Editing MPEG-1 (section)

===Entropy coding===
Several steps in the encoding of MPEG-1 video are lossless, meaning they will be reversed upon decoding, to produce exactly the same (original) values. Since these lossless data compression steps don't add noise into, or otherwise change the contents (unlike quantization), it is sometimes referred to as [[Source coding theorem|noiseless coding]].<ref name=mpeg1_audio/> Since lossless compression aims to remove as much redundancy as possible, it is known as [[entropy coding]] in the field of [[information theory]].

The coefficients of quantized DCT blocks tend to zero towards the bottom-right. Maximum compression can be achieved by a zig-zag scanning of the DCT block starting from the top left and using Run-length encoding techniques.

The DC coefficients and motion vectors are [[Pulse-code modulation|DPCM]]-encoded.

[[Run-length encoding]] (RLE) is a simple method of compressing repetition. A sequential string of characters, no matter how long, can be replaced with a few bytes, noting the value that repeats, and how many times. For example, if someone were to say "five nines", you would know they mean the number: 99999.

RLE is particularly effective after quantization, as a significant number of the AC coefficients are now zero (called [[wikt:sparse|sparse]] data), and can be represented with just a couple of bytes. This is stored in a special 2-[[dimensional]] Huffman table that codes the run-length and the run-ending character.

[[Huffman Coding]] is a very popular and relatively simple method of entropy coding, and used in MPEG-1 video to reduce the data size. The data is analyzed to find strings that repeat often. Those strings are then put into a special table, with the most frequently repeating data assigned the shortest code. This keeps the data as small as possible with this form of compression.<ref name=mpeg1_audio/> Once the table is constructed, those strings in the data are replaced with their (much smaller) codes, which reference the appropriate entry in the table. The decoder simply reverses this process to produce the original data.

This is the final step in the video encoding process, so the result of [[Huffman coding]] is known as the MPEG-1 video "bitstream."