<P> The earliest radio stations were radiotelegraphy systems and did not carry audio . For audio broadcasts to be possible, electronic detection and amplification devices had to be incorporated . </P> <P> The thermionic valve was invented in 1904 by the English physicist John Ambrose Fleming . He developed a device he called an "oscillation valve" (because it passes current in only one direction). The heated filament, or cathode, was capable of thermionic emission of electrons that would flow to the plate (or anode) when it was at a higher voltage . Electrons, however, could not pass in the reverse direction because the plate was not heated and thus not capable of thermionic emission of electrons . Later known as the Fleming valve, it could be used as a rectifier of alternating current and as a radio wave detector . This greatly improved the crystal set which rectified the radio signal using an early solid - state diode based on a crystal and a so - called cat's whisker . However, what was still required was an amplifier . </P> <P> The triode (mercury - vapor filled with a control grid) was patented on March 4, 1906, by the Austrian Robert von Lieben independent from that, on October 25, 1906, Lee De Forest patented his three - element Audion . It wasn't put to practical use until 1912 when its amplifying ability became recognized by researchers . </P> <P> By about 1920, valve technology had matured to the point where radio broadcasting was quickly becoming viable . However, an early audio transmission that could be termed a broadcast may have occurred on Christmas Eve in 1906 by Reginald Fessenden, although this is disputed . While many early experimenters attempted to create systems similar to radiotelephone devices by which only two parties were meant to communicate, there were others who intended to transmit to larger audiences . Charles Herrold started broadcasting in California in 1909 and was carrying audio by the next year . (Herrold's station eventually became KCBS). </P>

Most local radio stations utilize which type of broadcasting