History & CultureHow it works
How early radio put speech onto a wave.
A transmitter produces a steady carrier wave, say 1,000 kilohertz. In AM radio the microphone signal varies the strength of that wave in rhythm with the voice: loud means high amplitude, quiet means low. The receiver filters out the fast carrier, leaving the envelope, which is the sound. That's why a crystal and headphones sufficed in the 1920s, no battery needed.
Useful: Information often rides piggyback on something that carries no information itself.
Source: en.wikipedia.org