En fait me suis trompé de lien,
voirhttp://soundcheck-audio.blogspot.com/20 ... ox-20.htmll et scroll vers le bas pour voir le sujet sur le coax.
Il explique le probleme de la sortie coax :
2.3. SPDIF and Toslink
This is one of the key subjects. If you don't have your DATA link, the SPDIF link in particular, under control you'll face severe signal degradation and jitter on the receiver.
One word about Toslink. Toslink sounds pretty good if you use top quality glassfiber. (e.g. Van den Hul Optocoupler II). However, you still got another active stage, and a two stage conversion in the line. It's OK but not the final word.
A well done SPDIF implementation ( the whole link from sender chip to receiver chip) can beat any Toslink connection.
The real life problems with such an implementation are more then challenging though.
There are galvanic connection issues, rather poor 75R characteristic impedance implementations, unmatched terminations causing severe reflections, poor connectors, poor cables, able lengths discussions asf.asf.
All those above issues will cause extra jitter on the receiver side. And that jitter adds to the receiver jitter generated by the receiver itself..
As far as I can judge the Logitech part of that implementation looks pretty basic. They skipped a pulse transformer on the SPDIF output, as you'd find it on high quality SPDIF implementations and the RCA jack is not even 75R compliant . Those run roughly at 35-50R. This will cause reflections.These are the first very obvious shortcomings. A BNC would have been the better choice.
Here you'll find an interesting read about SPDIF by Steve Nugent.
To get a well working SPDIF connection in place must be considered a major
challenge. Without a scope at hand it's gonna be like gambling.
Advise1: Don't get fooled by super expensive SPDIF cables. I figured ( thx to
Robert Bastani from Bastanis speakers that one issue is caused by
HF bridge effects. Just attenuation the HF mess with his filters on his
filter-cables caused a serious improvement.
A pulse transformer costs far below 10$ and will solve quite some more issues.
In any case -- High-Price cables won't solve the actual key issues!!!!!
It's really the end-points causing all the trouble.
Some thoughts: Potenial SPDIF mods
On the SPDIF link there'll always be reflections jumping back and forth.
I hope you've read Steves article. (There are many more articles and
discussions on the net. Interesting to see that if 10 people measure
something they get 10 different results. )
Since not a single receiver is matched to the Touch output and i'd
assume that the perfect 75R termination won't exist, we all face
rather severe reflections on the link.
And those reflections will cause signal degradation thus jitter.
What to do? No, I don't have a solution. It needs an in-depth analysis of the Touch
SPDIF output and your DAC input. The RCA of the Touch must be removed, since it is not 75R
compliant. Crimped BNCs would be the much better choice here.
All this stuff would have to be measured properly. Otherwise it's really like fishing in the dark.
After scanning the net for a while and playing around with the stuff, reasonable improvements can be
achieved by
a. getting the characteristic impedance/termination right and
b. by increasing the slew rate of the signal slope, which will reduce the impact of reflections.
c. potentially introducing an RF attenuator ( google for jkenny M2Tech HiFace mods) not to
primarily lower the signal but to lower the 1st and following reflections amplitudes as well as other
HF mess on the link.
d. to get some galvanic isolation with a pulse transformer in place. (check your DAC if there is a
transfomer in place - quality DACs should have one in place. Note: Two transformers on one link
(input and output) might be too much!!
e. forget finding the right cable length, as mentioned by the "specialists". You'll find X different
recommendations from X different specialists. The "right cable length bingo" is meant for finding
the window for optimum signal/reflection ratio. I can tell you that without a scope, you'll gonna
end up fishing in the dark again.
That's about it. Looks like a piece of cake, doesn't it?
My current (DIY) SPDIF solution:
1. RCA on Touch removed, cable soldered right to the main-board.
(This makes a difference!) The best connector is no connector!
2. Cable length 12inch (Just look for a quality cable - no high-end stuff required)
3. Pulsetransfomer (Nevawa S22160 Digikey) on receiver, soldered to board.
4. Fixed soldered connection between Touch and receiver
