Audio Stream Input/Output (ASIO) is a computer sound card driver protocol for digital audio specified by Steinberg, providing a low-latency and high fidelity interface between a software application and a computer's sound card. Whereas Microsoft's DirectSound is commonly used as an intermediary signal path for non-professional users, ASIO allows musicians and sound engineers to access external hardware directly.
ASIO bypasses the normal audio path from a user application through layers of intermediary Windows operating system software so that an application connects directly to the sound card hardware. Each layer that is bypassed means a reduction in latency (the delay between an application sending audio information and it being reproduced by the sound card, or input signals from the sound card being available to the application). In this way ASIO offers a relatively simple way of accessing multiple audio inputs and outputs independently.
As of 2007 there is also an experimental ASIO driver for Wine, a Windows compatibility layer for Linux. This wineasio driver uses the JACK sound server as its audio back-end and allows many ASIO-aware applications to run with low latency under WINE.
Being a proprietary protocol, compatibility with Windows-based DAW and other recording software is not universal. For example, the user manual of the Audacity audio editor states: "Licensing restrictions prevent us including ASIO support in released versions of Audacity, but Audacity can be compiled with ASIO support for private, non-distributable use."
A popular alternative is to use the ASIO4All driver by German programmer Michael Tippach, which can often deliver low latency on soundcards that have not been designed with music production in mind.