# How many Word Clock devices on single cable?



## Peter Emanuel Roos (Feb 11, 2008)

Will my RME (M)ADI-648 be capable of driving 4 sampler PCs and an AD/DA unit daisy chained on one BNC cable? Are there different topologies? (using T-connector on the source for instance?)

TIA!

Peter


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## Frederick Russ (Feb 11, 2008)

I think the T splitter would be fine and terminate the the final device. Don't go "in" then "out" of the RME. Use the "T" to split the the input. If you've got 2 or 3 digital devices this should work fine. If you've got more get a good distribution box like Apogee, Lucid or Aardvark. 

Apogee states, "When the word clock input of connected devices is un-terminated, it's acceptable to chain a few devices with a BNC "T" connector on the word clock input." Their position is that the "T" connector more reliably maintains Word Clock's 75-Ohm impedance than the "In/Out" ports on many audio devices. So it is possible to feed two or three devices from one line.

Be advised though - the best way is using a WC distribution box - more expensive solution though.


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## Peter Emanuel Roos (Feb 11, 2008)

I meant a T-splitter on the MADI WC source. Would that work? Or would that divide the impedance by two?

I had no problems with chaining 3 samplers with T-splitters, but now I am adding 2 additional devices.


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## ComposerDude (Feb 11, 2008)

Some other issues: termination and loading.

Cables should be terminated _at the ends_ to prevent reflections from the open end. Typically a resistor at or slightly below the characteristic impedance of the cable would be used.

The number of devices you can operate on one cable may be limited by how much power they pull from the WC cable.

Devices may be internally terminated - ideally, switchable termination. If not switchable, then such a device should go at the end of the line.

The conventional model is driving the cable at one end, with termination at the other end (there may be internal termination at the driving end too).

-Peter


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## Nick Batzdorf (Feb 11, 2008)

You're not supposed to T word clock. I don't know what's going to happen if you do, but I do know that it's frowned upon. The Best People use a WC distribution box.


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## Peter Emanuel Roos (Feb 11, 2008)

Thanks Nick, Peter,

I hope daisy chaining will work, with the terminator(s) I have after the last sampler. Distribution box is too expensive for now :-(


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## synthetic (Feb 11, 2008)

I disagree. Using a T-splitter is better than daisy-chaining. Daisy chaining (if it's not a true throughput) can introduce a delay in the signal. 

The ideal is to have a distribution box, but T-junctions are an acceptable alternative. I've been on network broadcast remote trucks with much worse word clock distribution than that!


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## ComposerDude (Feb 11, 2008)

+1 to Synthetic.

Also, going via daisy chain - where you actually loop through the equipment - may introduce a short piece of different-impedance conductor, which can cause signal reflections.

Reflections occur at impedance discontinuities such as end-of-cable, and also can occur within the cable if it is not made with consistent impedance, _or if it has been damaged in some way._

I knew a nuclear physicist who took extreme care of his coaxial cables that were carrying some pulse signal - he said if a door closed on and pinched them, the permanent impedance change made the cables worthless for his experiment.

Also apparently if Cat6 cable gets tightly bent and kinked, that section - reportedly - becomes dicey or useless for top-speed Gigabit Ethernet.

This is one of the reasons why better manufacturers and better cable can give better digital audio results: if they more precisely control cable impedance along the cable's length at time of manufacture, you have fewer signal-degrading reflections.

But I digress.

Anyway, it's much safer to do the BNC T connector thing, and keep the length of the T stub short. Good luck!

-Peter


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## Dave Connor (Feb 11, 2008)

I agree with Nick, I wouldn't play around with clocking it's too vital. A decent distribution amplifier is the way to go imho.


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## redleicester (Feb 12, 2008)

Likewise I'm afraid - I had no end of clocking problems in the early days until I gave in and bought a master clock - so I would recommend at least a distributor, if not a master clock.

If you hunt around you may be able to find one of the last Drawmer M-Clocks (recently discontinued) which were as little as £500 but have a very solid clock, 8 outputs and several channels of SRC.


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## Peter Emanuel Roos (Feb 12, 2008)

Thanks guys for all the input.

For now I have absolutely to money for yet another box, so I will keep my fingers crossed to get it working with a single line, T-splitters and a terminator.

BTW: RME actually recommend using the WC-out on the MADI/PCI card as a repeater because of its high quality. That would give me two completely synced WC sources!

I will let you know later this week.

Cheers,

Peter


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## Dave Connor (Feb 12, 2008)

You can get a distribution amplifier at an electronics store for very little money. The master clock has to be of good quality but my understanding is the distrubution amplifier can be very basic.

Hope your current solution does the job in any case.


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