Tech Tip:Studio Tech Tip - Digital Domain
By Dennis Kambury
More and more gear is hitting the streets with digital I/O, and there are some things you'll want to be aware of as you put it all together. Digital is less forgiving than analog, so it's important to pay attention to details such as wiring, clocking, formats, and other minutiae of the digital world.
Two of the most common digital formats are AES/EBU and S/PDIF (Sony/Philips Digital Interface). At their core, they are identical. The primary difference between the two formats is the subcode information. The pro format, AES/EBU, contains ASCII code for source and destination ID, while S/PDIF - the so-called consumer format - carries SCMS (copy protection) information.
Most of the gear that boasts digital I/O refers to S/PDIF. Usually, this takes the form of one or two RCA jacks - one for stereo output, and one for stereo input, when applicable. Although you can physically connect a standard RCA plug to these jacks, you'll want to avoid doing so. It may work, and it may not, but at the very least you'll degrade the digital signal. At worst, you'll get to experience first hand just how bad digital can sound! S/PDIF is designed to be transmitted over inexpensive 75 ohm coaxial cable fitted with RCA plugs - the same cable (with the yellow plugs) you use to hook up your VCR to your TV. You'll occasionally see optical S/PDIF connectors. They are identical in format, and use inexpensive lightpipe cabling.
Generally found on higher-end gear, AES/EBU uses mic-style XLR connectors. Like S/PDIF, it's physically possible to hook a standard mic cord to an AES/EBU jack - but don't do it! 110-ohm balanced cables are required for proper hookup. It's perhaps even more important than with S/PDIF connections, as signal degradation due to the severe impedance mismatching easily corrupts the digital data stream. In short - it'll sound terrible!
In order to avoid random clicks, pops, and unusual digital noises, all your digital audio needs to march to the same beat. There are several ways to accomplish this, but the primary rule is that only one device can be the clock master, and all other devices must be slaved to the master. Some devices, such as DAT machines, automatically slave to the clock signals transmitted through the digital audio cables. Other devices require the hookup of a separate word clock cable to function correctly.
If you are hooking up just two devices, connecting the digital audio cable and/or word clock cable from master to slave is all you need do. If your rig is a bit more complex, you should consider a word clock distribution amplifier such as the AardSync II or SyncDA from Aardvark. These devices allow you to send well-regulated word clock signals to several devices simultaneously, ensuring that all your audio keeps to the beat. Yes, you can daisy-chain word clock cables, but like the digital audio cables, it's not advisable. It might work, and it might not, but you don't want to risk your reputation on a maybe!
Even though connectors and such may look the same, digital is a vastly different beast than analog, with much stricter rules and requiring more attention to the details. If you keep this in mind, you'll keep your problems to a minimum and your satisfaction high.