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Using a coathanger as a digital cable

Discussion in 'Accessories, Cables, and Remotes' started by Chu Gai, Sep 1, 2003.

  1. Chu Gai

    Chu Gai Well-Known Member

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    The link where this was first done, http://www.magnani.net/~al/DigitalWireLabTest.html no longer exists. Thanks to http://web.archive.org it still lives on...at least for a few more years. What follows are the details found on the last link for that page.

     
  2. Kevin C Brown

    Kevin C Brown Well-Known Member

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    I also heard about a similar experiment where someone actually used chicken wire. Same results. But...

    In both cases there was no measurement of (uh oh...) jitter.

    But if it's true that there was no audible difference (A/B/X blind? Maybe not...), then the (any) jitter introduced might not matter.

    In this case, they only checked for errors, no mention of sound quality itself? Plus doesn't look like they looked at PCM either, which is usually a higher bit rate than DD.
     
  3. John Royster

    John Royster Well-Known Member

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    I call BS. Especially the "testing" methodology.

    When testing a digital cable you use an injector on one end and a receiver on the other. Then you run patterns and sweeps across the entire frequency range and determine if what-goes-into = what-comes-outta. You never rely on actualy end user equipment to test digital transmission. Its just not reliable.

    Do we know the sensitivity of the receiver? Was PCM tested? What was the bit-rate?
     
  4. Doug_H

    Doug_H Well-Known Member

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    I have always understood the need for good cables when sending an analog signal but I have a hard time grasping why they are needed for a digital audio signal such as AC3.

    I don't know the answer so I am not being a smart ass.

    To me you are sending 1's and 0's so I can't imagine how a cable could affect the sound. You either get the data or you don't. Any change in the sound would have to take place in the processing of the digital data once it is received.

    So where is my logic failing me?
     
  5. John Royster

    John Royster Well-Known Member

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    Doug,

    The problem stems from the fact that you aren't sending 1s and 0s on the wire. You're sending an analog signal that represents digital data.

    Sure, sending digital signals over a wire is no big deal. We do it everyday we pick up a telephone, use the internet or cell phone, etc. But from the prospect of sending digital data over a cable at speeds greater than 1 megabit/sec, the cable is everything.

    Now I'm not saying one needs some super duper cable. Heck, any RG59 or RG6 coax will work. Twisted pair will work. Its how well does it work?

    There are specifications on just how much impedance can vary, how much crosstalk is allowed, how much return loss is allowed. If a coat hanger meets these, then great.

    However I did not see it tested. Only a "hey our processor didn't show any errors". Which from a datacommunication standpoint is totally unreliable. There is more to data communication than a bit error. Bit errors are real easy to detect - if the checksum on the packet doesn't match the receivers calculated checksum then the packet is corrupt. This is the simplest test there is. But it isn't the only error.

    Jitter, clock slips, bit error in the encoding but not the payload come to mind.

    Just wanted to point out the common misconception of "its just 1s and 0s". Because it is not 1s and 0s. Its ones and zeros encoded into a bit stream that is put on a analog wire (hence an analog waveform).
     
  6. Matt_Doug

    Matt_Doug Well-Known Member

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    I'm confused [​IMG]
    according to the quick start guide for the DP562 on Dolby lab's website the cyclic redundancy checksum (CRC) is used to check audio & video timing errors in the mpeg decoding process not errors within the data blocks. Please correct me if I'm wrong?

    shielded twisted pair (stp) is the standard transmission line of the computer industry. Ridiculous pricing aside, I find it curious that when A/v cable manufacturers implement stp it its considered voodoo & market speak by some including said EE's
     
  7. Neil Joseph

    Neil Joseph Well-Known Member

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    Ah yes, the ol' coat hanger.

    I am willing to bet that it would work but there would be intermittant dropouts.
     
  8. John Royster

    John Royster Well-Known Member

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    As an after thought maybe if I get time or bored I could make up some kind of twisted pair-coat hangar-twisted pair cable and scan it.

    Generally even a foot of no-twist causes the cable to fail above 1 Mhz.
     
  9. Chu Gai

    Chu Gai Well-Known Member

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    Well maybe Neil. They didn't have any dropouts though. Might not be my first choice in a connector, but it's pretty interesting.

    John, if you've read some of the postings I've made over the past several months, where I cited some of the work done by the late Julian Dunn, a partial summary is that so long as the cable can transmit the information without any errors, the jitter is a non-issue. And really we're not talking about 100's or 1000's of feet where the effects become cumulative. We're talking a couple of meters. The reason why the impedance is overblown here as an issue is related to the length of the signal compared to the length of the wire. I mean, you may think HDTV is something fantastic, but let's get real now. It doesn't compare to a hi rez calibrated monitor now.

    Got a coat hanger and some spare RCA's laying around along with a soldering iron? Knock yourselves out gentlemen [​IMG]
     
  10. Matt_Doug

    Matt_Doug Well-Known Member

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    As I read it a valid checksum only means certain parameters like the number of audio blocks in the frame are valid. It does not mean all the the bits that make up the frame are valid. So you can't say that there weren't any bit dropouts but you can say there were no invalid frames
     
  11. Cees Alons

    Cees Alons Moderator
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    Yes, a valid checksum and a few more ECD bits do not guarantee a 100% that everything is OK. But on the other hand, that certainly doesn't mean it isn't.

    Millions of bytes are sent over all sorts of transmission channels each day, and when was the last time you received an e-mail in which a letter was changed during transmission? Or in a post like this?

    I'm sorry, but jitter is not a problem of the transmission cable between player and receiver. Once the bitstream is established and baked onto the CD or DVD, all that the equipment has to do (including the cables) is: make sure that same pattern is transferred to the place where it's transformed to analogue again. And as long as it does exactly that, without adding or deleting bits, there will be no discernible differences.

    Of course there's a real-time aspect: if the stream is interrupted waaaay too long, it will be impossible to recreate the proper sound in time, but now we're talking about quite different time-lags than caused by the cables we're discussing.

    Just suppose I would claim that the CDs I brought to my home by foot (from the shop where I bought them) sound so much better "to my ears" than the ones I carried in a car or a bus or train. And that those latter only started to sound more clear and transparent, with much better defined bass regions and more punch, after I took them back to the shop and this time walked them to my home. Would anyone believe that? Why not?

    Cees
     
  12. Chu Gai

    Chu Gai Well-Known Member

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    I believe you Cees. You da administrator!
     
  13. Cees Alons

    Cees Alons Moderator
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    And you knew, of course, Chu, that I believed you, even though you're not an administrator here [​IMG].

    Cees
     
  14. Chu Gai

    Chu Gai Well-Known Member

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    Sheesh, italics for emphasis too!

    I hope everyone understands that I'm not advocating coathangers, or statues as interconnects, just looking to put a little perspective on one of those engineering axioms, 'engineering is 2/3 knowing what you can get away with'. But you don't have to be a soldering pro to try the idea out.
     
  15. Cees Alons

    Cees Alons Moderator
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    Chu,

    I happen to know Al Magnani personally (even met him in SF). He's an electro-technical engineer and a very intelligent and critically thinking person.
    He did the coat-hanger experiment partly for fun, partly because he's a born scientist and partly to make a practical point that could be checked in real life by his HT-friends.

    Cees
     
  16. John Royster

    John Royster Well-Known Member

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    I really do wish you guys would brush up on your EE, packets and digital clocking before you make broad strokes of "if its digital it will get there".

    The analogy of E-mail and what not has no bearing in a real-time stream. That works on the concept of if there is a problem (and there very frequently is) the packet will be retransmitted by the sender. The e-mail application is actually totally unaware the there was a problem.

    In our audio systems there is no retransmission, no way for the receiver to say to the sender "hey, that last piece wasn't right...can you resend it so I can add it to the total message and not be left hangin?"

    This is dealt with everyday in digital transmission, from voice to video where you deal with real-time streams. And believe it or not every single receiver I've ever worked with actually measures jitter, signal strength, CRC errors, etc. Tap into the DA converter in your cell phone and the software - I can guarntee jitter is measured and stored. If jitter gets too high it will slow down the transmission. Ever hear an echo on your cell phone? That's jitter.

    So, yes, jitter matters. Is it audible? Now that is golden question.
     
  17. Cees Alons

    Cees Alons Moderator
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    John,

    You're talking about cell phones. That's a different environment: both ends are RT, the generating and the receiving ends. Ether transmissions and relays in between.

    DVDs and CDs are recorded. Buffers can be added, "read ahead" is possible. Retransmission too, if the designer chooses so.

    But apart from that: jitter isn't a topic in a 5-10' (or even 45') long cable.

    Cees
     
  18. John Royster

    John Royster Well-Known Member

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    Jitter and the other parameters I've described for sending a digital signal apply no matter what the media.
     
  19. Chu Gai

    Chu Gai Well-Known Member

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    Don't forget the coathanger now!
     
  20. Cees Alons

    Cees Alons Moderator
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    John,

    But heavily dependent in magnitude by the physical length and complexity of the travel-path. And the effect of tiny jitter is effectively annihilated by buffering. It's the ultimate result that counts, not the intermediate problems.

    Cees
     

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