In order to generate, starting from an input MPEG bitstream, an output MPEG bitstream having at least one entity chosen among syntax, resolution, and bitrate modified with respect to the input bitstream, first portions and second portions are distinguished in the input bitstream, which respectively substantially do not affect and do affect the variation in bitrate. When at least one between the syntax and the resolution is to be modified, the first portions of the input bitstream are subjected to the required translation, then transferring said first portions subjected to syntax and/or resolution translation to the output bitstream. When the resolution is left unaltered, the second portions are transferred from the input bitstream to the output bitstream in the substantial absence of processing operations. When the resolution is changed, the second portions of the input bitstream are subjected to a filtering in the domain of the discrete cosine transform.