An RF modulator (or 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.
In order to display picture information on early analog televisions, the data must be modulated, or conditioned, to the format that the television expects. 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. Technically, this usually means combining the data with a carrier wave at a standardized frequency.
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, 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 on old televisions.
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, in older video game consoles such as the Atari 2600, NES, or Sega Master System, and in older home computers such as the Commodore 64, Atari 800, and Sinclair Spectrum.
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. One reason for this is that a device which outputs an RF signal must in general be certified by regulatory authorities – in the US, the FCC – and thus by having an external RF modulator, only the modulator itself needed to be certified, rather than the entire video game system.
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.
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.
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