Computer Audio Asylum: USB cables and sound by John Swenson Music servers and other computer based digital audio technologies.

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Posted by John Swenson (A ) on January 18, 2008 at 13:40:40

In Reply to: RE: Anyone Here Try Any After Market USB Cords??? posted by JimOfOakCreek on January 18, 2008 at 07:14:05:

I have worked on the internals of several USB chips, I can guarantee you that a USB bus is very definitely an analog signal! Everything those pulses go through degrade said pulses. What hits the receiver doesn't look anything like a theoretical square wave. Dielectric dispersion, skin effect and reflections due to impedance discontinuities all mess up the pulse edges. Those pulse edges are what the PLLs in the receiver see, the more degraded they are the harder it is for the PLLs to lock onto the signal and the more jitter they generate in the clock signal coming out.

Getting a USB chip to work well is actually much harder than some of the other much higher data rate busses I've worked with. The higher speed busses have much higher tolerances on every part of the system, the tolerances in USB are so lax (and frequently those are not even met in the real world) it gets very difficult to make it all work for all the corner cases. For example exactly how the wires in the cable get connected to the connectors make a huge difference in the impedance of the cable. Many cables do not get this right, causing significant impedance discontinuities which cause reflections at the connectors. These reflections significantly change the edge timing, causing jitter on the recovered clock. Interestingly some cheap cables do it better than the expensive ones, cost seems to have relatively little to do with whether a cable "gets it right".

The connection between the USB connector and the USB chip is another black hole. Very rarely is the impedance properly maintained on the board, again giving rise to reflections.

Now you may say that this shouldn't make any difference if the equipment is designed right. To some degree thats true, if asynchronous mode is used then as long as the bits get across correctly then it doesn't matter. BUT there are darn few devices out there using asynchronous mode (or some other custom mode that syncs the data transfer to the local clock). The reality is that most USB implementations out there use adaptive mode which IS affected by what is going on with the bus. In this case there is a big difference between SHOULD and reality.

With the asynch code that Gordon has written and reclocking devices such as the ones Steve has these problems will occur in fewer and fewer pieces of gear, so long term there is hope that this state of affairs will go away, but for now, most USB DACs out there WILL be sensitive to cables, connectors, power supplies in computers, whats going on IN the computer, whether your dog is laying on the cable etc.

John S.

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