iPhone audio analysis

Question

I'm looking into developing an iPhone app that will potentially involve a "simple" analysis of audio it is receiving from the standard phone mic. Specifically, I am interested in the highs and lows the mic pics up, and really everything in between is irrelevant to me.

Is there an app that does this already (just so I can see what its capable of)? And where should I look to get started on such code?

Thanks for your help.

Answer 1

Check the 'SpeakHere' sample code. it will show you how to record audio using the AudioQueue API. It also contains some code to analyze the audio realtime to show a level meter.

You might actually be able to use most of that level meter code to respond to 'highs' and 'lows'.

Answer 2

Look in the Audio Queue framework. This is what I use to get a high water mark:

AudioQueueRef audioQueue; // Imagine this is correctly set up
UInt32 dataSize = sizeof(AudioQueueLevelMeterState) * recordFormat.mChannelsPerFrame;
AudioQueueLevelMeterState *levels = (AudioQueueLevelMeterState*)malloc(dataSize);

float channelAvg = 0;

OSStatus rc = AudioQueueGetProperty(audioQueue, kAudioQueueProperty_CurrentLevelMeter, levels, &dataSize);
if (rc) {
    NSLog(@"AudioQueueGetProperty(CurrentLevelMeter) returned %@", rc);
} else {    
    for (int i = 0; i < recordFormat.mChannelsPerFrame; i++) {
    channelAvg += levels[i].mPeakPower;
    }
}
free(levels);

// This works because one channel always has an mAveragePower of 0.
return channelAvg;

You can get peak power in either dB Free Scale (with kAudioQueueProperty_CurrentLevelMeterDB) or simply as a float in the interval [0.0, 1.0] (with kAudioQueueProperty_CurrentLevelMeter).

Answer 3

The AurioTouch example code performs Fourier analysis on the mic input. Could be a good starting point:

https://developer.apple.com/iPhone/library/samplecode/aurioTouch/index.html

Probably overkill for your application.