Quality of PAL colour television pictures impaired by random noise: stability of subjective assessment

An investigation has been made of the subjective impairment introduced into PAL colour television by random noise of flat or triangular spectrum. Results are given in terms of a logistic surface model relating mean opinion score to noise power measured separately through luminance- and chrominance-weighting networks. The parameters given by the surface model (embracing all noise spectra) and by a simpler logistic model (applied to each spectrum in turn) are compared with those found in a closely similar study conducted some 14 years earlier. Although some parameters are significantly different at the 5% level, differences do not approach 1% and are, for all practical purposes, quite small. In general, a slightly increased sensitivity of observers to noise like impairments is apparent, particularly in the presence of triangular noise, and probable reasons are given for such changes. The results reported here are considered together with results from several earlier studies and indicate that the subjective assessment methodology of CCIR Recommendation 500¿¿1, taken in conjunction with Pt. 1, Annex IV of CCIR Report 405¿¿3, is well able to provide consistent results for experiments repeated at intervals ranging from a few weeks to many years.