This is a phenomenon caused by the propagation channel of the signal. When the channel affects the amplitudes of the different frequency components of the transmitted signal differently, this is said to produce amplitude distortion. When there is a phase shift depending on the frequency in a non-linear form, there is said to be phase distortion, in such a way that the frequency components of the spectrum of the signal reach their destination in different time slots. Both amplitude distortion and phase distortion result in changes to the shape of the signal transmitted. Phase distortion is particularly disturbing to the transmission of digital signals, producing intersymbol interference when a component of the signal of one bit is displaced to the time slot reserved for another bit.