1. A method of predicting subjective quality ratings for a processed video sequence comprising the steps of: obtaining perceptual difference scores for a worst quality video training sequence and for a best quality video training sequence; assigning to the worst and best perceptual difference scores corresponding subjective quality rating scores on a subjective quality rating scale that account for compression at the extremes of the subjective quality rating scale; and calculating from the worst and best perceptual difference scores and heuristically determined constants of a conversion function for converting perceptual difference scores to subjective quality rating scores on the subjective quality rating scale.