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Music servers and other computer based digital audio technologies.

RE: An insolent provocation .... if a usb dac is very well design (and built) the pc quality should not matter

Don't even consider isolating the USB signal. Were you to do so, the packet processing overhead on the DAC side of the isolator would still nail you as John Swensen discovered. Treat the entire USB section as "dirty". Do the isolation at much lower data rates, e.g. I2S rates. Send the clock from the DAC through its isolator to the USB converter section, synchronize it with the packet buffer, and send back the audio data in the DAC clock domain through a separate isolator. You can easily reclock (flip flop or shift register) in the DAC clock domain. Repeat the isolate and buffer mantra as necessary...

A single stage flip-flop will eliminate some noise (phase noise and amplitude noise) but not all because signals will leak around open switches and closed switches because the flip flops aren't completely "hard". However, multiple stages can be ganged in a shift register given appropriate clock phasing to get more isolation. The limit will be power rail noise shared by the stages of the shift register. Hence, the possible need to use a second stage of isolator, and reclocking to remove its noise.

Note that multiple stages will not produce a perfectly clean digital signal. There will still be noise. However, multiple stages of isolation will provide as much decorrelation as desired from the original signal. The problem of getting good analog to digital conversion will remain. :-)


Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar


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