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In Reply to: RE: Cable effects same regardless of USB mode posted by John Swenson on November 30, 2009 at 13:18:47
Do I understand correctly:
In principle there is no difference between adaptive and asynchronous mode other than in the first case the sender does the timing, and in the second case it is the receiver doing the timing.
If the quality of the clock is the same in both cases, the result will be the same.
The advantage of asynchronous is that one can improve on the sound quality by using a better clock.
So asynchronous mode is not better by design but by implementation because you can implement a top quality (low jitter) clock in the DAC.
The Well Tempered Computer
Follow Ups:
Essentially yes. Both cases have two clocks in the DAC, the "USB clock" which runs at the USB bus frequency and gets data off the bus, and the "audio clock" which pulls audio data out of the buffer. In adaptive mode the audio clock is adjustable so it can be set to the average of the audio data rate. In async mode the audio clock is fixed and the computer adjusts the rate at which it sends the data so the buffer does not over/under flow.As far as jitter is concerned the difference is in how well you can do between a fixed and an adjustable clock. Most adaptive implementations do not use a particularly good implementation. It is possible to implement an adjustable clock that is better than what is in most adaptive USB chips, and this has been undertaken by a couple companies. But you can always get lower jitter with a good fixed frequency clock.
There is actually a good example of this case of its the implementation of the clock thats important, not the asyncness itself that is important. The recent inexpensive Musiland devices use an asynchronous protocol but then use a frequency synthesizer to generate the local clock rather than use a fixed frequency oscillator. The result is jitter that is actually worse than some of the better adaptive implementations!
John S.
Edits: 12/02/09
Nice example
Thanks for the clarification
The Well Tempered Computer
But isn't it true (all else being equal) that logically the best async implementation will perform better than the best adaptive implementation, given that the async doesn't have to deal with the large amount of jitter from the USB frame that the adaptive one does?
Additionally, in terms of absolute clock frequency, adaptive can only be as good as the PC, whilst in async it can be as good as the DAC manufacturer wishes it to be?
your friendly neighbourhood idiot
Yes, the best async implementation will be better than the best adaptive.
The adaptive doesn't have to be as bad as you mention, the local clock does not have to be slaved directly to the frame. It can have a local adjustable clock and a controller that just looks at how full the buffer is and adjusts the local clock so the buffer stays partially filled. If the buffer is many frames long there should not be a direct relationship with jitter on the frame.
BUT as was the topic of my original post in this thread, such jitter CAN wind its way into affecting the jitter on the local clock in both adaptive and async modes.
And yes, the absolute accuracy of the sample clock is determined by the computer in adaptive mode since the local clock has to match the average data rate otherwise the buffer over/underflows.
John S.
Hi John,
thanks for your reply, it pretty much confirms my opinions - you can mess up no matter how clever the original scheme is if you don't pay attention to all the details!
your friendly neighbourhood idiot
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