Short note: Distortion and interference

Distortion is the unwanted change in the shape of a signal caused by the system or channel itself; the output is no longer $K\,x(t - t_d)$.

• Amplitude distortion: gain not constant across the signal band.
• Phase (delay) distortion: phase not linear with frequency, so components arrive at different times (causes ISI).
• Non-linear distortion: non-linear devices generate harmonics and intermodulation products.
• It exists only when the signal is present and is deterministic, so it can be reduced by equalizers, linear amplifiers and pre-distortion.

Interference is contamination of the wanted signal by unwanted signals from other sources, mostly man-made.

• Examples: co-channel and adjacent-channel interference, crosstalk between cables, image-frequency signals, power-line hum.
• It is present even without the wanted signal and usually has a definite frequency.
• Reduced by good filtering and selectivity, shielding, frequency planning, directional antennas and spread spectrum.