Fig. 1. All MOTU I/O boxes use a quasi-star configuration for distributing master clock throughout the system. The MOTU 24I/O is shown here.
IF YOUR recording rig is digital, you need to sync every device in your system to the same clock. Ignore this precept, and your sessions will be plagued by clicks, pops, and drop-outs—if your gear even passes signal at all. But that’s not the end of the story. The specific clock you choose as master—and the way in which you distribute clock to all your digital devices—can make a big difference in sound quality.
An inferior clock and poorly configured clock distribution cause jitter. In essence, jitter is dynamic deviation from the true sampling frequency of a digital clock. When that deviation causes digital 1’s and 0’s to arrive at a downstream device just a hair sooner or later than its nominal sampling frequency specifies it should, audio quality suffers. The stereo image narrows, localization in the stereo field becomes imprecise, bass instruments lose focus and get flabby-sounding, high frequencies sound harsh and glassy, and the overall sound becomes slightly veiled. On the other hand, a rock-solid clock distributed optimally makes for a tight, crystal-clear, warm, silky-smooth sound with pinpoint imaging. In this article, I’ll offer some tips for firming up the clock throughout your system. The first step is to choose the best device to serve as clock master.
Pick the Timekeeper If you’ve got the greenbacks to spare, buy a dedicated master clock; Apogee Electronics and Antelope Audio both offer excellent products. If that’s not an option, determine which of your digital boxes has the most stable clock. Make test recordings of acoustic guitar or some other delicate, full-range instrument, using each of your digital devices as clock master in turn. (It will help if you play the same simple, short phrase for each test recording.) Play back your recordings. The clock that produced the best-sounding recording should be used henceforth as master clock for your entire system.
Don’t Scrimp on Cables Use cables purpose-built for digital audio to carry all your clock (and digital audio) signals. Standard coax and mic cables are bad substitutes for S/PDIF and AES/EBU cables because they don’t have the bandwidth and impedance digital audio requires. Using analog cables for clocking is a recipe for runaway jitter and dropouts.
Use Star Configuration Daisy-chaining word clock from one device to another produces progressively amplified jitter as you move downstream in the signal chain. You’ll achieve the lowest jitter throughout your system by distributing master clock in a star configuration. In such a setup, master clock is distributed directly to each digital device in your system using discrete outputs.
Most digital devices provide only one word clock output, making it hard to use them to forge a star configuration. If, however, your designated clock master has an unused AES/EBU or S/PDIF output, you can use it together with its word clock output to get the job done. For example, you can patch the AES/EBU output on your master-clock device to your I/O box, and connect the master device’s word clock output to your digital mixer. The clocks for the I/O box and mixer will be phase-locked to the master clock and to each other. In this case, the AES/EBU cable doesn’t send any audio but only clock signal.
Fig. 2. In the MOTU PCI Audio Setup software utility, an HD192 I/O box is slaved to AES/EBU clock provided by an external converter. The HD192 automatically passes the clock on to all other I/O boxes connected to the PCI-424 card via their respective Audiowire cables.Syncing Multiple I/O Boxes MOTU implements a proprietary, quasi-star configuration for distributing master clock to two or more of its I/O boxes. In a PCI system (like the MOTU 24I/O and HD192 I/O boxes use), the box chosen as master sends clock signal over Audiowire cable—FireWire cable that uses MOTU’s proprietary data format—to a PCI-424 card installed in an expansion slot in your computer. The card then distributes the clock to each of the other I/O boxes in your system over discrete Audiowire cables.
MOTU’s FireWire/USB I/O boxes (such as the UltraLite-mk3 Hybrid and 896mk3 Hybrid) work slightly differently. They use an aggregate driver to distribute word clock. As with PCI-424-based systems, you choose one I/O box to be clock master. But instead of an expansion card distributing the clock, the computer itself distributes its host clock to the other boxes in the system via their FireWire or USB bus.
If you own a very high-end A/D converter with a clock that’s superior to those in your MOTU I/O boxes, slave one of the I/O boxes to the converter’s word clock, AES/EBU or S/PDIF output. The recipient box will send the clock on to all the other MOTU I/O boxes in the system. Any unused digital output on the converter can be used to slave a digital mixer to the master clock, thereby automatically phase-locking the mixer to your MOTU boxes as well.
Use What You Have You’ll always get the least amount of jitter and the best-quality sound by using a high-end master clock specifically built for the purpose. But if you own a potpourri of high-end and downmarket converters, I/O boxes and other digital hardware, take the time to discover which has the best clock and figure out how to get the closest to a star configuration in distributing clock throughout your system. You might just give your system’s audio fidelity a boost without spending a dime.
Michael Cooper is the owner of Michael Cooper Recording in Sisters, Oregon (myspace.com/michaelcooperrecording), and a contributing editor for Mix magazine.