Timing Is Everything (Bonus)

Larry the O explains the principles of clocking in the digital studio and offers solutions to common synchronization problems.
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This online bonus material supplements the "Timing Is Everything" feature story in the August 2007 issue of EM.

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credit: Chuck Dahmer

Switched on Clock

Armed with your new knowledge about clocking, you can even have some fun. For example, I have always been a fan of off-speed recording, which the Beatles commonly employed. Such recording became considerably more difficult when I migrated to a DAW years ago. I really missed being able to do off-speed recording until Korg''s Dan Phillips gave me a nifty trick for accomplishing it.

Among other gear, my studio has a MOTU PCIe card, which can clock from ADAT Sync as well as from the audio interfaces. Phillips suggested that I connect the ADAT Sync output from my ADAT to the PCI-424 card, set the card to sync to it, and then use the ADAT''s varispeed capabilities to provide an off-speed clock. It worked beautifully with no clicks or pops, so I took it one step further and connected the word-clock out of my MOTU 2408 (which is now clocking from the ADAT via the PCI-424) to the word-clock input of the Lucid SSG192 master clock that usually drives the 2408. This setup allowed every device in my studio to run off-speed.

Here''s one way of achieving varispeed with a DAW. The ADAT''s pitch control varies the rate of the clock from the ADAT Sync port. The PCI-424 is set to clock from ADAT Sync, causing it to follow the source''s rate variations. The 2408, which gets its clock from the PCI-424, passes the rate variations on to the SSG192 master clock device in the form of a word-clock signal; the SSG192 then clocks all the other devices in the studio. There is a limit as to how much speed can be varied before devices start rejecting the clock as too off-speed.

Seven Steps to Better Clocking

1. One — and only one—digital device must be designated as the master for the studio, and all others must be slaved to it.
2. Use the appropriate low-capacitance cable for clock and digital interface connections. Keep cable runs as short as possible.
3. Be sure that word-clock lines are properly terminated. If you''re in doubt about internal termination with some pieces of gear, call the manufacturer and ask.
4. Avoid daisy-chaining as much as possible. A star distribution scheme is ideal.
5. Phase-locked loops and clocks are important. Ask manufacturers to describe their PLL and clock structures and provide jitter specifications.
6. Because jitter matters most in A/D conversion, use the highest-quality A/D converter you can get your hands on.
7. Check that all devices are set to the proper sampling rate and clock source, especially whenever one of those changes or when starting a different computer application.

Recommended Reading

“Get in Sync” by Dan Phillips, February 1999 issue of EM

“Spare Interchange” by Jeff Baust, April 2000 issue of EM

“Digital Audio Clocking Primer” by Universal Audio

“Jitter Specification and Assessment in Digital Audio Equipment” by Julian Dunn

“Everything You Always Wanted to Know About Jitter but Were Afraid to Ask” by Bob Katz

“Big Ben Termination: How and Why” by Apogee Digital

“Clocking, Jitter and the Digidesign 192 I/O Audio Interface” by Gannon Kashiwa, Digidesign

“AES3-2003: AES standard for digital audio engineering — Serial transmission format for two-channel linearly represented digital audio data (),” by the Audio Engineering Society

“AES11-2003: AES recommended practice for digital audio engineering — Synchronization of digital audio equipment in studio operations” by the Audio Engineering Society