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Re: I don't think any USB audio devices have a master clock

Posted by BFitz (A) on September 7, 2005 at 19:53:29

In Reply to: I don't think any USB audio devices have a master clock posted by Monkey Bones on September 7, 2005 at 17:46:36:

My assumption was based on reading about a few high end designs like the Wavelength here
http://www.wavelengthaudio.com/usbdac.html
From that site

"USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away. "

It was also based on my conversations with local designers of PCI based audio systems. In general the PCI based stuff has two crystals, one to support 44.1, the other for 48kHz. I know the SB2 also has this, as I read the jitter tests done by the company CEO, where he removed one crystal and the jitter dropped slightly. I can't imagine where the USB card would get the time base from without a local crystal oscillator. It seems to me to be similar to PCI based sounds cards, just using USB instead of PCI for data transfer. All my PCI cards have crystals onboard.

The definition of Isochronous from here
http://www.webopedia.com/TERM/I/isochronous.html is
"Time-dependent. Pronounced eye-sock-ra-nuss, it refers to processes where data must be delivered within certain time constraints...
Isochronous can be contrasted with asynchronous, which refers to processes in which data streams can be broken by random intervals, and synchronous processes, in which data streams can be delivered only at specific intervals. Isochronous service is not as rigid as synchronous service, but not as lenient as asynchronous service."

If I understand this, it says not as rigid as syncronous, which I assume S/PDIF to be. I can't imagine you could transmit an embedded clock with a non-syncronous system, which you would have to do if there is no local clock, right? Not really sure how you could do it otherwise.

Bob

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