Effect of observer adaptation on the results of television quality-grading tests

`Adaptation¿ is known to affect the results of television quality-grading tests, in the sense that observers become more critical when the average picture quality is high and vice versa. The magnitude of the effect of adaptation for a particular type of impairment is dependent on the value of an `adaptation coefficient¿ ; for the two types of impairment tested, flat random noise and a single echo of 8 ¿s delay, the adaptation coefficient was found to be larger in the former case. A method of adjusting test results for a standard `conditioning situation¿, defined by a grand mean score of 0.5, is described. The magnitudes of the adjustments required are given for values of the grand mean typical of those that might occur in practice.