Building better positional audio [AudioQueue manipulation]

Question

Hey folks,

I'm building an app that has a requirement for really accurate positional audio, down to the level of modelling inter-aural time difference (ITD), the slight delay difference between stereo channels that varies with a sound's position relative to a listener. Unfortunately, the iPhone's implementation of OpenAL doesn't have this feature, nor is a delay Audio Unit supplied in the SDK.

After a bit of reading around, I've decided that the best way to approach this problem is to implement my own delay by manipulating an AudioQueue (I can also see some projects in my future which may require learning this stuff, so this is as good an excuse to learn as any). However, I don't have any experience in low-level audio programming at all, and certainly none with AudioQueue. Trying to learn both:

a) the general theory of audio processing

and

b) the specifics of how AudioQueue implements that theory

is proving far too much to take in all at once :(

---

So, my questions are:

1) where's a good place to start learning about DSP and how audio generation and processing works in general (down to the level of how audio data is structured in memory, how mixing works, that kinda thing)?

2) what's a good way to get a feel for how AudioQueue does this? Are there any good examples of how to get it reading from a generated ring buffer, rather that just fetching bits of a file on-demand with AudioFileReadPackets, like Apple's SpeakHere example does?

and, most importantly

3) is there a simpler way of doing this that I've overlooked?

Answer 1

1. I think Richard Lyons' "Understanding Digital Signal Processing" is widely revered as a good starter DSP book, though it's all math and no code.
2. If timing is so important, you'll likely want to use the Remote I/O audio unit, rather than the higher-latency audio queue. Some of the audio unit examples may be helpful to you here, like the "aurioTouch" example that uses the Remote I/O unit for capture and performs an FFT on it to get the frequencies.
3. If the built-in AL isn't going to do it for you, I think you've opted into the "crazy hard" level of difficulty.

Sounds like you should probably be on the coreaudio-api list (lists.apple.com), where Apple's Core Audio engineers hang out.

Answer 2

Another great resource for learning the fundamental basics of DSP and their applications is The Scientist and Engineer's Guide to Digital Signal Processing by Steven W. Smith. It is available online for free at http://www.dspguide.com/ but you can also order a printed copy.

I really like how the author builds up the fundamental theory in a way that very palatable.

Furthermore, you should check out the Core Audio Public Utility which you'll find at /Developer/Extras/CoreAudio/PublicUtility. It covers a lot of the basic structures you'll need to get in place in order to work with CoreAudio.