USB isochronous mode (async is one of the variations of isochronous mode) does NOT have error correction or re-transmission, it does have error detection, but you can't fix the data with that, just know it happened.

Isochronous means constant time, it was designed for streaming applications where it was presumed that there was no time for re-transmissions, thus there was no provision for it in the standard.

Note that disk drives etc do NOT use isochronous mode, they use a different mode which DOES allow for re-transmission etc

Most modern USB implementations in DACs use high speed mode which runs at 480 mega-bits per second. A cable which is only good to 0.5GHz is almost useless with that, it would only allow a sine wave at 480MHz, the variations in timing which define the data would be almost completely gone, you need much higher bandwidth to be able to get any useful data out of it at all.

I agree with Gordon, there are major impedance issues all over the place in USB implementation (BTW it's no better in FW), there are quite a few motherboards that don't come close to proper impedance matching from chip to connector, a LOT of DACs that don't get it right from connector to chip and of course cables that don't get it right. This gives rise to a major mish mash of reflections on the bus. That is what those input receivers have to try and deal with and why they have to work so hard at it.

The remarks from Gordon seemed to say that he thinks there are massive amounts of actual data errors happening all the time, I disagree with this, I certainly don't see this with the DACs that I build.

The issue of the async back channel is an interesting one. THIS is where I do see things getting very interesting. There are computer/software combinations that do have a hard time dealing with the speed change packets sent from the DAC to the host, if these happen often enough it can cause the infamous "clicks and pops" that sometime seem to plague some computer/DAC combinations. Sometimes changing cables CAN help these packets get properly dealt with.

JohnSwenson.