I record guitar using a Line 6 Pod Pro going into a Roland VMC7200 mixing desk via S/PDIF. While the guitar is plugged in, every so often a little audio spike comes through the monitors from the Pod. It's not particularly loud but it is loud enough to ruin a perfectly good take. I have tried several different guitars, so that is not the problem. Can you tell me what this noise could be and how to stop it?
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Technical Editor Hugh Robjohns replies: The clicks you're hearing are most likely the result of an asynchronous clock problem. The timing of the individual samples in a digital audio signal is governed by what is called the 'word clock'. S/PDIF and AES-EBU signals both carry an embedded word clock, and the idea is that the device receiving the signal uses the embedded clock data to accurately synchronise itself to the source, and thereby receive and decode the digital signal correctly. However, if the receiving device is configured to use an entirely different clock source as its reference (perhaps its internal clock, or the clock from a different source) then the two devices will effectively be working independently of one another and will drift in and out of sync with each other.
If you are lucky, most of the time the two clocks will be close enough that when the receiver goes looking for a sample from the source, it will find one. However, sooner or later the receiving device will look for a sample and not find one, and that's when you'll hear the click! The rate at which the clicks occur is an indication of the rate at which the two clocks are drifting relative to one another. I came across one professional installation with asynchronous clocks where the clicks only happened once every several days!
What you need to do is make sure that the Pod and VMC7200 are reading off the same page, as it were — you need to make sure that both devices are governed by a single word clock. There are several possible solutions. The most appropriate one will depend on your precise setup, but any and all of these solutions will be effective.
The simplest option is to configure the VMC7200 to take its clock reference from the S/PDIF output of the Pod: set the Pod to run on its internal clock, and in the VMC's Digital I/O menu, set the Digital Clock Source to 'Digital In' and select the appropriate sample rate. This will stop the clicks from the Pod, but if you are trying to mix the Pod's signal with other sources at the same time they will all start clicking instead, unless they themselves are synchronised to the Pod's word clock! The downside to this solution is that it places the clock stability (and therefore jitter) of the entire system in the hands of the Pod's crystal clock circuit, which may not be a particularly wise idea!
If you're using the VMC7200 as the clock master (or if it is being clocked from something other than the Pod) the next possibility is to take a word clock signal from the VMC (or the master clock source) and use this to clock the Pod. Simply switch the Pod to accept this new external clock, connect the digital output from the VMC to the Pod's digital input (either AES-EBU or S/PDIF) and away you go. This is probably the best long-term solution, but it may be impractical from the point of view of cables and connections.
Finally, the last option would be to simply ditch the S/PDIF connection completely and run a good old analogue signal to the desk instead. There are no sync problems with analogue — it's the ultimate clock-free interface — and the quality of modern converters is such that there won't be any significant difference in quality anyway.