Computer Audio Asylum

Posted by John Swenson (A) on February 25, 2008 at 12:05:10

In Reply to: RE: But John that is USB jitter which we do not care about posted by Gordon Rankin on February 24, 2008 at 12:27:08

Hi Gordon,
I did a lot of experimenting and testing the recovered mclk with a 2706 and found that the jitter on the bus itself has a significant effect on the mclk jitter. Since the hub chips do retransmit the data based on the 48MHz clock, lowering jitter will improve things at leat for a 270x receiver. I have not done these tests with a 1020 so I'm not sure how it will respond to this.

John S.

• RE: But John that is USB jitter which we do not care about - Gordon Rankin 08:16:59 02/26/08 (0)
  In Reply to: RE: But John that is USB jitter which we do not care about posted by John Swenson on February 25, 2008 at 12:05:10

  John,
  
  Basically the way the PCM270x works is that it links the Spact to the SOF frame. Now since the SOF frame has no acknowlegments to it. There is also a VSOF (virtual) timer in the PCM part in case the real SOF frame is missed. The problem is the VSOF timer will always fire after the expected SOF. Which actually throws off the Spact unit, because now the expected SOF time differential is lost.
  
  I have seen on allot of ocasions especially with my crappy HUSH pc that SOF frames are the first to corrupt for some reason and maybe this will lead to what you are saying.
  
  Or maybe this has nothing to do with Jitter on the USB link at all.
  
  I did some really in depth testing with ASYNC mode running 1KHz sine wave from the MAC (Faber Acoustics Signal Suite 2) to the emulator running on the TAS1020 board. In a 24 hour period running at 44.1/16 I did not recieve any packet errors.
  
  This of course would have the same effect on any Adaptive part not only the PCM270x series.
  
  Only packet errors would effect ASYNC mode.
  
  Thanks
  Gordon
  J. Gordon Rankin
  
  

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