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RE: I have to Agree With Fmak and Carcass

Posted by Tony Lauck (A) on July 10, 2012 at 11:04:15

In Reply to: RE: I have to Agree With Fmak and Carcass posted by Dynobot on July 10, 2012 at 09:33:45:

Have you verified that both O/S configurations are bit perfect? If so, how did you do this? Note that 32 and 64 bit operating systems require different instructions and this means that after compiling and loading there may be different results. It is even conceivable that there are CPU bugs affecting one mode of addressing. (Unlikely, but if so it wouldn't be the first time that Intel had a bug in CPU design.) The phrase is "trust but verify" but when it comes to computer audio my experience is that should forget about trust and go straight to verification unless one personally knows and trusts the design engineers.

Incidentally, it's not clear to me how to verify that USB output is bit perfect. One way would be to use an external USB to SPDIF adapter and record the SPDIF output by feeding it into another computer. However, even this won't be perfect, because the USB "enumeration" process sends configuration from the USB device back to the operating system and this will cause different code paths to execute in the computer, which may or may not matter. (It won't matter if the code is bug free, but then is there much bug free code?) If one gets similar sonic results when comparing the two OS through a USB to SPDIF converter to your DAC and if this converter is powered separately from the computer (e.g. has a source of power other than the USB) then this may be a useful way to see if you are getting the same bits from the two operating systems. The advantage of looking at the USB output stream is that you can easily verify that this stream is bit perfect by recording it on another computer and comparing files. If the results only appear with USB, then getting to the bottom of the problem is likely to be very difficult due to the complexity of USB. If I were designing a USB DAC I would certainly do this, but it would require specialized equipment that creates and records USB protocol. I assume this equipment exists, but (fortunately) I haven't found the need to use it. Some "USB protocol analyzers" showed up that were priced under $1000 in a Google search, but I don't know if these are any good.

I looked at the Wadia web site to see how their USB input works. I didn't get a good feeling for what I read. It looks like copy written by the marketing department. (When this happens, usually it means that the product feature is missing and the marketing department doesn't really want to ask the engineering department how the product works, lest they then be guilty of writing misleading copy. I've been on both sides of the marketing-engineering fence, so I know how this game is played.) I would not be surprised if the Wadia 121 USB implementation is not really asynchronous. Perhaps someone has looked carefully at this product and done some degree of reverse engineering and can comment more about this.

Small differences in sound will be much harder to track down as they won't show up reliably in listening tests, creating a constant source of noise in experimental results that make troubleshooting difficult. (Anyone who has diagnosed a broken electronic component will appreciate how much difficult it is to diagnose an intermittent device.) In your case, you've said the differences are large, so isolation may be (relatively) easy, particularly if you have a variety of equipment you can mix and match for listening tests.


Tony Lauck

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

• http://wadia.com/technology/technicalpapers/WadiaStream_USB.   (Open in New Window)

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