A UHF Modulator, also called RF Modulator, is an electronic circuit, which modulates audio and video signals into an RF signal compatible for input through a television aerial socket. Earlier televisions did not have any means to feed audio or video signals other than through an aerial socket, therefore, modulators were widely used. The circuit consists of an ultra high frequency (UHF) oscillator, which produces the radio frequency (RF) carrier. The video frequency, which is the base frequency, modulates the carrier signal, producing an RF signal that is compatible with a television receiver set. This signal usually feeds through the aerial socket where the tuner section of the television treats it exactly as if it was a broadcast signal. According to the FCC part 15, subpart H, the carrier power is limited to 3 mV rms, and fed through a 75 Ω cable.
The earliest modulators could modulate both audio and video, and there are some early examples of video recorders used by the BBC, which had this capability. In the 80s and 90s, the home computer revolution brought these modulators into computers. The earliest ones such as the Sinclair ZX80 and ZX81 did not have sound, and therefore their modulators were just for video modulation. Later computers had sound, and their modulators could modulate both. The sound transmitted on an inter-carrier of 4.5 MHz and was frequency modulated. The Aztec UM1082 LA2/3 and UM1233 were commonly used and available in almost every corner electronics store, however in 2012 they are a little hard to find.
Currently, with the switchover to digital, all modern televisions have a digital tuner, which operates on a completely different principle. This type of tuner is incompatible and cannot understand an analogue signal produced by a vintage computer console.
Discrete Component Modulator
The circuit shown above came from a 1980s Pong games console called the Binatone TV Master Mark IV, and modulates the video signal. As you can see, it is an incredibly simple single transistor circuit, which requires some resistors and capacitors.
This circuit operates with Vcc as little as 9 V DC. The copper wire winds around a ¼-inch former and is easy to construct. This circuit works for a wide range of televisions that were available at that time.
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