1. The Delta 44 interface from M-Audio features a PCI card that mounts inside a computer, and then connects to an external box with audio inputs and outputs. 2. The Avid Mbox Mini connects to a computer via USB and is a very compact interface for portable use. 3. The Focusrite Saffire PRO 40 FireWire interface is rackmountable and has multiple ins and outs.

Last month’s column [“How to Build a Recording Rig, Pt. 1,” March 2012] focused on determining the best possible centerpiece for your rig—the actual recording device. We looked at using computers as a primary recording device.

But a computer alone will not get the job done. You need to have a way to route your audio in and out of your machine. It’s true that many computers have rudimentary audio inputs and outputs, and it’s possible to use them to get some sounds down. But for quality recording, you must have an audio interface. This is a device that accepts incoming signals from microphones, instruments, or other sources, and then converts them into digital data that the computer can digest.

Audio interfaces come in many, many flavors. In fact, interfaces and microphones are probably the two categories of gear for which I’m most often asked, “Which is best?”

Consider the following questions to help you narrow the choices:

How does the interface connect to the computer? There are three options (with a fourth, Thunderbolt, now rising on the horizon). Some computers support all of them, while others support just one or two, which can help narrow things down.

An internal card slot is a type of audio interface that is usually not compatible with laptops, as many do not have card slots. Some desktop computers, like Apple’s iMacs, also don’t have card slots. Audio interfaces that are cards themselves, or that connect to a card in the computer, offer some advantages. One is higher throughput, and some have extra features for reducing latency, powering plugins, and for expanding the interface system. However, you have to be comfortable opening up your computer to install the card (or have a computer-savvy friend do it for you).

USB interfaces are external boxes that connect to the computer via a USB cable. These are easy to connect and can be used with most types of modern computers— laptops, desktops, and towers. The original USB 1.0 interfaces were limited in some cases, and tended toward the low end. But today’s USB 2.0 interfaces offer great quality and capability.

Like USB, FireWire interfaces are external boxes that connect to the computer using a FireWire (surprise, surprise) cable into a FireWire port. The two types of FireWire—400 and 800—are primarily distinguished by transmission speed. While the more common FireWire 400 is the slower of the two, don’t take this to mean that FW400 isn’t capable of recording and playing tons of channels— it can handle more than most of us need.

A few of the newer interfaces support FireWire 800, which is the only type of FireWire connector on today’s Macs. However, FW800 is backwards compatible with FW400 through the use of an adapter, so you can still use an FW400 interface with an FW800- equipped computer. Be aware that the FireWire bus will drop in speed to FW400 rates, which could affect performance if you also have FW800 hard drives connected in the same chain with the interface. This shouldn’t be a big deal unless you are really pushing the limits of what FireWire can do, which few of us ever do.

A USB or FireWire interface is the most convenient. If you’re like me and have both laptop and desktop/tower computers, you can easily move the interface from computer to computer.

How does the form factor in? A compact and portable interface is excellent if you don’t have much space. It’s also a great option if you want to record on location and capture rehearsals, gigs, or performances in unique acoustical spaces (like a classical guitar performance in a beautiful-sounding church). Throw it into your laptop bag and you can record just about anywhere, with many being able to draw power from the computer, thus eliminating the need to be plugged in. However, there are compromises. A small interface means there won’t be room for many inputs and outputs, limiting the number of sources you can record at once. Likewise, you won’t have much flexibility when it comes to multiple-headphone feeds.

The second interface form-factor is desktop. These units can range from fairly compact with a number of inputs and outputs, all the way up to larger units that resemble mixing consoles, chock-full of sliding faders for controlling volume and knobs for controlling other mix parameters. While generally too large to fit into a laptop bag, many are portable enough if you need to have a good number of inputs and outputs for a location recording and you don’t mind hauling a larger piece of gear around. Larger desktop interfaces often have monitor controls, multiple headphone outputs, more extensive metering, and other handy features.

The third form-factor is rackmountable. These interfaces have flanges on their sides that bolt into a rack. While a rack can certainly be portable, most rackmounted interfaces end up being used in the studio. These generally have the most inputs and outputs, and like desktop interfaces, may have controls for monitor speakers, more than one headphone output, bigger/more meters, and other features that are useful for larger-scale recording situations.

Next month we’ll explore other essential studio gear as we continue our quest for the perfect recording rig!