Abstracting XACT 3D Audio

Today I worked out the most of the major kinks of getting positional audio working in Space Combat Sim. Well, I guess it's yesterday now since it's 12:45 AM and I finished sound at about 6:00 PM, but I digress.

My main experience in audio middleware has been working with FMOD and FMOD Ex. Both versions of FMOD are extremely powerful yet very easy to use. If I were doing this project using C++ I would not hesitate to use them. But I'm not using C++. Instead I'm using XACT, Microsoft's Cross-platform Audio Creation Tool.

Overall XACT is somewhat less powerful (no module support, no enviornmental effects, less portable etc.) but it's still reasonably simple to work with and supports the features I'm interested in. Specifically I want to support panning and fading of sound based on the position of the object producing it and I want to support doppler shifting based on how fast the object is moving.

Despite XACT being simple it proved to not quite be simple enough to use directly with my game objects. Ideally what I'd want to do is be able to associate a sound with an object then start, stop and alter it without having to worry about stuff like which specific sounds should play and so on. Sounds kinda like FMOD Ex's method of dealing with channels and voices independently eh?

The system I have now is a step in that direction but isn't there yet. For now I'm sticking with it since it works and, for the most part, sounds in Space Combat Sim are really simple so I don't need anything more advanced. My audio class stores an array of some arbitrary number of so-called tracked sounds. Each of these contains a Cue which may or may not be playing, a magic number, and some additional state data.

When I wish to play a sound that will loop, (and therfore need to be stopped manually,) or that I just want to keep track of because it's long, I specify a flag in my PlaySound function that it should be added to the tracked sounds. PlaySound will then choose a slot in the tracked sound array and replace its cue with whatever I wish to play. The PlaySound function then returns a handle, made by mashing the sound's array index and magic number together. When I decide to stop or alter the sound I simply pass the handle to access it. The index and magic number are extracted and, if the magic number matches the one in the tracked sound, the operation is performed on the Cue object.

How does this help? Well, by using a handle with a magic number I can eliminate the possibility of accidently messing with a Cue instance that has been squelched due to a more important/louder sound starting. Howzat? The magic number. My audio class is implemented as a GameComponent and is updated every game tick. If it notices a tracked sound whos Cue has stopped for whatever reason it will increment the magic number and null the Cue reference. If an operation were to be attempted on the, now null, Cue nothing would happen since the magic number has been changed. The only requirement here is to keep the number of magic numbers high enough that numbers aren't recycled until after they're no longer in use, otherwise very hard to diagnose bugs could appear.

Of course now that I've had things working for a while I've learned better ways of doing this which I might try out later. Particularly removing the need to track handles and send position/velocity updates manually from the object playing the sound. But, for now, this is the way things work in the project.