Editing MPEG-1 (section)

===Quantization===
[[Quantization (image processing)|Quantization]] is, essentially, the process of reducing the accuracy of a signal, by dividing it by some larger step size and rounding to an integer value (i.e. finding the nearest multiple, and discarding the remainder).

The frame-level quantizer is a number from 0 to 31 (although encoders will usually omit/disable some of the extreme values) which determines how much information will be removed from a given frame. The frame-level quantizer is typically either dynamically selected by the encoder to maintain a certain user-specified bitrate, or (much less commonly) directly specified by the user.

A "quantization matrix" is a string of 64 numbers (ranging from 0 to 255) which tells the encoder how relatively important or unimportant each piece of visual information is. Each number in the matrix corresponds to a certain frequency component of the video image.

An example quantization matrix:
:<math>
\begin{bmatrix}
 16 & 11 & 10 & 16 & 24 & 40 & 51 & 61 \\
 12 & 12 & 14 & 19 & 26 & 58 & 60 & 55 \\
 14 & 13 & 16 & 24 & 40 & 57 & 69 & 56 \\
 14 & 17 & 22 & 29 & 51 & 87 & 80 & 62 \\
 18 & 22 & 37 & 56 & 68 & 109 & 103 & 77 \\
 24 & 35 & 55 & 64 & 81 & 104 & 113 & 92 \\
 49 & 64 & 78 & 87 & 103 & 121 & 120 & 101 \\
 72 & 92 & 95 & 98 & 112 & 100 & 103 & 99
\end{bmatrix}
</math>

Quantization is performed by taking each of the 64 ''frequency'' values of the DCT block, dividing them by the frame-level quantizer, then dividing them by their corresponding values in the quantization matrix. Finally, the result is rounded down. This significantly reduces, or completely eliminates, the information in some frequency components of the picture. Typically, high frequency information is less visually important, and so high frequencies are much more ''strongly quantized'' (drastically reduced). MPEG-1 actually uses two separate quantization matrices, one for intra-blocks (I-blocks) and one for inter-block (P- and B- blocks) so quantization of different block types can be done independently, and so, more effectively.<ref name=Didier_MPEG/>

This quantization process usually reduces a significant number of the ''AC coefficients'' to zero, (known as [[wikt:sparse|sparse]] data) which can then be more efficiently compressed by entropy coding (lossless compression) in the next step.

An example quantized DCT block:
:<math>
\begin{bmatrix}
 -26 & -3 & -6 & 2 & 2 & -1 & 0 & 0 \\
 0 & -2 & -4 & 1 & 1 & 0 & 0 & 0 \\
 -3 & 1 & 5 & -1 & -1 & 0 & 0 & 0 \\
 -4 & 1 & 2 & -1 & 0 & 0 & 0 & 0 \\
 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0
\end{bmatrix}
</math>

Quantization eliminates a large amount of data, and is the main lossy processing step in MPEG-1 video encoding. This is also the primary source of most MPEG-1 video [[compression artifacts]], like [[blockiness]], [[color banding]], [[noise]], [[Ringing (signal)|ringing]], [[discoloration]], etc. This happens when video is encoded with an insufficient bitrate, and the encoder is therefore forced to use high frame-level quantizers (''strong quantization'') through much of the video.