Audio Interfaces: The Basics

by Pablo Bellinghausen

Nowadays it seems so intuitive to record and listen to music through computers it’s hard to believe that the very concept of computer audio was considered a fad as recently as a couple of decades ago. During the recording of Nirvana’s Nevermind, both the engineers and the band still recall using Sound Tools software (the precursor to the now industry-standard Pro Tools), which at the time they nicknamed “Slow Tools” and considered as little more than a time-wasting gimmick.

Things have of course dramatically changed since, and now anyone with a recent computer is easily able to record sound with a quality that used to be reserved for professional studios. But the multi-track recording software is incapable of “hearing”audio by itself; it requires an audio interface, or sound card, to send real-world audio in and out of the software.


Anatomy of an Audio Interface

What a sound card does is fairly simple: it converts an electrical audio signal into a stream of numbers, routes them into the computer, and converts an output stream back into an electrical signal to go into speakers, headphones, or any other monitoring device. Most motherboards come with a small one (the familiar audio-in and headphone-out sockets), but as we have seen in previous articles these are usually of rather low quality and should not be used in a pro setup.

The different components can either be bought separately, or in a product that bundles them together, depending on your needs. Here is a diagram with all the required conversion and routing stages:


The average audio input goes through the following steps:

–        Electrical conversion into a standard “line level”signal (e.g. the turntable’s phono preamp)

–        Analogue-to-digital conversion from electrical signal into binary digits

–        Routing into the computer via a digital connection (USB, Firewire, PCI Express, Thunderbolt)

The digital audio output goes through the interface chip into digital-to-analogue converters, again transforming binary digits into a line-level signal, which will then undergo final amplification for speakers or headphones.

These components are most of the time bundled up in one way or another at the whim of the manufacturer. There are guitars (Behringer iAXE393), microphones (sE Electronics USB2200a), turntables (Stanton T.92 USB), synthesisers (Novation Ultranova), mixers (Yamaha MG12XU) and speakers (M1 Active 320 USB) –all of these products have a built-in sound card, with converters, a USB chip and even small headphone preamps. These products are very useful “one trick ponies”and remain a cheaper option to a separate sound card, but they will usually remain at the low end of the market, and besides lacking in versatility they lock you into a set “combo” from which it is more expensive to upgrade afterwards.



USB microphones are great, as long as you dont need to upgrade later on

What people usually refer to as an “audio interface”will normally consist of a few channels containing a DI and a microphone preamp (instrument/mic inputs), sometimes some more line-level inputs and outputs, as well as a headphone amplifier. Here are the main things to look out for when choosing one for your setup.


Channel count

The first question to ask when looking for a sound card is the number of inputs and outputs that will be required. Although there are a few models with only a single input (like the barebones ART TConnect USB cable and the aptly-named Apogee ONE), most of the time small interfaces will have at least two mono ins, so they can also work as a single stereo input. Good examples are the Roland Duo Capture EX and the PreSonus AudioBox USB, with the M-Audio M-Track Plus giving you an extra digital in/out via S/PDIF. These are great for musicians and songwriters who need at most a vocal and a guitar at the same time.


The Apogee ONE only has a single input, but the sound quality is top-notch

Four and six-channel input interfaces usually offer a few more studio-standard options (like the insert inputs on the Alesis iO4, the USB hub on the AKAI EIE Pro or the dual headphone output of the Focusrite Scarlett 6i6) but keep the home studio in mind. Many of them will increase the number of inputs but like the 6i6 still only offer two microphone preamps; to plug in four mics you will either need separate preamps to plug into the line inputs, or to upgrade to a bigger model, like their excellent Scarlett 18i8.


The Akai EIE Pro is packed with features at a very reasonable price

Models with eight inputs will usually be aimed at the project studio and are designed to capture full band performances, or at least drum kits; they will mostly be rack-mountable and have internal routing facilities to quickly adapt to different workflows, particularly with regards to monitoring, so that musicians can hear a different mix than the engineer; good examples are the Presonus FireStudio Project, the Tascam US-16X08, the Steinberg UR824and the new MOTU 8M. Many of these are made with expansion in mind and include ADAT digital inputs, which make it easy to add more inputs to the aforementioned models by connecting them to stand-alone converters; the track count at that point can go much higher than the physical one, although the limits will often be set by the throughput of the digital connection.


The new MOTU range can be linked with Ethernet cables to achieve impressive track counts

Next time we’ll take a look at the different types of connections as well as whether you need built-in effects or not. Until then, happy recording!