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-   -   XMOS-based Asynchronous USB to I2S interface (https://www.diyaudio.com/forums/digital-line-level/188902-xmos-based-asynchronous-usb-i2s-interface.html)

barrows 31st October 2011 02:58 PM

Two issues...
 
in other words: it's complicated.

1. GMRs on the I2S lines add jitter, this is fact, not speculation. As I recall the amount of added jitter for the best ones is something like 30-90 pS. Of course this will depend on how one measures, as measuring jitter is not straight forward.

2. Noise, from the computer transmitted by the USB cable, and noise from the XMOS processor itself can increase jitter at the DAC-this is why people want galvanic isolation from the computer. How much jitter can this cause? I have not seen or heard of anyone measuring this, and the jitter caused will be entirely dependent on the computer set up, and how much noise the computer system allows to come over the USB bus, and how the USB lines are handled at their termination point on the USB interface (are they filtered?)

So, we see that adding isolation causes additional jitter of its own, but will also reduce jitter by reducing the computer borne noise. Then the answer to the question of whether to use isolation will come down to whehter the reduced jitter is higher than the added jitter.

Additionally-no isolation is perfect. Isolation schemes allow some, higher frequency (and computers have 400 mHz noise), noise products to go through. So the question of whether or not to use isolators is not simple. Some commercial DACs with async USB do not use isolation, as their designers feel the isolation does more harm than good. Right now I am using a non-isolated USB interface, with a relatively low noise laptop and am getting great sound-would the sound be better if it was isolated? I do not know. My understanding is that Lorien's board has both isolated and non isolated I2S outputs, so one could try both and listen test for differences.

rsdio 1st November 2011 02:05 AM

I also believe that it would be important to determine the source of the noise transmission from the computer. Is it really coming over the data lines only? It seems more likely to me that the significant noise is coming over the USB power and ground lines.

It should be possible to lay out the PCB such that the USB data lines completely avoid vias and connect from the USB jack directly to the processor. If no other traces are near the USB data lines, then noise from that source should be minimized.

Power supply noise (whether coming directly from the USB power and ground lines or by nature of the switching circuitry inside the USB function causing current draw on the filter board supply) should be controllable by changing the quality of the power supply itself and the local filtering.

In other words, you have to know exactly how the noise is getting through if you want to eradicate it without undue tradeoffs (such as increased jitter).

abraxalito 1st November 2011 02:30 AM

Noise on the USB data lines can be mitigated with a common-mode choke designed for such an application. Murata makes one.

I agree its much more likely that the USB power lines (ground especially) are responsible for noise.

counter culture 1st November 2011 02:34 AM

Quote:

Originally Posted by barrows (https://www.diyaudio.com/forums/digital-line-level/188902-xmos-based-asynchronous-usb-i2s-interface-post2764968.html#post2764968)
Noise, from the computer transmitted by the USB cable, and noise from the XMOS processor itself can increase jitter at the DAC-this is why people want galvanic isolation from the computer. How much jitter can this cause? I have not seen or heard of anyone measuring this

So how do you know it's there? The only way of telling that it's occurring is by comparative measurement.

abraxalito 1st November 2011 02:40 AM

I think people notice sound quality differences and assume they're caused by jitter. That's because they haven't yet caught on to the degradations introduced by common-mode noise. Probably its a bit of both that results in sub-standard sound.

counter culture 1st November 2011 02:44 AM

People are welcome to do whatever they like in private. This is a public forum.

If I can't depend on the rationality of that statement, how do I know I can depend on the accuracy of his other assertions?

abraxalito 1st November 2011 03:02 AM

You can't. This is DIYaudio, not Hydrogen audio - did you take a wrong turn somewhere?

barrows 1st November 2011 05:22 AM

counter culture...
 
Not sure what you mean here, please read closely, the following statement is carefully worded, and as such entirely rational:

"Noise, from the computer transmitted by the USB cable, and noise from the XMOS processor itself CAN increase jitter at the DAC-this is why people want galvanic isolation from the computer. How much jitter can this cause? I have not seen or heard of anyone measuring this..."

The word CAN does not equal DOES. I am pointing out that I have not heard of anyone measuring both this noise level, and then the jitter level at the DAC chip (or on the I2S out from the interface). My point is, that we cannot be sure what is going on until someone with some serious RF gear and knowledge publishes some measurements correlating the noise from the computer, with the jitter level at the output of the interface. I do know that RF phobic designer Charles Hansen of Ayre uses optocouplers for the I2S lines from his USB receiver to DAC board in the QB-9 USB DAC, despite the fact that the optocouplers themselves will add some jitter.

Some additional thoughts: computer grounds are very noisy, I have this on the good authority of accomplished engineers who have measured. The USB ground connects to the interface board, and the V+ is used by most USB interfaces, at least for handshaking. How much damage does the computer borne noise cause, that is a question I would like to see answered. In the mean time, I do try and reduce noise factors from the computer any reasonable way I can, and usually note subjective sound quality improvements with computer tweaks which reduce computer activity.

Turbon 1st November 2011 05:31 AM

If it's a 1 it should show itself as a 1 and if it's a null it should show itself as a null. Otherwise it's a bug...

Brgds

Turbon 1st November 2011 05:32 AM

Lorien! Are you busy?


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