Audio Recording on iOS

Question

I've just started working on a project that requires me to do lots of audio related stuff on iOS.

This is the first time I'm working in the realm of audio, and have absolutely no idea how to go about it. So, I googled for documents, and was mostly relying on Apple docs. Firstly, I must mention that the documents are extremely confusing, and often, misleading.

Anyways, to test a recording, I used AVAudioSession and AVAudioRecorder. From what I understand, these are okay for simple recording and playback. So, here are a couple of questions I have regarding doing anything more complex:

• If I wish to do any real-time processing with the audio, while recording is in progress, do I need to use Audio Queue services?
• What other options do I have apart from Audio Queue Services?
• What are Audio Units?

I actually got Apple's Audio Queue Services programming guide, and started writing an audio queue for recording. The "diagram" on their audio queue services guide (pg. 19 of the PDF) shows recording being done using an AAC codec. However, after some frustration and wasting a lot of time, I found out that AAC recording is not available on iOS - "Core Audio Essentials", section "Core Audio Plug-ins: Audio Units and Codecs".

Which brings me to my another two question:

• What's a suitable format for recording, given Apple Lossless, iLBC, IMA/ADPCM, Linear PCM, uLaw/aLaw? Is there some chart somewhere that someone might be able to refer to?
• Also, if MPEG4AAC (.m4a) recording is not available using an audio queue, how is it that I can record an MPEG4AAC (.m4a) using AVAudioRecorder?!

Super thanks a ton in advance for helping me out on this. I'll super appreciate any links, directions and/or words of wisdom.

Thanks again and cheers!

Answer 1

For your first question, Audio Queue services or using the RemoteIO Audio Unit are the appropriate APIs for real-time audio processing, with RemoteIO allowing lower and more deterministic latency, but with stricter real-time requirements than Audio Queues.

For creating aac recordings, one possibility is to record to raw linear PCM audio, then later use AV file services to convert buffered raw audio into your desired compressed format.