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Is there Really a True 75 Ohm RCA Plug?
Characteristic Impedance and Video Connectors
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.
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. 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. But as we all know, most consumer video equipment doesn't use these connector types for baseband video signals. 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.
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. 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.
Characteristic 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. Unfortunately, where RCA plugs are concerned, this just doesn't work out. 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.
How'd it get this way? Well, the RCA plug didn't originate as a video connector at all. 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. 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. Until the VCR came along, consumers had little use for baseband video hookups, and the RCA plug continued to play its traditional role as an audio connector; along came the VCR, and for whatever reason, the RCA plug quickly became the standard for a composite video connection on consumer gear--a bad decision, when 75 ohm BNCs were readily available, but there it is.
So, the dimensions of the RCA plug and jack are all wrong for 75 ohm characteristic impedance. What's to be done? The best answer to the problem is to limit the damage. The shorter the distance over which an impedance mismatch occurs, the less likelihood there is that it will result in perceptible image degradation. This is where the Canare design--though it can't remedy the basic faults of the RCA plug dimensions--really shines.
Practically all RCA plugs are designed to be soldered onto cable. When one opens up a solder-type RCA plug, one sees a center pin contact to which the center of the coaxial cable has to be joined, and then usually some sort of outer contact/cable clamp structure to which the braid is soldered. Over this, typically, goes a screw-on plug shell, which naturally has to be large enough to accommodate the contact hardware. What this means is that the plug departs both from the symmetrical coaxial geometry and from 75 ohm dimensions not only at the point where it meets the jack, but for some distance prior, to allow soldering and assembly.
The Canare RCAP plugs are a different breed, and have more in common with the crimp BNC plugs widely used in the broadcast industry than they do with solder-type RCA plugs. In the Canare RCAP, the coaxial cable, still encased in its dielectric, enters the rear of the connector through a perfectly round tunnel of nickel-plated brass; because this tunnel is sized for the specific cable diameter (unlike solder-type plugs which often are "one size fits all," the RCAP is made in numerous different sizes) and is crimped to the coax shield, it effectively replaces the stripped-away shield of the coax, continuing the cable's 75 ohm impedance along toward the connector tip. At the end of the cable dielectric, the center conductor is crimped to a gold-plated pin which fits a socket centered in the plug body to make contact with the RCA plug pin.
Where an ordinary solder-type RCA plug becomes asymmetrical, and departs dramatically from the required dimensions to maintain a 75 ohm impedance, well before the "business" end of the plug, the Canare RCA plug maintains the coaxial geometry through its entire length, and maintains a 75 ohm impedance as far as possible into the plug. While it's inaccurate to call this a "true 75 ohm" RCA plug, it's accurate to say that no other plug on the market does such a good job of minimizing the effects of the impedance mismatch caused by the RCA plug's unfortunate dimensions. The RCAP's reliability, durability, high pull strength, and excellent spring-grip outer contact complete the package, making it, in our opinion, far and away the best RCA plug available for video use.
