Can someone please explain how it is possible for a digital CABLE to induce jitter?

Discussion in 'Cables & Switches' started by NicolasB, May 24, 2003.

  1. NicolasB

    NicolasB
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    This is a point which which has always troubled me.

    It's fairly well established, now, that even a rather poor digital interconnect won't be poor enough to introduce any actual bit errors. (There was the famous case of someone getting no bit errors at all while using a wire coat hanger as a digital interconnect).

    I think we've also managed to establish that, for movie soundtracks (DD, DTS, etc.) jitter is completely irrelevant if it's higher up the chain than the processor, because the data is compressed and therefore must, by definition, be buffered and reclocked before it hits the processor's DACs. (So jitter in the player or interconnect can't have any influence on the eventual sound).

    But jitter certainly can have a big impact on the performance of a processor playing back a PCM stream, e.g. when a DVD player is acting as a CD transport and the processor's DACs are doing the conversion. This fact is often quoted as the reason why a more expensive digital interconnect can have an influence on sound quality.

    But I have yet to come across any explanation as to how a cable can introduce jitter. How is it possible for one pulse to pass through it at a different speed from the next one?
     
  2. hornydragon

    hornydragon
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    This has troubled me too. How ever as an undergrad electronics engineer it seems to be a mix of potential problems.
    PCM (Pulse code modulation) on a cable can result it the cable impeadance changing with signal and the use of less than perfect 75ohm terminations can cause signal reflection at the interface. this i beleive is where the jitter comes from the interferance of the reflected signal on the following signal. I think it is all buried deep in the modulation. as un modulated signal are very much easier to transmikt but also more expensive. I have no idea if PCM has any real advantage for Music replay or whether it was just the "best" system available at the development of the media. Hope this helps nicholas.
     
  3. LV426

    LV426
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    I suspect that the cause of this phenomenon in the same as the phenomenon of Plasma TVs losing 'gas' and needing to be recharged.


    Fiction.
     
  4. Jeff

    Jeff
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    That isn't my experience. In my view DVD players vary quite a lot in their DD/DTS passthrough performance, the same with sound cards in PCs. In fact with PCs even the software decoder makes a difference even though the sound is supposed to be untouched. I think jitter is still very relevant with DD and DTS.
     
  5. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Slartibartfast has posted information about this on another forum that you frequent Nicholas. You could also read TAG McLarens white paper on how they designed their digital interface and cables for DVD32r and AV32r.

    Gordon
     
  6. Nic Rhodes

    Nic Rhodes
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    I have already posted the link but some has seen fit to remove it :(
     
  7. NicolasB

    NicolasB
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    A lot of PC soundcards mess things up by trying to convert sampling frequencies on the fly (ugh) so I'm not sure how comparable that is....
     
  8. Jeff

    Jeff
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    How do you resample DTS without killing it?
     
  9. Gordon @ Convergent AV

    Gordon @ Convergent AV
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  10. Nic Rhodes

    Nic Rhodes
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    I did realise a link needed moderating :(
     

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