Digital Video

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Compaction

Compaction Encoding

The last step in compressing individual frames (intraframe compression) is a sequence of three standard text file compression schemes. Run-length encoding (RLE), Huffman coding, and arithmetic coding. RLE replaces sequences of identical values with the number of times the value occurs followed by the value (e.g., 11111000011111100000 ==>> 51406150). Huffman coding replaces the most frequently occurring values|strings with the smallest codes. Arithmetic coding similar to Huffman coding, codes the commonly occuring values|strings using fractional bit codes.

Automatic Talking-Head Compression

At 30 fps very little changes from one frame to the next. Interframe compression takes advantage of this fact to achieve dramatic compression. Instead of storing complete information about each frame only the difference information between frames is stored. MPEG stores three types of frames. The first type I-frame, stores all of the interframe compression information using no frame differencing. The second type P-frame is a predicted frame two or four frames in the future. This is compared with the corresponding actual future frame and the differences are stored (error signal). The third type B-frames, are bidirectional interpolative predicted frames that fill in the jumped frames.

When storing differences MPEG actually compares a block of pixels (macroblock) and if a difference is found it searches for the block in nearby regions. This can be used to alleviate slight camera movement to stablize an image. It is also used to efficiently represent motion by storing the movement information (motion vector), for the block.

Video Capture

There are two methods for capturing video. The first method compresses the video in real-time as it is received. It requires a fast processor capable of compressing frames at the same rate as the video source. The second method breaks up the capture and compression phases into separate steps. It first captures the raw video data to the hard drive which is compressed later when time allows. This method requires a fast large hard drive to store the huge raw video data files. In either process if the hardware cannot keep pace with the video source frames are dropped (skipped), which causes signal degradation.