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