Method and apparatus for encoding and decoding video

This concerns an apparatus and method for encoding and/or decoding video signals for use in a compatible high definition television system or in other applications. In a form of the disclosure there is provided an encoder which includes means (300) for storing an input frame as an array of digital pixel values, and means (600) for interrogating the stored array and for generating a sum signal (I) and at least one difference signal (X, Y, Z). The sum signal is representative of the sum of the pixel values in a group of pixels. The difference signal (e.g. Xn) is representative of the difference between thetotal of the pixel values in some of the pixels (Aon, Bon) in the group and the total of the pixel values of other pixels (Ain, Bin) in the group. The sum signal and the difference signal are generated for a multiplicity of groups which cover the video frame. The sum signal is generated at a relatively high information refresh rate and the difference signal is generated at a relatively low information refresh rate. The decoder includes means for storing the sum signal and each of the difference signals, and means for combining the sum and difference signals to obtain an output pixel value for each pixel of an output video frame. In another form of the invention (Fig. 20), electronic processing is performed to obtain low and high spatial frequency component representations of the image represented by the video signal, and these are stored in a digital memory in the encoder. Output frames of the low spatial frequency component representation are produced at a fast frame refresh rate, and output frames of the high spatial frequency component representation are produced at a slow frame refresh rate.