Image data multiplexing apparatus for multiplexing digital luminance and chrominance data of a representation of an image into multiplexed data blocks of a predetermined length, along with a corresponding demultiplexing apparatus, is described. Each of the multiplexed data blocks comprises a luminance data section comprising luminance data from a portion of the representation, a chrominance data section comprising chrominance data from the portion of the representation, a portion address indicative of the position in the representation of the portion, and a block offset pointer indicative of the position in the multiplexed data block of the boundary between the luminance data section and the chrominance data section.