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An RF modulator (for radio frequency modulator) is a device that takes a baseband input signal and outputs a radio frequency-modulated signal.

This is often a preliminary step in transmitting signals, either across open air via an antenna or transmission to another device such as a television.

The main board of an early microcomputer- the Microtan 65 - showing the silver-cased UHF TV modulator at top left

In order to display picture information on a television, the data must be modulated, or conditioned, to the format that the television expects. This usually means combining the data with a carrier wave at a standardized frequency.

TV Modulators take the audio and video signal from a composite video, RGB, YUV or internal source, and generates a PAL or NTSC broadcast signal that can be fed into a television's aerial/coaxial connector.

Internal RF Modulators are commonly found in VCRs, and older video game consoles such as the Atari 2600, NES, or Mega Drive. During the 1980s and early 90s, it was common for systems that did not have internal RF modulators to provide external units that connected to the antenna jacks of a television; however, as television designs gained composite, S-Video, and component video jacks, manufacturers stopped including the modulators as technology advanced. RF modulators are now largely a third-party product, purchased primarily to run newer equipment such as DVD players on old televisions. RF modulators produce a relatively poor picture, as image quality is lost during both the modulation from the source device, and demodulation in the television. However, the aerial connector is standard on all TV sets, even very old ones. As digital television becomes ubiquitous, the future of the RF modulator is grim.

RF modulators generally output on channel 3 or 4, although the Atari consoles offer channels 2 & 3.

Multi Channel RF Modulators are commonly used in home audio/video distribution. These devices have multiple audio and video inputs and one RF output. Audio/Video outputs from source devices such as a DVD player, VCR, or DSS receiver are connected to the Audio/Video inputs on the modulator. The modulator is then programmed to broadcast the signals on a certain frequency. That RF broadcast is then received by the connected TV. When the TV is tuned to the programmed channel, the audio/video signal of the source device is accessed. RF modulation can become difficult in a CATV system. High pass, low pass, and notch filters must be used to block certain frequencies, or channels, so that the modulator can broadcast the audio/video signal of the source device on that channel.

Different RF modulators that are audio only, and output on FM Radio frequencies are also used in car audio to connect such devices as add-on CD changers and video systems. These FM modulators also suffer from loss of quality and interference issues.

An example of RF Modulation in use can be found here.