A hypothetical decoder that is conceptually connected to the output of the encoder. Its purpose is to provide a constraint on the variability of the data rate that an encoder or editing process may produce (ISO13818-2 Annex C). This postulates the existence of a buffer in the receiver and a prediction mechanism in the encoder. This mechanism will predict the buffer fullness due to the constant fill from the constant bit rate (CBR) stream and the variable empty due to the variation in decoder bit demand. This latter factor can be controlled at the encoder by varying the quality of the encoding process (quantization factor, mainly).