A radio frequency modulator is a device that allows consumers to connect external devices to radios and televisions, adding an audio or video signal to a carrier wave, transmitting the audio, or displaying the picture in a manner similar to an over-the-air broadcast. RF modulators are useful for cars without auxiliary audio inputs and television sets that only have antenna hookups.
In car audio applications, professional installers or consumers wire RF modulators between the head unit and the antenna. Signals from the antenna pass through the RF modulator to the head unit, similar to how a radio station transmits its signal, but at much lower power. An auxiliary input allows external CD players, MP3 players and smartphones to connect to the RF modulator, and transmit their signals to carrier waves. This allows these devices to connect to a car's audio system without a direct auxiliary input into the head unit.
RF modulators for video applications are either integrated into VCRs and video game consoles, shipped as external units, or sold as third-party devices. The audio and video data combines with the carrier signal at a standardized frequency which is sometimes selectable. In North America, RF modulators typically send a signal to channel 3 or 4. Matching the channel on the television with the channel on the RF modulator allows for the video and audio signal to appear, without the need for composite, S-Video, component or HDMI jacks.