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Word Clock distribution and sync
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Old 18th October 2006   #1
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Talking Word Clock distribution and sync

Hello

I'm posting a quetion here after I've been pondering it for some time and for lack of pratical experiences on it (due to either unavailability of the appropriate machines or occasion).

We know that in an all digital setup word clocking to a single master is paramount. It's also well known that each device should take its clock from a direct and dedicated output from the clock master. Now, suppose this last option isn't possible and that you have to daisy chain all of your devices to a single clock output from the master and link them with BNC T connectors . Is the clock sinal going to remain stable or can we suppose it will be degraded? If so, can we figure out an average number of units that could be locked that way without a significant clock degradation?
I'm asking this because I'm in the need to resolve my entire remote rig to a balck burst signl fed from the video crew that would be converted to WC via something like a Digidesign Universal Slave Driver, which would also accept timecode (whatever format) and convert it to MTC for my recorders. Space and budget limitations won't allow to get two units like (e.g.) a Rosendhal Nanosync and the USD, so I'll have to make do with just the one that can handle both clock and TC.
On a last note, how will all of this affect audio quality and converters behaviour?

Thanx

L.G.
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Old 18th October 2006   #2
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To keep it short. Wordclock was designed a long time ago to be daisy-chained.

One master.
All cabling and other things is the correct type of coaxial cable (there are several different inherent impedances to choose from, only one of them is right).
Each slave has a T-connector on the IN connector.
Daisychaining through the T-connectors.
Exactly one termination resistor, on the last in the chain.

Given short interconnects 10 unit should be able to be interconnected.

All this gives no interference on the sound. (Except that just about any quality card sounds better on the internal oscillator).

Of course some sound card designers has forgotten this and added all kinds of automatic terminations and stuff. You have to read the manuals on the cards.

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Old 18th October 2006   #3
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While some generators have multiple outputs, many people use a VDA to distribute black burst or clock. (sorry, I shoula have already equated VDA=Video Distribution Amplifier.) There are many dedicated products made to distribute TC anc WC, as well as reasonably priced, dedicated VDAs that do a great job. No need for the T's, and as you might find, at some point machines will start to drift from the impedance change. There is nothing worse than some amount of drife making your machines pop out of record at an opportune moment!!! The voice of experience, sadly.
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Old 19th October 2006   #4
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Thanx for the replys

As I can see, two quite different points of view.

So, can we assume that using strictly 75 Ohm cabling throughtout the chain and using a simple termination on the last device everything should work, right?
I was also thinking about using the WC output from the devices that can lock to an incoming signal and regenerate the same at their WC output, locking the following devices to that...am I going to get things worse? I'm also going to stay within, say 5 or 6 daisy chained devices, all within the same rack.

Jim, I know there's several dedicated video distribution devices, but I just wouldn't want to get another box (that costst and weights), I'd like to try and find a simpler approach that works with what I have. I understand your point though. Obviously, a testing pahse would be in order before going out for a real gig, should things go south with the T connected boxes I'd have to make up my mind and think about that dedicated box...

Thanx

L.G.
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Old 19th October 2006   #5
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Just a little note,
The worst clock in your studio should usually be the master ...
I thought it was the other way round but after an explanation from Graham (Prism) i understood ...good clocks deal with problems better then bad clocks, hope it makes sense.
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Old 19th October 2006   #6
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Quote:
Originally Posted by Gerax View Post
should things go south with the T connected boxes I'd have to make up my mind and think about that dedicated box...
Totally understood. I did not get my distro system solid until the daisy-chained system failed during live performance. Even Yamaha, with the PM1Ds, used "mil-spec" T's until they had enough problems to say, FINE. GET THE DISTRO!

Best of luck! May your daisy chain be rock solid and your pocket full!
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Old 9th December 2006   #7
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Love my Big Ben. Although according to Dave Hill at Crane Song there isn't yet a formal standard for Wordclock, so its best to try every configuration you can and use what sounds best. I have my Big Ben clocked to the Hedd and distributed henceforth.
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