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Horloge externe

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Horloge externe

Message par Geoff Emerick de Fake le Mer 21 Oct 2009, 02:02

http://akmedia.digidesign.com/news/docs/WhitePaper_ClockJitter_30863.pdf

Un white paper un peu aride de Digidesign qui démontre que l'amélioration de qualité prétendument apportée par l'utilisation d'une horloge externe n'est pas attribuable à une diminution du jitter. Bien au contraire, sur les trois horloges externes testées (les modèles ne sont pas cités mais on peut se faire une idée d'après les commentaires), deux introduisent une augmentation conséquente du jitter.

Geoff Emerick de Fake
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Re: Horloge externe

Message par madees le Jeu 22 Oct 2009, 00:22

C'est sur

http://audioaddict.fr/phpBB2/viewtopic.php?t=313
Dan Lavry, Lavry Engineering :

You are correct to say "Dan Lavry believes that
external clocks can RARELY (if ever) make any system sound better than
using a device's internal clock".

I would go much further then that: it is not about what I believe, it is about engineering.

I believe that people hear a difference between external clock boxes,
and between internal and external clocking. Any and everyone is
entitled to their subjective likes and dislikes.

It is a fact that jitter creates rather unpleasant distortions (non harmonic) and in most cases it increases the noise floor.
Yet, one can decide they like more noise, harsh distortions... all that
is rather surprising, but not arguable because it is subjective.

But those that push the notion that an external clock will improve the
sound, are not just saying that the sound will be altered, that some
people will like it, others will not. The claim here is that there will
be an improvement. That claim really translates to "more jitter is
good". Of course no one is ready to state publicly that "more jitter is
good", and most often the proponents of external clocking will go out
of their way to convince you that they are providing with less jitter.
Less jitter the internal clock? A good internal clock provides a lot less jitter then the best of all external clocks.

In the case of internal clocking, one relays on a local fixed crystal
aimed at keeping the frequency constant. That crystal circuit would be
positioned near the AD, to minimize interference.

For external clocking, one may Begin with an equal quality crystal
circuit inside the clock box, but the signal has to go through a
driver, cable, receiver and yet another crystal circuit, all with some
frequency locking circuitry...All of that is a lot more jitter.

So in order to overcome the above stated reality, there are stories
floating around, that a clock box sends signals that improve the AD
clocking. This is of course impossible, because one can not improve
without knowing what needs to be improved. The clock box gets to send
the signal to an AD, but it is a one way communication, and the AD does
not tell the clock box what it is doing. In other words, a doctor can
not heal a patient without seeing talking or having any communication
with the patient.

The clock box can not send a better word clock then a simple square
wave with the lowest possible jitter. Putting the same clock inside,
eliminates many of the elements piled up in series between the clock
source and the AD.

Of course there are times when one needs to use external clocks, and in
such cases, a mediocre clock (say 25ps jitter is about as good as a
theoretical 0ps clock, because by the time the clock shows up the AD
most of the performance will be determined by the ability of the
receiving AD to reject the jitter. Given the same quality level of
design, an internal clock will be better, so external clocking it is a
trade-off, not an improvement. With good gear the damage is small, but
it should not be viewed as an improvement.

Most of the same concerns I raised regarding external clock would apply
to distribution amps. The AD jitter rejection is the main factor in
external clocking.
Of course there is no harm in doing all that one can to reduce the
jitter on the way to the AD. I would go for a low jitter distribution
amp, I hope you find one with low jitter specs. I would also try my
best to keep cables short when possible.

And last but not least, word clock has some advantages when compared to
AES black (no audio). Word clock is much more forgiving regarding
transmission line properties (cable impedance and termination accuracy).

_________________
"Caminantes, no hay caminos, hay que caminar" (Luigi Nono)
"Working the room, keeping people happy and relaxed is half of mixing successfully" (Mixerman)
George Massenberg said, "Get a clue. There's a reason mixing seems simple: it is simple . . . Learn how to do something useful; learn how to write. Learn how to do electronic CAD, and give me a call"
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Re: Horloge externe

Message par Geoff Emerick de Fake le Jeu 22 Oct 2009, 06:17

Dans un article plus récent, Dan Lavry revient sur ce sujet et dit qu'il a vu beaucoup d'installations controllées en WC qui souffrent d'un problème de décalage lié à la polarité du WC (en fait il s'agit de la polarité du récepteur de WC dans les appareils esclaves). Comme l'AES ou l'IEEE n'ont jamais édicté de standard (front positif ou négatif), on a un peu de tout (un certain nombre d'appareils permettent de choisir), on peut avoir soit une inversion gauche/droite (ce qui n'est pas très grave car on s'en aperçoit en général), soit un décalage d'un demi échantillon, ce qui n'est pas vraiment grave non plus mais devrait faire grincer les dents de plus d'un puriste!
Il attire l'attention sur le fait que du coup la synchro à partir de l'AES black représente une alternative intéressante, sous réserve d'utiliser de bons cables.

Geoff Emerick de Fake
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Re: Horloge externe

Message par madees le Jeu 04 Nov 2010, 16:52


_________________
"Caminantes, no hay caminos, hay que caminar" (Luigi Nono)
"Working the room, keeping people happy and relaxed is half of mixing successfully" (Mixerman)
George Massenberg said, "Get a clue. There's a reason mixing seems simple: it is simple . . . Learn how to do something useful; learn how to write. Learn how to do electronic CAD, and give me a call"
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madees
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Re: Horloge externe

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