Video Decompression - Video Decompression - 1.1 English - PG313

Video compression standards often use block-based motion estimation. To decode a 16x16 pixel block in the current frame, the decoder can receive a motion vector relative to a past or future reference frame. It can use this frame to fetch the best approximation of the current block. As a result the majority of DRAM access done by a video decoder is in the form of rectangular block fetch from an image frame buffer, with arbitrary offset or alignment. The result depends on the speed and direction of the motion in the video.

The following figure shows video decode frame buffer tiling. In the buffer, 50 8x8 blocks are randomly placed to model a video decoder motion-compensated block fetch.

Figure 1. Video Decode Frame Buffer Tiling

Notice that most blocks fall within a single tile and therefore exhibit very efficient DRAM access. Some blocks span two tiles either horizontally or vertically, and exhibit somewhat reduced efficiency. On rare occasions, a block might fall on a corner of four tiles and exhibit yet lower efficiency.

Note that such mapping imposes restrictions on how the video frame buffer is allocated and used. Key requirements are as follows:

• The frame buffer start address must be page-aligned.
• If the video decode line size is not a power-of-2 value (as in HD: 1920), the line length is rounded up to the next power-of-2 value (2048 in this example), and therefore some portion of the line is unused. This is referred to as Line Stride, being larger that the line length.