Streaming Versus In-Memory Playback

A minute-long 44.1 kHz 16-bit stereo wave file is 10MB. 10MB isn't a big deal when you run a stand-alone DirectMusic file on a contemporary computer, but it is a very large file in the world of game audio. Do not forget that the rest of a game's resources need to reside in memory as well. If the game is written to run on a system with as little as 64MB of RAM, you are already in way over your head. Console games are even more unforgiving. Consider yourself lucky if you get 4MB for your entire sound budget! You can use audio streaming to alleviate these restrictions. Streaming is a technique that works a lot like a cassette player. In a cassette player, audio data is moved across the play head a little bit at a time. The play head reads the data as it comes and plays the appropriate sound. With streaming, a small area in memory called a buffer is created. Wave files are moved, bit by bit, through the buffer and read by DirectMusic. The CD player in your PC uses streaming for playback. If it weren't for streaming, you'd have to load the entire music file into your computer's RAM, which in most cases simply isn't an option.

DirectMusic uses the following rule for streaming: The wave streams with 500 msec readahead if it is more than five seconds long. If it is shorter than five seconds, it resides and plays from memory (RAM). Readahead determines the size of the buffer. For 500 msec, our 44.1 kHz 16-bit stereo wave file will use 88.2KB of memory (.5 sec x 44100 samples/sec x 2 bytes/sample x 2 channels), a big difference when compared to 10MB! Memory usage is reduced by a factor of more than 100! You can override this behavior, choosing to load all wave data to memory, or specify a different readahead value in the Compression/Streaming tab of a wave's property page in DirectMusic Producer. To get to any Track's property page, simply right-click on it in the Track window.