(iPhone) Live FFT from iPod

Question

Okay guys, I've read many things about the FFT stuff, but it seems to be a bit more complicated than building a tableView.

I am searching for a way to analyze the playing audio (from iPod Library) in three ranges (low, mid, high). I think FFT is doing the job, but I'm not sure if I could filter (Lowpass, Bandpass and Highpass) the playing audio and analyze the peaks as well. So if anyone knows what is the best (by best I mean, fastest (CPU) way to do so, please help me. There will be no front-end, so I won't draw the FFT in a Window (I guess the drawing does eat a lot of the cpu).

Then I have no idea how I could analyze the audio. All the FFT Sample Codes I found are using the mic. I do not want to use the mic. I saw something getting the Audio File and exporting it to a uncompressed file, but I need a live-analysation. I've had a look at aurioTouch2, but I don't get how I could change the input from the mic to the iPod Library. I think, the part I'm searching for is here:

    // Initialize our remote i/o unit

inputProc.inputProc = PerformThru;
inputProc.inputProcRefCon = self;

CFURLRef url = NULL;
try {   
    url = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, CFStringRef([[NSBundle mainBundle] pathForResource:@"button_press" ofType:@"caf"]), kCFURLPOSIXPathStyle, false);
    XThrowIfError(AudioServicesCreateSystemSoundID(url, &buttonPressSound), "couldn't create button tap alert sound");
    CFRelease(url);

    // Initialize and configure the audio session
    XThrowIfError(AudioSessionInitialize(NULL, NULL, rioInterruptionListener, self), "couldn't initialize audio session");

    UInt32 audioCategory = kAudioSessionCategory_PlayAndRecord;
    XThrowIfError(AudioSessionSetProperty(kAudioSessionProperty_AudioCategory, sizeof(audioCategory), &audioCategory), "couldn't set audio category");
    XThrowIfError(AudioSessionAddPropertyListener(kAudioSessionProperty_AudioRouteChange, propListener, self), "couldn't set property listener");

    Float32 preferredBufferSize = .005;
    XThrowIfError(AudioSessionSetProperty(kAudioSessionProperty_PreferredHardwareIOBufferDuration, sizeof(preferredBufferSize), &preferredBufferSize), "couldn't set i/o buffer duration");

    UInt32 size = sizeof(hwSampleRate);
    XThrowIfError(AudioSessionGetProperty(kAudioSessionProperty_CurrentHardwareSampleRate, &size, &hwSampleRate), "couldn't get hw sample rate");

    XThrowIfError(AudioSessionSetActive(true), "couldn't set audio session active\n");

    XThrowIfError(SetupRemoteIO(rioUnit, inputProc, thruFormat), "couldn't setup remote i/o unit");
    unitHasBeenCreated = true;

    drawFormat.SetAUCanonical(2, false);
    drawFormat.mSampleRate = 44100;
    (...)

But I'm quite new to all of these AudioUnits, so I can't understand where an input is loaded. Then, the code mentioned above uses AVAudioSession. A little birdie told me, this will be deprecated, so what is the alternative?

So, basically:

1. How can I get the currently playing audio in order to do an analyzation? Can I just use a MPMusicPlayerController and get the samples? Or do I have to build a entire AudioUnit which plays the Library?

2. What is the fastest way (CPU) to analyze lows, mids and highs? Filtering? FFT? Something else?

3. Will I get in trouble with the Copyrights of bought music? Because I tried to convert the playing file to PCA Samples and sometimes I have this error:

   VTM_AViPodReader[7666:307] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[AVAssetReader initWithAsset:error:] invalid parameter not satisfying: asset != ((void *)0)'

4. What is the "new" way to do an FFT if the whole AVAudioSession stuff won't work in the future?

Answer 1

You can't get the currently playing audio (security sandbox prevents this) on iOS, unless your app is the one playing the audio using certain select APIs (Audio Queue, RemoteIO, etc.)

3 bandpass filters (made with IIR biquads) will be faster than an FFT. But even a full FFT will use a very small percentage of CPU time.

An app can't convert or play protected music from the iTunes library in a form where samples can be captured.

The FFT is in the Accelerate framework, not in the audio session.