We used to own a beautiful 1940's Hallicrafters shortwave receiver, which among its features offered an input for a phonograph-and the RCA jack on the back of the Hallicrafters looked in every way like the jacks one sees on home video equipment today. Its original function was simply as an analog audio connector, used to connect phonographs to other equipment (hence the term "RCA phono plug"), and its design predates broadcast video. How'd it get this way? Well, the RCA plug didn't originate as a video connector at all. The plug's dimensions don't present a 75 ohm characteristic impedance, and obviously these dimensions are pretty well fixed by the standard sizing of the jacks the plugs must fit. Unfortunately, where RCA plugs are concerned, this just doesn't work out. At the same time, a look at the construction of these plugs shows that they are easily the best plug on the market for a good impedance match with 75 ohm cable.Ĭharacteristic impedance of a coaxial cable, or of a connector with a coaxial (that is, two conductors sharing a common axis) construction, is determined by the size of the conductors, the distance between them, and the type of dielectric that separates them. Maintaining 75 ohm impedance on mini-DIN and HD-15 plugs is a lost cause but is there such a thing as a true 75 ohm RCA plug? Not really Canare's RCAP-series plugs, which we feel are the best RCA plugs available for video, are often referred to as "true 75 ohm" plugs, but that's not quite accurate. Composite and component video generally run through RCA connections, while s-video is run through four-pin mini-DIN plugs, and many projectors and HDTV receivers these days use 15-pin VGA-type plugs. But as we all know, most consumer video equipment doesn't use these connector types for baseband video signals. BNC connectors made for video use (but not those made for other uses!) are designed for 75 ohm characteristic impedance, as are F-connectors, so it's easy enough to keep a consistent impedance in your lines when using these connector types. The solution, it would seem, is to use only true 75 ohm connectors all throughout your system but this is not as easy as it might appear. One hears a lot, in home a/v cable marketing, about "true 75 ohm RCA plugs." Ideally, all connectors used in handling video signals ought to present a true 75 ohm characteristic impedance as impedance mismatches in a system pile up, they become more and more a factor in signal quality, and the complexity of a modern home theater system can result in signals being fed through multiple cables along their way from the original source to the display, so impedance mismatches, even if short, can add up. Is there Really a True 75 Ohm RCA Plug? Characteristic Impedance and Video Connectors
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