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Open-source USB interface: Audio Widget
Open-source USB interface: Audio Widget
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Old 23rd August 2011, 06:15 AM   #161
alexlee188 is offline alexlee188  Canada
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Will be interesting to compare SQ between Salas shunt and LiFePO4 :-)

Alex
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Old 23rd August 2011, 06:40 AM   #162
soundcheck is offline soundcheck  Germany
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Hi guys.

Did you have a look at the "Asynchronous I2S Fifo" thread over at the Digital Line Level forum section?
Lot's of interesting and related info. He claims that different inputs (he's using a DIR9001 as SPDIF receiver afaik) wouldn't
make a difference with his solution anymore.

I'm wondering if something like that Fifo functionality is already or could be introduced to your
design!?!? One wouldn't need a seperate FPGA board if the receiver chip would be able
manage it all. However...

...on the other hand a 2-stage approach "USB-receiver + seperated I2S Fifo" would allow for isolators in between.
The intrisic jitter of those isolators wouldn't really matter anymore, because the FIFO would be able to clean up this jitter.

The guy uses batteries to power his device. USB receiver and FIFO supplies could be kept 100% seperate using that approach.
You might be able to use bus-power for the receiver in this case and batteries for the FIFO and clock..

All that sounds really promising to me. You might get in touch with him -- at least on certain parts.
I do have a feeling that a combination of both approaches would be the potentially best solution
around and would solve a lot of issues.


Cheers

Last edited by soundcheck; 23rd August 2011 at 07:09 AM.
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Old 23rd August 2011, 10:29 AM   #163
UnixMan is offline UnixMan  Italy
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Quote:
Originally Posted by soundcheck View Post
To me above thinking incorporates a big mistake.

Jitter sources must be avoided as far upstream as possible.
this is just plain nonsense. Perhaps you have not clearly understood how digital audio systems works and what can be their pitfalls. Let me try to explain.

Except perhaps at huge, pathological levels, jitter does NOT affect digital data transmission integrity (and this is not what we're talking about). Even with a link as bad as s/pdif and a jittery source, if the source sends "1234" the receiver still gets "1234".

The only places where jitter does matter are those places where data samples AND timing information are somehow combined together, either to calculate new data sample values (ASRC) or to recreate an analog signal (DAC).

In many (most) existing solutions the master clock is either at the data stream source or somewhere in the middle of the stream path.

Examples could be the typical CD transport + DAC (master clock at the source) and USB interfaces such as the hiFace (where the master clock is in the interface itself, that is "in the middle").

Of course, with this arrangement the DAC clock have to be somehow slaved to that master clock.

This is inherently, conceptually wrong.

In such situations, what you say can be true: the jitter of the master clock is propagated down the line and combined with that of the slave clock(s) (e.g. by PLLs) and/or embedded into the new data samples by ASRCs.

The resulting jitter on the clock used by the DAC and/or errors in the data introduced by jittery input to ASRC will eventually affect the reconstructed analog signal.

Audio Widget is different. It is designed from the ground up in the right way: the master clock is right on the DAC side and everything else is slaved to it.

It is NOT the I2S clock (and its jitter) that rules the DAC, but the other way around: it's the DAC clock that "drives" the I2S. Jitter on the I2S bus does not affect the timing information used for d/a conversion. That is, jitter on I2S does not (can not) affect the d/a conversion in any way.

That's why, as BÝrge suggested, you may safely add optocouplers (which are inherently jittery...) in the middle of the I2S bus. In most other systems, doing so would likely be a solution worse than the problem they'd called to solve. But NOT on the AW.

Here only the local DAC clock provides the timing information for d/a conversion. Thus it's only the local clock jitter that matters.

The only remaining pitfalls can be EM noise and any other unwanted interaction between different part of the circuit which may somehow affect the local DAC clock oscillator and/or the DAC itself.

That's why layout, good and independent local power supplies, maximum possible isolation of the sensible parts (DAC and its clock) from the "dirty" side (host computer, USB, MCU), etc, are paramount.

And that's why the optocouplers idea is IMHO a very good one.

If properly implemented, completely isolating the sensible parts from all the rest should solve the remaining problems and thus may make a huge difference.

P.S.: now that I think about it, I have a question. AFAIK the Sabre input uses an ASRC. Is there an internal FIFO to buffer and reclock the I2S according to the local clock before passing the stream to the internal ASRC? otherwise, an external FIFO reclocker must be added on the I2S bus! (even if not using optocouplers!)
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Last edited by UnixMan; 23rd August 2011 at 10:39 AM.
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Old 23rd August 2011, 10:42 AM   #164
cviller is offline cviller  Canada
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Quote:
Originally Posted by UnixMan View Post
P.S.: now that I think about it, I have a question. AFAIK the Sabre input uses an ASRC. Is there an internal FIFO to buffer and reclock the I2S according to the local clock before passing the stream to the internal ASRC? otherwise, an external FIFO reclocker must be added on the I2S bus! (even if not using optocouplers!)
Isn't that only required if the ASRC derives its incoming clock from data? If master clock is used, then the ASRC interpolation will be based on master clock and thus "reclocked" correctly.
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Old 23rd August 2011, 10:43 AM   #165
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by UnixMan View Post
That's why layout, good and independent local power supplies, maximum possible isolation of the sensible parts (DAC and its clock) from the "dirty" side (host computer, USB, MCU), etc, are paramount.
Yep.

Quote:
If properly implemented, completely isolating the sensible parts from all the rest should solve the remaining problems and thus may make a huge difference.
Complete isolation? So that no signals get through at all? Surely you don't mean that - rather you mean maximum possible isolation commensurate with still functioning properly. In which case agreed - but that's what people have been working towards for many years. The isolation factor is paramount and its never going to be complete, hence the ongoing discussion.
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Old 23rd August 2011, 11:04 AM   #166
borges is offline borges  Norway
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Complete electrical isolation is worth trying out.

Anybody who wants to try out this (or other analog consepts) is very welcome to do so. Tweak the AB-1.1 with all you've got and tell us about the result.

Cheers,
BÝrge

Quote:
Originally Posted by abraxalito View Post
Yep.
Complete isolation? So that no signals get through at all? Surely you don't mean that - rather you mean maximum possible isolation commensurate with still functioning properly. In which case agreed - but that's what people have been working towards for many years. The isolation factor is paramount and its never going to be complete, hence the ongoing discussion.
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Old 23rd August 2011, 11:54 AM   #167
UnixMan is offline UnixMan  Italy
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Quote:
Originally Posted by soundcheck View Post
Did you have a look at the "Asynchronous I2S Fifo" thread over at the Digital Line Level forum section?
nice one.

Quote:
Originally Posted by soundcheck View Post
...on the other hand a 2-stage approach "USB-receiver + seperated I2S Fifo" would allow for isolators in between.
no need for such complication: AW is already 100% asyncronous. Optoisolation can be safely added in the middle of the I2S bus. All that may be needed (if it's not already included in the Sabre input) is a simple FIFO buffer (actually just a latch).
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Old 23rd August 2011, 12:20 PM   #168
UnixMan is offline UnixMan  Italy
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Quote:
Originally Posted by cviller View Post
Isn't that only required if the ASRC derives its incoming clock from data? If master clock is used, then the ASRC interpolation will be based on master clock and thus "reclocked" correctly.
not sure about that, but I'm no ASRC expert at all. If someone has better cue and explanation, please tell us!

AFAIK, ASRCs makes no errors only if both the incoming and the local ("output") clocks are synchronous with each other and jitter free (that is, when they are basically useless... ).

Indeed, given that in AW the I2S signal is synchronous with the local clock and thus the ASRC can be (it is) clocked from the clean local clock, perhaps jitter on the I2S (data) lines does not matter.

Hints, anyone?
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Old 23rd August 2011, 12:31 PM   #169
borges is offline borges  Norway
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Hi Paolo,

I believe jitter on the I2S lines don't matter. Period. As long as digital signal integrity is OK (i.e. signals latched on correct clock edges), the FIFO / SIPO / other digital stuff in the DAC should be doing just fine. In fact it may even be good with a bit of jitter to spread the spectrum.

Where jitter really does matter is when the DAC moves its internal digital signal onto a switched current generator or R2R ladder. This is the first time the signal becomes analog, and this is a process which must happen with the smallest possible time-domain variations. (And of course, the current generators etc. must be precise in the current or voltage domain.)



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Old 23rd August 2011, 12:44 PM   #170
UnixMan is offline UnixMan  Italy
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Quote:
Originally Posted by abraxalito View Post
Complete isolation? So that no signals get through at all?


of course, I mean complete electrical isolation. That's what an optoisolator would (almost perfectly) do.

A relatively long optical link using optical fiber would be even better (it would completely avoid any parastitic coupling...), but it would be complex, expensive and not very practical to use on the many I2S lines.

A great thing would be to use an optical USB3 link, if it will ever be available (perhaps in addition to the optoisolators on the I2S bus to keep also most of the locally generated noise out of the way).
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Last edited by UnixMan; 23rd August 2011 at 12:54 PM.
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