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brainz2000
11-10-2009, 20:33
Looking for a piece of advice .....

My Beresford DAC is a good 4m from the DVD player I want to feed it occasionally.

If I run a coaxial digital out to the DAC, circa 5m once it has gone around the room, will I be losing much sound quality ?

If this is not recommended, then is the optical interface a better option?
(Can I get 5m of SPDIF optical cable?)

I am also hoping to feed a digital signal from a PC on the other side of the room.

I was planning to put a USB sound card near the DAC, and use a 5m USB to move the signal in the digital domain to the soundcard, thus minimising the signal path between the sound card and the DAC. Is this likely to be better than keeping the soundcard near the PC and then using long SPDIF to move the signal to the DAC?

Any guidance would be gratefully received ...

tim

:scratch:

DSJR
11-10-2009, 20:38
Am I right in suggesting a very low loss UHF co-ax cable at the very least?

There are wires out there (Ecoflex 10 and the air-gapped 7mm one they sell) that are measured in losses per Km at RF frequencies, so a few metres *shouldn't* cause any attenuation.

The most important thing is for the cable to retain a "clean" waveform to the DAC I think.

Themis
11-10-2009, 20:43
If you need a long digital cable, I would suggest a 6meter 75ohm coax cable. ;)

(there's even a theory saying that coax of 6m are better than the ones of 1m... go figure)

brainz2000
11-10-2009, 20:50
Dave

Was thinking of purchasing a custom made Van Damme cable designed for the purpose ....something like

http://cgi.ebay.co.uk/5m-Neutrik-Van-damme-SPDIF-Digital-Phono-SP-DIF-Cable_W0QQitemZ320096021522QQcmdZViewItemQQptZUK_C omputing_CablesConnectors_RL?hash=item4a8735ac12&_trksid=p3286.c0.m14

It isn't going to break the bank, but should be reasonably well made and uses the right materials I think ...

To date, I've used a short digital interconnects sold by Stan - but nothing 5m long ....

Lots of people seem to say that you can "hear" different co-ax cables, although I've none that I can compare in A/B mode so I cannot validate this ... just wondered if I'm about to downgrade the sound or whether it is not likely to make much difference ...

Is this the kind of thing you were suggesting ....

Thanks for the input

tim

Themis
11-10-2009, 21:00
Lots of people seem to say that you can "hear" different co-ax cables, although I've none that I can compare in A/B mode so I cannot validate this ... just wondered if I'm about to downgrade the sound or whether it is not likely to make much difference ...
Hi tim,

well, strangely enough, the better the equipment design (of player and dac) the less it benefits of "perfect" coax cables.

MartinT
12-10-2009, 06:53
If you need a long digital cable, I would suggest a 6meter 75ohm coax cable

Seconded, and make sure it is properly 75ohm cable. It will perform much better than 6m of optical as the jitter levels will be a lot lower (light bouncing around the side-walls of optical cable create smear in the waveform rather than nice sharp edges).

Labarum
12-10-2009, 16:46
Dave

Was thinking of purchasing a custom made Van Damme cable designed for the purpose ....something like

http://cgi.ebay.co.uk/5m-Neutrik-Van-damme-SPDIF-Digital-Phono-SP-DIF-Cable_W0QQitemZ320096021522QQcmdZViewItemQQptZUK_C omputing_CablesConnectors_RL?hash=item4a8735ac12&_trksid=p3286.c0.m14
tim

Anyone tried the analogue cables?

http://stores.shop.ebay.co.uk/Pro-Audio-Supplies__W0QQ_armrsZ1QQ_fsubZ6767566

Good prices. QED cables start around £20.

Chippy_boy
14-10-2009, 15:42
Seconded, and make sure it is properly 75ohm cable. It will perform much better than 6m of optical as the jitter levels will be a lot lower (light bouncing around the side-walls of optical cable create smear in the waveform rather than nice sharp edges).

Because of friction presumably. Light rubbing against things etc.

What a load of tosh.

The (alleged) higher jitter in an optical connection is due to the electical>optical and then optical>electrical converion going on at both ends, not what happens in between. For long runs you are actually better off with an optical interconnect! A good toslink cable is good for 100ft or more.

MartinT
14-10-2009, 16:55
Because of friction presumably. Light rubbing against things etc.

What a load of tosh

Err no, it's because different packets of light take different paths as they travel along the fibre. Plastic fibre (which is used in most Toslink cables) is worse than glass in this respect. It's a well-known phenomenon. But what do I know, I'm only an electronics engineer who took opto-electronics as a degree module all those years ago.

Chippy_boy
03-11-2009, 15:00
Err no, it's because different packets of light take different paths as they travel along the fibre. Plastic fibre (which is used in most Toslink cables) is worse than glass in this respect. It's a well-known phenomenon. But what do I know, I'm only an electronics engineer who took opto-electronics as a degree module all those years ago.

Sorry for the delay in replying.

Your quals have perhaps clouded your common sense.

Different packets of light don't take different paths. How on earth could they. Is the emitter to send some of them out sideways? Yes, it is true that each and every packet will go through many internal reflections along the fibre and I can also accept that this will lead to a smearing of the signal and a narrowing of an eye diagram. But that smearing will apply to each and every packet equally.

i.e. the temporal smearing of the data stream will not introduce bit-to-bit timing errors (i.e. jitter) because any time smearing is consistent across data packets. The fibre would have to be vibrating wildly in order to introduce randomness into the path of each packet for that to be the case.

As I said above, most of the jitter is caused by the electro-optical conversion and vice-vera, not by the cable.

MartinT
03-11-2009, 15:37
Group delay of an optical fibre causes pulse broadening at the receiving end. This is, in effect, jitter. It can and does affect the accuracy of the D to A process and therefore the analogue end result.

Your

Different packets of light don't take different paths

Conflicts with

Yes, it is true that each and every packet will go through many internal reflections along the fibre

The many internal reflections ARE different paths.

More to the point, I have heard the effect of a long optical fibre compared with a long co-ax cable, all other things being equal, and it is quite audible. This demonstration was given by Paul Miller, a man who knows a thing or two about audio reproduction.

NRG
03-11-2009, 21:21
Fibre optic cable 101....

http://www.americantechsupply.com/howfiberopticsworks.htm

Chippy_boy
04-11-2009, 09:28
Group delay of an optical fibre causes pulse broadening at the receiving end. This is, in effect, jitter.
No, it isn't.


It can and does affect the accuracy of the D to A process and therefore the analogue end result.
Debateable. This could be possibly be true, depending on the gear, but is not necessarily true.


The many internal reflections ARE different paths.
You miss the point I think. Although each packet is reflected many times, each packet follows *the same* path. Therefore there is no packet-to-packet path variation, nor timing variation, which is what your original post (and to a lesser extent your subsequent one) implies.

Therefore there is no increased jitter in the incoming signal, merely a smearing of the signal, which may or may not induce jitter in the d-a process, depending on how it operates.

MartinT
04-11-2009, 10:22
Although each packet is reflected many times, each packet follows *the same* path. Therefore there is no packet-to-packet path variation, nor timing variation

If the opto-sender was a laser and the light was coherent, you might have a point. However, typically the opto-sender is an LED and therefore its light takes different paths. The end result is a squarewave with rounded sides and frequency domain noise which causes the typical Phase Locked Loop to experience timing jitter.

I shall move on from this debate as it's getting a little tired. People should judge for themselves as the final arbiter is how it sounds, and co-ax usually wins.

Chippy_boy
04-11-2009, 10:46
If the opto-sender was a laser and the light was coherent, you might have a point. However, typically the opto-sender is an LED and therefore its light takes different paths.

I don't know why you keep saying this: it's nonsense. How coherent or not the light source is has b*****r all to do with it. The fact is that each packet follows the same path. It is ABSOLUTELY NOT a case that one packet goes one route and the next one takes another route. That notion is just plain wrong however you look at it.


The end result is a squarewave with rounded sides and frequency domain noise which causes the typical Phase Locked Loop to experience timing jitter.

This part has more sense in it. The cheap opto-electrical receivers that are commonly used have a wide tolerance of circa 20ns of signal "rounding" as you put it and they are notoriously prone to introducing jitter as the point at which they will trigger is not well controlled. This is where the jitter comes in.

Marco
04-11-2009, 11:37
Hi Chippy,

I'm not sure that I like the tone of your last post to Martin. It is rather condescending, which unfortunately seems to be a feature of the way you sometimes communicate with people.

Please remove the "probably best" remark and try not to be as patronising in future. If you think someone is wrong, then by all means say so, but in a more considered and respectful way, please.

Marco.

MartinT
04-11-2009, 13:07
I don't know why you keep saying this: it's nonsense. How coherent or not the light source is has b*****r all to do with it. The fact is that each packet follows the same path. It is ABSOLUTELY NOT a case that one packet goes one route and the next one takes another route.

Look, I'll have one more go at this, in the spirit of lively debate :)

A laser emits coherent light: look at it from an angle and you can't see any light coming from it. An LED's light is not coherent: look at it from an angle and you will still see some light coming from it. Butt it up to an optical fibre cable and light will enter it at lots of different angles. It stands to reason that different 'packets' of light will take different paths!

Chippy_boy
04-11-2009, 14:43
Look, I'll have one more go at this, in the spirit of lively debate :)

A laser emits coherent light: look at it from an angle and you can't see any light coming from it. An LED's light is not coherent: look at it from an angle and you will still see some light coming from it. Butt it up to an optical fibre cable and light will enter it at lots of different angles. It stands to reason that different 'packets' of light will take different paths!

Martin - In response to Marco's post, I offer my humble apology if my tone was in any way offensive. I don't know how I could have better explained forcefully what I was trying to say without a similar form of words, but there you go.

Now, to allow me to answer your point with a question: Why do you think the direction of light output from the led would be different from one packet to the next? That's the critical part: from one packet to the next. I am not disputing that light will come out of the led at all sorts of angles and hit the insides of the fibre all over the place, but why would this vary from one packet to the next?

The answer is, it would not. Whatever physical property of the led that sends the light down the fibre in a particular direction so that it hits the internal walls of the fibre in particular places, does not vary from one packet to the next. How and why could it? Sure individual photons go in apparently random directions, but there are billions of photons in each packet and the behaviour of each packet is essentially identical. This isn't an argument about coherence or heisenberg... it's an argument about geometry. The geometry of the emitter, the housing and the cable dictate the internal reflections and this is static. It is certainly not packet-dependent.

(By the way - I have an honours degree in Physics from Imperial College and although most of what I have learned has long since been forgotten, I do remember enough about electromagnetism, coherence and basic optics.)

Themis
04-11-2009, 14:49
I wonder whether, the right question would be "does the optical media influences the moment each packet arrives, comparatively to when it is emitted". Then, if the answer is "yes", we could ask whether this interval difference comes from the way signal is propagated through the media.
What do you think ?

MartinT
04-11-2009, 15:07
Perhaps we are talking at cross-purposes? Are you referring to a packet as a bit of data? I am thinking more fundamental than that, i.e. particle/wave. Your reference to billions of photons made me realise that we are talking at different scales!

By the way, no offence was taken!

Chippy_boy
04-11-2009, 15:25
Perhaps we are talking at cross-purposes? Are you referring to a packet as a bit of data? I am thinking more fundamental than that, i.e. particle/wave. Your reference to billions of photons made me realise that we are talking at different scales!

By the way, no offence was taken!

I see, perhaps we are at cross purposes then. Of course individual photons will go different routes.

But although the fibre may be carrying millions of bits per second, nevertheless each bit - each optical "high" state - comprises billions of photons, for which the average paths have been the same.

It's a bit like the old two slit diffraction experiment where you turn the laser intensity down to single photons at a time. You can never be sure where any individual photon will go, but you know what the aggregate picture will look like, averaged over billions of photons. You will always get the same pattern.

Same thing in the toslink example. Each "bit" of information coming down the fibre will be identical in terms of the smearing that has occurred. Each will be compromised of billions of photons that have all gone different routes, but on average the make up of each bit is the same.

That is why you don't get any jitter in the fibre per se.

However, to a certain extent, I am arguing over semantics. The smearing of the waveform *does* cause the receiver to induce jitter because for each packet it has to do it's best to work out when exactly the transition from 1 to 0 or 0 to 1 is supposed to have occurred... and the cheap Toslink receivers (they only cost a few cents each) do a pretty bad job of it.

Marco
04-11-2009, 16:30
Hi Chippy,


Martin - In response to Marco's post, I offer my humble apology if my tone was in any way offensive. I don't know how I could have better explained forcefully what I was trying to say without a similar form of words, but there you go.


You could start by removing the rather patronising "probably best" remark from your post #15, as I requested earlier ;)

{Edit: s'ok, I've done it myself to save you the bother}.

Marco.

Joe
04-11-2009, 18:01
Scientists, eh! What are they like?

Marco
04-11-2009, 18:09
Indeed - they're generally full of themselves, or full of something anyway! ;)

Marco.

Barry
04-11-2009, 18:12
Indeed - they're generally full of themselves! ;)

Marco.

Oi Marco! - I'm a retired scientist myself! :eyebrows:

Marco
04-11-2009, 18:19
Yes, but anyone with a penchant for good claret is ok in my book :eyebrows:

Marco.

Spectral Morn
04-11-2009, 18:20
Oi Marco! - I'm a retired scientist myself! :eyebrows:


A Gentleman scientist....:)



Regards D S D L

Themis
04-11-2009, 18:34
"Imagination is more important than knowledge"
A.Einstein

"Incorrect fact observing is more perfidious than incorrect reasoning"
Paul Valéry

chrism
04-11-2009, 18:56
"Bulls pool baffles brains"
chrism.

Marco
04-11-2009, 19:00
"May all your ups and downs be between the sheets".

The mad Marco-Boy 2009.