Note, however, that a gain may vary with time independently
of to yield a linear time-varying filter. In this case,
linearity may be demonstrated by verifying

to show that both scaling and superposition hold. A simple example of a
linear time-varying filter is a tremolo function, which can be
written as a time-varying gain,
. For example,
would give a maximally deep tremolo with 4 swells
per second.