Subjective Effect of Substituting Lines in a Video-Telephone Signal

In processing video-telephone signals to reduce bandwidth requirements, lines may occasionally be lost due to buffer overflow or DPCM channel errors. The subjective degradation introduced by replacing deleted lines by averaging or repeating adjacent lines (substitution) has been measured. In the test, both skilled and unskilled observers were asked to add white noise to an unimpaired picture until the quality was equal to the same picture in which a number of lines had been randomly substituted. It was found that the more critical skilled observers deemed a picture, degraded by substituting 1 averaged line per frame, comparable to a signal-to-noise ratio of 38.6 dB, a rating of "definitely noticeable but not objectionable" on a 5-point impairment scale. Futhermore, it was found that repeating lines rather than averaging produced more degradation, equivalent to a 4 dB lower signal-to-noise ratio.