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.
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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. Its main strength lies in its method of bypassing the inherently high latency and poor-quality mixing and sample rate conversion of Windows audio mixing kernels (KMixer), allowing direct, high speed communication with audio hardware. Unlike KMixer, an unmixed ASIO output is "bit identical" or "bit perfect"; that is, the bits sent to or received from the audio interface are identical to those of the original source, thus having higher audio fidelity. In addition, ASIO supports true 24-bit samples, unlike Windows MME and DirectSound which only pass the upper 16 bits. True 24-bit samples offer the potential for a higher signal-to-noise ratio.
Interface support is normally restricted to Microsoft Windows. Starting with Windows Vista, KMixer has been removed and replaced by WASAPI and a new WaveRT port driver. WaveRT cannot provide synchronized audio to multiple devices and does not support external clocks.[1]
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.