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FROM USER audioengr
USB cables CAN make a difference, depending on the USB protocol used by the converter. For instance, Synchronous adaptive mode of isochronous transport relies on the frequency of the incoming stream in most chip implementations (mostly TI chips). If you read their treatise on their difficulty in desinging this interface for audio streaming, you will understand how difficult this problem is. In any case, the two design groups Burr-Brown and TI both have implementations that work, however the audio quality that results from each is quite different due to the jitter. The buffering is minimal in most TI chips and the digital PLL locks onto the incoming stream frequency in adaptive mode. It is quite resistant to jitter in this stream, however not completly insensitive. The PLL does react differently to different USB cables.
The reason for this is similar to that on S/PDIF cables:
1) differences in dielectric absorption - insulation materials differ
2) differences in skin-effect - some USB cables are 26 gauge and some are 28-gauge
3) differences due to ferrites or lack thereof
Ferrites are a bad idea IMO and can lead one down the garden path. They are nothing more that low-pass filters that turn signal into heat. On a real-time digital transmission-line it is never a good idea to do filtering rather than impedance matching etc.. The resulting slowing of edge-rates will introduce more jitter at the receiver because of increased uncertainty in the edge detection accuracy.
Just because one listener likes the cable does not mean that it is doing a positive thing. There are lots of tube systems that listeners like because they roll-off all the highs. This does not make them more accurate. There are even forms of jitter that seem to be euphonic, but that does not make them better. Lower jitter is best in my book, and in my system.
Steve N. Principal Engineer
Empirical Audio
