CPAL - Cross-Platform Audio Library

Low-level library for audio input and output in pure Rust.

This library currently supports the following:

• Enumerate supported audio hosts.
• Enumerate all available audio devices.
• Get the current default input and output devices.
• Enumerate known supported input and output stream formats for a device.
• Get the current default input and output stream formats for a device.
• Build and run input and output PCM streams on a chosen device with a given stream format.

Currently supported hosts include:

• Linux (via ALSA)
• Windows (via WASAPI by default, see ASIO instructions below)
• macOS (via CoreAudio)
• iOS (via CoreAudio)
• Emscripten

Note that on Linux, the ALSA development files are required. These are provided as part of the libasound2-dev package on Debian and Ubuntu distributions and alsa-lib-devel on Fedora.

ASIO on Windows

ASIO is an audio driver protocol by Steinberg. While it is available on multiple operating systems, it is most commonly used on Windows to work around limitations of WASAPI including access to large numbers of channels and lower-latency audio processing.

CPAL allows for using the ASIO SDK as the audio host on Windows instead of WASAPI. To do so, follow these steps:

1. Download the ASIO SDK .zip from this link. The version as of writing this is 2.3.1.

2. Extract the files and place the directory somewhere you are happy for it to stay (e.g. ~/.asio).

3. Assign the full path of the directory (that contains the readme, changes, ASIO SDK 2.3 pdf, etc) to the CPAL_ASIO_DIR environment variable. How to set persisting Environment Variables on Windows. The existence of the CPAL_ASIO_DIR environment variable determines whether or not CPAL will attempt to build the ASIO SDK and generate bindings to it.

4. Download and install LLVM from here under the "Pre-Built Binaries" section. The version as of writing this is 7.0.0.

5. Add the LLVM bin directory to a LIBCLANG_PATH environment variable. If you installed LLVM to the default directory, this should work in the command prompt:

   setx LIBCLANG_PATH "C:\Program Files\LLVM\bin"
   

6. If you don't have any ASIO devices or drivers available, you can download and install ASIO4ALL. Be sure to enable the "offline" feature during installation despite what the installer says about it being useless.

7. Loading VCVARS. rust-bindgen uses the C++ tool-chain when generating bindings to the ASIO SDK. As a result, it is necessary to load some environment variables in the command prompt that we use to build our project. On 64-bit machines run:

   "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64
   

   On 32-bit machines run:

   "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x86
   

8. Select the ASIO host at the start of our program with the following code:

   let host;
   #[cfg(target_os = "windows")]
   {
       host = cpal::host_from_id(cpal::HostId::Asio).expect("failed to initialise ASIO host");
   }
   

   If you run into an error along the lines of "no variant Asio in HostId", make sure that CPAL_ASIO_DIR is set correctly and try cargo clean.

9. Make sure to enable the asio feature when building CPAL:

   cargo build --features "asio"
   

   or if you are using CPAL as a dependency in a downstream project, enable the feature like this:

   cpal = { version = "*", features = ["asio"] }
   

In the future we would like to work on automating this process to make it easier, but we are not familiar enough with the ASIO license to do so yet.

Updated as of ASIO version 2.3.3.