The audio interface is at the centre of every modern studio, but how do you choose the right one for you?
Thirty years ago, control rooms contained big mixers and racks of audio processing equipment. Today, the computer is the cold robot heart of the studio, and software has replaced the console and outboard. This change means that one piece of equipment has become indispensable. You can't run a computer-based studio without an audio interface — but how do you know which interface is right for your computer-based studio?
Getting sound into a computer is a two-stage process. First, an analogue signal is converted to a stream of numbers. Then this stream of numbers is fed into the computer. This second stage is the core function of an audio interface, but nearly all of them do both.
In other words, most interfaces can accept analogue signals directly. What's more, they can often take two different types of analogue input. A 'line' input expects signals at a standard studio level such as is generated by synthesizers, mixers or mic preamps. However, we also want to plug microphones and electric guitars directly into our audio interfaces. These put out feebler and less predictable signals, which have to be preamplified before they can be digitised. Most project-studio interfaces thus feature analogue inputs with mic preamps and/or sockets to plug a guitar into. Often, you'll find dual-purpose sockets which combine an XLR for mic input and a quarter-inch jack for line input.
So one of the most basic questions to ask yourself is: how many analogue inputs do I need, and of what type? As a first step towards answering this question, count up the number of sources you're going to want to connect simultaneously. If you only ever record yourself, you might only need one or two inputs, even if you're building complex tracks by overdubbing. If you record bands live, you may require enough inputs to cover a multi-miked drum kit, several other instruments and vocals.
In a studio where the audio interface is the only item of hardware, you'll need it to have as many mic preamps as you're ever likely to want to connect mics. But if you plan to use your audio interface with a hardware mixer, or you only ever record synths, or you have other equipment already that can amplify the signals from your mics, you might well prefer to get an interface that has only line-level I/O.
This only gets us so far, because there are a lot of audio interfaces that seem to offer identical analogue input facilities. If you have to choose between several models with the same features, you should also consider the following:
- The audio performance of interfaces varies, and some of these differences can be important. Read the 'Which Specs Matter?' box for more details.
- Some interfaces have separate mic and line sockets for the same inputs. This allows you to leave mics and line-level sources permanently connected and switch them on the fly as needed, rather than having to re-plug.
- On some interfaces, mic preamp gain is adjusted digitally. This is more precise than using an analogue control and means that settings can be fully recalled and sometimes even stored with your DAW project.
- If you use capacitor microphones, you'll need your interface to offer phantom power. Nearly all do so, but sometimes this is only switchable globally or in groups. This can be relevant if you want to connect things like ribbon mics, which shouldn't encounter phantom.
Audio interfaces perform a comparable two-stage job at the 'back end' of the system, spitting digital audio out of the computer and turning it into an analogue signal. The most basic reason for this is so that we can connect speakers or headphones to hear sound coming out of it! Nearly all interfaces therefore feature at least one pair of line-level outputs and at least one stereo headphone socket.