Data Compression


Criteria

Survey Formats

Basics

Compression Methods

Data Formats

Files

Audio Data

MPEG-1 Audio

MPEG-1 Layer I to III

Sampling and Bit Rate

Variable Bit Rates

Principle of the coding

Filter

Acoustic Model

Entropy Coding

Encoding (Diagram)

Image Data

Video Data


Glossary

Index


Download


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Principle of the coding


The fundamental parameters are set up by selecting bit rate and layer. It is the task of the encoder to achieve the optimum under these circumstances.


The actual encoding procedure is not defined by the standard. It only specifies the abilities of the decoder and the data format. The realisation of the encoding process and corresponding quality aspects are a matter of the specific encoder technology.


As in case of video coding it is necessary to find a format that allows a differentiated rating of the effect on the sound impression. Such an estimation is impossible with conventional, uncompressed formats (PCM: Pulse Code Modulation; sequential consequence of samples).


Therefore the MPEG specification defines a conversion of the original data. The data resulting from it afterwards will be evaluated by a so-called psychoacoustic model, in order to remove only portions, which have a small effect on the sound impression.


Depending on layer and desired data transfer rate, of course this is only possible in insufficient measure. In general a higher quality can be achieved with a higher-order layer. The consequence of the data reduction is not predictable in a deterministic manner. It depends on the specific type of contents.


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MPEG-1 Audio Variable Bit Rates Filter