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.