This paper describes source models of bit rate for digital TV codecs based on the Discrete Cosine Transformation as decorrelation technique. These models are based on cyclostationary processes. Properties of cyclostationary processes are such that the process is preserved over the whole TV transmission. Therefore, the influence of image Statistics is presented on the coders, the transmission networks, and, the decoders. An emphasis is given on variable bit rate codecs transmitting on ATM networks.