English: Block diagram of a crystal radio, the simplest type of radio receiver which was invented in the early 1900s and widely used until vacuum tube receivers replaced it. It consists of an antenna attached to a tuned circuit, which functions as a bandpass filter which allows through the frequency of the desired station while rejecting all the other radio signals picked up by the antenna, followed by a detector consisting of a semiconductor diode which extracts the audio modulation signal (sound) from the radio frequency carrier wave. The audio is applied to earphones which convert it to sound waves.