DCT. Like JPEG, MPEG is DCT-based. The codec calculates a DCT on each 8x8 block of pixel or difference information of each image. The frequency information is easier to sort by visual importance and quantize, and it takes advantage of regions of each frame that are unchanging.
Quantization. Quantization in MPEG is different for different block types. There are different matrices of coefficient-specific values for intra and non-intra macroblocks, as well as for color and intensity data. There is also a scale applied across all matrices. Both the scale and the quantization matrix can change each macroblock.
For intra blocks, the DC, or zero-frequency, coefficient is quantized by dropping the low 0 to 3 bits; that is, by shifting it right by zero to three bits. The AC coefficients are assigned into quantization steps according to the global scale and the matrix. The quantization is linear.
For non-intra blocks, the DC component contains less important information and is more likely to tend toward zero. Therefore, the DC and AC components are quantized in the same way, using the non-intra quantization matrix and scale.
Huffman Coding. For reduced entropy in the video data to become a reduced data rate in the bit stream, the data must be coded using fewer bits. In MPEG, as with JPEG, that means a Huffman variable-length encoding scheme. Each piece of data is encoded with a code the length of which is inversely related to its frequency. Because of the complexity of MPEG-2, there are dozens of tables of codes for coefficients, block types, and other information.
For intra blocks, the DC coefficient is not coded directly. Instead, the difference between it and a predictor is used. This predictor is either the DC value of the last block if present and intra, or a constant average value otherwise.
Two scan matrices are used to order the DCT coefficients. One does a zig-zag pattern that is close to diagonally symmetric for blocks that are not interlaced; the other does a modified zig-zag for interlaced blocks. These matrices put the coefficients in order of increasing frequency in an attempt to maximize lengths of runs of data.
The encoder codes run-level data for this matrix. Each run-level pair represents the number of consecutive occurrences of a certain level. The more common pairs have codes in a Huffman table. Less common codes, such as runs of more than 31, are encoded as an escape code followed by a 6-bit run and 12-bit level.
