16 Jun 2014

UPDATED: Is this USB cable really worth £400 (US$ 680)(€502)?

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£400 USB cable £400 USB cable Computer Audio Design/RedShark

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£400 for a USB cable? Madness, surely? Or perhaps not...

Update: We've heard from Scott Berry, the designer of the cable, who's written a reply to this piece. You can see what he says at the end of the article.

How can a USB cable cost £400? Well, it probably doesn’t cost anything like that to make but you can charge whatever you like for a product and if you can sell them at that price - you’ve got a pretty good business.

There is some method to this madness, but it’s just not explianed very well either by the compnay that makes the cable, or by the journalists who have reviewed it.

Digital cables are not digital

There is more than meets the eye to digital cables.

The first thing to understand is that digital cables are not digital. Not even slightly. In fact, they’re 100% analogue. You simply can’t send numbers down a cable: a number is a concept, and cables don’t carry concepts. What they do carry is voltages and current, and the job of a “digital” cable is to carry signals effectively so that the numbers (and the timing of the numbers) that the voltages and currents represent can be accurately extracted at the other end.

And that’s where the problem is: quite often, the pulses that represent the numbers get mangled.

There is an intrinsic problem with cables carrying digital signals. The easiest way to represent ones and zeros is with a square wave or pulse. It makes sense: in a binary system you only have two possible signal levels. There really should be nothing in between “on” and “off”. So, ideally, all the receiving circuit should have to do is tell the difference between on and off, high or low, etc.

But remember - digital cables are actually analogue cables, and in the analogue world, things are far from perfect. Cables have resistance, inductance and capacitance, all of which can degrade a signal whether it’s carrying digital information or not. The longer the cable, the worse it gets.

The problem is timing

None of this matters much initially, because even if the square wave ends up looking more like a sine wave, it still has a top and a bottom, and as long as it’s possible to detect these, it’s easy enough to extract the meaning from it. The real problem comes with timing.

Digital video and audio signals, as opposed to timeless data (like a spreadsheet or like this document as it is being typed) needs to arrive in time, in a constant fashion. If it doesn’t, it’s likely that the signal, when it’s reconstructed at the other end, will be distorted. It’s easiest to see how this might happen with audio.

Think of a nice, even, crisp square wave. And now imagine what it’s going to look like at the far end of a poor quality cable. It will, effectively, be a wonky sine wave, or a combination of several of them. Sine waves get narrower towards the top, unlike square waves, whose sides are vertical.

This shouldn’t be a problem, and it won’t be, as long as the recieving circuit measures exactly at the point where the waveform crosses zero. If it doesn’t, in other words, if it measures the waveform above or below the zero line, then it’s effectively going to be measuring the upside and the downside of the waveform at a point where it’s not symetrical. Which means that the resulting output, instead of being a regularly spaced pattern of ones and zeros, is going to be more like two pulses and then a gap, then two more pulses and another gap.

 

Jitter.jpg

 

This is one example of a phenomenon called Jitter, and it’s one of the worst things that can happen to your digital signal. If a jittery digital signal is feeding an audio digital to analogue converter, the result will be distortion ranging from subtle to severe. If it’s not severe, you can still hear it, although you might not be able to put your finger on it: I find that jitter-affected audio just doesn’t sound quite right.



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David Shapton

David is the Editor In Chief of RedShark Publications. He's been a professional columnist and author since 1998, when he started writing for the European Music Technology magazine Sound on Sound. David has worked with professional digital audio and video for the last 25 years.

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