks. In the example, a sixteen-pixel-wide pad has been used—other values may be adequate or more appropriate for use with different DVC encoders. Generally, sixteen pixels to the left of a block will be copied to the left of the block in the mapping, and sixteen pixels to the right of a block will be copied to the right of a block in the mapping. If the block resides at the left edge of both the QVGA image and mapping  232 , no left copy is used. If the block resides at an edge of the QVGA image, but not at the corresponding edge of mapping  232  (e.g., blocks A 0 , 3  and A 4 , 0 ), no data is available for copy at that edge; instead, block data is mirrored about that edge. Finally, if a block is flipped in the mapping, the copied padding pixels are flipped with it. Note that this padding data will result in some additional video segment data related to the QVGA image; this data is discarded in the data selector. 
   The desirable properties illustrated in the preceding embodiments include: 1) full usage of any video segments that carry QVGA information; 2) duplication of QVGA data for two frames to reduce buffer usage; 3) distribution of QVGA data across the video segment output stream to reduce latency and transmit/receive buffer requirements; 4) ordering of the raster order of QVGA superblocks with the raster order of DVC 525-60 superblocks to reduce input and output buffer latency and buffer size; and 5) padding of data to reduce artifacts in the reconstructed image. Taking these properties into account, other mappings are equally possible and may be preferable for some specific systems. 
   For example,  FIGS. 21   a  and  21   b  show A frame/B frame tilings  240  and  250 . These tilings use smaller superblocks of three different types (1×6, 2×3, and 1×3 macroblocks). In addition, the superblock tiling in the A frame is different from the B frame. This allows the mapping  260  shown in  FIG. 22  to be used. This mapping fills requirements 1–4 outlined in the preceding paragraph, while spreading the valid video segment output across the encoder output (or decoder input). In even frames, every set of five DVC superblocks will produce six valid “A” video segments, at the fourth through ninth video segment positions. In odd frames, every set of five DVC superblocks will produce six valid “B” video segments, at the 19 th  through 24 th  video segment positions. This allows the transmit and receive buffer latency and size to be reduced even further than in the preceding examples. Note that mapping  260  can be padded, using the principles illustrated in mapping  232 , if desired. 
   As a final example,  FIG. 23  illustrates a tiling  270  for a reduced horizontal resolution image (192×240 pixels). The corresponding mapping  280  of  FIG. 24 , including padding, illustrates how the invention can be applied to other image sizes. 
   Some choices of parameters and frame size may result in a non-integer number of video segments required for image mapping, i.e., some blank macroblocks resident in the “valid” video segments, and resulting compression inefficiency in the output. These parameter choices fall within the scope of the invention, as long as valid video segments are substantially filled, i.e., on the average contain about 75% valid data or more. 
     FIG. 25  shows a transmit/receive system  290  using an embodiment of the invention. A video source  292  (e.g., a digital image capture device, tape reader, video broadcast tuner, etc.) supplies data to an encoding system  294  according to an encoding embodiment of the invention. Encoding system  294  outputs video segments corresponding to a selected format to transmit buffer  296 . Transmitter  298  reads video segments from buffer  296  and relays them to a receiver  300  using a suitable relay method (optical, radio frequency, twisted pair or coax cable, etc.) and format (the transmission channel could be dedicated to system  290 , or shared as in a time-multiplexed or packet-based channel). Receiver  300  supplies the transmitted video segments to a receive buffer  302 . Decoding system  304 , according to a decoding embodiment of the invention, reads video segments from receive buffer  302  and produces video frame output form video display  306 . 
   System  290  has several benefits. It allows use of off-the-shelf DVC coders, decreasing system cost. By using a format such as QVGA, data rate can be held to a reasonable range. But data rate can also be traded for image quality, by allowing the encoder and decoder to communicate using various numbers of video segments per frame and the same DVC coders. 
   This last benefit is illustrated in encoding system  310  of  FIG. 26 . Encoding system  310  has a mode select capability  318 . Mode select capability  318  may be a user setting, or can be automatically adaptable to an achievable data rate under given transmit conditions. Mode select capability  318  controls two switches  316  and  322 , and may also provide input to DVC coder  320  and/or QVGA preformat  314 /postformat  324  (inputs not shown). Video frame input  312  is provided to switch  316  and to QVGA preformat  314 . Switch  316  is configured to provide either video frame input  312  (for full-frame DVC conversion) or the output of QVGA preformat  314  (for subframe DVC conversion) to DVC coder  320 . DVC coder  320  provides video segments to switch  322  and QVGA postformat  324 . Switch  322  provides a video segment output  326  that is either the full output of DVC coder  320 , or selected video segments as output by QVGA postformat  324 . Note that QVGA pre- and post-formatters may have other possible resolutions selectable by mode select  318 , such as the reduced horizontal resolution of  FIG. 23 . 
   With encoding system  310 , common-format video segments can be produced efficiently at several data rates. The current data rate can be selected to match the characteristics of a particular transmission channel. Or, video resolution can be traded for record time if video is being recorded to storage media. Note that a corresponding decoding system, although not shown, can be similarly configured. 
   One of ordinary skill in the art will recognize that the concepts taught herein can be tailored to a particular application in many advantageous ways. Special-purpose hardware, software running on a digital signal processor or general purpose microprocessor, or some combination of these elements can be used to construct an encoding system or a decoding system according to embodiments of the invention. The particular superblock size or shape selected for a subimage is not critical, as long as it allows efficient distribution of the subimage amongst DVC video segments—indeed, superblock size can be as small as a DVC macroblock. If a system has a full-DVC and a subimage mode, one of the DVC frame buffers needed for full-DVC mode can be utilized for other buffers in subimage mode. Input to the DVC coder may be in raster form, by passing a pointer to a block of frame data, or other common methods. Such minor modifications are encompassed within the invention, and are intended to fall within the scope of the claims.