Form Factor
The vast majority of computers today come with USB ports, making USB audio interfaces an attractive option for musicians and producers. USB 1.1 is a good choice for working with two channels at once and creating CD-quality tracks.

USB 2.0
Are you turning a spare room into a project studio, or are you looking to make music on the road? Audio interfaces come in all different shapes and sizes, some designed for permanent rack-mount installations, and some designed for mobile music production. If you want something portable—or if desk space is an issue—check out a small, bus-powered interface like the M-Audio® ProFire® 610 or Pro Tools® Mbox®.

Other audio interfaces, such as the M-Audio ProFire 2626 or 003 Rack+ Factory, are primarily designed for in-studio applications. The rack-mount form factor is great for hooking up tons of extra gear and hiding the cable mess behind the desk. If you have the room and want expanded I/O capabilities, a larger interface like the M-Audio ProjectMix I/O offers FireWire connectivity, expanded I/O choices, and a built-in control surface.

Other Factors to Consider: Audio Quality
Visit any online audio forum and you'll no doubt find a heated discussion about which audio interface sounds the best. Everyone seems to have a different opinion on the subject, so it's hard to separate fact from the fiction. One thing is for sure—when it comes to overall audio quality, internal components play a big role. But it's also important to select the right sample rate and bit depth for your project in order to capture the entire dynamic range of the performance.

Sample Rate Explained
A thorough discussion of digital audio sampling would probably encompass several volumes of text. However, it is possible to understand the basics with a brief overview…

Higher sample rates sound better because more information is captured. When the interface converts the digital data back to analog audio, the empty space between the samples is "smoothed over" (see figure). When recording at higher sample rates, there is less empty space between samples, which results in more accurate smoothing and better-sounding audio.

Higher sample rates also capture a wider range of frequencies. While it's true that the human ear can't hear past 20 kHz, there are harmonics and overtones that reach much higher into the frequency range. These harmonics, when captured by an interface running at 96 kHz or higher, can impact the frequencies that we do hear.

Bit Depth Exposed
While the sample rate determines how often the interface captures audio, the bit depth determines how much information is captured by each sample. It's kind of like a digital photograph—an eight-megapixel photo captures more detail and has higher resolution than a three-megapixel shot. Likewise, 24-bit recordings capture better detail than 16-bit recordings (while 8-bit audio sampling should be left to old-school Nintendo games!). Higher bit depths also allow you to capture a greater dynamic range and record hotter signals.

Selecting the Right Audio Quality
Many wonder why it's worth recording at high sample rates and bit depths—especially since those formats consume extra system resources and hard drive space. The answer depends on what's being recorded, as well as what the final output medium will be. If the final destination is CD, the audio will have to be down-sampled to 16-bit/44.1 kHz—at which point the extra information captured at 24-bit/96 kHz will be lost. However, as mentioned earlier, those upper-range frequencies do impact what we hear in the lower bands. Recording at high sample rates can be especially beneficial when recording acoustic instruments, cymbals, and other sources with high-frequency "air-like" qualities. Any EQ, effects, and other processing will be done at a higher level of audio quality, resulting in better-sounding audio even when down-sampled to CD format.

If the final output will be on DVD, it's definitely a good idea to record at higher rates. DVDs, video games, and other multimedia formats typically use far higher sample rates than standard CDs. The M-Audio ProFire 2626 and ProFire 610 audio interfaces allow you to record at rates up to 24-bit/192 kHz.

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