Coaxial cable seems simple, doesn't it? And yet, when high bandwidth is to be delivered, it's anything but simple to actually manufacture a consistent, precision quality cable. Why is that?

Any transmission line has a characteristic impedance, and in the case of SDI cable, the specified impedance is always 75 ohms. The characteristic impedance of a cable is a function of its physical geometry and materials: especially the dielectric, that inconsequential-looking layer of plastic which insulates the center conductor from the shield. Characteristic impedance isn't load impedance: you can take your ohmmeter to video cable and measure all you like, but you won't find 75 ohms anywhere. Rather, the characteristic impedance of a line is a way of describing how its impedance affects the impedance of the load: the load should look like a 75 ohm load (not just in terms of resistance, but in terms of reactance) when it's hooked up directly to the source, and it should look like a 75 ohm load if it's hooked up through a run of cable, no matter the length. A balance of inductive and capacitive reactance – from the inductance and capacitance of the cable – achieve this end.

But, alas, it's not so simple as looking at the massed values of inductance and capacitance for the cable run. Signals propagate down a cable over time – traveling at a significant fraction of the speed of light – and react to local conditions at every point along the way. Changes in the impedance as seen at any point along the line cause signal reflections – what we call "return loss" – and these add up, not only diminishing but actually interfering with the intended signal. Above all, what a cable needs, to minimize these reflections, is consistency: consistency of dimensions, even down to microscopic levels, and consistency of materials. If the density of bubbles in the dielectric varies, or a wire has been drawn over a wheel that's out of round and so gets slightly bigger and smaller along the way, or any of a number of other little manufacturing issues, return loss rears its ugly head.

While a precision video cable might look superficially a lot like, say, a ten-cents-a-foot CATV drop cable, the engineering and the manufacturing resources that go into it are a world apart. Equipment is carefully maintained to eliminate small periodicities and inconsistencies in dimensions. Dielectric extrusion is continuously monitored during production to catch even small departures from tight tolerances. Everything from tension on braider wires to the consistency of foil overlaps to the amount of nitrogen going into the polyethylene foam is watched, and checked, and the final product is sweep-tested just in case something, somewhere, has been missed. The result is a cable of tremendous physical and electrical consistency.

To that one adds connectors, and the cable-connector interface is where a lot of impedance excursions take place -- deviations from spec that bump impedance up or down, leading to return loss. Control of impedance through these junctions requires a precisely made connector, with dimensions that internally mimic the cable itself despite the physical transitions at the interface -- too tight a tunnel, or too loose, or anything else in the way of dimensional variation, will cause trouble. The Canare BNCs we use are designed for this cable, sized exactly for consistent impedance, tip to tip.

Lots of people can make you an SDI cable -- but at Blue Jeans Cable, we have made and sold literally millions of feet of these over the last couple of decades. Our assembly process -- below -- ensures high consistency, from one cable to the next.

Blue Jeans Cable has been in the cable assembly business since 2002, with our primary manufacturing operation in Seattle, Washington. Today we occupy a small industrial building in the Interbay neighborhood where we employ about a dozen people. From the very beginning, we've emphasized assembly, not import: if we can build the cable here, rather than offshoring it, we do, and in addition to conventional assembly techniques we've introduced novel materials, methods and processes, like our ultrasonically-welded speaker cable. We can't underprice our foreign competition, but what we can do is produce goods of consistent higher quality; to do that we use the highest quality cable and the highest assembly quality standards. To do that, you need experienced workers, and so we try to be the sort of place where people want to work; our employee retention is high and that means that, likelier than not, the technician assembling your cable is someone who's been doing this work for at least ten years. American manufacturing isn't dead, and neither are good American manufacturing jobs; and we thank our customers for making this possible.