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Digital cable length: Does it matter?

Discussion in 'Hi-Fi Stereo Systems & Separates' started by WhyAyeMan, Mar 30, 2003.

  1. WhyAyeMan

    WhyAyeMan
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    I am considering buying a Trichord Pulsewire cable to connect my DVD player up to my DAC, as it is said that my exisiting cable (QED P75) might not have the correct impedance due to the RCA connections, but also to get rid of the RCA > BNC adapter I use to connect it to the DAC. The Trichords are said to be a true 75ohm impedance, and are available in 0.5m lengths.

    However, I found this little pearl of wisdom on diyaudio forum...

    The full thread can be found here: http://www.diyaudio.com/forums/showthread.php?threadid=12839

    So, is there any truth in what is being said there, and also has anyone tried a 0.5m cable vs a 1m cable?
     
  2. nwgarratt

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    It sounds like a load of rubbish to me. There shouldn't be any difference between a 0.5m and 1.5m or even 5m.

    Also, with coaxial digital, you can just use a normal phono cable. I do think some Hi Fi shops do take the advantgae of some people saying "to get decent sound you need the really fancy cable".

    I just use decent 'but not over-priced' phono cables for my DVD and Hi Fi digital sound and it sounds fine. It is the equipment being used, that makes the difference between bad and good sound.
     
  3. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    You can use any phono cable you want however if you want the best sound quality invest a small amount in a correctly specified digital interconnect. The difference is obvious.

    I have played a .5m against a 1M and subscibe to the idea that .5m wasn't as good.

    Sorry if this sounds like snake oil but quite frankly for the £25-30 it costs to do it right I don't care!

    Gordon
     
  4. Nic Rhodes

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    As Gordon has say to do it right doesn't cost much, say £25. The Trichord is decent cable but there is much better and cheaper. Their phonos are not 75 ohm but there BNCs are. Cable okay, much better for less around.

    It is not correct to say you can just use an normal phono cable. I can get a SPDIF signal down wet sring, doesn't mean it is any good though. These cables and connectors greatly effect the signal.

    I would keep lead lengths as short as possible and then you will minimise risetime / jitter related problems.
     
  5. MikeK

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    Good points on both sides!! :)


    My own opinion is that theoretically perhaps it could make a difference (although personally I feel his claim re: reflection time on a short cable, be it 0.5 or 1m etc to be highly dubious).

    In practice, I've not been able to detect any perceivable difference in the digital interconnects I've tried.
    True, I haven't tried them all (nowhere near).


    Also have to confess to being something of a cynic over most of the claims made by the makers of this or that cable - be it digital or analogue.

    And then there is the argument that if you think there'll be a difference, you'll probably hear one, placebo effect etc etc (which is to be honest pretty much my opinion).
    However, I also accept there is a possibility that for those like me who think cables don't really make any difference (within reason), that the opposite psychological effect could be at work - if you don't believe a difference can exist, then you'll probably never hear one, although if it were as obvious as some claim, I find that a bit hard to swallow! :)


    BTW - I don't think all cables sound the same - they don't (at least IMO), it's just that I think for those few that do sound different , either they've been deliberately engineered to sound different, or else one cable is revealing a flaw in the system, due to it's electrical makeup, which another doesn't reveal.
    Even then, I'm really not sure that I'd always describe such differences as an improvement (usually the more expensive one is deemed to be the better).

    Still it's each to their own - if this 0.5m cable is what you want, then get it. Personally I doubt you'll hear any difference over your current one, but you never know - I certainly don't think it'll be any worse for you!


    EDIT - When I wrote above that IMO cables don't make any real difference, I did of course mean reasonable quality cables (doesn't mean expensive) designed for purpose.
    As BeeKeeper says, while a normal "50ohm" audio phono cable may well work as a digital audio interconnect, it's no more expensive to give it the 75ohm interconnect it was really designed for. Same goes for video cables.
    For analogue audio, well that's another story :) :) :)
     
  6. Dubbing Mixer

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    From memory the critical length is more like 10-15metres. Then you can get complete cancellation of the signal. I vaguely remember a very impressive demo of this about twelve years ago.
     
  7. WhyAyeMan

    WhyAyeMan
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    What do you suggest (ideally Phono > BNC)?

    Cheers :)
     
  8. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    For the nearst to ideal cable PM or email X3ELS for a crimp 75ohm phono to crimp 75ohm bnc...If he refuses to supply you contact me for one.

    Or try and find some other forum member (MARK GRANT) or a dealer (none I know of) who can do this

    Gordon
     
  9. Nic Rhodes

    Nic Rhodes
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    pbirkett

    Gordon is smack on, Canare crimp 75 ohm RCA to crimp 75 ohm BNC with 75 ohm well screen coax. The coax needs to have a linear CI along it's whole length which most coax doesn't. This does separate out the good from the ultimate which is what you are after?

    75 ohm CI plugs are important as are the removal of solder. I like crimps but there are other solutions to this as well (Tag again). The cable is the easy bit!

    Elliot can sort you out with the best and you will save money over the trichord (which I have here) and it will be that little bit better.
     
  10. WhyAyeMan

    WhyAyeMan
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    Cheers for all your help guys, I intend to get one of these said cables you speak of :D
     
  11. MikeK

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    Just to throw a spanner in the works, there's no such thing as a 75ohm RCA connector - it's physical dimensions prevent it.
    Even if there were such a thing, it's highly unlikely that the RCA socket you plug it into could be 75 ohms - it's dimensions make this even more improbable.
    Canare may be as close as you can get, but I really can't see why these would be any closer to 75ohm than numerous others.
    No matter how you design it, it still has to fit a standard RCA socket, and that means c.3mm OD centre conductor, with c.8mm ID for the outer contacts. Given these unchangeable parameters, I can't see how it's possible to get above about 50ohms max (and in reality, probably nearer 40).
    Unless someone knows different!

    No cable has uniform characteristic impedance - it's a design goal, but the realities of manufacturing prevents it. The return loss (VSWR) spec will give an average indication of the likely impedance mismatches along a typical piece of said cable, but it's only a guide.

    So, IMO

    Yes, use good quality 75ohm cable.
    Yes, do take care with terminations (crimp are arguably better, but in reality.....well!! One of the big probs with solder is melting the dielectric if you aren't very careful, causing a change in it's geometry).

    And errr.........that's about it really! :) :)
     
  12. Nic Rhodes

    Nic Rhodes
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    MikeK

    A few comments echoing your excellent note and a few more general info stuff thrown in for good measure.

    basically correct, though the Canare does measure at 75 ohm CI. With conventional ‘RCA’ plugs most are WAY off, 25 – 40 Ohm CI is about the average I think.

    smack on, the plugs are never 75 Ohm, it is however about limiting problems here. The use of 75 Ohm connectors where available will limit problems. This is why I bang on about BNCs which as well as being cheap have 75 Ohm plugs and sockets.

    Speaking to a manufacturer today they say the UK market is more tolerant to BNCs than the Americans! Learn something new every day. They expressed real enthusiasm for my want only to use BNCs. In fact they were quite animated in their email!

    Beware that some BNCs are 50 Ohm CI and some manufacturers fit 50 Ohm CI connectors as this will take both 50 and 75 ohm connectors (pins different size). This is not good practice IMHO.

    I don’t and full agree with you! Not sure how Canare do it, but it works and we are not talking silly money either. Canare are not the £20 / plug syndrome. This is not the norm however with RCA plugs. Don’t be fooled into thinking nice shinny WBT plugs are great because they are most expensive, look great. Digital signals do not like them.

    I agree this is the norm for most coax and the reason why I have highlighted several times this issue here. I do know of a couple of cables that do this however, at least as far as testing can allow / show. Elliot supplies one which a rival manufacturer of cables has measured at 75 Ohm CI linear across the cable. They have said openly they thought this cable to be excellent. Sonic reports from Elliots customers have also been EXCELLENT. This was the same cable I GAVE away on this site about 2 years ago, with very few takers! I agree it is the norm not to have linear CI but when you see how cheap this cable is you wonder why people bother with other types which are less suitable.

    When this TOP flight cable is so cheap and crimp BNCs are about the cheapest there are (I bought 100 today for £77, 77p each! You can get down to <50p!!) you wonder why people spend so much on poorly performing digital cables (He says with the nation digital lead collection at home, inc Chord, Kimber, Van den Hul, QED etc :blush: ).

    Re soldering / making your own leads. I have found most people do NOT solder correctly and alter the geometry of the plug and cable. Temp controlled irons are a real benefit here. Many people make a mess of this. The other area that goes wrong is the cable clamp, they use a mega tight one, which squashes the cable and changes those important characteristics. This again is common among DIYers. I have been guilty on all accounts in the past but hopefully I have learned by my mistakes and my purchases.
    :blush:
     
  13. MikeK

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    I think we've all done just the same at some point :)


    One last point :)

    Every single RCA plug in existence will have a 75ohm impedance at some given frequency - it's whether that matches the cable's CI where I have my doubts.


    I doubt I've tried as many Digital interconnects as you, but I've never been able to detect any differences (all the ones I've tried have been 75ohm links). As you pointed out, as it's no dearer to use a 75ohm cable, I've never bothered listening properly to an audio cable used in this way - I know it works though, just not how well!

    Your point about limiting problems is taken, and is generally good advice IMO, providing the cost is in proportion to the advantaage gained (if any :) ).
     
  14. WhyAyeMan

    WhyAyeMan
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    Interesting, well I should shortly be the recipient of said cable, I have sent a cheque off today, and will let you guys know honestly whether I can tell any difference between it and my QED P75.

    I did consider a Cambridge CD4SE as transport (to go with the Cambridge DACMAGIC 3), as this would have given me BNC throughout, but in the end, it went for a price higher than I thought worth paying, and also I like the flexibility of the DVD drive, as my hifi is also a "2 channel" home cinema (insufficient space for extra speakers, and insufficient funds to buy them). FWIW, I feel the DVD makes a reasonable quality transport in the context of my relatively inexpensive hifi.

    Anyway, I'll let you know when it comes if its any better. The only test up to now I've done is to compare the QED P75 with an Ecosse CA-1 analogue interconnect, and there was only the tiniest of differences, I doubt I would even have been able to tell under an ABX test. Personally, I come from the 1/0's are 1/0's stable, so am not expecting a huge difference (if any), but I always keep an open mind. It wont be a great loss anyway - I need another digital cable anyway.

    Cheers
     
  15. WhyAyeMan

    WhyAyeMan
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    This morning, I was the recipient of a digital cable courtesy of Mark Grant (cheers BTW)

    The difference is incredible over the QED cable I was using before. It seems to have better bass performance, lots more detail, and also sounds smoother, its much more musical, and has made more difference to the sound than any interconnect I've heard thus far. Thanks to that, the sound I get from my cheapo Pioneer DV-350 and £60 second hand DAC matches or beats the sound quality I have heard from many £500 seperate decks that I've heard.

    Thanks for all your advice guys :D
     

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