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RF modulators are used to convert signals from devices such as media players, VCRs and game consoles to a format that can be recognised by a device that accepts an RF signal, such as a radio or television.
Prior to the introduction of specialised video connector standards such as SCART, TVs were designed to only accept signals through the aerial connector: signals originate at a TV station, are transmitted over the air, and are then received by an antenna and fed into the TV. Thus, other equipment, such as a VCR, DVD player, or video game console, which wishes to send a signal to such an old TV must replicate this process, in effect "faking" an over-the-air signal.
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, which skip the modulation and demodulation steps, modulators are no longer included as standard equipment, and RF modulators are now largely a third-party product, purchased primarily to run newer equipment such as DVD players with an old television set.
However one should note that the direct video signal setups differ in signal quality.
Technically, RF modulation usually means combining the data with a carrier wave at a standardized frequency.
In North America, RF modulators generally output on channel 3 or 4 (VHF), which may be selectable, although the Atari consoles offer channels 2 and 3. In Europe standard modulators use channel 36 (UHF) by default but are usually tunable over part or all of the UHF band.
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. Some used on very early home computers had no sound capability at all. Most cheaper modulators (i.e. not intended for professional use) lack vestigal sideband filtering.
TV modulators generally feature analog passthrough, meaning that they take input both from the device and from the usual antenna input, and the antenna input "passes through" to the TV, with minor insertion loss due to the added device. In some cases the antenna input is always passed through, while in other cases the antenna input is turned off when the device is outputting a signal, and only the device signal is sent onward, to reduce interference.
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.
Integrated modulators 
Some systems were supplied with an external modulator unit that connected to both the system and to the antenna jacks of a television. One reason for this is that a device which outputs an RF signal must in general be certified by regulatory authorities—such as the U.S. Federal Communications Commission (FCC) — and thus by having an external RF modulator, only the modulator itself needed to be certified, rather than the entire video game system.
Broadcasting modulators 
RF modulators may also be used to take the audio and video signal from a PAL or NTSC composite video, RGB, YUV or other composite AV source, and generate a broadcast RF signal that can be fed into a television's aerial/coaxial connector.
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 video and audio 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 that is played through the car radio. Car FM modulators suffer from loss of quality and interference issues.