A device that provides A/D converters and D/A converters for interfacing instruments and output devices to a computer. (A few high-end units provide conversion in one directly only, but most audio interfaces provide both.) This is a necessity for hard disk recording and for computer processing of signals generated externally to it, as well as for listening to signals generated within the computer. Early model audio interfaces took the form of expansion boards that had to be installed inside the computer, and typically could only be used with software provided by the board’s manufacturer. However, open interface standards such as ASIO and Core Audio have opened up interfaces to different software programs in the past few years.

Since about 2010, the trend in professional audio interfaces has been towards mounting the circuitry in an external box and interfacing to the computer via one of its standards buses. As of 2019, USB 3 is now widely used for the smaller interfaces of up to 4 audio channels, while larger interfaces (8 or more channels) are using Thunderbolt. (A few interfaces using Firewire are still on the market; a Thunderbolt interface can easily be adapted to Firewire with an inexpensive converter.)

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