Short time distortion amplitudes are not generally quoted directly as a percent of the transition amplitude but rather are expressed in terms of an amplitude weighting system which yields “percent-SD.” This weighting is necessary because the amount of distortion depends not only on the distortion amplitude but also on the time the distortion occurs with respect to the transition. The equation for NTSC Systems is SD = at0.67 where “a” is the lobe amplitude and “t” is the time between transitions and distortions. In practice, screen graticules eliminate the need for calculations.

Refer to the figure below. Also see the discussion on Short Time Distortions.