Computer Audio Asylum

Posted by audioengr (M) on February 23, 2008 at 11:06:03

In Reply to: RE: USB hub posted by pryamomimo on February 22, 2008 at 15:39:24

The fact that there is a crystal on this board may indicate that it is probably using a PLL internally to reclock. The datasheet is not much help. One thing that you could try is to replace the crystal with a low-jitter oscillator.

Steve N.

• RE: USB hub - pryamomimo 12:40:47 02/23/08 (0)
  In Reply to: RE: USB hub posted by audioengr on February 23, 2008 at 11:06:03

  Thank you for your comments. Another reason I have a feeling this chip might reclock is the way it simultaneously handles USB 1.1 and 2.0 "streams" ("split transaction", "transaction translation", etc.). Do not pretend to have an in-depth understanding of USB protocol, but my guess is that it might involve reclocking.
  
  So far I find an audible improvement when powering the hub from an external battery. Will have to listen more. I will be also trying to remove the D2 diode to disconnect for sure the downstream USB bus power from the PC (in this case the only connection to the PC power bus is via an RC circuit to the Vbusm pin - second from the borrom on the left side of the chip - used ONLY for monitoring whether there is a computer attached).
  
  Will also try other things to bring the picture to a resonable ideal - only the differential USB pair running to the USB chip, and power and ground lines isolated (or filtered) as much as possible.
  
  Then might try the oscillator route...
  
  

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