Design
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.
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