Hi
I could not get my head around the fact why a USB cable should be of any influence.This is a shorter, translated version of a dutch article I found. See for the source: http://www.alpha-audio.nl/2011/06/de-usb-kabelmythe/
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USB transfer modes
isochronous (asynchronous), interrupt, bulk and control.
Let's start with the most common: bulk. This method is used for data transfer. The bandwidth is not guaranteed, but that's not important. The bus is trying to get it to the highest throughput. With this transfer mode the host controller and the client (the USB drive / stick) perform CRC checks. When the CRC does not match, the data packet is transmitted again. It can not go wrong.
The interrupt mode is for machines that have a low 'latency' require: quick response. Think of a mouse. The control mode is used to initialize a device.
Finally, the isochronous transfer mode. This is used for audio and video streaming, as USB dacs do. The main advantage of this mode is the guaranteed bandwidth on the bus. This rate is agreed between the controller in the PC and the USB receiver in the DAC. If the speed is not feasible for some reason - too many other devices on the bus, or a bad cable - it will not work.
Control
In bulk mode (data transfer) crc packets are transmitted. This ensures that errors in transmission are not possible. Or the PC gives an error. In isochronous mode is a crc-check is performed, but the controller does not send a packet again if something goes wrong. The package is therefore 'dropped'. Is that bad? no, it's not audible, as it is a constant flow of data and we talk about a millisecond. But is there a chance a packet gets lost in the first place? More on that later.
How is the flow of data is now governed by usb? Very different - much easier and more controlled - than spdif. Every millisecond (1000 Hz) The host sends a USB SOF (start of frame) token. Using this package, with a unique 11bit ID, the receiving side can Synchronize the data. Both sides know exactly what was sent and where the package belongs in the chain.
Testing
But, skeptics will say, in isochronous mode is no-crc correction. No, that's right. In theory, data loss is possible. To find out how much we did a test.
The following software tool shows the data over a USB connection: http://www.usblyzer.com/download.htm. And it shows if packets arrive, crc-checks are correct, etc.
U.S. 300 euro 5 euro
We purchased a AudioQuest Coffee USB cable, sized 1, 5 meters (ca. 300 euros). Yes: we too were convinced of the usefulness of a high-end USB cable. The AudioQuest entered the fight with a standard cable of 5 euros. This of course meets the USB 2.0 standard.
We played a complete song by Massive Attack, twice. Once over the AudioQuest Coffee and once over the budget cable.
In both cases, error free data is transferred. A song of more than 5 minutes, more than 300,000 milliseconds ... error free in both cases.
Logical
And that is very logical when you consider that there is no other information going from a to b then data and synchronization data. There are packets of data going through the cable, supplemented with an ID token which indicates where the packet belongs. The receiver chip in the DAC creates spdif.There it can go wrong: not with the USB transfer.
Conclusion
Do not be fooled by expensive USB cables. It's a waste of money. We have believed, but are now convinced of the nonsense. with a thorough digital protocol it is simply impossible to ruin the data transfer. The cable is broken or simply does not meet the standard.
