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In Reply to: OK, I'll try again..What are they supposed to do?..........nt posted by Cut-Throat on September 7, 2014 at 14:13:48:
What it does is to provide electrical isolation. Among other things it will break ground loops, something that won't be of much significance if you don't have one. Another thing it will do is replace one source of power with a different source of power (hopefully cleaner power). It will also provide some common mode noise rejection and possibly some signal noise rejection. I don't believe any of these devices retime the bits in a USB frame and they certainly don't retime the start of the 1 msec ischronous USB frame rate.
I would expect that if USB noise were causing sonic problems then these devices might help. They would likely reduce the difference between clean USB ports on clean computer systems vs. noisy USB ports on noisy computer systems. I believe these may also be connected in a chain, in which case there may be additional levels of isolation possible. If a DAC came with good USB isolation then the effect of these devices might be minimal. They might also be minimal with a cheap DAC that has horrible sound, with or without the benefit of a clean USB signal. YMMV.
Because they don't affect the rate at which the USB frames are transmitted, these devices will not remove timing effects, i.e. if the computer sends frames at a different rate then they will arrive at the DAC at a different rate. If the DAC uses an async protocol then does not directly affect the timing of the DAC master clock. However, the processing of USB frames is fairly complex and all of the logic involved will create noise inside the DAC and the timing of this noise may affect the DAC clock and analog circuits if the DAC doesn't have its own isolation from the "evil computer like USB receiver" and the critical DAC circuitry.
OF course, one of these devices may make the sound worse, even while reducing the dependance of the upstream computer.
My personal belief is that these devices are a bandaid. The DAC should be built with a very low power USB receiver on a separate supply and then provide an I2S interface, clocked from the DAC to the receiver. This I2S interface is where the real isolation should be. The advantage of this is to keep all the noisy USB electronics away from the DAC proper. For this approach to work there has to be careful consideration of how power supplies and power wiring, ground planes, etc., are handled as well as how the clock architecture works.
With USB, unlike SPDIF, the signal path itself is potentially all digital, with no clock coming from the source. Unfortunately, with USB the signal path is much more complex than USB and this creates a lot of extra logic and complexity, all of which provide paths around any isolation coming from the signal path itself.
There are several DAC vendors who claim to have made DACs that are transparent to transports and some of them have customers (shills?) supporting their claims. Personally, I am skeptical. None have come up with any measurements to show how their products have more isolation than competitors and we know that bits aren't just bits. They have been supposed to have been just bits since the early 90's when two box transports first hit the market, but it has been known since these two box transports first hit the market they this was not the case. (As with my two box Proceed CD player from this vintage, where it was clearly evident that the coax interconnect sounded better than the ST fiber interconnect.)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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- RE: OK, I'll try again..What are they supposed to do?..........nt - Tony Lauck 09/7/1418:43:54 09/7/14 (0)
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