Editing RF modulator (section)

== Design ==
{{Unreferenced section|date=December 2018}}
[[File:C64 opened RF modulator on ASSY NO 250427 motherboard 1984.jpg|upright=0.75|thumb|RF modulator inside the [[Commodore 64]] manufactured in 1984, [[PAL]] system]]

RF modulation puts the desired information on to a [[carrier signal]] at a standardized frequency. [[Amplitude modulation|Amplitude]] or [[frequency modulation]] may be used, as required by the receiving equipment.

Modulating a TV signal with stereo sound is relatively complex; most low-cost home TV modulators produce a signal with monaural audio. Even some units that have two or more 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 [[Cable television|professional use]]) lack [[vestigial 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.

Consumer audiovisual devices sold in [[North America]] typically shipped with a modulator allowing for use on analog channels 3 and 4. This channel option was provided because [[television channel]]s 3 and 4 were rarely assigned in the same market due to [[adjacent channel]] [[Adjacent-channel interference|interference]], and the use of channel 3 overall was mostly confined to medium or smaller markets. The choice allowed the user to select the unused channel in their area so that the connected device would be able to provide video and audio on an RF feed to the television without excessive interference from a broadcast signal. 

Both channels being in use and receivable were rare and confined to the outskirts of markets (for instance, channel 3, [[WISC-TV]] in [[Madison, Wisconsin]] and [[WTMJ-TV]], channel 4 in [[Milwaukee]] being both available in eastern [[Jefferson County, Wisconsin|Jefferson County]]); in this case unless the viewer received cable television service where either channel could be chosen, they would have to tune to the channel with less power to their set, an alternate input, or use an alternate modulator which instead modulated on a different channel.

Other countries had the RF output for video equipment on different groups of frequencies. For example, equipment sold in Europe, South Africa and Hong Kong used UHF channels 30–39 for this purpose. Equipment sold in Japan used channel 1 or 2 (Channel 13–16 is for cable converters). With other channels being used for RF modulation function in other regions, channel 3/4 output is a misnomer for those regions.

Recently, some RF demodulators have been marketed on obscure online markets to allow channel 3 inputs from legacy devices to work on RCA composite in.<ref name=Gaming/>