Context adaptive variable length code decoder for decoding macroblock adaptive field/frame coded video data

This application claims priority to Provisional Application for U.S. Patent, Ser. No. 60/573,107, entitled “Context Adaptive Variable Length Code Decoder for Decoding Macroblock Adaptive Field/Frame Coded Video Data”, filed May 21, 2004, by Hellman, which is incorporated herein by reference in it entirety for all purposes.

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[Not Applicable]

Encoding standards often use recursion to compress data. In recursion, data is encoded as a mathematical function of other previous data. As a result, when decoding the data, the previous data is needed.

An encoded picture is often assembled in portions. Each portion is associated with a particular. region of the picture. The portions are often decoded in a particular order. For decoding some of the portions, data from previously decoded portions is needed.

A video decoder typically includes integrated circuits for performing computationally intense operations, and memory. The memory includes both on-chip memory and off-chip memory. On-chip memory is memory that is located on the integrated circuit and can be quickly accessed. Off-chip memory is usually significantly slower to access than on-chip memory.

During decoding, storing information from portions that will be used for decoding later portions in on-chip memory is significantly faster than storing the information off-chip. However, on-chip memory is expensive, and consumes physical area of the integrated circuit. Therefore, the amount of data that on-chip memory can store is limited. In contrast, decoded video data generates very large amounts of data. Therefore, it may be impractical to store all of the decoded data on-chip.

The data needed for decoding a portion is typically contained in the neighboring portions that are decoded prior to the portion, such as the left neighbor. In many decoding orders, the left neighboring portion is decoded either immediately prior to the portion or shortly prior to the portion. In such a case, it can be possible to store some of the information from each portion for use in decoding the portions right neighbor.

However, the information needed from the left neighboring portion may not be determinable until after decoding the right neighboring portion.

Other limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.

Described herein is a context adaptive variable length code decoder for decoding macroblock adaptive field/frame coded video data.

In one embodiment, there is presented a decoder for decoding symbols. The decoder comprises a symbol decoder, and a neighbor buffer. The symbol decoder decodes frequency coefficients associated with a portion of a picture. The neighbor buffer stores information about another portion of the picture, the another portion being adjacent to the portion. The symbol decoder decodes the frequency coefficients based on the information about the another portion of the picture and overwrites the information about the another portion with information about the portion.

In another embodiment, there is presented a method for decoding symbols. The method comprises storing information about a first portion of a picture, decoding frequency coefficients associated with a second portion of the picture based on the information about the first portion of the picture, and overwriting the information about the another portion with information about the second portion.

In another embodiment, there is presented an integrated circuit for decoding symbols. The integrated circuit comprises a symbol decoder, and a neighbor buffer. The symbol decoder is operable to decode frequency coefficients associated with a portion of a picture. The neighbor buffer is operable to store information about another portion of the picture, the another portion being adjacent to the portion. The neighbor buffer is connected to the symbol decoder. The symbol decoder decodes the frequency coefficients based on the information about the another portion of the picture and overwrites the information about the another portion with information about the portion.

These and other features and advantages of the present invention may be appreciated from a review of the following detailed description of the present invention along with the accompanying figures.

FIG. 1 is a block diagram of an exemplary frame;

FIG. 2A is a block diagram describing spatially predicted macroblocks;

FIG. 2B is a block diagram describing temporally predicted macroblocks;

FIG. 2C is a block diagram describing the encoding of a prediction error;

FIG. 3 is a block diagram describing the encoding of macroblocks for interlaced fields in accordance with macroblock adaptive frame/field coding;

FIG. 4 is a block diagram of a video decoder in accordance with an embodiment of the present invention;

FIG. 5 is a block diagram describing the decoding order for a video decoder in accordance with an embodiment of the present invention;

FIG. 6 is a block diagram describing top neighboring partitions;

FIG. 7 is a block diagram describing left neighboring partitions; and

FIG. 8 is a block diagram describing a CAVLC decoder in accordance with an embodiment of the present invention.

Referring now to FIG. 1 , there is illustrated a block diagram of a frame 100 . A video camera captures frames 100 from a field of view during time periods known as frame durations. The successive frames 100 form a video sequence. A frame 100 comprises two-dimensional grid(s) of pixels 100 (x,y).

For color video, each color component is associated with a two-dimensional grid of pixels. For example, a video can include a luma, chroma red, and chroma blue components. Accordingly, the luma, chroma red, and chroma blue components are associated with a two-dimensional grid of pixels 100 Y(x,y), 100 Cr(x,y), and 100 Cb(x,y), respectively. When the grids of two dimensional pixels 100 Y(x,y), 100 Cr(x,y), and 100 Cb(x,y) from the frame are overlayed on a display device 110 , the result is a picture of the field of view at the frame duration that the frame was captured.

Generally, the human eye is more perceptive to the luma characteristics of video, compared to the chroma red and chroma blue characteristics. Accordingly, there are more pixels in the grid of luma pixels 100 Y(x,y) compared to the grids of chroma red 100 Cr(x,y) and chroma blue 100 Cb(x,y). In the MPEG 4:2:0 standard, the grids of chroma red 100 Cr(x,y) and chroma blue pixels 100 Cb(x,y) have half as many pixels as the grid of luma pixels 100 Y(x,y) in each direction.

The chroma red 100 Cr(x,y) and chroma blue 100 Cb( x,y) pixels are overlayed the luma pixels in each even-numbered column 100 Y(x, 2y) between each even, one-half a pixel below each even-numbered line 100 Y(2x, y). In other words, the chroma red and chroma blue pixels 100 Cr(x,y) and 100 Cb(x,y) are overlayed pixels 100 Y(2x+½, 2y).

If the video camera is interlaced, the video camera captures the even-numbered lines 100 Y(2x,y), 100 Cr(2x,y), and 100 Cb(2x,y) during half of the frame duration (a field duration), and the odd-numbered lines 100 Y(2x+1,y), 100 Cr(2x+1,y), and 100 Cb(2x+1,y) during the other half of the frame duration. The even numbered lines 100 Y(2x,y), 100 Cr(2x,y), and 100 Cb(2x,y) what is known as a top field 110 T, while odd-numbered lines 100 Y(2x+1,y), 100 Cr(2x+1,y), and 100 Cb(2x+1,y) form what is known as the bottom field 110 B. The top field 110 T and bottom field 110 T are also two dimensional grid(s) of luma 110 YT(x,y), chroma red 110 CrT(x,y), and chroma blue 110 CbT(x,y) pixels.

Luma pixels of the frame 100 Y(x,y), or top/bottom fields 100 YT/B(x,y) can be divided into 16×16 pixel 100 Y(16x−>16x+15, 16y−>16y+15) blocks 115 Y(x,y). For each block of luma pixels 115 Y(x,y), there is a corresponding 8×8 block of chroma red pixels 115 Cr(x,y) and chroma blue pixels 115 Cb(x,y) comprising the chroma red and chroma blue pixels that are to be overlayed the block of luma pixels 115 Y(x,y). A block of luma pixels 115 Y(x,y), and the corresponding blocks of chroma red pixels 115 Cr(x,y) and chroma blue pixels 115 Cb(x,y) are collectively known as a macroblock 120 . The macroblocks 120 can be grouped into groups known as slice groups.

The ITU-H.264 Standard (H.264), also known as MPEG-4, Part 10, and Advanced Video Coding, encodes video on a frame by frame basis, and encodes frames on a macroblock by macroblock basis. H.264 specifies the use of spatial prediction, temporal prediction, DCT transformation, interlaced coding, and lossless entropy coding to compress the macroblocks 120 .

Spatial Prediction

Referring now to FIG. 2A , there is illustrated a block diagram describing spatially encoded macroblocks 120 . Spatial prediction, also referred to as intraprediction, involves prediction of frame pixels from neighboring pixels. The pixels of a macroblock 120 can be predicted, either in a 16×16 mode, an 8×8 mode, or a 4×4 mode.

In the 16×16 and 8×8 modes, e.g, macroblock 120 a , and 120 b , respectively, the pixels of the macroblock are predicted from a combination of left edge pixels 125 L, a corner pixel 125 C, and top edge pixels 125 T. The difference between the macroblock 120 a and prediction pixels P is known as the prediction error E. The prediction error E is calculated and encoded along with an identification of the prediction pixels P and prediction mode, as will be described.

In the 4×4 mode, the macroblock 120 c is divided into 4×4 partitions 130 . The 4×4 partitions 130 of the macroblock 120 a are predicted from a combination of left edge partitions 130 L, a corner partition 130 C, right edge partitions 130 R, and top right partitions 130 TR. The difference between the macroblock 120 a and prediction pixels P is known as the prediction error E. The prediction error E is calculated and encoded along with an identification of the prediction pixels and prediction mode, as will be described. A macroblock 120 is encoded as the combination of the prediction errors E representing its partitions 130 .

Temporal Prediction

Referring now to FIG. 2B , there is illustrated a block diagram describing temporally encoded macroblocks 120 . The temporally encoded macroblocks 120 can be divided into 16×8, 8×16, 8×8, 4×8, 8×4, and 4×4 partitions 130 . Each partition 130 of a macroblock 120 , is compared to the pixels of other frames or fields for a similar block of pixels P. A macroblock 120 is encoded as the combination of the prediction errors E representing its partitions 130 .

The similar block of pixels is known as the prediction pixels P. The difference between the partition 130 and the prediction pixels P is known as the prediction error E. The prediction error E is calculated and encoded, along with an identification of the prediction pixels P. The prediction pixels P are identified by motion vectors MV. Motion vectors MV describe the spatial displacement between the partition 130 and the prediction pixels P. The motion vectors MV can, themselves, be predicted from neighboring partitions.

The partition can also be predicted from blocks of pixels P in more than one field/frame. In bi-directional coding, the partition 130 can be predicted from two weighted blocks of pixels, P 0 and P 1 . According a prediction error E is calculated as the difference between the weighted average of the prediction blocks w 0 P 0 +w 1 P 1 and the partition 130 . The prediction error E, an identification of the prediction blocks P 0 , P 1 are encoded. The prediction blocks P 0 and P 1 are identified by motion vectors MV.

The weights w 0 , w 1 can also be encoded explicitly, or implied from an identification of the field/frame containing the prediction blocks P 0 and P 1 . The weights w 0 , w 1 can be implied from the distance between the frames/fields containing the prediction blocks P 0 and P 1 and the frame/field containing the partition 130 . Where T 0 is the number of frame/field durations between the frame/field containing P 0 and the frame/field containing the partition, and T 1 is the number of frame/field durations for P 1 ,

w 0=1 −T 0/( T 0+ T 1)

w 1=1 −T 1/( T 0+ T 1)

Transformation, Quantization, and Scanning

Referring now to FIG. 2C , there is illustrated a block diagram describing the encoding of the prediction error E. With both spatial prediction and temporal prediction, the macroblock 120 is represented by a prediction error E. The prediction error E is also two-dimensional grid of pixel values for the luma Y, chroma red Cr, and chroma blue Cb components with the same dimensions as the macroblock 120 .

A transformation transforms 4×4 partitions 130 (0,0) . . . 130 (3,3) of the prediction error E to the frequency domain, thereby resulting in corresponding sets 135 (0,0) . . . 135 (3,3) of frequency coefficients F 00 . . . F 33 . The sets of frequency coefficients are then quantized and scanned, resulting in sets 140 (0,0) . . . 140 (3,3) of quantized frequency coefficients, F 0 . . . F n . A macroblock 120 is encoded as the combination of its partitions 130 .

Macroblock Adaptive Frame/Field (MBAFF) Coding

Referring now to FIG. 3 , there is illustrated a block diagram describing the encoding of macroblocks 120 for interlaced fields. As noted above, interlaced fields, top field 110 T(x,y) and bottom field 110 B(x,y) represent either even or odd-numbered lines.

In MBAFF, each macroblock 120 T in the top field is paired with the macroblock 120 B in the bottom field, that is interlaced with it. The macroblocks 120 T and 120 B are then coded as a macroblock pair 12 TB. The macroblock pair 120 TB can either be field coded, macroblock pair 120 TBF or frame coded, macroblock pair 120 TBf. Where the macroblock pair 120 TBF are field coded, the macroblock 120 T is encoded, followed by macroblock 120 B. Where the macroblock pair 120 TBf are frame coded, the macroblocks 120 T and 120 B are deinterlaced. The foregoing results in two new macroblocks 120 ′T, 120 ′B. The macroblock 120 ′T is encoded, followed by macroblock 120 ′B.

Entropy Coding

Referring again to FIG. 2C , the macroblocks 120 are represented by a prediction error E that is encoded as sets 140 (0,0) . . . 140 (3,3) of quantized frequency coefficients F 0 . . . F n . The macroblock 120 is represented by side information, such as prediction mode indicators, and identification of prediction blocks.

Context Adaptive Variable Length Coding (CAVLC) is used to encode the frequency coefficients F 0 . . . F n . In H.264 the CAVLC used to encode the quantized frequency coefficients F 0 . . . F n , for a 4×4 partition 130 (x,y) depends on the quantized frequency coefficients F 0 . . . F n of its top neighboring partition 130 (x−1,y) and left neighboring partition 130 (x, y−1).

The frames 100 are encoded as the macroblocks 120 forming them. The video sequence is encoded as the frame forming it. The encoded video sequence is known as a video elementary stream. The video elementary stream is a bitstream that can be transmitted over a communication network to a decoder. Transmission of the bitstream instead of the video sequence consumes substantially less bandwidth.

Referring now to FIG. 4 , there is illustrated a block diagram describing an exemplary video decoder 400 in accordance with an embodiment of the present invention. The video decoder 400 includes a code buffer 405 for receiving a video elementary stream. The code buffer 405 can be a portion of a memory system, such as a dynamic random access memory (DRAM). A symbol interpreter 415 in conjunction with a context memory 410 decode the CABAC and CAVLC symbols from the bitstream. The context memory 410 can be another portion of the same memory system as the code buffer 405 , or a portion of another memory system.

The symbol interpreter 415 includes a CAVLC decoder 415 V. The CAVLC decoder 415 V decodes CAVLC symbols, resulting in the sets 140 of quantized frequency coefficients F 0 . . . F n .

The symbol interpreter 415 provides the sets of scanned quantized frequency coefficients F 0 . . . F n to an inverse scanner, quantizer, and transformer (ISQDCT) 425 . Depending on the prediction mode for the macroblock 120 associated with the scanned quantized frequency coefficients F 0 . . . F n , the symbol interpreter 415 provides the side information to either a spatial predicter 420 (if spatial prediction) or a motion compensator 430 (if temporal prediction).

The ISQDCT 425 constructs the prediction error E. The spatial predictor 420 generate the prediction pixels P for spatially predicted macroblocks while the motion compensator 430 generates the prediction pixels P, or P 0 , P 1 for temporally predicted macroblocks. The motion compensator 430 retrieves the prediction pixels P, or P 0 , P 1 from picture buffers 450 that store previously decoded frames 100 or fields 110 .

A pixel reconstructor 435 receives the prediction error E from the ISQDCT 425 , and the prediction pixels from either the motion compensator 430 or spatial predictor 420 . The pixel reconstructor 435 reconstructs the macroblock 120 from the foregoing information and provides the macroblock 120 to a deblocker 440 . The deblocker 440 smoothes pixels at the edge of the macroblock 120 to prevent the appearance of blocking. The deblocker 440 writes the decoded macroblock 120 to the picture buffer 450 .

A display engine 445 provides the frames 100 from the picture buffer 450 to a display device. The symbol interpreter 415 , the ISQDCT 425 , spatial predictor 420 , motion compensator 430 , pixel reconstructor 435 , and display engine 445 can be hardware accelerators under the control of a central processing unit (CPU).

As noted above, the symbol interpreter 415 includes a CAVLC decoder 415 V that decodes the quantized frequency coefficients F 0 . . . F n for partitions 130 . However, the CAVLC coding of the quantized frequency coefficients F 0 . . . F n for a partition 130 (x,y) depends on the quantized frequency coefficients F 0 . . . F n from the top neighbor partition 130 (x−1, y), and left neighboring partition 130 (x, y−1). In the case of the row of partitions 130 (0,y) at the top of a macroblock, information from the bottom row 130 (3,y) of partitions of the above macroblock is needed. In the case of the column of partitions 130 (x,0) at the left of a macroblock, information from the right edge 130 (x,3) of the right neighboring macroblock is used.

Referring now to FIG. 5 , there is illustrated a block diagram describing the decoding order of the video decoder, in accordance with an embodiment of the present invention. For interlaced fields 110 T, 110 B with MBAFF encoding, the video decoder 400 decodes the macroblocks in pairs, starting with the macroblock pair 120 T(0,0), 120 B(0,0) at the top corners of the top field 110 T and bottom field 110 B and proceeding across the top row of macroblocks 120 T(0,n), 120 B(0,n). The video decoder 400 then proceeds to the left most macroblock of the next row of macroblocks 120 T(1,0), 120 B(1,0) and proceeds to the right, and so forth.

The macroblock pairs represent 32×16 pixel blocks of the frame 100 . However, where the macroblock pairs are frame coded, such as macroblocks 120 TBf, the reconstructed macroblocks 120 ′T(0,0), 120 ′B(0,0) represented the top and bottom halves of macroblocks 120 T(0,0) and 120 B(0,0) deinterlaced. Macroblock 120 ′T(0,0) includes the first eight lines of pixels from macroblocks 120 T(0,0) and 120 B (0,0) . Macroblock 120 ′B (0,0) includes the last eight lines of pixels from macroblocks 120 T(0,0) and 120 B(0,0).

As noted above, the CAVLC coding for frequency coefficients F 0 . . . F n for partitions 130 is dependent on the frequency coefficients of the top and left neighboring partitions. The CAVLC coding of frequency coefficients F 0 . . . F n of the top row and left column of partitions in a macroblock 120 depends on the bottom row of partitions of the top neighboring macroblock and the right column of partitions of the left neighboring macroblock.

The location of the bottom row of partitions of the top neighboring macroblock and the right column of partitions of the left neighboring macroblock depend on whether the macroblock pair, the left neighboring macroblock pair, and the top neighboring macroblock pair are frame or field coded.

Referring now to FIG. 6 , there is illustrated a block diagram describing the top neighboring partitions for the top row of partitions in macroblock pairs 120 TB. In case 1 , the macroblock pair 120 TB and its top neighbor macroblock pair 120 TB are both frame coded. The top neighbor partitions for partitions A, B, C, D are as indicated by the arrows.

In case 2 , the macroblock pair 120 TB is frame coded while its top neighbor macroblock pair 120 TB is field coded. The top neighbor partitions for partitions A, B, C, D are as indicated by the arrows.

In case 3 , the macroblock pair 120 TB is field coded while its top neighbor macroblock pair 120 TB is frame coded. The top neighbor partitions for partitions A, B, C, D are as indicated by the arrows.

In case 4 , the macroblock pair 120 TB and its top neighbor macroblock pair 120 TB are both field coded. The top neighbor partitions for partitions A, B, C, D are as indicated by the arrows.

Referring now to FIG. 7 , there is illustrated a block diagram describing the left neighboring partitions for the left column of partitions in macroblock pairs 120 TB. In case 1 , the macroblock pair 120 TB and its left neighbor macroblock pair 120 TB are both frame coded. The left neighbor partitions for partitions A, B, C, D are as indicated by the lines.

In case 2 , the macroblock pair 120 TB and its left neighbor macroblock pair 120 TB are both field coded. The left neighbor partitions for partitions A, B, C, D are as indicated by the lines.

In case 3 , the macroblock pair 120 TB is field coded while its left neighbor macroblock pair 120 TB is frame coded. The left neighbor partitions for partitions A, B, C, D are as indicated by the lines.

In case 4 , the macroblock pair 120 TB is frame coded while its left neighbor macroblock pair 120 TB is field coded. The left neighbor partitions for partitions A, B, C, D are as indicated by the lines.

Referring now to FIG. 8 , there is illustrated a block diagram describing certain aspects of the CAVLC decoder 415 V, in accordance with an embodiment of the present invention. The CAVLC decoder 415 V includes a left neighbor buffer 930 and a symbol decoder 935 .

The symbol decoder 935 decodes the CAVLC symbols encoding the frequency coefficients F 0 . . . F n for partitions 130 . The CAVLC decoder 415 V receives the video data on a macroblock by macroblock 120 basis.

When the CAVLC decoder 415 V decodes the CAVLC coding of the frequency coefficients for a column of partitions 130 (x,N) in macroblock 120 T, the CAVLC decoder 415 V writes the number of frequency coefficients n for each partition 130 (x,N) to one half 930 a of the left neighbor buffer 930 . The symbol decoder 935 uses the foregoing information, as well as information regarding the top neighboring partition (fetched from DRAM) to decode the CAVLC coding of the frequency coefficients F 0 . . . F n for the next column of partitions 130 (x, N+1) in the macroblock 120 T.

The half of the left neighbor buffer 930 a overwrites the number of frequency coefficients n for the previous column with the number of frequency coefficients n for the current column for use in decoding the next column. The foregoing continues until the CAVLC coding of the frequency coefficients F 0 . . . F n is decoded for each partition 130 of a macroblock 120 .

The CAVLC decoder 415 V then decodes the CAVLC coding of the frequency coefficients for the next macroblock 120 B. If the next macroblock 120 B is the bottom macroblock 120 B of a macroblock pair 120 TB, the CAVLC decoder 415 V decodes the CAVLC coding for the frequency coefficients F 0 . . . F n of the macroblock 120 B, using the other half of the left neighbor buffer 930 b.

After decoding a macroblock pair 120 TB, the left neighbor buffer 930 includes the number of frequency coefficients for the right column of partitions 130 (x,3) of the macroblocks 120 T and 120 B. When decoding the next macroblocks 120 T 1 and 120 B 1 , the information from the right column of partitions 130 (x,3) of macroblocks 120 T and 120 B, the left neighbors of partitions 130 (x,0) of macroblocks 120 T 1 and 120 B 1 , is stored in the left neighbor buffer 930 .

The symbol decoder 935 uses the information stored for the particular ones of the right column partitions 130 (x,3) of macroblocks 120 T and 120 B to decode CAVLC coding for partitions 130 (x,0) of macroblock 120 T. The particular ones of the right column partitions 130 (x,3) of macroblock 120 T and 120 B are selected based on whether the coding type (field/frame) of each macroblock pair, as indicated in the FIG. 7 .

As the symbol decoder 935 generates the frequency coefficients for the left column of partitions 130 (x,0) for macroblock 120 T 1 , the information regarding the left column of partitions 130 (x,0) is written to the left neighbor buffer 930 . If the macroblock pairs 120 T, 120 B, and 120 T 1 , 120 B 1 correspond to cases 1 or 2 of FIG. 7 , the information about the partitions 130 (x,0) in macroblock 120 T 1 overwrites the selected partitions 130 (x,3). If the macroblock pairs 120 T, 120 B, and 120 T 1 , 120 B 1 correspond to cases 3 or 4 of FIG. 7 , the information about the partitions 130 (x,0) in macroblock 120 T 1 , overwrites the remaining partitions from the selected partitions 130 (x,3).

When the symbol decoder 935 decodes the CAVLC coding of the frequency coefficients for the left column of partitions 130 (x,0) of macroblock 120 B 1 , the information regarding the left column of partitions 130 (x,0) overwrites the partitions 130 (x,3) opposite from the partitions 130 (x,3) overwritten when generating frequency coefficients for macroblock 120 T 1 .

The embodiments described herein may be implemented as a board level product, as a single chip, application specific integrated circuit (ASIC), or with varying levels of the decoder system integrated with other portions of the system as separate components. The degree of integration of the decoder system will primarily be determined by the speed and cost considerations. Because of the sophisticated nature of modern processor, it is possible to utilize a commercially available processor, which may be implemented external to an ASIC implementation. If the processor is available as an ASIC core or logic block, then the commercially available processor can be implemented as part of an ASIC device wherein certain functions can be implemented in firmware. Alternatively, the functions can be implemented as hardware accelerator units controlled by the processor.

While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. For example, although the embodiments have been described with a particular emphasis on the H.264 standard, the teachings of the present invention can be applied to many other standards without departing from it scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.