In order to display picture information on a pre-digital 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 video game systems that did not have internal RF modulators to provide external units that connected to the antenna jacks of a television. RF modulators produce a relatively poor picture, as image quality is lost during both the modulation from the source device, and the demodulation in the television, but the aerial connector is standard on all TV sets, even very old ones. Since later television designs include composite, S-Video, and component video jacks, game manufacturers have stopped including modulators. RF modulators are now largely a third-party product, purchased primarily to run newer equipment such as DVD players on old televisions.
RF modulators generally output on channel 3 or 4, although the Atari consoles offer channels 2 & 3. In Europe standard modulators use channel 36 (UHF).
Modulating a TV signal with stereo sound is relatively complex and most low-cost home TV modulators produce a signal with monaural audio. Even some units that have two audio inputs simply combine the left and right audio channels into one mono audio signal.
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.
Audio RF modulators are used in low-end car audio to add devices like CD changers without requiring dashboard hardware upgrades. For example, a portable CD player's headphone jack is connected to the modulator, which outputs a low-power FM radio signal, which plays through the car radio. Car FM modulators suffer from loss of quality and interference issues.