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Microphone Month 3

Master Clock in Digital Audio: Daisy Chain or Not?

The application of digital audio devices in the home and pro studio has created its own set of unique technical issues and questions. Many of these have to do with clock sources and their use, as this one does:

Q: “I have three audio interfaces that each have Word Clock In and Word Clock Out connectors on the back. I know I need one master clock to control them all. Can I daisy chain the clock signal from one device to the next?”

A: Our definitive, authoritative answer is, “No, but…” The concept of a master clock source for digital audio is to ensure jitter-free performance during recording and playback. Anything that causes deterioration in the clock signal path – whether it’s voltage loss or variations in impedance in the clock signal – will increase jitter and result in a reduction of quality at the converter. So most clock generator manufacturers strongly advise that you not daisy chain using the in-to-out connectors on your audio devices. One tech rep put it this way: “Every (passive) daisy chaining of clock lines degrades the quality of your clock signals regarding their jitter performance and voltage level.”

Think of it this way: it makes no sense to spend money for a high-end clock generator and then degrade its performance by daisy-chaining the lines between the remaining devices. Apogee states in their manuals: “Each digital device should be connected directly to the Master Clock Source with the shortest possible cable, in a ‘star network‘ configuration.”

However, Apogee goes on to say, “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.

A number of available master clock sources have multiple Word Clock output jacks so you don’t need to daisy chain. Among these are Apogee’s Big Ben and Aardvark’s AardSync II.

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