An audio interface is a device that transmits audio signals to and from a computer. They essentially replace a computer’s sound card but provide better sound quality with more inputs, outputs, and features. Interfaces come in all shapes and sizes with various feature sets, including everything from portable USB-powered boxes to massive rack-mounted professional systems.
Inputs and Outputs
Also known as I/O, inputs and outputs are all of the ways that audio signals go in and out of an interface. They can include inputs for microphones, guitars, and line-level sources; outputs for headphones, monitors, and auxiliary sends; and even optical, ethernet, or SPDIF connections for digital audio.
Different combinations of I/O are useful in different scenarios. For example, a small bedroom studio may only need headphone and speaker outputs with one or two microphone and guitar inputs, while a commercial studio may need 32 or more line inputs, main and secondary monitor outputs, and multiple headphone jacks.
Digital Audio Converters
from A/D and D/A conversion
Digital audio converters are chips inside of an interface that convert analog audio signals into digital audio (A/D) and vice versa (D/A). Converters can have a huge impact on sound quality, but technology has advanced so much that even entry-level modern interfaces feature high-quality converters. Most interfaces feature controls to adjust the converters’ sample rate (similar to a video’s frame-rate) from the CD standard of 44.1 kHz all the way up to 192 kHz for audiophile quality.
Unlike purely analog equipment, converters take a few milliseconds to process audio into digital information, introducing an audible delay called latency, which can be distracting when overdubbing to pre-recorded tracks. For this reason, interfaces usually include an on-board monitoring system to route signals directly to headphones or monitors, circumventing the latency problem.
Preamps
Not all audio interfaces feature on-board preamps, but they're included on many models as a convenience in order to boost weak signals up to a useable level. Preamps can have just as much impact on sound quality as converters, but they also greatly influence the price of an interface, so it’s important to determine how many are actually needed.
Singer/songwriters may only need two for a guitar and vocal, small studios often require 8 to 16 for recording drum kits and live bands, and professional studios may have enough outboard and console preamps that they don’t need any more from their interface.
Data Connections
While the actual analog/digital conversion happens inside the interface, those digital signals must travel to and from the computer to be recorded and played back. This happens along a single cable—typically USB, Firewire, or Thunderbolt.
The speed and bandwidth of this connection also influences how much latency will be heard. Older interfaces using USB 1 or 2 will cause significant latency, while modern standards such as USB 3.0 and Thunderbolt have almost imperceptible latency.