coaxial digital cable :what to buy? [Archive]

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nbaptista

05-02-2010, 22:39

Hello all,
I wonder if you a 10£ cable work or if I should spend more...my idea is to connect a Dac to a transport through digital out.Sorry,but I don´t understand much of digital cables,and I wonder if anyone could help me!How much I should spend?The Dac costs £200

Alex_UK

05-02-2010, 23:49

If the interconnects are anything to go by, I'd try a Mark Grant one at £15...

http://markgrantcables.co.uk/shop/index.php?main_page=product_info&cPath=40_3&products_id=5

Stratmangler

05-02-2010, 23:55

Coax or optical ?

Stan can provide either - I have one of his coax's in my system at the moment, and it's very good indeed.

http://www.homehifi.co.uk/PP/DIGITAL_COAX_CABLE_.html

http://www.homehifi.co.uk/PP/TOSLINK_CABLE_XQ_.html

The Mark Grant cables are also well liked here - I have no personal experience of them yet.

Chris:)

nbaptista

06-02-2010, 00:19

What about Ixos or Profigold cables?

Stratmangler

06-02-2010, 00:23

What about Ixos or Profigold cables?

:confused:

Never used 'em.

Chris:)

Themis

06-02-2010, 07:00

Rare Bird

06-02-2010, 19:16

I use QED 'Performance' coax & Opticals, brilliant qualiy & sound.
http://www.qed.co.uk/100/gb/product/performance/digital_audio.htm

The Vinyl Adventure

06-02-2010, 19:29

I use QED 'Performance' coax & Opticals, brilliant qualiy & sound.
http://www.qed.co.uk/100/gb/product/performance/digital_audio.htm

if you gonna spend that much, you are muh better off with one of marks grants or mike homars.

my experience of qed performance cables is certainly not a bad one ... but mikes was a def upgrade, im yet to swop in my g1000hd digi cable but based on the interconnects im sure its pretty bloody decent

not wanting to repeat previous info...links...
if i was looking at buying a cable at the stated budget i would stretch it by a touch and go for this for coax

http://markgrantcables.co.uk/shop/index.php?main_page=product_info&cPath=40_3&products_id=3

optical

http://www.beresford.me/others/products.html

Rare Bird

06-02-2010, 19:47

if you gonna spend that much, you are muh better off with one of marks grants or mike homars.

Sorry Hamish don't buy that for one second.If there is anything overrated it's a digital cable, as long as it's built correctly with true impedance cable it's perfectly fine. The difference with QED performance & Grants is the QED is far better looking & quality, i'm not saying Grants arnt quality but they don't look good to my eyes. thats all that is important to me in this instance.

Themis

06-02-2010, 22:29

Mark uses Canare coaxial RCAs, true 75ohm. These are about the best RCAs you can find on the market, and the Belden 1694A cable he uses is considered one of the best too. ;)

Better looking, I don't know. Better quality, certainly not.

Rare Bird

07-02-2010, 00:22

What ever i know fuck all

Themis

07-02-2010, 06:52

What ever i know fuck all:lol: :lolsign:

Marco

07-02-2010, 08:12

Guys,

I'm sure that the QED works well for Andre, so why should he change? :)

As for the MG Belden 1694A - it is very good (and superb value for money), but I've no reason to think that it would be any better than the QED Andre uses.

If you want the best performance in this application from a Mark Grant cable, and can afford to spend a bit more, then use a G1000HD: http://markgrantcables.co.uk/shop/index.php?main_page=product_info&cPath=43_1&products_id=165

It doubles up as a digital cable and uses the exact same Canare plugs as the 1694A! ;)

Marco.

Themis

07-02-2010, 08:21

Marco, for coaxial digital, impedance matching is essential. The use of 75ohms connectors and cable is mandatory if you want to match the digital equipments' designer specifications. And the cable used for digital is not the same for audio: the frequencies involved differ.

G1000HD are 75ohm ? It is not specified... :scratch:

Andre's cable is very good. I didn't advice him to change, he would gain probably nothing of such a change.
My advice was for the OP. ;)

Marco

07-02-2010, 08:37

Hi Dimitri,

I'm not doubting what you're saying - it makes perfect sense - but I've got both 1694A and G1000HD, and can say absolutely quite categorically that the G1000HD sounds much superior used either in a digital or analogue context :)

Look - Mark himself even recommends them here:

http://markgrantcables.co.uk/shop/index.php?main_page=product_info&cPath=40_3&products_id=183

The above are miles better than the 1694A as a digital cable - no question :cool:

Marco.

DSJR

07-02-2010, 15:31

if you can make something up yourself, the Aircel 7 wire should be easy to terminate into some decent phono plugs (which I thought were 50 ohm anyway.....). It has a very wide bandwidth and is designed for very long runs with negligible attenuation. Surely more than good enough for the 0's and 1's in a metre or two of digital interconnect.

http://www.diodecomms.co.uk/index.php?cPath=58&osCsid=70e12ef28c2ffed707d5deb31c8d55b6

If evrybody used decent connectors designed for a wide bandwidth transmission, there are some chunky looking connectors too.

Bloody phono plugs...................................

P.S. Impedance matching usn't necessarily an issue as many manufacturers used to have their own agendas on this, from Meridian to Linn and back.....

hifi_dave

07-02-2010, 15:56

I thought digital incons had to be 75 ohm and that they all sound the same. It's been measured, so it must be true....:scratch:.........:lol:

Kris

07-02-2010, 16:49

If there is anything overrated it's a digital cable, as long as it's built correctly with true impedance cable it's perfectly fine

:exactly:

A co-ax digital cable is transmitting ones and zeros. Either it gets it right (sounds good) or it gets it wrong (sounds bad). IMO.

S/PDIF was designed to use 75ohm cable and ordinary phono plugs. No need for 75ohm plugs.

I'm waiting for someone to say gold plated Toslink connectors sound better than non gold plated ones . . . .

Steve Toy

07-02-2010, 17:44

You'll have a long wait. There are good and bad digital interconnects though. Whatever the reductionist theory you cling to, the process of subjective listening iss not avoidable for digital i/cs.

Themis

07-02-2010, 18:03

:exactly:

A co-ax digital cable is transmitting ones and zeros. Either it gets it right (sounds good) or it gets it wrong (sounds bad). IMO.
S/PDIF emitters/receivers are designed for 75 ohms connections (cable+connectors).
If there's an impedance mismatch (and there's always one), then you have reflections. Some zeros and ones don't arrive when they should.

Ali Tait

07-02-2010, 18:21

Don't quite get this "zeroes and ones" thing.The cable is still passing an electrical current.

Themis

07-02-2010, 18:28

Don't quite get this "zeroes and ones" thing.The cable is still passing an electrical current.
Yes, but the receiver doesn't receive the bit emmited...
http://en.wikipedia.org/wiki/Reflection_%28electrical%29

Ali Tait

07-02-2010, 19:31

Yes,but you are never going to eliminate that.

Themis

07-02-2010, 19:50

Yes,but you are never going to eliminate that.
You are right, we can't, because neither receivers nor cables are exactly 75ohm.

If I'm not mistaken, some errors are corrected with CIRC correction, others are scattered in the signal, in an attempt to mask them.

In any case, even if life is not perfect, as I was saying, best is to keep to 75ohms when possible. Of course, any cable can do, but why "tempt fate" ? ;)

Ali Tait

07-02-2010, 19:54

Indeed.I believe a manufacturer was on here recommending using at least a 6m i/c,as apparently it minimises the relections.Anyone tried this?

Themis

07-02-2010, 20:09

I have two 6m coax cables (one made by Mike, here).
They are perfect with the Denon and most of the dacs.
They are less crucial for the Northstar, probably because of its ASRC receiver. ;)

To tell you the truth, as I use a variety of dacs and I don't want to get bothered with this (small) "problem" of impedance matching, I don't use sub-2m coaxes anymore (the price difference is not worth the trouble).

Kris

07-02-2010, 20:25

Themis

07-02-2010, 20:35

Lol Kris, I was talking about digital (S/PDIF) plugs not... phono (analog) plugs... :lol:
(the impedance is the receiver impedance, not the plug one)

Barry

07-02-2010, 20:36

For all those who insist on 75ohm phono plugs, don't forget to replace your phono chassis sockets with 75 ohm ones, other wise you'll have an impedance mismatch.

Oh dear, I just remembered, you can't buy 75ohm phono chassis sockets . . . :doh: :ner:

Neither can you buy 75 Ohm phono plugs. The geometry of the phono plug is such that it is impossible for it to have an impedance of 75 Ohms.

The specification for the Sony/Philips Digital InterFace (S/PDIF) calls for a 75 Ohm impedance cable, yet also specifies RCA phono connections. I would much prefer to see 75 Ohm BNC connectors used.

Regards

Themis

07-02-2010, 20:48

Neither can you buy 75 Ohm phono plugs. The geometry of the phono plug is such that it is impossible for it to have an impedance of 75 Ohms.
http://www.bluejeanscable.com/pages/technicaldocs/canarercap.pdf

Marco

07-02-2010, 20:59

Hi Dimitri,

Those are precisely the plugs Mark Grant uses on his G1000HD (and 1694A)....

For one of the best digital interconnects available (at ANY price), use the former with confidence :)

Marco.

Themis

07-02-2010, 21:16

Those are precisely the plugs Mark Grant uses on his G1000HD (and 1694A)....

For one of the best digital interconnects available (at ANY price), use the former with confidence :)

Marco.
I know Marco. I just didn't realize that Mark had a new 1000 model for coax.
Mark's cables are among the best, in my opinion. Independent of price. ;)

I know I'm a bit of exception here, I'm embarrassed to say that I buy digital coaxes on sheer specifications (contrarily to audio analog cables)... :o And Mark's are of the best pedigree.

Barry

07-02-2010, 23:16

http://www.bluejeanscable.com/pages/technicaldocs/canarercap.pdf

Canare may claim that their RCA phono plugs have a characteristic impedance of 75Ω, but if they conform to the usual geometry and construction, it is physically impossible for them to have such impedance. Being of a coaxial construction the characteristic impedance is determined by the geometry and dielectric filling just as it does for coaxial cables.

Specifically the characteristic impedance of an air filled coaxial line is given by:

60*ln (diameter of outer conductor/diameter of inner conductor).

For an RCA phono plug the diameter of the inner pin is 3.1mm and that of the outer conductor is 8.25mm. The impedance of air filled plug is thus 58.7Ω. Now there has to be some dielectric insulating support of the centre pin, and the presence of this will locally reduce the impedance by the square root of the dielectric constant of the material used for the support. Canare use Noryl PPO (polyphenylene oxide) for this. This material has a dielectric constant of 2.69, so reducing the overall characterisic impedance to 35.8Ω.

The only way the impedance can be raised to 75Ω is for the design to deviate from a coaxial geometry. Bullet plugs do this but have their own shortcomings, and when mated with a coaxial socket the impedance is reduced for the reasons explained above.

As to whether it is desirable to rigorously maintain a characteristic impedance of 75Ω throughout the digital interface, the following threads and posts might be of interest:

http://theartofsound.net/forum/showthread.php?t=3519&highlight=Digital+cable&page=5
posts: 46, 208 and 213,

and
http://theartofsound.net/forum/showthread.php?t=3994&highlight=75Ohm+%2C
posts: 8, 9, 30, 44, 46 and especially 52 and 53.

Initially I was sceptical that 75Ω connectors were necessary, however in learning that the sampling waveforms can have rise and fall times as short as 25ns, I changed my mind.

Clearly the use of phono plugs as connectors will work, but I believe that the use of (75Ω) BNC connectors would be far better.

Regards

Themis

08-02-2010, 06:54

Clearly the use of phono plugs as connectors will work, but I believe that the use of (75Ω) BNC connectors would be far better.

I agree. ;)

Unfortunately some drives/dacs don't have BNC connectors...

Mike

08-02-2010, 08:57

Oh dear... are we back to the 75ohm phono connector again? :doh:

Ali Tait

08-02-2010, 09:26

If you know what the actual impedance is,how about inserting a suitable resistor to make it 75 ohms?

Themis

08-02-2010, 09:35

Characteristic impedance is not something that is measurable with a volt-ohmmeter. It is an AC characteristic that is defined as: sqrt(L/C), where L is the inductance per unit length and the C is the capacitance per unit length of the transmission-line.
http://www.positive-feedback.com/Issue14/spdif.htm

Mike

08-02-2010, 09:35

If you know what the actual impedance is,how about inserting a suitable resistor to make it 75 ohms?

http://www.theartofsound.net/forum/images/misc/progress.gif

Shhhh.... :eyebrows:

Marco

08-02-2010, 09:40

If you know what the actual impedance is,how about inserting a suitable resistor to make it 75 ohms?

Isn't that something else for the signal to navigate and introduce distortion and/or coloration through increased resistance, no?

It's surely far better using the right type of connector in the first place to achieve the correct result - I hate applying 'bandages'! ;)

Marco.

Mike

08-02-2010, 09:42

Isn't that something else for the signal to go through and introduce coloration though increased resistance, no?

Better to use the right type of connector in the first place - I hate applying 'bandages'! ;)

Marco.

What a clown! :D

We've done this before, Marco. You should 'get it' by now. :rolleyes:

Marco

08-02-2010, 09:48

Clown? Thanks - no need to be so cheeky!

No I'm afraid that I don't get it, so perhaps you could explain it to me then clever clogs? ;)

How is fitting a resistor in-line better than actually having a true 75Ω connector present in the first place?

I await to be suitably enlightened :)

Marco.

Ali Tait

08-02-2010, 10:36

....or use a pulse transformer.

Marco

08-02-2010, 10:41

Wots one of dem, then - and how precisely would you use it beneficially in this particular application? :)

Marco.

Stratmangler

08-02-2010, 11:13

http://www.theartofsound.net/forum/images/misc/progress.gif

Shhhh.... :eyebrows:

You've got me intrigued here Mike.
I always thought that the impedence was relative to the frequency at which the signal was transmitted, but this implies other............

Chris:)

Barry

08-02-2010, 13:25

Oh dear... are we back to the 75ohm phono connector again?

Sorry, I didn't raise this spectre.

If you know what the actual impedance is,how about inserting a suitable resistor to make it 75 ohms?

It doesn't work like that. Inserting a series resistor, of say 39Ω , in an attemp to raise the local characteristic impedance to 75Ω will just introduce series resistive loss; resulting, in this case, in an attenuation of ~2dB. (20*log[1+(39/(2*75))] )

How is fitting a resistor in-line better than actually having a true 75Ω connector there in the first place?

It's not, for reasons explained above.

Frankly I'm puzzled why Sony/Philips chose a characteristic impedance of 75Ω and specified phono connectors when they wrote the specification for the digital interface (S/PDIF). I can only assume that the specification was written in the days when the sampling rate was 44.1KHz. With the advent of higher sampling rates (192KHz) and faster rise times of the sampling waveform, the strict maintenance of the 75Ω characteristic impedance becomes much more important.

I have seen rise times as short as 25ns (10% to 90%) quoted for the sampling waveform. This implies effective frequencies in the MHz region. I have also read that as long as the digital cable is shorter than 1.5m, or longer than 6m, there ought not to be any problems with timing issues (aliasing?) created by the impedance mismatch reflections. Unfortunately I don't know enough about digital techniques to comment.

Regards

Ali Tait

08-02-2010, 17:41

Me either,though I was told the resistor did improve the sound.

Themis

08-02-2010, 18:22

The problem with digital is that sometimes "better sound" is actually "less accurate"... :lol:

(joke)

Ali Tait

08-02-2010, 18:44

Well in that case,the sabre dac I heard with a PP 6n6p output stage is one of the least accurate dacs I've ever heard!

Themis

08-02-2010, 19:58

Joke apart, there's always a resistor (of about 100ohm) between ground/live on s/pdif outputs, if I'm not mistaken.

Mykhailo M

22-12-2015, 21:53

In my experience coaxial cables DO make a difference especially on Mid to High-End systems as they are responsible to deliver a sound wave between audio systems. I wouldn't advise on compromise and get to audition several top coaxial cables in a price range £200 + and get what YOU like. :)

CageyH

22-12-2015, 23:32

Getting better.
This one is only 5 years old. :eyebrows:

What is your favourite coax?

walpurgis

22-12-2015, 23:46

What is your favourite coax?

Those I make myself. They are much better sounding than the fairly costly Townsend item and others I had been using.

Oh. And if anybody says there can be no difference in the effect a digital cable has on the sound. Just experiment. The differences are not subtle at all. Just don't ask me why!! I didn't believe it either.

struth

22-12-2015, 23:52

I had one of Geoff's coax cables(bev has it now as it went with the timebase) and it was excellent. only reason it went was it was too small for my needs after selling the dac etc. I aint using any at moment but their are improvement to be had, and money aint everything. the fisual optical Ive still got is another superb bit of kit for little money

walpurgis

23-12-2015, 00:02

If, or when you need another. Let me know. :)

struth

23-12-2015, 00:06

If, or when you need another. Let me know. :)

Might be wanting a set of long rca's in future if youve still got that nice cable. Not sure where I'm going yet :D Maybe a digi is a possible too.but it would be longer

Time will tell as I'm getting confused with all the tech thats hitting this old yin:scratch:

walpurgis

23-12-2015, 00:19

Might be wanting a set of long rca's in future if youve still got that nice cable. Not sure where I'm going yet :D Maybe a digi is a possible too.but it would be longer

Time will tell as I'm getting confused with all the tech thats hitting this old yin:scratch:

No probs! Loads of spare cable here. :thumbsup:

Reffc

23-12-2015, 08:41

Sorry, I didn't raise this spectre.

It doesn't work like that. Inserting a series resistor, of say 39Ω , in an attemp to raise the local characteristic impedance to 75Ω will just introduce series resistive loss; resulting, in this case, in an attenuation of ~2dB. (20*log[1+(39/(2*75))] )

It's not, for reasons explained above.

Frankly I'm puzzled why Sony/Philips chose a characteristic impedance of 75Ω and specified phono connectors when they wrote the specification for the digital interface (S/PDIF). I can only assume that the specification was written in the days when the sampling rate was 44.1KHz. With the advent of higher sampling rates (192KHz) and faster rise times of the sampling waveform, the strict maintenance of the 75Ω characteristic impedance becomes much more important.

I have seen rise times as short as 25ns (10% to 90%) quoted for the sampling waveform. This implies effective frequencies in the MHz region. I have also read that as long as the digital cable is shorter than 1.5m, or longer than 6m, there ought not to be any problems with timing issues (aliasing?) created by the impedance mismatch reflections. Unfortunately I don't know enough about digital techniques to comment.

Regards

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.

Gazjam

23-12-2015, 09:31

Recently played around with getting the best sounding connection between my Transporter and Dac.
Glass optical were very good, better than RCA cox, but best of all (noticeably so actually) was properly terminated 75 Ohm BNC.
Importantly I think 75 Ohm impedance at both the Dac and Transporter end played a big part, no real benefit to be gained if this wasn't the case.
So it's not a BNC connection all on its own that makes the difference, it's what it's being plugged into as well.

Interestingly,
Cable choice can make a difference here too.

Initially had a Bluejeans BNC cable using Belden 1694a and Canare 75 Ohm connectors.
Sounded great, thought it was all I needed.
Then I took a punt on something I'd read to be "exceptionally good", which I posted about here:
http://theartofsound.net/forum/showthread.php?41208-Black-Cat-digital-cable-holey-moley!

A lot better sounding across the board, which kinda goes to show that it's not the BNC connection itself that makes the difference, but how it's implemented.

Oh, and digital cables CAN sound different and do make a difference, sorry sceptics! :)

mikmas

23-12-2015, 18:57

Gazjam

23-12-2015, 20:08

If the length is unimportant just get a 1.5m one!
:)

PaulStewart

23-12-2015, 21:33

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.

Reffc

24-12-2015, 07:22

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:rolleyes:. 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.

Gazjam

24-12-2015, 09:36

Good post Paul.

NRG

24-12-2015, 10:20

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.

mikmas

24-12-2015, 10:36

PaulStewart

24-12-2015, 12:48

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.

RMutt

24-12-2015, 15:04

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?

Gazjam

24-12-2015, 19:06

Stratmangler

24-12-2015, 19:22

It can't all be down to cost

D'ya wanna bet?
Just ask Jez :eyebrows: