How can I do metering/average peak power level in OpenAL?

Question

I'm in the process of switching from AVAudioPlayer to OpenAL using the Finch sound engine. I need to do metering, i.e. get the average peak levels. Finch sound engine does not provide this, and I'm completely new to OpenAL. How can I do this? Any examples would be really appreciated.

Answer 1

I'm assuming you're looking for a drop-in replacement of AVAudioPlayer's peakPowerForChannel: method. Unfortunately, there is none. You'll have to roll your own.

OpenAL "sounds" are a combination of a "buffer" (your sample data, loaded in memory) and a "source," which represents something like properties you want applied to your sample data.

The easy approach to OpenAL playback is to load the entire file into memory and just play the whole thing in one call. However, you can use an NSInputStream to read in a chunk of PCM sample data from a file into an OpenAL buffer, use alBufferData() to compute your peak power using your own function, play the chunk using your source, and then repeat until EOF.

I know you are intending to use Finch, but you should give AudioQueues a real close lookover (if metering is a critical feature for you). It is much better designed for this type of application. In particular, the kAudioQueueProperty_CurrentLevelMeterDB property will provide you with either peak RMS (mPeakPower) or average RMS levels (mAveragePower), which you can read as often as you like.

Good luck and happy coding!

Some resources that might be helpful:

• http://kcat.strangesoft.net/openal-tutorial.html
• http://connect.creativelabs.com/openal/Documentation/OpenAL_Programmers_Guide.pdf
• http://www.hydrogenaudio.org/forums/index.php?showtopic=78578
• http://developer.apple.com/mac/library/documentation/MusicAudio/Reference/AudioQueueReference/Reference/reference.html