Audio interfaces are essential for connecting various devices that can be used in the process of recording audio such as microphones, guitars, and keyboards. Yet an interface does not only connect to these devices but also controls them through software such as DAWs (Digital Audio Workstation). The majority of modern home studios use a sound card with a USB or Firewire output that provides both functions; however, it is possible to combine two cards—one for audio input and one for output—with different hardware configurations using either Firewire or USB cables with ADAT optical outputs.
An interface usually includes a built-in digital mixer with faders, knobs, buttons and sometimes patchbay to manage the input channels. Some interfaces provide additional hardware inputs and/or outputs using proprietary connectors. Examples include XLR connectors for microphones on the Tascam US-800, in addition to the standard RCA jacks; and both balanced TRS phone connectors with +4 dBu line level input as well as unbalanced RCA jacks on the Focusrite Scarlett series.
The choice of audio interface depends on specific hardware used in the studio, e.g., if an audio engineer uses an analog mixing console. The choice of AD/DA conversion technology depends on the desired audio quality. The main parameters of an audio interface include bit depth, sample rate and channel count.
Not all devices that connect to and record audio are considered audio interfaces. Microphones and drum machines are common input devices, such as in digital audio workstations (DAW), most notably in the recording of vocals at home or in a professional studio. For the sake of simplicity, the terms "audio interface" and "soundcard" are used interchangeably by some audiophiles.
For budget home studios, generic output cards from well-known manufacturers can be used with any computer's built-in sound card to provide connections for line-level analog outputs from an analog mixer with a variety of channel counts, inputs for two stereo pairs and couple of more mono inputs, all three features being available via the same interface.
This is a common feature in low-cost consumer audio equipment such as the Zoom H1 digital recorder.
In the music business, which mainly uses analog mixing consoles, the corresponding analog outputs from the mixing console—which is an analog device that generates non-digital signals—are given to monitor speakers and subsequently to microphones. Therefore, a special "audio interface" is required for recording music.
Below is a description of various audio interfaces and their features. Note that not all interfaces are available in all countries and not every device supports all features of an interface. Also, some devices have internal processing or effect units which can be used with other input and output devices (e.g. a preamplifier with line-level inputs can be used in a digital mixer).
Interfaces for use with analog audio devices typically operate at 48 kHz or 32 kHz samples per second. Analog-to-digital converters (ADC) are found in most digital audio interfaces. They convert an analog signal into a digital signal, taken from the highest sampling rate that the device can provide to internally generated stored digitized audio (sampled at 48 kHz or 32 kHz). These conversions take place in real time, so that a program is not interrupted when converting any signal. This means that levels are generally better adjusted after recording than during recording. Some interfaces have only one ADC.