Subjective quality of television pictures impaired by long-delayed echoes

Echoes are a well known form of impairment in television reception, but precise information on the subjective effect they cause has been scanty, and the limits for echo in specifications of television transmission systems have been set on a somewhat arbitrary basis. Using a previously proposed quality-grading method, tests have therefore been made of the effect of single undistorted echoes, having delays in the range 1¿13¿s, on still monochrome 625-line pictures typical of scenes in television broadcasting. The magnitude of echo corresponding to a given quality of picture decreases by about 5 dB as the delay is increased from 1 to 13¿s. The effect of variation of delay over the range 5¿13¿s is less than 1dB; within this range, when echo-sensitive pictures are viewed under specified conditions, 50% favourable opinions are obtained with a signal/echo ratio of 23dB and 95% favourable opinions with a ratio of 28dB.