Determine the frequency response of the channel

Question

Solve the following problem:

A channel is said to be distortion less if the response y(t) to an input x(t) is K x(t - t₀), where K and t0 are contants. Show that if the frequency response of the channel is A( f )e j_θ( f ) , where A( f ) and θ( f ) are real, the necessary and sufficient conditions for distortion less transmission are A( f ) = K and θ( f ) = 2π f t₀ ± nπ, n = 0, 1, 2,.