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RE: USB isolation hub (Gordon, audioengr, John - input wanted)

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In Reply to: RE: USB isolation hub (Gordon, audioengr, John - input wanted) posted by pryamomimo on February 22, 2008 at 07:59:59

Very interesting post, I've actually been thinking along these lines for awhile.
First off, inductor in the GROUND line is very bad, almost certainly going to make things worse.
A few general thoughts on ground loops:
first notice the word LOOP, in order to have a ground loop you must have a loop, a single ground wire does not a loop make, in order to have a ground loop you have to have at least two ground paths. So if the ONLY electrical connection between the laptop and the DAC is the USB cable, you cannot have a ground loop and you are fine. So if your laptop is running off batteries and there is no other connection to anything else, just the USB cable you can't have a ground loop.
The problem occurs if you have the laptop plugged into the wall, and the ground of the USB cable is connected to either the neutral or the safety ground, then presto, ground loop. Laptops actually have an advantage here because a lot of the power supplies do not connect to the safety ground and are isolated from the neutral. If yours has a three pin plug then things get a bit more complicated. Use the meter and measure the resistance from the ground on the USB port to the ground pin on the supply. If the resistance is high you are fine, if its low then you are going to have a ground loop through the power line.
For long distances and or ground isolation I like the optics cable, its not cheap but really does work well, BUT as Gordon said you need to get rid of the supply that comes with it and make your own high quality supply. Its not hard, the thing barely uses any current so a big honking supply is NOT needed.
On the reclocking and hubs, this is an interesting subject that I have been looking into recently. It turns out that all hubs that I could find DO reclock, but they use a PLL to generate the clock. They use either a 6 or 12MHz crystal and a PLL based frequency multiplier on chip to generate a 48MHz signal used to read the input and clock the output. Since the power supplies are rerely done to "audiophile" standards, and they usually get driven off VBUS anyway, the feed to these PLLS is usually noisy as all get out so the clock jignal has very bad jitter.
I searched and searched and found a hub chip that allows you to bypass the on chip PLL and feed in an external 48MHz clock. So it looks like its possible to build a decent reclocking hub using a really good PS and a very low jitter 48MHz clock. With good board layout it should be possible to make a hub whoose output jitter is at least an order of magnitude less than any other USB connection. The only problem is I forgot to write down the chip number! (you'd think I'd remember my memory is now the proverbial steel sieve) If I can ever find this again I'll post it.
John S.
I found the chip! Its the TI TUSB2046B, digikey has them.

Edits: 02/22/08

Follow Ups:

But John that is USB jitter which we do not care about - Gordon Rankin 12:27:08 02/24/08 (2)
John,

Jitter as it applies to audio is the clock associated with the data. Since the 48MHZ clock is merly the USB clock and the relaying of data it has nothing to do with the quality of audio since there is no real "clock" associated with the audio data.

In adaptive mode the dac will still be goverened by the computer not the hub.

But a well designed hub could isolate the power from the PC which might be a good thing.

Thanks
Gordon
J. Gordon Rankin
- RE: But John that is USB jitter which we do not care about - John Swenson 12:05:10 02/25/08 (1)
  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)
    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
USB hub - pryamomimo 15:39:24 02/22/08 (2)
I thought it might be curious to see this hub design (prior to my modification). It seems at least some sort of consideration went into designing the power supply. If you read the NEC 720114 datasheet, it appears that Vbus can be used just for "handshake", and the rest of the chip is powered by the +5 V supply through the EH11A regulator. In my case, it runs off a rechargeable battery. I might eventually remove D2 for the extra assurance. Thus, I did not see it relevant to install the inductor on the Vbus line. The way I have it now is in hope it alleviates some hi-frequency currents between my hi-fi and my laptop via USB ground. By the way, ALL of my power supplies are TWO pin, for better or for worse.
- RE: USB hub - audioengr 11:06:03 02/23/08 (1)
  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)
    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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