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Thread: coaxial digital cable :what to buy?

  1. #61
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    Quote Originally Posted by Reffc View Post
    Yes...there's an urban myth born from a few internet articles which purport to be fact that you need a 1.5m digital coax to avoid problems of reflection caused by impedance mis-match.
    Interesting post, if it were accurate, it would be even better. To cut a long story short, if all the components of the connection are true 75Ω and the lead is made to a high quality, then the reduced reflection of a 1.5m one give a better SQ than a shorter one, with 1.0m being about the worst length to choose. As Nugent says in the article quoted by Mike, if ALL components in the chain are 75Ω, one should get no reflections. As this is not often the case, the 1.5m cables are a quickand easy solution if using co-ax. the best possible option is to go optical in my opinion.

  2. #62
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    I'm not familiar with his writing or work but go by what I learnt from a signal transmission engineer and from my own knowledge, which I am offering here for your derision or otherwise which seems for some to be their raison d'etre on AoS these days. It may well still be accurate too as it seems that even experienced signal transmission engineers do not agree on this subject. If 1.5m is a quick and easy solution, so be it but I have yet to be convinced that this is a real problem in audio as when using sources capable of hi-rez sampling, I have never heard any differences between half a metre or 1.5m of the same cable terminated in RCAs. I'm sure that there are cables that do create audible issues and even drop-outs, but I don't use them.

    Anyway, for those with perhaps a more enquiring mind than quoting verbatim the 1.5m rule, then here's a few other things to consider when considering high speed digital data packet transmission. For those of a non nerdy disposition, switch off now:

    Jitter: The rise time in signals can be slowed by losses which cause the receiving DAC input to detect edges with less certainty, but even so, there can be up to 8x data packet redundancy and with modern DACs and transports, Jitter correction has improved. Whilst jitter cannot be totally eliminated, the data redundancy comes into play as do jitter correction circuits. For a cable which is made of poor quality conductors (with metallurgical boundary loss issues affecting very low level Jitter), then signal propagation may be affected;

    Impedance mis-match: This may arise from the cable itself or from connectors used. Whilst this is affected by using non-optimal RCAs, even some BNCs won't cure the issue if the receiving circuit isn't precisely 75 Ohms at input. There are cables and there are cables. Pick a cable stamped as 75 Ohms but in fact from poor Far Eastern manufacture, the characteristic impedance can be anything but...it can be plus/minus 10% as in some tested cases, so the longer the cable, the WORSE the reflection issues become. Such lack of care with some cheaper cable manufacture can also hint at perhaps non-ideal dielectrics and perhaps non-ideal metallurgy being thrown into the mix too, so first and foremost CABLE QUALITY MATTERS.

    Dielectric: As hinted above, changes in dielectric properties have an effect on propagation by absorption and later release causing reflections which bounce back and forth until absorbed by cable losses. Jitter is the result where differences in dielectric charge occur (which is the case usually) causing differences in rise and fall of each edge, and there is no mechanism for the signal to overcome these small charges. Again, this can be controlled by using high quality dielectrics with very low dielectric absorption.

    Metallurgy: As mentioned above, poor crystalline metallurgical structure with lots of impurity/discontinuities and changes to boundary transmission qualities may not have pronounced effects for analogue signals, but they can affect lower level S/PDIF digital signals with rise times typically as low as 1 to 5nS. As with impedance mismatches, when signals of that speed are transmitted, lossy boundaries cause small reflections resulting in increased Jitter.

    Point is, there is more to prescribing cable than just length which is what I am trying to point out, and as someone with more than a smattering of experience in this field, I was merely trying to offer further explanation and suggest that 1.5m is NOT the panacea to improved signal transmission. Double blind tests using the same cable but of different lengths is one way of discovering for yourself where the optimal cable length may lay. It could be that 0.3m or 0.5m is fine, or that 2m is possibly the best minimum length. The amount of jitter from your transports, the jitter correction circuits, signal rise times and a host of other non-cable related issues will all play a part here, hence the danger of one generalisation to cover all eventualities.

    Generally, most people will find that in most systems, using a HIGH quality cable, properly terminated, even using higher rate sampling that 1m is usually fine. If it allows you to sleep at night, then why stop at 1.5m? Rise times may still be as low as 15nS at 1.5m so going to 3m would (if cable length were ALL that mattered) be better, but as most of us have experienced, in most cases, it isn't necessary. Hopefully re-reading the above will allow some to see that it's all really common sense stuff and a good reason to say why just one specified cable length is not necessarily the answer...the issues are complex and numerous so getting fundamentals like cable quality right are a no brainer before one starts to look at other sources of impedance mismatch and sources of jitter.

    Point made, and now I'll bow out of the debate but for those who this helps, then job done.
    Last edited by Reffc; 24-12-2015 at 08:11.

  3. #63
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    Good post Paul.
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  4. #64
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    Quote Originally Posted by Reffc View Post
    Yes...there's an urban myth born from a few internet articles which purport to be fact that you need a 1.5m digital coax to avoid problems of reflection caused by impedance mis-match. A few signal transmission engineers I speak to have confirmed that this is nonsense. For longer lengths, there are some issues with noise and with reflections, but the reflections due to connector mis-matched impedance become less of an issue as lengths increase, although for any mis-match, increased rise time with faster sampling rates is more testing of cable and connector specification. Other factors come into play then which will only confuse the debate here for domestic hifi. For hifi, they suggest keeping coax lengths as short as possible for digital interfaces. A well known and respected current hifi designer (no names to save him embarrassment) recommended to me a length of 300mm for a digital coax based on proper research into digital signal transmission undertaken for North Sea oil rig digital data transmission.

    Long story short, data packet contains redundancy and its when this redundancy is used up an interpolation takes over that signal corruption starts to become an issue, and as I understand it is where most DACs can start to "sound different" (as well as other areas of noise affecting S/N in the DAC output stages) until such time as signal drop outs occur. Just because the signal locks doesn't mean it isn't corrupted.

    Hifi buffs make way too much of cable length as the final arbiter of SQ because they mostly have little to go on, so urban myths prevail. The advice is not to be overly concerned about cable length but most certainly to be concerned about cable quality as mis-matches in characteristic cable impedance over longer runs are almost certainly as destructive to the signal as that of using a 54 Ohm connector (typical for many RCAs). I have tested various cables and been quite surprised at how poor QC has been on some imported cables which vary quite a lot in impedance due to variation in dielectric thickness or conductor spacing which is why I only use very high quality cable for making up digital interconnects and use plugs which are fully shielded and keep impedance mis-match to a minimum.
    There's a very well know RF engineer who proved it does make a difference and at one point was selling extra length coax cables, I've certainly not found it to be nonsense as have many others. BTW there are no data packets with redUndancy in SPDIF, just simple parity.
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  5. #65
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    Quote Originally Posted by Reffc View Post
    I'm not familiar with his writing or work but go by what I learnt from a signal transmission engineer and from my own knowledge, which I am offering here for your derision or otherwise which seems for some to be their raison d'etre on AoS these days.
    I wasn't deriding your points in the original post per-se but pointing out your dismissal of the 1.5m *recommendation* as an 'urban myth' with no basis in anything credible. This kind of attitude I find more destructive of debate and symptomatic of the very tendency you are lamenting with the above sentence. What you have now added is very interesting and far more worthy of consideration but is actually also covered in Nugent's article (albeit in a different wording)

    That there are differences of opinion on matters of hifi technology between very qualified experts goes without saying but what is not helpful for anyone looking to learn or find solutions is when posters set themselves up as some kind of definitive authority and dismiss other equally valid viewpoints out of hand as rubbish or (as you put it) fallacies based on rumour and chinese whispers when, in fact, they are nothing of the kind.
    Last edited by mikmas; 24-12-2015 at 11:27.

  6. #66
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    Paul, you know a bit I'll grant you, but a little knowledge is a dangerous thing. Do the maths, read the literature and use your ears. Most of this stuff is available from the AES.

  7. #67
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    What I don't get, is why manufacturers don't use these little tweaks to improve their sales. Neither my Squeezebox Touch nor my Rega use BNC, both use RCA for S/PDIF. My Cambridge Audio cables are all 1 metre not 1.5. Nothing comes with a great thick mains cable or oak feet or whatever. It can't all be down to cost. It makes you wonder what they're doing in research and development.......or do they think it makes no difference?

  8. #68
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    Quote Originally Posted by RMutt View Post
    What I don't get, is why manufacturers don't use these little tweaks to improve their sales. Neither my Squeezebox Touch nor my Rega use BNC, both use RCA for S/PDIF. My Cambridge Audio cables are all 1 metre not 1.5. Nothing comes with a great thick mains cable or oak feet or whatever. It can't all be down to cost. It makes you wonder what they're doing in research and development.......or do they think it makes no difference?
    Now THAT is a great question.....
    AC POWER
    Hardwired 10kVA balanced mains powering entire system
    AMPS
    Meridian 557 power Amp (Modded) / PS Audio BHK Preamp (Modded)
    SPEAKERS
    Wharfedale Evo 4.4
    DAC
    PS Audio Directstream (Modded)
    TURNTABLE
    Pro-Ject X8 balanced output via XLR / Ortofon Quintet Blue cartridge
    PHONOSTAGE
    Pro-Ject DS3 B balanced Input (TT and Phonostage powered by Pro-Ject Power box RS2 linear psu)
    DIGITAL
    OPPO 203 (Modded: Linear PSU, i2s output to Dac) - Roon Endpoint, HDMI input used for all things Streaming/ PS5 /AppleTV ... also good for movies apparently?
    MUSIC PLAYBACK
    Tweaked AP-Linux based Roon Server into Oppo 203 as Roon endpoint
    Ipad Roon Remote.
    Apple Music/ YouTube via AppleTV, fed to Dac via Oppo HDMI input/i2s output to Dac.
    SPEAKER CABLES
    Biwired: Duelund DCA10GA (Bass) Duelund DCA16GA (mid & treble) Duelund 12DCA used as jumpers (On
    "Blackcat Cable" Chris Sommivigo's advice - yup, even with biwire it sounds better - and it does)
    INTERCONNECTS
    All Balanced: Ghost+ recording studio XLR cables

  9. #69
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    Quote Originally Posted by RMutt View Post
    It can't all be down to cost
    D'ya wanna bet?
    Just ask Jez
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