I have a question about sound.
I have used sound libraries like OpenAL in my projects before.
What I need, is insight as to what underling OS APIs these libraries use.
Even if each library provides an easy way to manipulate the input file
according to format, the very basic "raw byte-to-byte, send to the driver"
function has to exist.
I mean, surely there has to be a default api (one for windows, another for Linux)
that these libraries use. I don't suppose they use directly each sound
card's drivers, so the OS has to somehow do the magic. Am I correct?
Now, I know DirectX supports sound (Although I have never used it), but
DirectX isn't installed by default on windows, so I suppose it doesn't count,
and I have no idea what happens on Linux, and I would like to know about both.
I know it's probably impractical not to use a dedicated library, and I don't
really intend not to, but I'm curious about this subject.
So please indulge me.
So, for basic graphics it's OpenGL and DirectX... But what about sound?
Thank you in advance.
Each major platform has a number of API's that allow you to work with sound, On Windows and Mac there are Native Sound Api's that are used by default by the OS as well as well as others that are either Non-Standard or deprecated.
Have A look at he diagram HERE, it has a useful breakdown of many of the major sound API's across major platforms.
In addition to each platform having native sound API's there are also many cross platform API's that encapsulate the way that each native API works in order to allow you to write portable audio software.
For example there is: PortAudio which is a well know C language API there is also RtAudio which is a C++ API for sound, but it is somewhat of an older C++ style in my opinion(Does not take advantage of post C++11 features).
I am currently working on my own more modern C++11 audio API which can be found HERE. At the moment my API is a thin wrapper around PortAudio that allows you to work with audio in a more modern C++ way.
Keep in mind, the library that you choose will also depend on what kind of audio work you intend to do. All of the libraries I have listed above deal with real-time audio processing and do not deal with audio files. If working with audio files is what you are trying to do you could use libsndfile which is a popular open source sound file manipulation API.
From the context of your question it sound like you have been dealing with sound primarily in a Game Dev related context. It's worth mentioning that working with sound at the level that most of the libraries I have suggested thus far will be at a much lower level than simply calling one function to play a sound file.
Trying to answer the question of how OpenAl interacts with the OS is an answer best left to reading the OpenAL documentation.
I would also suggest looking into basic digital audio theory as well as digital signal processing. There are many resources available for free online on either subject.
In regards to how audio API's work... The average audio api works off of several layers of abstraction between the programmer and the sound card. Typically the programmer is given a buffer of audio data stored as an array of values. The programmer will have requested a specific set of parameters that the system will use for playback (sampling rate, buffer size, number of channels). The programmer will do their work with the audio data and hand the output buffer over to the api which in turn will eventually hand the buffers data over to a device driver written specifically for the installed sound card. The driver for that sound card will have been implemented based off of an interface specified by the platform that the driver is targeting. That is why when you install a new sound card on a machine you may be required to install drivers for it, by installing the drivers you are giving the OS level api a means of communicating with the device.
(There is a lot more that goes on than is possible to easily explain, and i'm sure i have missed a few steps in the process. But I hope that should be a good enough explanation to get started)