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jolon

Why digital seems to be affected by power and cables

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I2S isn't affected by jitter like SPDIF is.

 

 

I2S is most certainly affected by jitter.   However, because in I2S the data and the clock are carried in separate wires, then the practicalities are different.

 

In practise.   All DAC chips receive a signal which is I2S (or very similar).    For SPDIF, there is a part which translates the SPDIF into I2S, before it enters the DAC chip.

 

One very big issue for I2S and DIYers  ..... is a lack of standards to follow.      In SPDIF (or USB, or Ethernet, etc.), it is built to expect a specific impedance at each end of the cable.

 

In I2S, we don't know what terminating impedance is required at the end of the cable.   There is no standard.   We need to actually measure our specific system and work out what impedance terminates the cable properly.   Get it wrong, and the signla reflects up and and down the cable, and doesn't perform very well.    It's hard to make it so bad the data becomes corrupt, but a skewed signal makes the clock poor.

 

 

EDIT:  Sorry had this reply window open half the afternoon

Edited by davewantsmoore

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OK, "no bidirectional communication".... and all "the DAC can do, is mute the output", so do some DACs do this better than others? Or, is it some DACs don't do this at all?

 

Most do.   All this means is that if you input a digial signal which is nonsense.... instead of making a screeeeching noise .... The DAC goes to mute.

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Well yeah - do what I did and search for the term "jitter" and just read those bits.  Although there is quite a lot on the subject!

 

No insult was intended brother, honestly. I am going to read that interview with Malcolm Hawksford, it does look interesting (I skimmed it).

 

I know the head engineer at Naim very well (I recruited him from University 25 years ago into Wharfedale, when I was CTO there).  His signature design is the Naim Statement (google it), for which you need house mortgage deep pockets to buy.

 

Last time we talked was at the Bristol Audio show, and he was describing the internal reclocking in the DAC he had designed and had just launched.  Basically there were several very accurate, low jitter clocks, closely spaced in frequency, and a FIFO.  The input data had its clock roughly restored with a PLL, and that was used to clock the data into the FIFO.  Then the accurate clock was chosen which kept the stack pointer of the FIFO at about half full.  If it was emptying, a slower accurate clock was chosen, and if it was getting too full a faster accurate clock was chosen.  The switching between accurate clocks was inaudible - it was the ultra-low jitter of these that was the important thing. I forget what the type number for the product was, but most reviewers did not really want to send the review sample back.

 

During my research for a better power supply for the Pi, I had came across Naim Audio... I scoured over their website looking at not only power supplies but also DACs and read as much as I could. I will do a search for "low jitter clocks" for the Raspberry Pi later.

 

What is a PLL?

 

The bold stuff above looks interesting....

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Phase locked loop

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Yes other dacs do it better, ess saber have a time domain jitter patent implementation that rebuilds lost samples by over sampling and rebuilding in an intelligent way.

 

For the purpose of this discussion, what ESS do is essentially "reclocking".

 

All incoming sample rates are converted to a new (very fast) rate.   The new timing is provided by a clock in the DAC.    This allows the DAC to (try to) decouple its performance from the input data.

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"(ii) it needs galvanic isolation of both clock and data and..."

Like the DIGI hat has on it's coax line, galvanic isolation.

"...ess saber have a time domain jitter patent implementation that rebuilds lost samples by over sampling and rebuilding in an intelligent way."

I thought ESS Saber made a DAC for the Pi, have to check on that.

What about this, USB from the Pi to an external DAC, then off to my AVR for amplification. This would depend on the brand and model of DAC, and the DAC's communication with the AVR, yes? What about USB cables, are they an issue, same or otherwise?

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ESS Saber is a DAC chip...

 

"New version of our ES9023 I2S DAC specially made to fit the Rasperry Pi model B. LT1763 ultra low noise regulator and on board asynchronous master clock."
http://hifimediy.com/i2s-raspberry-pi-dac

 

"Notice that the DAC chips used are the ones that can cope without a Master Clock. RPi I2S does not Master Clock, so the DACs synch on bitclock and generate their own master clock."
https://hifiduino.wordpress.com/2014/11/12/raspberry-pi-version-b/

 

This sounds ominous.... "RPi I2S does not Master Clock".

Edited by rastus

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No insult was intended brother, honestly.

 

 

Absolutely none taken!  

 

It is a very interesting interview, although long.  Prof Hawkesford is an elder statesman of audio, at Essex University, and jitter is one of his long-term research interests.  One of his research students, Richard Bews, has run his own company called LFD for many years http://www.lfd-audio.co.uk/

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Why on earth is there a university in Essex?

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I take back what I said a few pages ago, that I disliked how this thread was going. I am very, very thankful to all the kind people that contributed with their extremely informative answers to my many, many questions. And thank you for your patience when I didn't 'get it', and further answers when I did 'get it' and asked even more questions. I think the subject, jitter, needed me to 'engage' by asking those questions, rather than just reading. I had already read so much about jitter that made no sense. Now, I think I have 'an idea' of what it is, causes, and solutions, thanks to you guys.

 

Oh and by the way, I do have more questions... owning a Raspberry Pi I would, hey. But that's for another day.... maybe tomorrow. :emot-bang::party:sorry::na::rolleyes:

Edited by rastus

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