A method of evaluating a video signal comprising a plurality of video frames, the method comprising: (a) in the plurality video frames, analysing the number of pixels changed from one frame to the next; (b) identifying a frame as being a repeat if a set proportion of the pixels present in the current frame match those present in the previous frame; (c) recording a pattern of repeat frames in the video signal; (d) scanning the pattern to determine whether or not a given repeat frame is part of a temporally repetitive pattern; characterized by (e) scanning the pattern by establishing a first window of multiple frames referenced in the pattern to a first frame and a second window of equal size in the pattern; (f) arranging the second window at an offset (a') relative to the first window; (g) comparing the patterns of repeat frames in the first and second windows and recording the number of mismatches between the patterns; (h) repeating the comparison of step (g) for each value of offset (a', a") over a range of offset values and recording the results; (i) selecting a new value of window size for the first and second windows, repeating the comparisons over the range of offset values and recording the results; (j) determining a target window size (X') that provides the smallest number of mismatches over the range of offset values; (k) using sections (77, 78) conforming to the target window size to scan the pattern in the scanning step (d); generating an indicator of quality by identifying those repeat frames not determined to be part of the temporally repetitive pattern; (m) in which the indicator of quality is generated by comparing the pattern of repeat frames in a first section whose size is determined by the size of the target window with the pattern of repeat frames in a number of second sections of the same size and recording the number of times a mismatch occurs between the pattern in the first section and the patterns in each of the second sections. (a) in the plurality video frames, analysing the number of pixels changed from one frame to the next; (b) identifying a frame as being a repeat if a set proportion of the pixels present in the current frame match those present in the previous frame; (c) recording a pattern of repeat frames in the video signal; (d) scanning the pattern to determine whether or not a given repeat frame is part of a temporally repetitive pattern; (e) scanning the pattern by establishing a first window of multiple frames referenced in the pattern to a first frame and a second window of equal size in the pattern; (f) arranging the second window at an offset (a') relative to the first window; (g) comparing the patterns of repeat frames in the first and second windows and recording the number of mismatches between the patterns; (h) repeating the comparison of step (g) for each value of offset (a', a") over a range of offset values and recording the results; (i) selecting a new value of window size for the first and second windows, repeating the comparisons over the range of offset values and recording the results; (j) determining a target window size (X') that provides the smallest number of mismatches over the range of offset values; (k) using sections (77, 78) conforming to the target window size to scan the pattern in the scanning step (d); generating an indicator of quality by identifying those repeat frames not determined to be part of the temporally repetitive pattern; (m) in which the indicator of quality is generated by comparing the pattern of repeat frames in a first section whose size is determined by the size of the target window with the pattern of repeat frames in a number of second sections of the same size and recording the number of times a mismatch occurs between the pattern in the first section and the patterns in each of the second sections.