How to check the FFT results in iphone?

Question

I have done FFT of a audio file using OouraFFTl.How to check whether the Sampled output is right or wrong.Whats the better and easy way to check it.This is my code.

MyAudioFile  *audioFile = [[MyAudioFile alloc]init];
OSStatus result = [audioFile open:var ofType:@"wav"];
int numFrequencies=16384;
int kNumFFTWindows=10;

OouraFFT *myFFT = [[OouraFFT alloc] initForSignalsOfLength:numFrequencies*2    andNumWindows:kNumFFTWindows];
for(long i=0; i<myFFT.dataLength; i++)
 {
myFFT.inputData[i] = (double)audioFile.audioData[i];
} 
 [myFFT calculateWelchPeriodogramWithNewSignalSegment];
NSLog(@"the spectrum data 1 is  %f ",myFFT.spectrumData[1]);
NSLog(@"the spectrum data 2 is  %f",myFFT.spectrumData[2]);
NSLog(@"the spectrum data 8192 is  %f ",myFFT.spectrumData[8192]);

I have created MyAudioFile class which contains

    -(OSStatus)open:(NSString *)fileName ofType:(NSString *)fileType{
OSStatus result = -1;

 CFStringRef filePath=fileName;

  CFURLRef audioFileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault,      (CFStringRef)filePath, kCFURLPOSIXPathStyle, false);
 //open audio file
 result = AudioFileOpenURL (audioFileURL, kAudioFileReadPermission, 0, &mAudioFile);
 if (result == noErr) {
 //get  format info
 UInt32 size = sizeof(mASBD);

result = AudioFileGetProperty(mAudioFile, kAudioFilePropertyDataFormat, &size, &mASBD);

UInt32 dataSize = sizeof packetCount;
result = AudioFileGetProperty(mAudioFile, kAudioFilePropertyAudioDataPacketCount, &dataSize, &packetCount);
NSLog([NSString stringWithFormat:@"File Opened, packet Count: %d", packetCount]);

UInt32 packetsRead = packetCount;
UInt32 numBytesRead = -1;
if (packetCount > 0) { 
    //allocate  buffer
    audioData = (SInt16*)malloc( 2 *packetCount);
    //read the packets
    result = AudioFileReadPackets (mAudioFile, false, &numBytesRead, NULL, 0, &packetsRead,  audioData); 
    NSLog([NSString stringWithFormat:@"Read %d  bytes,  %d packets", numBytesRead, packetsRead]);
}
}
  else
  NSLog([NSString stringWithFormat:@"Could not open file: %@", filePath]);


CFRelease (audioFileURL);     
return result;
 }

Answer 1

The easy way to check FFT is take FFT of sinusoidal signal. The output should be all zeros except dozen of non-zero values.

Answer 2

You need to plot the magnitude of the output of the FFT. I'm not familiar with your programming language, but in Python you would use something like plot(abs(fft(a))). For a silent input, the output should be all zeros. For a sine wave input, you should see two spikes:

For a real signal, the spikes will be symmetrical from left to right. If you're doing a real FFT, though (which is more computationally efficient) you'll only get the left half of the plot as your output, since it ignores the redundant mirror image.

If the frequency is higher, the spikes will be closer to the center. If the frequency is perfectly in sync with the chunk size, the spike will only be one point wide and everything else will be exactly 0. Otherwise it will have a tapering "skirt" like above.