= Carrier wave
{wiki}
Early transmitters such as the <spark-gap transmitter> could only send noises to send <Morse code>.
To send voice and music, <amplitude modulation> had to be developed. And a key ingredient of this is the carrier wave.
The problem is, the carrier wave needs to have somewhat high frequencies, in the hundreds of kHz TODO why. But as you might imagine, that is hard to achieve by mechanical means such as a hand cranck like <Hippolyte Pixiis alternator>!
Interestingly, some of the first carrier wave generators were actually mechanical, e.g. the <Alexanderson alternator>.
But clearly such mechanical machines were not very scalable, and soon more <electronic> devices were introduced, notably the <vacuum tube>.