English: Animated diagram illustrating the difference between radio waves modulated by amplitude modulation and by frequency modulation. The vertical axis represents the amplitude (voltage or current) of the signals and the horizontal axis represents time. The top graph, labeled "Signal" is the modulation signal applied to the transmitter, in this simple example a sinusoidal wave. For example it could be an audio signal from a microphone. The middle graph (red) labeled "AM", is the output of an amplitude modulated radio transmitter. The magnitude (voltage or current) of the sinusoidal carrier wave varies with the modulation. The bottom graph (blue) labeled "FM", is the output of an frequency modulated (FM) radio transmitter. The frequency of the carrier wave varies with the modulation.