Transient gain distortion, also referred to as transient nonlinearity, is present when abrupt changes in APL temporarily affect signal amplitudes. The error is defined as the maximum transient departure in amplitude of a sync pulse from the amplitude it had before the change to the amplitude after the change. Measurement of transient gain distortions is done as an out-of-service test and should be checked for transitions of low-to-high APL and high-to-low APL. If the transient gain distortion only affects the sync pulse and then not so severely as to cause the sync to be unusable, then the viewable picture would not be affected.

However if the sync pulse is affected, the rest of the picture is also normally affected; when transient gain distortions affect the picture, it appears as abnormal transient brightness effects. A test signal generator capable of producing a “bouncing” flat-field signal is used to test for transient gain distortions. A typical signal is shown below. The time between bounces (APL level changes) must be longer than the transient effects so that all the transient effects can be viewed before the next APL change occurs.