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In Reply to: Re: Asynchronous adaptive USB Audio device posted by ThomasPf on March 23, 2005 at 08:53:34:
I found some details on Tom's Hardware, it uses the Philips ISP1581 general purpose USB2.0 interface chip. This talks to Creatives own custom chip, which talks to some effects processing chips and a sigmatel multi-channel DAC/ADC chip via I2S. I only see one crystal on the board, I wasn't sure which chip its directly connected to or what the frequency was.What would be interesting to know is the sample rate to the DACs. If they are running aynchronous or adaptive mode then its going to be sync'd to the main crystal frequency, which means its tough to do 44.1 and 48.
If there is a fixed phase relationship between the I2S wordclock and the crystal then we can easily use it, send in a very low jitter clock instead of the main crystal an reclock the I2S signal with same clock before sending to our own DAC.
The Philips chip is a general purpose USB chip, not specifically an audio chip, its designed to be used for things like disk drives etc, so it makes sense that it uses asynchronous mode. The USB chip talks to the Creative chip via a standard microprocessor bus, my guess is there is an embedded processor and memory on this chip that grabs the data off the USB chip when it needs it and sets up its own special hardware to clock out the data to the DACs. Its not obvious what the chip is using as a clock to clock the data out. My GUESS is that its using a fixed frequency to talk to the DACs and that is determined by the main crystal. This would mean there would have to be a sample rate conversion either in the chip or in software to support that. If true this means we could take advantage of the asynchronous mode and get a very low jitter I2S signal, BUT would have to live with sample rate conversion.
John S.
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Follow Ups
- Audigy 2 NX details - John Swenson 01:19:05 03/25/05 (1)
- Re: Audigy 2 NX details - dwk 10:28:53 03/25/05 (0)
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