Detecting blowing on the iPhone microphone? - iphone

Question

I am trying to detect when the user is blowing into the mic of an iPhone. Right now I am using the SCListener class from Stephen Celis to call
if ([[SCListener sharedListener] peakPower] > 0.99)
in an NSTimer. However, this returns true sometimes when I'm not blowing. Anyone have any sample code to check if the user is blowing into the mic?

Answer 1

I would recommend low-pass filtering the power signal first. There is always going to be some amount of transient noise that will mess with instantaneous readings; low-pass filtering helps mitigate that. A nice and easy low-pass filter would be something like this:
// Make this a global variable, or a member of your class:
double micPower = 0.0;
// Tweak this value to your liking (must be between 0 and 1)
const double ALPHA = 0.05;
// Do this every 'tick' of your application (e.g. every 1/30 of a second)
double instantaneousPower = [[SCListener sharedListener] peakPower];
// This is the key line in computing the low-pass filtered value
micPower = ALPHA * instantaneousPower + (1.0 - ALPHA) * micPower;
if(micPower > THRESHOLD) // 0.99, in your example
// User is blowing on the microphone

Answer 2

when run on iPhone, you should add the following code after [recorder prepareToRecorder]
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:nil];
[[AVAudioSession sharedInstance] setActive:YES error:nil];

Answer 3

Use return as lowPassResults is bigger than 0.55. This is working fine:
-(void)readyToBlow1 { NSURL *url = [NSURL fileURLWithPath:#"/dev/null"];
NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat: 44100.0], AVSampleRateKey,
[NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
[NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
[NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,
nil];
NSError *error;
recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];
if (recorder) {
[recorder prepareToRecord];
recorder.meteringEnabled = YES;
[recorder record];
levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.01 target: self selector: #selector(levelTimerCallback1:) userInfo: nil repeats: YES];
} else
NSLog(#"%#",[error description]);
}
-(void)levelTimerCallback1:(NSTimer *)timer { [recorder updateMeters];
const double ALPHA = 0.05;
double peakPowerForChannel = pow(10, (0.05 * [recorder peakPowerForChannel:0]));
double lowPassResults = ALPHA * peakPowerForChannel + (1.0 - ALPHA) * lowPassResults;
if (lowPassResults > 0.55) {
lowPassResults = 0.0;
[self invalidateTimers];
NextPhase *objNextView =[[NextPhase alloc]init];
[UIView transitionFromView:self.view
toView:objNextView.view
duration:2.0
options:UIViewAnimationOptionTransitionCurlUp
completion:^(BOOL finished) {}
];
[self.navigationController pushViewController:objNextView animated:NO];
**return;**
}
}

Answer 4

http://mobileorchard.com/tutorial-detecting-when-a-user-blows-into-the-mic/
this tutorial works fine with simulator but its not working in iphone there is no response from iphone mic

Answer 5

Try this It is working fine for me. Thanks #jinhua liao
- (void)viewDidLoad {
[super viewDidLoad];
lowPassResults = 0.0;
[self readyToBlow1];
NSURL *url = [NSURL fileURLWithPath:#"/dev/null"];
NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat: 44100.0], AVSampleRateKey,
[NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
[NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
[NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,
nil];
NSError *error;
recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];
if (recorder) {
[recorder prepareToRecord];
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:nil];
[[AVAudioSession sharedInstance] setActive:YES error:nil];
recorder.meteringEnabled = YES;
[recorder record];
levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.03 target: self selector: #selector(levelTimerCallback:) userInfo: nil repeats: YES];
} else
NSLog([error description]);
}
- (void)levelTimerCallback:(NSTimer *)timer {
[recorder updateMeters];
const double ALPHA = 0.05;
double peakPowerForChannel = pow(10, (0.05 * [recorder peakPowerForChannel:0]));
lowPassResults = ALPHA * peakPowerForChannel + (1.0 - ALPHA) * lowPassResults;
NSLog(#"lowpassResult is %f",lowPassResults);
if (lowPassResults > 0.95){
NSLog(#"Mic blow detected");
[levelTimer invalidate];
}
}