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An <u>'''audio interface</u> ''' is a device that is used to transfer audio streams (usually analog) to and from a computer, allowing recording and playback for DAW software. They typically include microphone, instrument-level, RCA, SPDIF, and AES-EBU inputs, as well as monitor, headphone, and auxiliary/line-out outputs. Often they will interface [[MIDI]] in addition to audio.
Interfaces vary in form factors and how they connect to the computer. Early interfaces were PCI cards that had to be inserted into the computer's motherboard. Modern ones may use a PCI or PCI-e card, or use USB 2, USB 3, or a Firewire connection. Some early interfaces consisted of nothing but a PCI card and a break out cable that split into various jacks to connect to audio cables - all the controls for the inputs were controlled via software. Most interfaces nowadays consist of a hardware box meant to either sit on a desktop or live in a rack - the box provides various controls in addition to the input/output jacks.
=== Inputs/Mixer ===
The mixer or input tab of the control panel typically resembles a mixing board. This includes the <u>'''physical inputs</u> ''' on your interface as well as <u>'''software return inputs</u> ''' from your DAW which your DAW will see as ''outputs''. Your DAW should automatically see all of your interface's physical inputs without having to change anything here. Different interfaces will provide your DAW software with different numbers of virtual outputs. The mixer page determines how the physical and software inputs are routed to the physical outputs on the interface. Let's look at a few examples.
# Guitar > Mic'ed Amp/Cab > Interface Mic Physical Input > DAW > Software Return Inputs 1/2 > Interface Mixer Settings > Interface Outputs 1/2 > Monitors/Headphones
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Communication between the interface and the rest of the computer is not instantaneous or reliably timely. For a slower computer or one bogged down with other tasks, it may not be timely at all. This results in crackles and pops in the audio signal, as the stream is constantly being interrupted and dropped. To prevent this, the interface can store the audio stream in an <u>'''input buffer</u>''', a temporary memory location, where it can be reliably transported to the rest of the computer when the computer is ready to receive it. In turn, this means that it takes longer for the audio to get to where it is supposed to go, resulting in <u>'''latency</u> ''' between the input into the interface and the DAW's ability to record, process, or monitor it.
Interfaces often allow you to set the buffer size in terms of samples or miliseconds. You should set it as small as possible so long as the audio never drops out or pops.
While the DAW is smart enough to account for this latency, it may make it difficult for the musician to record if using software monitoring. This latency can be avoided by using <u>'''direct monitoring</u> ''' (routing the interface's physical input directly to a physical output). It can also be minimized by using proper low-latency drivers (typically ASIO) and setting up your OS and DAW software to efficiently work with audio. Or you can use a more powerful computer.
Besides driver and OS settings, certain interfaces may be able to transport its data more quickly to the computer depending on the bus with which it uses to communicate. USB 2.0 or 2.1 can sometimes run into issues, although theoretically fast enough. Firewire is typically faster; however, this depends on the Firewire controller in the computer - some interface manufacturers recommend certain controller chips over others for best reliability and speed. Interfaces that use a PCI or PCI-e card may benefit from being seated in higher slots on the motherboard to obtain a higher priority for its interrupt requests. USB 3.0 is relatively new but should be more than fast enough to achieve imperceptibly low latency.