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In Reply to: Why USB? posted by ketchup on October 18, 2004 at 16:39:51:
"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.What happens is... On power up of the computer the 2 devices negotiate services. In this case the Cosecant [or any other USB DAC - detail added] tells the computer it can do 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the separate DAC IC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the dac has to sync to that frequency). Therefore the jitter problems of SPDIF almost go away using USB. So using USB we have a zero error protocol to link the computer to the DAC and very low jitter what else"I was going to try and paraphrase this myself, but Gordon has already done a good job, so credit where credit is due. One thing I would add is that USB2.0 seems to sound nicer than 1.0, and I don't know why. There is enough bandwidth on 1.0 to cope with digital audio (CD data) so why the extra bandwidth should make a difference is a mystery. In fact, I'm not even sure its the badnwidth that does make the differece - perhaps there is something else different about the 2.0 protocol that is having a positive effect?
I'm currently playing with a TwinDACPlus! and am enjoying very much. *Much* more than I would have expected, even for a NONOS DAC, and much more than the CD7 or the SCD-1 that I used to own.
Big J.
Follow Ups:
J.Thanks for the nice comments... I don't know how the Twindacplus works but there are 3 USB Audio 1.1 formats (well six but most of those include eq and other stuff like vol control). Only one of them has error correction. The others work similar to SPDIF in that the output is time corrolated.
It maybe that the reason the 2.0 is better is because it has much better throughput do to buffering and this maybe why it sounds better on your system.
I use type 3 Audio format with error correction and have not found much difference in using an older iBook 500 with 1.1 controllers and the 2.0 controller on my newer iBook using the Cosecant.
Gordon,how do you specify which type of protocol you want to choose on the USB 1.1?
Just happy to have access to information like this, which seems well ahead of the curve.Tell me, weren't you in communication with Apple about the direction and feature-list of their new digital wares? Any progress?
Lastly, do you know of any decent USB cables or is this too 'old hat' in the PC age? Are they all pre-formed and moulded?
Big J.
J.Apple has their hands full right now and none of my ideas are getting past DTS. So all that is a stall right now...
I send good quality 2.0 cables with my stuff. It really should not matter as much because the specification is soo much better than SPDIF.
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