Subjective assessment of flicker in visual-telephone pictures

Subjective tests have been made of the effect of the 100 Hz modulation, caused by 50 Hz lighting, on the pictures displayed by two visual telephone systems of equal bandwidth and line frequency, but differing picture frequencies. In a 50 fields/s 319-line system the flicker due to the modulation is normally at a rate of less than one per second and much less objectionable than the inherent flicker of the picture at field rate. In a 60 fields/s 267-line system the modulation flicker is at a rate of about 20 per second and can be much more objectionable than the inherent flicker at field rate. A comparison-type subjective test showed that the two systems were judged to be of equal overall quality for typical pictures of highlight luminance 500 cd/m2 with 2.1% modulation; when the highlight luminance was reduced to 200 cd/m2, equality, was achieved with 1.3% modulation. A quality-grading test with the 60 fields/s 267-line system indicated that, for typical pictures, subjective impairments of 1 imp and 1/8 imp are obtained with 10.3% and 1.6% modulation, respectively, for either value of highlight luminance.