Abstract: A high definition image transmission system comprising a stage for the transmission of encoded data representative of images and, after transmission of these data at a specific field frequency and by a first channel having a limited passband, a stage for receiving the transmitted data. The transmission stage comprises an image encoding device having three processing channels in parallel for the purpose of transmission, at three separate transmission rates, of one or another of these three channels depending on the output signal of a decision circuit. It further comprises a device for decoding the encoded data, comprising three processing branches located in parallel and each receiving the sequence of the encoded data and a circuit for routing the outputs of the said branches according to the output signal of the decision circuit transmitted to the decoding device by a second channel called the digital assistance channel.