ios: Play sound outside the main thread

Question

I made a simple app that plays an alarm when a certain noise level is exceeded. So therefore I have an AudioQueue that records sound and meters the level of the recorded sound (only important code parts shown below):

#import "AudioRecorder.h"

#include <AudioToolbox/AudioToolbox.h>

#include <iostream>

using namespace std;

@implementation AudioRecorder

@synthesize sp; //custom object SoundPlayer
@synthesize bias; //a bias, if the soundlevel exeeds this bias something happens

AudioRecorder* ar;    

//callback function to handle the audio data contained in the audio buffer.
//In my case it is called every 0.5 seconds
static void HandleInputBuffer (...) {
    ...
    char* levelMeterData = new char[size];
    AudioQueueGetProperty ( inAQ, kAudioQueueProperty_CurrentLevelMeter, levelMeterData, &size );
    AudioQueueLevelMeterState* meterState =  reinterpret_cast<AudioQueueLevelMeterState*>(levelMeterData);
    cout << "mAveragePower = " << meterState->mAveragePower << endl;
    cout << "mPeakPower = " << meterState->mPeakPower << endl;
    if( meterState->mPeakPower > ar.bias )
        [ar playAlarmSound];

}
...
//The constructor of the AudioRecorder class
-(id) init {
    self = [super init];
    if( self ) {
        ar = self;
        sp = [[SoundPlayer alloc] init];
    }
    return self;
}

-(void)playAlarmSound {
    [sp playSound];
}

So what basically happens here is the following:

I tap on the iphone screen on a button that causes the audioqueue to record sound from the mic in the iphone. When a queue is full the callback function "HandleInputBuffer" gets called to handle the data in the audio buffer. Handling the data means in my particular case that I want to measure the sound intensity. If the intensity exceeds a bias the method "playAlarmSound" get invoked.

So this invocation happens outside the main thread i.e. in an extra thread.

The object "sp" (SoundPlayer) has the following implementation:

//callback function. Gets called when the sound has finished playing
void soundDidFinishPlaying( SystemSoundID ssID, void *clientData ) {
    NSLog(@"Finished playing system sound");
    sp.soundIsPlaying = NO;
}

-(id)init {
    self = [super init];
    if(self) {
        self.soundIsPlaying = NO;
        srand(time(NULL));
        soundFilenames = [[NSArray alloc] initWithObjects:@"Alarm Clock Bell.wav", @"Bark.wav", @"Cartoon Boing.wav", @"Chimpanzee Calls.wav", @"School Bell Ringing.wav", @"Sheep Bah.wav", @"Squeeze Toy.wav", @"Tape Rewinding.wav", nil];
        [self copySoundsIfNeeded];
        sp = self;
        [self playSound]; //gets played without any problems
    }
    return self;
}

-(void)playSound {
    if( !self.soundIsPlaying ) {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        int index = rand() % [soundFilenames count];
        NSString* filePath = [ [self getBasePath] stringByAppendingPathComponent: [soundFilenames objectAtIndex:index] ];

        NSFileManager *fileManager = [NSFileManager defaultManager];
        if( [fileManager fileExistsAtPath:filePath] )
            NSLog(@"File %@ exists", [soundFilenames objectAtIndex:index]);
        else
            NSLog(@"File %@ NOT exists", [soundFilenames objectAtIndex:index]);

        CFURLRef url = CFURLCreateWithFileSystemPath ( 0, (CFStringRef) filePath, kCFURLPOSIXPathStyle, NO );
        SystemSoundID outSystemSoundID;
        AudioServicesCreateSystemSoundID( url, &outSystemSoundID );
        AudioServicesAddSystemSoundCompletion ( outSystemSoundID, 0, 0, soundDidFinishPlaying, 0 );
        self.soundIsPlaying = YES;
        AudioServicesPlaySystemSound( outSystemSoundID );
        [pool drain];
    }
}

Actually the code works fine, so there should be no errors in the code, also the format of the wave files is correct. And this is what happens:

iPhone Simulator: Everything works fine. When the object SoundPlayer is created the call to [self playSound] in the constructor causes the simulator to play a random sound, so that proves that the method is correctly implemented. Then I start soundrecording and when a certain soundlevel is exceeded the method gets invoked from AudioRecorder (in a separated thread) and the sounds are also being played. So this is the desired behavior.
Actual iPhone device: When the object SoundPlayer is created a random sound is played, so this works fine. BUT when I start audio recording and a noise level is exceeded the method playSound in Soundplayer gets invoked by AudioRecorder but no sound is played. I suspect this is because this happens in a separated thread. But I have no idea how this could be repaired.

I already tried to repair it by using notifications form the default notification center but it didn't work either. How can I manage that the sound gets played?

Florian Florian · Accepted Answer · 2011-08-08T15:21:04

a couple of things to consider:

1) make sure you enable play&record:

UInt32 category = kAudioSessionCategory_PlayAndRecord;
AudioSessionSetProperty(kAudioSessionProperty_AudioCategory,
                        sizeof(category), &category);

2) check the return values of the various AudioServices* calls in -(void)playSound to see where it actually fails.

3) make sure that the sp property in class AudioRecorder is properly initialized (i.e. that you call the correct init method of AudioRecorder)

4) It is possible that (some) iOS devices do not support different sampling rates for simultaneous record & playback. If your .wav files have a different sampling rate than your recording AudioQueue, it's worth a try to adapt either one.

5) instead of calling [sp playSound] use this statement to call it on the main thread:

[sp performSelectorOnMainThread:@selector(playSound) withObject:nil waitUntilDone:NO];

Also note that most likely, your alarm sound will be heard on the recording (though iPhone 4 can filter it out).

ios: Play sound outside the main thread

1 Answers