According to DIN 44300, a decoder is a converter or transducer that reconverts digital or analogsignals as a functional unit. In circuit terms, a decoder is a device that reverses coding, i.e. decodes.
In the case of a code converter, the reconversion takes the form of assigning a unique character from another character set to a character from one character set. The output of a decoder should contain the signal that was at the input of the codec. However, since there are lossy processes, there may well be deviations between the output signal of the decoder and the input signal of the coder.