1. A method of concealing an error in a video stream, the video stream comprising a current frame having a lost image region, and a reference frame, the method comprising: using a motion vector to identify a reference image region in the reference frame; calculating a motion compensated temporal activity of an image region in the current frame based on a comparison between the image region in the current frame and the reference image region in the reference frame; calculating a spatial activity of the image region in the current frame and selecting to use either a spatial error concealment module or a temporal error concealment module to conceal the error based on a comparison of the calculated motion compensated temporal activity and the calculated spatial activity; and using the selected module to conceal the lost image region, and wherein a calculation using the motion compensated temporal error concealment module to identify a replacement motion vector and replacement pixels is performed prior to the calculation of the motion compensated temporal activity and the selection of which module to use.