If you own outboard gear that supports digital I/O (or if you plan on expanding your setup), it's good to have an interface that offers digital inputs and outputs. Transferring audio in the digital domain has several advantages. You avoid unnecessary conversions that can degrade the quality of your signal. You can send as many as eight digital audio signals over a single ADAT cable. And since digital audio is simply a series of zeros and ones, you never have to worry about setting levels or clipping the signal. There are several digital audio formats currently in widespread use; the most common are S/PDIF (Sony/Phillips Digital InterFace) and ADAT (Alesis Digital Audio Tape).

S/PDIF

The S/PDIF format is capable of sending a stereo audio signal over a single RCA-type or ADAT TOSLINK cable. Many effects processors, A/D-D/A converters, microphone preamps, CD players, keyboards, and samplers support S/PDIF.

ADAT

The ADAT optical standard supports multitrack digital I/O over a fiber optic cable. A single ADAT port is capable of transferring either eight channels of 44.1/48 kHz audio, four channels of 88.2/96 kHz audio, or two channels of high-definition 192 kHz audio. Many digital audio converters, digital mixers, and multichannel preamps are ADAT-compatible.

MIDI I/O

MIDI, or Musical Instrument Digital Interface, is a communication protocol that enables electronic musical instruments, computers and other digital equipment to communicate and share data. Nearly all modern keyboards, synthesizers, and DAW applications support MIDI. If you want to integrate synths, effects processors, and other MIDI gear into your setup, look for an audio interface that offers at least one MIDI input and one MIDI output.

It's a Keyboard… and an Audio Interface

When you're putting together a mobile rig or just tight on space, you might appreciate the efficiency of a MIDI keyboard controller with an audio interface built right in. Check out the M-Audio® KeyStudio™ 49i, ProKeys Sono 61, and ProKeys Sono 88. They even have built-in sounds including a stereo-sampled grand piano.

Word Clock

Your computer-based DAW stores and manipulates music as digital samples (more on sampling later). Each device in your interconnected digital world must share the same timing in order to communicate correctly—that is, their clocks must be synchronized.

Word clock makes this possible by designating one device as the timing "master," and all other connected devices as "slaves," locking the slaves to the master. This ensures that all digital gear is running at a common sample rate. Without word clock, your digital audio signal will be filled with clicks and pops (if, in fact, it plays at all). Many Avid® interfaces can send and receive word clock data via S/PDIF, ADAT, or coaxial BNC cable—allowing you to synchronize with A/D converters, outboard preamps, or other digital gear.

For more information on choosing an audio interface, download Overdub Vol 2.