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Compression circuitry for generating an encoded bitstream from a plurality of video frames

Compression circuitry for generating an encoded bitstream from a plurality of video frames description/claims

This application is a divisional of U.S. Application for patent Ser. No. 10/391,442, filed Mar. 17, 2003, which claims priority from European Application for Patent No. 02251932.6 filed on Mar. 18, 2002, the disclosures of which are hereby incorporated by reference.

1. Technical Field of the Invention

The present invention relates to motion picture compression circuits for pictures such as television pictures, and more particularly to a compression circuit complying with H.261 and MPEG standards.

2. Description of Related Art

FIGS. 1A-1C schematically illustrate three methods for compressing motion pictures in accordance with H.261 and MPEG standards. According to H.261 standards, pictures may be of intra or predicted type. According to MPEG standards, the pictures can also be of bidirectional type.

Intra (“I”) pictures are not coded with reference to any other pictures. Predicted (“P”) pictures are coded with reference to a past intra or past predicted picture. Bidirectional (“B”) pictures are coded with reference to both a past picture and a following picture.

FIG. 1A illustrates the compression of an intra picture I1. Picture I1 is stored in a memory area M1 before being processed. The pictures have to be initially stored in a memory since they arrive line by line whereas they are processed square by square, the size of each square being generally 16 by 16 pixels. Thus, before starting to process picture I1, memory area M1 must be filled with at least 16 lines.

The pixels of a 16 by 16-pixel square are arranged in a so-called “macroblock”. A macroblock includes four 8 by 8-pixel luminance blocks and two or four 8 by 8-pixel chrominance blocks. The processes hereinafter described are carried out by blocks of 8 by 8 pixels.

The blocks of each macroblock of picture 11 are submitted at 10 to a discrete cosine transform (DCT) followed at 11 by a quantization (Q). A DCT transforms a matrix of pixels (a block) into a matrix whose upper left corner coefficient tends to have a relatively high value. The other coefficients rapidly decrease as the position moves downwards to the right. Quantization involves dividing the coefficients of the matrix so transformed, such that a large number of coefficients which are a distance away from the upper left corner are cancelled.

At 12, the quantified matrices are subject to zigzag scanning (ZZ) and to run/level coding (RLC). Zigzag scanning has the consequence of improving the chances of consecutive series of zero coefficients, each of which is preceded by a non-zero coefficient. The run/level coding mainly includes replacing each series from the ZZ scanning with a pair of values, one representing the number of successive zero coefficients and the other representing the first following non-zero coefficient.

At 13, the pairs of values from the RLC are subject to variable length coding (VLC) that includes replacing the more frequent pairs with short codes and replacing the less frequent pairs with long codes, with the aid of correspondence tables defined by the H.261 and MPEG standards. The quantification coefficients can be varied from one block to the next by multiplication by a quantization coefficient. That quantization coefficient is inserted during variable length coding in headers preceding the compressed data corresponding to macroblocks.

Macroblocks of an intra picture are used to compress macroblocks of a subsequent picture of predicted or bidirectional type. Thus, decoding of a predicted or bidirectional picture is likely to be achieved from a previously decoded intra picture. This previously decoded intra picture does not exactly correspond to the actual picture initially received by the compression circuit, since this initial picture is altered by the quantification at 11. Thus, the compression of a predicted or intra picture is carried out from a reconstructed intra picture I1 rather than from the real intra picture I1, so that decoding is carried out under the same conditions as encoding.

The reconstructed intra picture I1r is stored in a memory area M2 and is obtained by subjecting the macroblocks provided by the quantification 11 to a reverse processing, that is, at 15 an inverse quantification (Q−1) followed at 16 by an inverse DCT (DCT−1).

FIG. 1B illustrates the compression of a predicted picture P4. The predicted picture P4 is stored in a memory area M1. A previously processed intra picture I1r has been reconstructed in a memory area M2.

The processing of the macroblocks of the predicted picture P4 is carried out from so-called predictor macroblocks of the reconstructed picture I1r. Each macroblock of picture P4 (reference macroblock) is subject to motion estimation (ME) at 17 (generally, the motion estimation is carried out only with the four luminance blocks of the reference macroblocks).

This motion estimation includes searching in a window of picture I1r for a macroblock that is nearest, or most similar to the reference macroblock. The nearest macroblock found in the window is the predictor macroblock. Its position is determined by a motion vector V provided by the motion estimation. The predictor macroblock is subtracted at 18 from the current reference macroblock. The resulting difference macroblock is subjected to the process described with relation to FIG. 1A.

Like the intra pictures, the predicted pictures serve to compress other predicted pictures and bidirectional pictures. For this purpose, the predicted picture P4 is reconstructed (P4r) in a memory area M3 by an inverse quantification at 15, inverse DCT at 19, and addition at 19 of the predictor macroblock that was subtracted at 18.

The vector V provided by the motion estimation 17 is inserted in a header preceding the data provided by the variable length coding of the currently processed macroblock.

FIG. 1C illustrates the compression of a bidirectional picture B2. Bidirectional pictures are provided for in MPEG standards only. The processing of the bidirectional pictures differs from the processing of predicted pictures in that the motion estimation 17 consists in finding two predictor macroblocks in two pictures I1r and P4r, respectively, that were previously reconstructed in memory areas M2 and M3. Generally, pictures I1r and P4r respectively correspond to a picture preceding the bidirectional picture that is currently processed and to a picture following the bidirectional picture.

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