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In Reply to: Re: How do these modded USB Dacs really work? posted by ThomasPf on October 27, 2004 at 17:45:27:
Thomas,From what I have read AUDIOENGR uses the TI PCM2707, this is a DAC with SPDIF output. He probably made up a small board with this on it and the supporting circuitry and links the TTL level SPDIF signal directly into the SPDIF reciever at the same level.
The clock into the PCM2707 is not as critical because it is going into a quality clock generator for all the internal stuff (sigma-delta output filters, SPDIF clocking, internal USB, etc). Therefore there is no need to have a secondary PLL for this. The SPDIF is already to go out of the PCM2707.
My dac does use the ASYNC mode and I2S out to the dac is rock on and some of the best sqaure wave preformance I have seen from a digital receiver.
Follow Ups:
I though audioengr typically modded M-Audio Transits which support 24/96 which (unless I'm misreading the data sheet or M-Audio is fibbing) the 2707 doesn't support. Does something like the transit use a custom/discrete approach?
The Transit uses a TI tas1020a and a CODEC. And yes, I use the Transit board to mod because it passes 24/96.
Thanks GordonNow we are getting closer to what I was hoping for. So in your case you do actually only use the local clock to drive the flow of data from the PC? Do you use the standard system supplied USB audio driver or do you ship with your own?
After quickly scanning the PCM2707 manual it looks like this solution is using isochronous mode and extracts the clock from this signal with the changes moderated by an analog PLL. Then it outputs a synchronous S/PDIF signal which then will have to be extracted and reclocked again before handed to the actual DAC chip. What exactly is the purpose of this approach? This is not liekly to reduce jitter, is it?
Cheers
Thomas
Thomas,You can use the PCM in several ways though kinda of hidden in the data sheet. It can be used ASYNC as well as isosynchronous.
In ISO mode the clock is pretty stable as the USB controllers on the computer side have a large enough buffer as to not conflict with the speed. Therefore the SPDIF could go directly to a SPDIF receiver and out in the format needed for the particular dac chip required. No other clock needs to be applied.
Jitter in this regard is reduced because there is no relationship to the dive mechanism and position of the head which is more of an issue with jitter that I have tested. This is what led me to do a USB dac. When I was working on a secondary PLL for a crystal receiver I was shocked at how much the clock changed in relationship to the position of the head. So too the amount of jitter. That and crappy MCLK's in even expensive transports.
Therefore USB to SPDIF will result in lower Jitter than say transport to DAC directly.
This is also true (at least with iTunes) that when a CD is played it is buffered to the tune of a few buffer lengths of the USB buffer required by the controller.
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