My impression, having seen a lot of Chinese cable assembly companies at a lot of trade shows, is that there are a lot of people who just do not really care what the spec is. A lot of these companies have rather primitive systems, and once made products that were not very spec-critical. For example, one factory I recall corresponding with had originated as a telephone-wire business. Telephone wire is very easy to make. Twist rates aren't especially critical, impedance isn't especially critical, and so if you've got the ability to insulate wire, twist it and jacket it, you're a telephone wire company. Companies like this will take a look at a coax, or a high-speed data cable, and just copy it, with highly variable results. Most of the customers don't know enough about the product -- and this is true at the importer/wholesaler level, too -- to actually do any independent evaluation of quality, so it goes to market. But what is the impedance of that coax? What's the impedance of those pairs? Are the UL codes printed on the jacket real, or are they fraudulently using some other factory's UL codes? On those subjects, it's pretty much guesswork.
Making a coax, to the uneducated eye, may seem like the easiest thing in the world. So, you've got a wire; it's got insulation over it; it's got a foil wrap and a braid shield, and a jacket. If you and I had an extruder, a braider, and some spools of wire (most cable factories don't draw their own wire), we could make coax with very little setup time. But what could go wrong? Well, lots of things.
(1) The wire: how well was it drawn? How round is it? How well was it annealed? Has it still got lube on it from the wire drawing line? If it was washed, is it now dry? When we pull it over pulleys and into our extruder, is the tension high enough to slightly knock it out of round? Is there any out-of-round or other inconsistency in pulleys that will cause such a change in wire shape to repeat over a regular distance? Tiny, tiny things--but these tiny things matter.
(2) The dielectric: what are we foaming it with? Do we have a foamer with consistent bubble size? As the foamer warms up, or as the dielectric warms up, does it affect the consistency of the bubble size?
(3) The dielectric extrusion: how consistent is the outer diameter of the dielectric? It's being drawn through a die but it's foamy stuff and it expands. How much? How well centered is the wire in the dielectric? If there is a minute amount of lube, or washing fluid, or water, on the wire, is this affecting the dielectric? And, very important for any high-bandwidth cable: do we have in-process monitoring that will detect small changes in the product and correct them on the fly (or, at least, alert the operator)?
(4) The winding of the extruded dielectric onto a production spool: shield application runs much slower than dielectric extrusion, so you've got to collect your "core" on production spools. Wind too tight, and you crush the core together. Wind too loosely or too irregularly, and the core crosses over itself causing bumps and divots. Is the temperature of the material, and hence the final size of the bubbles, still changing at this point?
(5) Shield foil wrapping: your foil can easily fold up while being wound onto the core. How are you going to stop this from happening? The foil should have a shorting fold. Does your foil wrap machine consistently produce that shorting fold, without gaps?
(6) Braiding: tension is important. The coax has got to be fed through the braider at a rate consistent with the application of the braid. And the braider is a clattering nineteenth-century steampunk kind of an affair...do all the parts do what they're supposed to do? And do you have a nineteenth-century steampunk mechanic on hand if they don't?
(7) Jacketing: Ah, this is getting easier. But you do need the jacket to extrude on nicely so that it doesn't infiltrate the braid excessively but does bond to it well enough to keep the insides of the cable from slipping through it as through a tube when somebody goes to terminate the stuff. And the jacket material is critical to your UL rating and NEC rating...
So, you, I, and a buddy or two could knock out some coax if we were left in an abandoned cable factory for a while. But when we stuck some BNCs on the end of it and sweep-tested it, we'd probably see a lot of nice spiky return loss, and we'd probably find that its impedance bore no very close resemblance to whatever we were shooting for. And sure enough, I have seen a few test reports from Chinese coaxes (not a lot; they don't find them useful for bragging!) and that's what we see.
In paired cables, it's the same problem. Process control, impedance, consistency. USB ought to be easy to make, but the factory that can't make four good data pairs for a Cat 5e can't make one good data pair, either. And who's going to test it? Who has both the test gear and an incentive to test? For the vast majority of applications, a USB cable that's "good enough" truly is good enough. I hook the occasional printer up with a USB cable, and it works. Whether USB cables are frequently problematic for audio is something I don't know much about, but when we did look at producing a US-made, bonded-pair cable stock for USB a few years back I know that we came to the conclusion that it had a very, very limited market, mostly because USB has an inherent length limit (so we could not have a cable that works over substantially longer distances than others) imposed by its two-way protocol. Maybe I should revisit that -- but we don't sell a lot of USB cable.
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