The source encoder reduces or eliminates any redundancies in the input image, which usually leads to bit savings. Source encoding techniques are the primary focus of this discussion. The channel encoder increase noise immunity of source encoder’s output, usually adding extra bits to achieve its goals. If the channel is noise-free, the channel encoder and decoder may be omitted. At the receiver’s side, the channel and source decoder perform the opposite functions and ultimately recover (an approximation of) the original image
- Mapper: transforms the input data into a (usually nonvisual) format designed to reduce interpixel redundancies in the input image. This operation is generally reversible and may or may not directly reduce the amount of data required to represent the image.
- Quantizer: reduces the accuracy of the mapper’s output in accordance with some pre established fidelity criterion. Reduces the psychovisual redundancies of the input image. This operation is not reversible and must be omitted if lossless compression is desired.
- Symbol (entropy) encoder: creates a fixed- or variable-length code to represent the quantizer’s output and maps the output in accordance with the code. In most cases, a variable-length code is used. This operation is reversible.