Subjective quality of a 70 Mbit/s digital codec for colour television

Quality-rating tests have been conducted to determine the subjective quality of PAL- and SECAM-coded colour television pictures after passage through a 70 Mbit/s DPCM digital coder and decoder. The assessment methods employed comprised both the traditional single-stimulus quality-grading method and a double-stimulus continuous quality-rating method specifically intended for assessing small impairments, such as are introduced by digital codecs. The double-stimulus method is shown to be highly suited to the evaluation of such impairments. After removal of the residual impairment (due, for example, to scan structure and flicker) and assuming an error-free environment, the digital codec was found to introduce a basic impairment of about 1/20 imp for PAL coded pictures and about 1/10 imp for SECAM coded pictures, irrespective of the assessment method employed. These levels of impairment are compared with the performance of analogue links, of various types and lengths, presently in use for the distribution of television signals.