1. A method of determining a quality value of a frame, the method comprising the steps of: dividing the frame into a plurality of tiles: determining attributes of each said tile based upon (i) pixel values of the tile and (ii) pixel values of a corresponding tile of a preceding frame; and establishing the quality value of the frame by: testing the tile attributes of the frame against pre-determined criteria; and defining the quality value of the frame depending upon results of said testing.