Merging Wav Files in Objective C.

On a recent Mac Development project I had to merge two audio inputs into a single file which was an interesting exercise in understanding the rfc wav spec. While concatenation of wav files is simpler since you just have to remove the header from one file and update the header in the other file to reflect the new file size, merging two sounds so that they play simultaneously is a little bit trickier. I've decided to post my objC implementation here in case someone else ever runs into a similar need. Suggestions and comments as usual are always welcome. Especially in regards to clamping the new values uint values to a max which I've ignored in this implementation since it didn't seem to affect the output merged sound I was creating negatively.

    1 -(void)mixSkypeWavFile:(NSString*)wav1 withWavFile:(NSString*)wav2 intoNewWavFile:(NSString*)outWav{
    2         //the format of our wav files is ("Subchunk1Size = Hex 12/Dec 18" plus 2 bytes ExtraParamSize hex "0000"
    3       //instead of the standard ("Subchunk1Size = Hex 10/Dec 16")
    4     
    5         //a good breakdown for the wave spec bytes https://ccrma.stanford.edu/courses/422/projects/WaveFormat/
    6       //turns out the ones here are little endian for numbers and bigendian for strings. go figure.
    7     
    8         //once you understand that you can make sense of this nice comment on stack overflow from lucius
    9         //http://stackoverflow.com/questions/1540380/how-to-lower-sound-on-the-iphones-sdk-audioqueue
   10       // the strat is to add each 2byte sample together and then divide by two to get the average sound at that time.
   11         // this has the adverse effect of lowering the volume too but shouldn't bee to bad since we only need to do this once.
   12     NSData * wav1Data = [NSData dataWithContentsOfFile:wav1];
   13     NSData * wav2Data = [NSData dataWithContentsOfFile:wav2];
   14     
   15         //grab one of the 46 byte headers from these files (assume they are identical which they should be for this to work.)
   16         //grab data from wav1 without headers
   17       //grab data from wav2 without headers
   18         // you don't have to modify the header if you don't append the files one after the other and change the total file size!
   19         // divide each 2byte sample which is just a number then add them together to create the mixed outWav data. 
   20       //keep in mind these numbers need to be little endian. but at 2 byte blocks little endian and big endian seem to be equiv. (we'll test this now.)
   21     
   22     int wav1DataSize = [wav1Data length] - 46;  
   23     int wav2DataSize = [wav2Data length] - 46;
   24     if (wav1DataSize <= 0 ||  wav2DataSize <= 0) {
   25         NSLog(@"error merging wav sources for now just error out of sound mixing.");
   26         [[NSAlert alertWithMessageText:@"error merging sound sources" defaultButton:nil alternateButton:nil otherButton:nil informativeTextWithFormat:@"We apologize, please check for updates and contact support if this continues to be an issue."] runModal];
   27         return;
   28     }   
   29 
   30         //Use the header from the shorter of the two files.
   31     NSMutableData * headerBuffer;
   32     int outputWavDataSize;
   33     if (wav1DataSize < wav2DataSize) {
   34         outputWavDataSize = wav1DataSize;
   35         headerBuffer = [NSMutableData dataWithData:[wav1Data subdataWithRange:NSMakeRange(0, 46)]];
   36     }else{
   37         outputWavDataSize = wav2DataSize;
   38         headerBuffer = [NSMutableData dataWithData:[wav2Data subdataWithRange:NSMakeRange(0, 46)]];     
   39     }
   40     
   41 //  NSLog(@"are both headers identical? = %d", [headerBuffer isEqualToData:headerBuffer2]);
   42 //  NSLog(@"header is = %@",[headerBuffer description]);
   43     
   44         //read each 2 byte increment
   45         //get an average
   46         //store in a merged buffer
   47     NSData * wav1DataBuffer1; 
   48     NSData * wav1DataBuffer2; 
   49     NSData * wav2DataBuffer1; 
   50     NSData * wav2DataBuffer2;
   51     
   52     NSMutableData * littleEndianHexWav1Sample;
   53     short iValueWav1Sample;
   54     NSMutableData * littleEndianHexWav2Sample;
   55     short iValueWav2Sample;
   56     short iValueWavSampleAverage;
   57     
   58     for (int i=0; i<(outputWavDataSize/2); i++) {
   59             //simulate little endian by flipping the bytes
   60         wav1DataBuffer1 = [wav1Data subdataWithRange:NSMakeRange(46 + (i*2), 1)];
   61         wav2DataBuffer1 = [wav2Data subdataWithRange:NSMakeRange(46 + (i*2), 1)];
   62         wav1DataBuffer2 = [wav1Data subdataWithRange:NSMakeRange(46 + (i*2) + 1, 1)];
   63         wav2DataBuffer2 = [wav2Data subdataWithRange:NSMakeRange(46 + (i*2) + 1, 1)];
   64             //littleEndianHexWav1Sample = [[[wav1DataBuffer2 description] substringWithRange:NSMakeRange(1, 2)] stringByAppendingString:[[wav1DataBuffer1 description] substringWithRange:NSMakeRange(1, 2)]]; 
   65             //      littleEndianHexWav2Sample = [[[wav2DataBuffer2  description] substringWithRange:NSMakeRange(1, 2)] stringByAppendingString:[[wav2DataBuffer1 description] substringWithRange:NSMakeRange(1, 2)]]; 
   66     littleEndianHexWav1Sample = [NSMutableData dataWithData:wav1DataBuffer1];
   67         [littleEndianHexWav1Sample appendData:[NSMutableData dataWithData:wav1DataBuffer2]]; 
   68         littleEndianHexWav2Sample = [NSMutableData dataWithData:wav2DataBuffer1];
   69         [littleEndianHexWav2Sample appendData:wav2DataBuffer2];
   70         
   71             //      [littleEndianHexWav1Sample getBytes:&iValueWav1Sample length:2];
   72             //      [littleEndianHexWav2Sample getBytes:&iValueWav2Sample length:2];
   73         NSString* wav1HexString = [[littleEndianHexWav1Sample description] substringWithRange:NSMakeRange(1, 4)];
   74         NSString* wav2HexString = [[littleEndianHexWav2Sample description] substringWithRange:NSMakeRange(1, 4)];
   75     unsigned wav1Hexint;
   76         unsigned wav2Hexint;
   77         
   78         [[NSScanner scannerWithString:wav1HexString] scanHexInt:&wav1Hexint];   
   79         [[NSScanner scannerWithString:wav2HexString] scanHexInt:&wav2Hexint];   
   80         unsigned wavAverage;
   81             //wavAverage = ((wav1Hexint/sqrt(2))+(wav2Hexint/sqrt(2)));
   82             //      wavAverage = (pow(wav1Hexint, 2)+pow(wav2Hexint,2))/sqrt(2);
   83             //      wavAverage = (wav1Hexint+wav2Hexint)/2;
   84         wavAverage = (wav1Hexint+wav2Hexint);
   85         iValueWav1Sample = wav1Hexint;
   86         iValueWav2Sample = wav2Hexint;
   87         iValueWavSampleAverage = wavAverage;
   88             //      NSLog(@"wav1 bytes = %@, intVal = %d",littleEndianHexWav1Sample, iValueWav1Sample );
   89             //      NSLog(@"wav2 bytes = %@, intVal = %d",littleEndianHexWav2Sample, iValueWav2Sample );
   90             //  iValueWavSampleAverage = (iValueWav1Sample/2) + (iValueWav2Sample/2);
   91         
   92             //http://stackoverflow.com/questions/836681/iphone-int-to-nsdata
   93         NSData * bigEndian = [NSData dataWithBytes:&iValueWavSampleAverage length:2];
   94         NSMutableData * littleEndian = [NSMutableData dataWithData:[bigEndian subdataWithRange:NSMakeRange(1, 1)]];
   95         [littleEndian appendData:[bigEndian subdataWithRange:NSMakeRange(0, 1)]];
   96             //NSString * littleEndian = [[bigEndian substringFromIndex:[bigEndian length]/2] stringByAppendingString:[bigEndian substringToIndex:[bigEndian length]/2]]; 
   97             //      NSLog(@"the average val = %d, which becomes little endian %@", iValueWavSampleAverage, [littleEndian description]);
   98         
   99         
  100             //      char temp[lengthOfMessage];
  101             //      strcpy(temp, [littleEndian cString]);
  102         [headerBuffer appendData:littleEndian];
  103             //
  104             //      
  105             //      NSLog(@"header is = %@",[headerBuffer description]);
  106         
  107     }
  108         //  NSLog(@"header is = %@",[headerBuffer description]);
  109     [headerBuffer writeToURL:[NSURL fileURLWithPath:outWav] atomically:YES];
  110 }
  111 
  112