History
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.
Read more about this topic: RF Modulator
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