Add ASIO guide to README

README.md

@@ -16,7 +16,7 @@ This library currently supports the following:
 Currently supported backends include:
 
 - Linux (via ALSA)
-- Windows
+- Windows (via WASAPI by default, see ASIO instructions below)
 - macOS (via CoreAudio)
 - iOS (via CoreAudio)
 - Emscripten
@@ -24,3 +24,64 @@ Currently supported backends include:
 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](https://en.wikipedia.org/wiki/Audio_Stream_Input/Output) 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 backend on Windows instead of
+WASAPI. To do so, follow these steps:
+
+1. **Download the ASIO SDK** `.zip` from [this
+   link](https://www.steinberg.net/en/company/developers.html). The version as
+   of writing this is 2.3.1.
+2. Extract the files and place the `ASIOSDK2.3.1` directory somewhere you are
+   happy for it to stay (e.g. `~/.asio`).
+3. Assign the full path of the `ASIOSDK2.3.1` directory to the `CPAL_ASIO_DIR`
+   environment variable. [How to set persisting Environment Variables on
+   Windows](https://gist.github.com/mitchmindtree/92c8e37fa80c8dddee5b94fc88d1288b#file-windows_environment_variables-md).
+4. **Download and install LLVM** from
+   [here](http://releases.llvm.org/download.html) 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 availabe, you can [**download
+   and install ASIO4ALL**](http://www.asio4all.org/). Be sure to enable the
+   "offline" feature during installation despite what the installer says about
+   it being useless.
+7. **Use the correct command prompt** to build cpal and run examples. In my
+   case, I had to run a specific command prompt, otherwise rust-bindgen would
+   fail to find some of the necessary build tools. To do this, I went to the
+   `Start Menu > Visual C++ Build Tools > Visual C++ 2015 x64 Native Build Tools
+   Command Prompt` and ran this prompt. The exact prompt you need might differ
+   based on your machine's architecture and how you installed your Visual C++
+   tools. There must be an easier solution to this (especially as not everyone
+   wants to build projects from the command line).
+8. Select ASIO as the backend at the start of our program with the following:
+   
+   ```rust
+   #[cfg(target_os = "windows")]
+   {
+       cpal::os::windows::use_asio_backend().expect("Failed to select ASIO backend");
+   }
+   ```
+
+   If you run into this error:
+
+   ```
+   cpal::os::windows::use_asio_backend().expect("Failed to use asio");
+                      ^^^^^^^^^^^^^^^^ did you mean `use_wasapi_backend`?
+   ```
+
+   Make sure that `CPAL_ASIO_DIR` is set correctly and try `cargo clean`.
+
+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.