An audio interface is the bridge between the analog world of microphones and instruments and the digital world of computers and recording software. It performs two jobs: analog-to-digital conversion (ADC) when recording, and digital-to-analog conversion (DAC) when playing back audio through monitors or headphones.
For XLR microphones, the interface also provides a microphone preamplifier (preamp). The preamp amplifies the mic's small signal to a level the ADC can work with. The quality of the preamp significantly affects the noise floor and character of the recording.
Modern audio interfaces connect to computers via USB, USB-C, or Thunderbolt. USB interfaces are the most common for home studios. Thunderbolt interfaces offer lower latency and higher channel counts but cost more.
The Focusrite Scarlett range is the most popular entry-level choice globally. The Scarlett Solo has one XLR input and costs around $120. The Scarlett 2i2 adds a second input and higher headroom for about $170. Both include phantom power for condenser microphones.
When choosing an interface, the key specs are: number of inputs, preamp quality (measured by EIN — equivalent input noise), maximum gain, and bit depth/sample rate (24-bit/96kHz is standard for all modern interfaces).