Daisy‑chaining word clock connections.
I'm trying to set up my digital recording system so that I don't have to do any repatching. I'm using a Wami Rack 24 audio card and breakout box, Line 6 Pod Pro and Bass Pod Pro, for recording guitar and bass respectively. For the S/PDIF audio side, I've decided to buy a Midiman Digipatch 12x6 digital patchbay to avoid repatching of the Pods in turn to the single S/PDIF input of the WR24. I can also connect my CD‑ROM and future gear to this.
And now here comes my question. I like to make the WR24 the word clock master in all cases. To achieve this, I have to connect the word clock out of the Wami to the word clock in of the Pods. The Wami has one word clock out. The Pods both have a word clock input, not an output, so I would still have to repatch the BNC cables carrying the word clock signal when changing between Pods.
How could I avoid this patching? Can I just make a split lead, or use asplitter? Will the word clock signal be OK if I do? If this won't work, is there a (cheap) device that sends out multiple word clock signals?
Gert‑Jan van der Hout
SOS Technical Editor and digital audio specialist Hugh Robjohns responds: I'm pleased to say that there's an easy solution... in theory! Word clock is a pretty simple square‑wave signal running at the sampling frequency of the system, and it can be 'daisy‑chained' between virtually as much gear as you like, as long as you are a little careful.
For your applications, all you have to do is take the word clock out of your WR24 card from its BNC connector and run it to the first Pod's WC input. Make sure you use a proper 75Ω cable designed to handle digits or video, equipped with 75Ω BNCs. Anything less will distort the shape of the clock signal and you will get all sorts of problems.
At the clock input of the first Pod, connect the cable to a BNC T‑piece connector (available from most electronics hobby shops and mailorder catalogues such as Canford Audio) and plug that into the Pod's clock input. The master word clock is now available at the Pod and you should configure the unit to clock to this new reference signal.
Then take another length of 75Ω cable, connect it to the open terminal on the T‑piece and extend it to the second pod's clock input. Again, connect through a second BNC T‑piece and configure the second Pod for external reference clocking too. On the open connection of this second T‑Piece you need to plug in a 75Ω termination to stop reflections back along the cable. This is nothing more complex than a pair of 150Ω resistors soldered in parallel between the pin and body contacts of a spare BNC connector. You can buy ready‑made terminators if you don't want to make your own. Everything should now work exactly as you want it to (see diagram).
With properly designed clock inputs, this kind of daisy‑chaining between equipment can be extended almost indefinitely, as long as the 75Ω termination is connected at the end of the chain. However, there is a potential fly in the ointment. If the Pod clock inputs are already terminated — ie. the manufacturer has already fitted the termination resistors internally — the clock signal will be diminished in amplitude with every additional piece of equipment connected to the chain. Eventually, there will be insufficient signal for anything to clock reliably. Usually, however, the clock input circuitry is sufficiently sensitive to cope with a few doubleterminated connections, but not many! It shouldn't cause a problem with just a couple of Pods.
You could check the specifications for the Pods in their user manuals, to see if their clock inputs are terminated, but manufacturers often don't provide this kind of information. A better method is to connect an oscilloscope to the clock cable at the end and take a look at the shape and size of the signal. If it is halved in size when you plug in the second Pod, or if the square waves look triangular or rounded, take off the end termination and see if that helps. If the signal is very jittery and confused, put the termination back on! There is a certain amount of trial and error involved here, with just a dash of black magic!
If you suspect double terminations, just plug the linking cable directly into the second Pod's clock input and don't bother with the second BNC T‑piece or termination plug. If you are not sure, try both ways and see which works best, but if in doubt, leave the termination plug on at the end of the chain.
For bigger systems where daisy‑chaining word clocks gets messy, a better solution, employed routinely by professional installations, is a word clock distribution unit, although these are relatively expensive.