ASIO Audio Interface
|The proprietary ASIO interface standard is essential on Windows for low latency recording and playback. It is also usually the best way of making multi-channel recordings on Windows.
Latencies on Windows, Linux and Mac OS X
ASIO is a proprietary audio interface standard in use on Windows which bypasses the operating system's mixing kernel, so providing lowest latency direct communication between computer audio software and hardware.
- ASIO supports 24-bit sampling which is only otherwise available under Windows WASAPI or WDM-KS (Windows Driver Model Kernel Streaming). 24-bit sampling allows greater dynamic range, lower theoretical noise floor and greater resolution at lower audible volumes.
- An unmixed ASIO output is "bit identical" to the original source.
- Multiple physical input and output channels of the hardware are accessed over one single device.
On Linux, the standard ALSA audio API typically provides lower latencies than Windows under MME or Windows DirectSound. However, many Linux distributions now use PulseAudio by default for audio routing and mixing. PulseAudio sits between the sound source and the Linux kernel and thus has somewhat higher latency than direct use of ALSA. For lowest latencies, you can use the JACK API that provides both low latency audio communication and audio routing between applications. Current Audacity supports JACK fairly well, but with some limitations.
On Mac OS X, Core Audio is the standard API and is fully supported by Audacity. Core Audio also has lower latencies than Windows under MME and Windows DirectSound but Jack OS X can be used for lowest latency.
Audacity and ASIO
The ASIO technology was developed by German company Steinberg and is protected by a licensing agreement which prevents redistribution of its source code.
Audacity, as an open source program licensed under the GPL, is therefore currently unable to support ASIO, despite being ASIO-capable (providing the user's sound device is similarly capable). If ASIO support were distributed in Audacity builds this would either violate Steinberg's licence agreement if the code were included, or conversely would violate Audacity's GPL Licence if the code were withheld. There are persistent rumours of Steinberg opening up licensing, but without any apparent movement. Anyone who cares about this issue is invited to make their views known to Steinberg via their Contact page.
Non-distributable ASIO support in Audacity
Since legacy version 1.3.3, Audacity has provided ASIO support on Windows for individuals who are prepared to compile Audacity from source code using the optional Steinberg ASIO SDK.
ASIO support is provided strictly on the basis that it is NON-DISTRIBUTABLE, that is, you may NOT copy or distribute builds including ASIO support to anyone else. The build is strictly for your own personal (private or commercial) use. For the same reasons, Audacity can NOT distribute builds of Audacity including ASIO support, so please don't ask!
The following is an overview of compiling Audacity from source code including ASIO support.
- Install the free Microsoft Visual Studio Express Integrated Development Environment (IDE) and an appropriate Microsoft Software Development Kit (SDK).
- Download and install the ASIO SDK from Steinberg.
- Set the ASIOSDK_DIR environment variable to the directory where you installed the ASIO SDK.
- Download wxWidgets (Audacity's GUI toolkit that provides its interface) then build wxWidgets using Visual Studio.
- Set the WXWIN environment variable to the directory where you installed Widgets and restart the computer.
- Download and install the Tortoise SVN program and use it to checkout the latest Audacity source code.
- Build Audacity using Visual Studio.
Follow the instructions and download links on Developing On Windows for full details.