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An analytical quality measure is essential for the successful design of digital transmission systems. For images the processes involved in human quality perception are too complicated to be modelled in a mathematically tractable way. In order to obtain useful results we have given up generality and investigated only quality measures for achromatic still images which have been coded by Pulse-Code-Modulation (PCM), Pseudo-Random Noise PCM, or Differential PCM. For this class of distortions we have found an objective quality measure based on the mean squared error, that performs better than commonly assumed. Another powerful measure has the same form as the mutual information between original and distorted image under the assumption of jointly Gaussian signals. In this paper these measures are compared with subjective results for the same distortions. The correlation is sufficiently good to justify the use of such measures in coding and evaluation of digital image coders.