1. A method of generating a digital video fingerprint from a piece of digitally encoded video content, the video content having been encoded according to an encoding method which involves the generation of residual macroblocks of pixels and the generation of quantized transform coefficients of the residual macroblocks or of portions of the residual macroblocks, the method comprising: selecting a group of frames of the encoded video content; processing the digitally encoded video content to obtain a set of quantized transform coefficients of residual macroblocks or portions of residual macroblocks associated with each of the selected frames; identifying a set of residual macroblocks per frame whose transform coefficients satisfy a threshold criterion; and generating a digital video fingerprint for the encoded video content in dependence upon the identified macroblocks or some property thereof within each of the selected frames.