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

Posted by Monkey Bones on September 7, 2005 at 20:35:46

In Reply to: Re: I don't think any USB audio devices have a master clock posted by BFitz on September 7, 2005 at 19:53:29:

The reason that it's termed isochronous is that no clock is included, but the devices do need to establish a synchronous connection for the data to be transferred. But how that is accomplished can involve either synchronous, asynchronous or adaptive modes depending on whether the source and sink clocks on each end are locked to the USB clock or free running. The data is still sent out in timed packets that the receiver must sync up to, with a known number of samples per frame. Sure there is a local 12M oscillator for the USB receiver end, but the actual sync clocks are PLL derived, and without careful design there can be large amounts of jitter. You should read up on the design process for the PCM2702 - some very interesting setbacks along the way in the PLL design phase and it gives you a pretty good understanding of how the USB transfers work for audio data and what the SpAct two stage PLl is all about. If nothing else there are some good block diagrams. You're on your own with Gordon and the Wavelength info, but it doesn't sound quite right from my understanding of the bus. There is a really good book on USB by Don Anderson and Dave Dzatko if you are interested called simply Universal Serial Bus System Architecture that covers all the protocols and what is possible. From MindShare, Inc. Written in very easy to understand language with lots of pictures, both big plusses for me :-)

But all I was getting at is that it seems like there are a lot of misconceptions about how USB works, and what it can and can't do. It is definitely a synchronous system when transferring audio data, and the clock at the DAC end that is used for D/A conversion will be derived from the sample rate of the source via the timing of the data packets. It's just not a direct sync to the clock like in SPDIF.

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