Around 1920, vacuum tube technology had matured to the point where the availability of radio receivers made radio broadcasting viable. A technical challenge was that the vacuum tubes in the radio receivers required 50 to 250 volt direct current, but car batteries ran at 6V. Voltage was stepped up with a vibrator that provided a pulsating DC which could be converted to a higher voltage with a transformer, rectified, and filtered to create higher-voltage DC.
