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NJB's picture
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Optical Cable or Digital Coax
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I know that people love cable topics (:wall: ), so here is a question for all you experts. 

I want to connect my iPod dock to my DAC.  I have a choice of fibre optic cable or digital coax.  The distance between the components is measured in fractions of a meter.  Clearly, the signal that I wish to transfer is digital and so I just have to get a stream of '0' and '1' from one place to another.  Any errors will cause the DAC to apply error correction and thus 'make up' the music based upon its algorithms.

With digital coax I might have signal losses at the connectors and in the cable.  These are less of a problem (provided that the DAC can still distinguish between the '0' and '1') than any VSWR issues which could lead to bitstream errors and error correction in the DAC.  On a short cable the propogation losses do not really need to be factored in, but the effect of external interference does and so effective screening is important.

With fibre optic I have to convert the signal to a light stream and decode this at the other end.  This introduces a new process which will have its own error correction circuitry but the VSWR risk is much reduced.  Again, propogation losses are trivial, and this time interference is unlikely.

So, I suspect that a reasonable quality coax will provide a more accurate data stream at the DAC as the optical conversion process carries the most risk of creating errors.  Any views?

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RE: Optical Cable or Digital Coax

NJB wrote:

 The distance between the components is measured in fractions of a meter.

 

given that statement I'd be suprised you'd get any chance of any errors through either choice, other than a faulty cable.

 

Why not pick up a cheapy optical and co-ax cable and see if you hear anything, then take it from there, as without being in your house and listening to your system, i feel it's a bit of a shot in the dark to say one or the other...  To be honest, I'd be surprised if you head any difference between either.

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RE: Optical Cable or Digital Coax

Try both and see which you prefer, if you can hear a difference at all.

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RE: Optical Cable or Digital Coax

You're making a problem where none exists in reality.  Buy Fisual off the web and plug them in.  If the cable is a duffer the signal won't pass.  Otherwise the issue is pretty academic and not an audible one.

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RE: Optical Cable or Digital Coax

NJB wrote:

I know that people love cable topics (:wall: ), so here is a question for all you experts. 

I want to connect my iPod dock to my DAC.  I have a choice of fibre optic cable or digital coax.  The distance between the components is measured in fractions of a meter.  Clearly, the signal that I wish to transfer is digital and so I just have to get a stream of '0' and '1' from one place to another.  Any errors will cause the DAC to apply error correction and thus 'make up' the music based upon its algorithms.

With digital coax I might have signal losses at the connectors and in the cable.  These are less of a problem (provided that the DAC can still distinguish between the '0' and '1') than any VSWR issues which could lead to bitstream errors and error correction in the DAC.  On a short cable the propogation losses do not really need to be factored in, but the effect of external interference does and so effective screening is important.

With fibre optic I have to convert the signal to a light stream and decode this at the other end.  This introduces a new process which will have its own error correction circuitry but the VSWR risk is much reduced.  Again, propogation losses are trivial, and this time interference is unlikely.

So, I suspect that a reasonable quality coax will provide a more accurate data stream at the DAC as the optical conversion process carries the most risk of creating errors.  Any views?

In answer to your questions.

S/PDIF does not have error correction, but does have error detection.  As far as I am aware, most DACs just ignore frames with incorrect parity.

Over a short distance, both Coax and Optical will be 100% error free.

The light conversion process for Optical does not have error correction or detection.

With correctly terminated Coax, VSWR issues are negligable.

 

Having said all that, if you want to choose between them, the arguments are:

Optical provides galvanic isolation, but is more likely to introduce jitter than Coax

Coax may transport ground plane noise through the screen to the DAC, but has better jitter characteristics than Optical.

You pays your money, and you takes your choice.

 

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RE: Optical Cable or Digital Coax

The optical connection is more transparent. Smile

Joking aside, I've tried both between my Sonos Connect and DacMagic and couldn't tell a difference

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RE: Optical Cable or Digital Coax

So, I have spent some time today running back to back comparisons.  The back story, just so you don't think that I am a complete cheapskate, is that I bought an external DAC but have no proper cables.  I have a real cheapo optical cable which cost £3.99 off the web some years ago, and a digital coax that was £5 from the local electrical shop.  The afternoon was interesting. Both cables sounded far too bright at the start, but the hifi was cold and it takes a while to mellow.  From then one, it was a no brainer.  The optical link is far and away the better connection.  There is more bass, better balance and more detail.  I also spent the time trying to believe that they sounded the same as my brain says they ought to be identical, but they are clearly not.  How much is due to the cheap cables, and how much is due to the optical and coax circuits in the iPod Dock and DAC remains a mystery.

I have just ordered some better connections, I am in Switzerland and the Swiss do not seem quite as hifi-centric as the UK, so I have ended up going for some German Inakustik cables that are still a rarity in the UK but popular over here.  They have to be better than the ones that I have and I might need to repeat my comparison once I have a better coax link.

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RE: Optical Cable or Digital Coax

It would be great if you posted the result after trying with a better coaxial cable. I would have thought that differences are due to the electrical and optical S/PDIF implementations.

What devices are you connecting?

Arcam irDac • A19 • P38 • Kef R100

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RE: Optical Cable or Digital Coax

I am connecting a Pure i20 iPod Dock to a Beresford Bushmaster 2 DAC.  

Unsleepable, you comments match my thoughts. It is probably more to do with the electronics at either end rather than the cables.  However, that is a touchy subject on this forum because the world is split down the middle on whether digital cable performance matters at all. What I can say, stating the obvious really, is that the optical solution changes the digital stream to optical and back, which has 2 more chances to introduce errors. Also, if the DAC does not try to reconstruct faulty data and just ignores it, as it has been previously mentioned on this thread, then timing errors etc become a problem and the fidelity of the bitstream from the i20 dock is critical.

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RE: Optical Cable or Digital Coax

You know its a pity you cannot go up budget on both cables both coax and optical. The reason I say this is having tried both - cheapo optical cable, came free with XBOX 360elite - my preference is coax more body to the sound through my amps DAC saying that the coax cost from cheapo freebies upto 50£ - the best sounding cost 25£.

anyhoo let us know how you get on.

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RE: Optical Cable or Digital Coax

Said it before, say it again, Fisual.  Great build, cheap as chips, performance excellent.  I've got a QED optical cable that was about £20.  You couldn't tell them apart.  Seriously guys, wake up...    :O

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RE: Optical Cable or Digital Coax

If it is digital it makes xxxx all difference.

Chris

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RE: Optical Cable or Digital Coax

NJB wrote:

I am connecting a Pure i20 iPod Dock to a Beresford Bushmaster 2 DAC.  

Unsleepable, you comments match my thoughts. It is probably more to do with the electronics at either end rather than the cables.  However, that is a touchy subject on this forum because the world is split down the middle on whether digital cable performance matters at all. What I can say, stating the obvious really, is that the optical solution changes the digital stream to optical and back, which has 2 more chances to introduce errors. Also, if the DAC does not try to reconstruct faulty data and just ignores it, as it has been previously mentioned on this thread, then timing errors etc become a problem and the fidelity of the bitstream from the i20 dock is critical.

I believe the DAC cannot reconstruct data coming from a S/PDIF port if it gets corrupted. There isn't enough redundant information for that. There is a single parity bit that will allow the receiver to detect up to 1-bit erros, but that's all. I think andyjm summarized pros and cons very well in his previous post.

I also agree with him that errors are very unlikely in this kind of point-to-point links. When errors occur, they won't normally be single frames, and then should be clearly audible.

And for some reason it is generally assumed that jitter affects TOSLINK more than coaxial connections. And I am not saying that this is not true, but honestly, I don't know why… The dependance of the receiver on the clock of the source to recompose the audio for real-time playback is exactly the same whether electrical or optical.

Arcam irDac • A19 • P38 • Kef R100

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RE: Optical Cable or Digital Coax

unsleepable wrote:

And for some reason it is generally assumed that jitter affects TOSLINK more than coaxial connections. And I am not saying that this is not true, but honestly, I don't know why… The dependance of the receiver on the clock of the source to recompose the audio for real-time playback is exactly the same whether electrical or optical.

S/PDIF is sent as a self clocking bitstream, the clock is embedded in the data.  This is done using a technique called 'manchester biphase mark encoding'. In very simple terms the bitstream changes state at the end of every bit, but also changes state in the middle of every '1'. One implication is that '0's effectively have half the frequency of '1's.

Every system is band limited, and in a band limited system the detection point of a '0' can be different to the detection point of a '1', given the double frequency nature of the encoding. This can lead to jitter being introduced into the recovered clock. For those interested:

http://audioworkshop.org/downloads/AES_EBU_SPDIF_DIGITAL_INTERFACEaes93.pdf

Anyway, the upshot is that generally the TOSLINK implementations in audio equipment have worse frequency response than coax (of the datalink, not the resulting audio) because of the additional steps required and this worse frequency response translates into additional jitter on the recovered clock.

To make matters worse, this jitter is 'code correlated' which means it is related to the programme signal in some way, and is not an entirely random process.  Correlated jitter has been found to be more easily detected than random jitter.

The truth is that the S/PDIF interface was good for its time, but is now past its sell by date. The clock in a DAC needs to be next to the D2A converter chip, and that means that the DAC needs flow control on the incomming data. Async USB or some other protocol with flow control is the way to go. 

 

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RE: Optical Cable or Digital Coax

You forgot to say (andyjm) that the coax cable always seems more secure than the horrid oprical.. Smile

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http://www.whathifi.com/forum/home-cinema/lounge-hc-signature-update-bass-traps

 

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RE: Optical Cable or Digital Coax

Covenanter wrote:

If it is digital it makes xxxx all difference.

Chris

True for the data, but the clock which is embedded into the data is a delicate thing. See my note above.

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