Quantization

Factor applied to DCT coefficients as a part of the process of achieving a required amount of video compression. The coefficients relating to the least noticeable aspects of picture detail e.g. high frequencies with low amplitude, are progressively reduced so that the final data will fit into the specified data file space. This space is often fixed and relates directly to the quoted compression ratio for I-frame only schemes such as DV. Note that the required quantization will vary according to scene content. Given that too much data would cause problems by overflowing the allotted capacity of the file, compression schemes are cautious and designed to undershoot the file limit. To what extent the files are filled is a measure of the quality of a compression scheme, a reason why the quoted ‘compression ratio’ does not tell the whole story.