Mike
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Post by Mike on Sept 17, 2013 20:40:40 GMT
Hi Gentlefolk, After being directed to a few posts about this over on AoS, I thought I'd start a thread about cable length for digital interconnects. There's been much chatter about this over the years and wondered what the general consensus was... I have a hypothesis or two, but would appreciate your thoughts first. ATB, Mike.
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Javier
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Post by Javier on Sept 17, 2013 21:02:28 GMT
Hi Mike,
What kind of digital interconnects are you interested in? there are quite a few types like coaxial SPDIF, plastic Toslink, USB, FW, CAT5/6, glass optic fiber, HDMI, etc.
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Mike
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Post by Mike on Sept 17, 2013 21:15:14 GMT
Ah, good point!
I'm primarily talking about SPDIF cables...
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Dave
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Post by Dave on Sept 17, 2013 21:20:24 GMT
Hi Guys, I think the title 'Gentlefolk' is quite appropriate here - friendly and helpful folk . For the time being I will abstain from joining in this because my old friend Mike (I could have been his father, but that's another story, and a private joke between me and him ) knows my feelings on aspects of this topic and, like him, I'd like to hear what others have to say before joining in. This could be a very interesting topic I believe so pitch in with your views. Dave.
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Mike
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Post by Mike on Sept 17, 2013 21:23:04 GMT
Ah, there you are ya old bugger! I still find that avatar slightly disturbing even after all these years... Yuk!
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Dave
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Post by Dave on Sept 17, 2013 21:34:05 GMT
I still find that avatar slightly disturbing even after all these years... Yuk! Mmmm, not sure about the greeting but I know what you mean about the avatar. I've actually been thinking about replacing it with a picture of me in my prime (no laughing at the back there please ), just a case of getting round to it. Anyone know of a free program to reduce a A4(ish) scan down to avatar size please? Dave.
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solderdude
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Post by solderdude on Sept 17, 2013 21:42:41 GMT
The maximum length would depend on a few factors where the most important ones are bitrate and termination (impedance matching combined with the proper plugs and cable impedance) as well as sensitivity of the receiver (DAC) as well as the output impedance and voltage of the transmitter side (PC or other source).
Because of this the maximum length will vary depending.
When the length is too big you will experience all kinds of discomfort such as drop-outs e.t.c. As long as your data is received properly you are O.K.
Yes, some will start to moan about jitter and reflections and degrading sound e.t.c. but as explained it is all related to one or more factors mentioned above. The better quality 'link' you have the longer the maximum distance.
@ Dave... PM me the scan and I will 'avatar' it for you.
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Mike
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Post by Mike on Sept 17, 2013 22:13:54 GMT
What about short lengths... say, 50cm or less? Lots of chatter here and there about problems with reflections, and much talk about a minimum 1.5m cable. Any comments?
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solderdude
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Post by solderdude on Sept 18, 2013 5:06:18 GMT
proper termination is the answer.
Problem is to decide/determine when something is terminated 'properly' which is hard to do extremely exact without measurements of the eye pattern, preferably somehwere IN the front end of the 'receiving' unit just before the analog representation is converted to more digital-ish signals. At short distances a cable doesn't reach it's nominal impedance which by itself could cause (small) reflections as there is a slight mismatch there BUT the amplitude is high so reflections are 'buried' by the signal also the wavelength is relatively long for short cables so impact will be very small/negliable. At greater lengths (depending on the frequencies present which is determined by the source side) the nominal impedance of a cable becomes more important/determining.
Even attaching the capacitance of an oscilloscope may be enough to cause the cable to make small reflections or alter them so you might think, judging from a scope picture all is well while in reality things may be different. Perhaps with expensive active probes you may get a more accurate reading.
As long as bit's don't tumble over (are misinterpreted) and you use a proper (modern) DAC with good jitter rejection I don't see the problem though as there is none from an objective viewpoint.
Let me ask a question, perhaps you have already seen the answer in your searches.
Are there (aside from suspect occasional oscilloscope screenshots) any test results that are performed with different cable lengths with say a data analyser that shows timing errors or even bits tumbling over or jitter test measurements that are quite conclusive and show different cable lengths (or brands or types) have a serious impact on the 'received' data. That is on the output of various DAC's, those with and without jitter elimination circuits.
I don't like to read about this or that person or even noted people that have 'confirmed' differences with their ears between different cable (lengths). I am hoping for some real evidence performed by people who know how to properly test/measure this with the proper equipment or some of the more knowledgeable such as Hydrogen folks.
From experiments done even to 100Mb with terminated simple wiring I can't remember ever to have seen misinterpret bits due to copper cables (I have a data analyser at my disposal which keeps track of misinterpreted bits)
Now many people will scream ... ah but jitter and the all important edges and mutilated 'analog representations' of a 'digital yet analog represented bit' must have an impact. Well.. all I have to say about it is this will largely depend on the used DAC and HOW it handles incoming data.
In the early days engineers took a shortcut and let a local clock synchronise with the data clock as that was easy to implement and 'worked'. This could give a certain dependency on downstream induced jitter (regardless if it is source or cable induced). With a proper modern DAC this mutilation/jitter/timing factor is eliminated by smart circuitry and the stability of the used clock alone determines DAC output timing.
The people who continue to say that streaming is real time and this is the main problem as it is close to analog simply do not understand how signals are handled inside modern DAC chips. They aren't handled real time but a very short delay is present (micro-seconds) so transmission and downstream jitter is simply removed unless you use older DAC's that have been known for measurably having poor jitter rejection.
I say: Just connect any damn cable with the desired length and if it sounds good and doesn't show weird problems (clicks, ticks, drop-outs e.t.c.) on your system that does NOT occur with other lengths, you are fine (provided you have a DAC with proper jitter rejection) and can stop worrying about what other people have found, suspect or think what might be happening'. If it works... it works
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Mike
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Post by Mike on Sept 19, 2013 19:59:00 GMT
Oh, that's a good reply... I like that! I pretty much agree with everything you say, but the subject simply won't go away and people have started asking me for more cables. Again!!! I'm not in the HiFi industry (my background is telecommunications - although I seem to be spending most of my work-life in the RF domain these days) and this making cables for folk started by accident... and it doesn't seem to want to go away. A while back I even started playing about with digital cables containing RF attenuators (I'll let Dave tell the story if he can be bothered) but, err... well, something happened that made me take something of an internet sabbatical and I sort of forgot all about it, so don't really know what happened to the idea. Dave? Anyway, someone contacted me the other day wanting another cable, as the question of cable lengths had popped up again, so I thought I'd ask around and see what people thought... I've got another idea about using a directional RF attenuator, it has negligible forward effect but has >50dB attenuation in the reverse direction. However, I can't escape the feeling that I'm looking for a solution to a non existent problem! Cheers, Mike.
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Dave
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Post by Dave on Sept 19, 2013 21:09:01 GMT
OK Mike, here we go - might give Frans apoplexy though but we'll see . Most folk on here know that my hearing is 'shot' - audiologist says it's typical of industry related hearing loss - I've been in the iron and steel industry all my working life, much of it in the heavy end and more than that on the shop floor. Both ends of my hearing spectrum are well down on what is considered normal. Some time back Mike got a reputation on another forum for making bake-off winning i/cs and I treated myself to one or two. I was more than delighted with the improvement they made and told Mike so. Somehow that lead to Mike asking me to audition a couple of his new i/cs, externally identical other than the colour of a piece of insulation tape round one end . Who better to test new cables than someone who knows nothing about such things and therefore can have no pre-conceived ideas? . After playing just one track through both i/cs, all other bits being identical, I found the difference to be so apparent that I though it was a leg pull. I immediately told Mike what listening to one track had resulted in, to be greeted with the cryptic response "Very interesting". I listened to many more tracks and became more and more convinced that there was a 'night and day difference' between the two i/cs. Sorry about repeating that much over-used phrase but it really was very noticeable. Mike then confirmed that the one that I thought was the best was the one he had tweaked and subsequently provided me with a broad range of the ''tweaks' to apply myself to other i/cs that I had. I followed his instructions and found with each cable I tried I could reach the optimum tweak level - all were improved to a greater or lesser extent. IIRC around that point Mike took a sabbatical from the audio fora and I don't think either of us took it any further. I got involved in other audio projects so, without Mike calling the shots (I know nowt ) I didn't get round to taking it any further. I still have the bag of tweaks and I guess I could repeat some trials again if required. As I don't really understand the technicalities of what the tweaks were actually doing I have been deliberately vague to avoid anyone asking me any difficult questions so I'll defer to Mike on this one. Just let me repeat, the differences I heard and consistently repeated were totally impossible to ignore or deny. Over to Mike . Dave.
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solderdude
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Post by solderdude on Sept 19, 2013 21:28:36 GMT
However, I can't escape the feeling that I'm looking for a solution to a non existent problem! Cheers, Mike. You are, for most cases, yet for other cases where older and jitter sensitive DACs are used things may be somewhat differnt. DO note that even people with excellent DACs and sources will always here improvement after a cable has been swapped. Digital cable = RF cable, or at least it should be treated that way, though many audio cables can easily do 10MHz and more. And yes directional dampers can be used and to many WILL give audible improvements, yet no-one will ever be able to provide technical 'proof' based on analog signals from that DAC differing in any way... because it won't yet will most certainly be perceived as improvements by many, many, many people. Perhaps even a hole in the market if played well. edited...
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Dave
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Post by Dave on Sept 19, 2013 22:11:11 GMT
Frans, no one here has more respect for you and your knowledge of audio technicalities than I do and I hope you know that. If however you are inferring in your post that I was imagining the differences in Mike's two cables I have to tell you, with respect, that you are wrong, very wrong. I have never been more certain about anything as I was about that difference. No expectation bias, no preconceived ideas, no axe to gind, I could not have cared less which way the results went. I was almost literally selected at random off the street from people who 'happened to be passing at the time'. Accordingly I would ask you to accept it and spend some time on considering why it might be so, rather than dismissing it offhand as being imaginary, not possible and a figment of someone's imagination. I can not say it any clearer, it was fact, not fiction, and has been repeated many times to my complete satisfaction. The experiments are extremely easy to undertake so why not humour your friend(s) and repeat them rather than closing your mind to the possibility because it is not possible in your opinion? OTOH, if I have misunderstood your position on this (I know nowt and understand even less ) please forgive my rant. Cheers, Dave.
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Deleted
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Post by Deleted on Sept 20, 2013 6:26:55 GMT
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Dave
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Post by Dave on Sept 20, 2013 10:19:55 GMT
Mornin' Frans Yes I agree, let's keep it on a friendly discussion level - I would never set my opinions up in opposition to yours, believe me but I am not talking opinion here, I am talking about fact. I admit to being a numptie about these matters (no disagreement there then ), but my background of 50 years has been loosely in engineering, where I was often in a position to change the parameters of what we were doing to assess the changes that occurred, part of this time in one of the world's foremost steel research establishments, FWIW. The point I am trying to make is that I have some experience of carrying out trials and making decisions on the results, making further changes and checking for confirmation of the results. Now, regarding your statement: "Mike is talking about digital cables here and not about analog which is what I think you are referring to.", I can confirm that is one error in your interpretation. The two cables involved were both digital, each one being put in an otherwise unchanged set up and swapped. Yes I did know which cable was being tested at any one time but I did not know which, if any, was supposed to be better or even different - they both might even have been identical as far as I knew when I listened to them. Mike will confirm that I emailed him after listening only to one track (the same one) through each cable and told him that the difference was immediately apparent and told him which was best. After doing the same listening test over many tracks I again contacted him and said that in each case the same cable gave me the best listening experience, some tracks showing more improvement than others but without exception the same cable came out better. Now, a challenge for you: are you willing to put your money (or convictions ) where your mouth is? . I think I still have the same cables somewhere, if I can find them, and I could forward them both to you for you to test. I would accept your assurance that you would not do any destructive or non-destructive testing until after you had done extensive listening tests. All I would ask is that you report here on the forum what your opinion is, warts and all.. There, the gauntlet is on the floor in front of you - will you pick it up? All said in the spirit of this forum - as far as I am concerned we merely have a difference of opinion, not the 'seed' for World War III, OK? Cheers, Dave.
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