Video data compression with data target allocation

Video data compression apparatus for compressing images of an input video signal on a block-by-block basis comprises a block analyser for analysing each block of an image to be compressed to detect a degree of difference between pixels within that block. A data quantity allocator allocates target output data quantities for the compression of each block, the target data quantities being dependent on the detected degrees of pixel difference of the blocks. A data compression encoder for compresses each block in accordance with the target output data quantity associated with that block.