core audio how to generate a square wave with two channel ( stereo) - objective-c

Question

It's the code from Learning Core Audio http://www.amazon.com/Learning-Core-Audio-Hands-On-Programming/dp/0321636848
Just like the book says ,generating stereo should set asbd.mBitsPerChannel to 8 and asbd.mChannelsPerFrame to 2 . But the audio I get only has one channel . I don't know what's wrong with the code ,please help me . Thanks
#import <Foundation/Foundation.h>
#import <AudioToolbox/AudioToolbox.h>
#define SAMPLE_RATE 44100
#define DURATION 5.0
#define FILENAME_FORMAT #"%0.3f-square.aif"
int main(int argc, const char * argv[])
{
#autoreleasepool {
if (argc < 2) {
printf("Usage: CAToneFileGenerator n\n(where n is tone in Hz)");
return -1;
}
double hz = atof(argv[1]);
assert(hz > 0);
NSLog(#"generating %f hz tone", hz);
NSString *fileName = [NSString stringWithFormat:FILENAME_FORMAT, hz];
NSString *filePath = [[[NSFileManager defaultManager] currentDirectoryPath] stringByAppendingPathComponent:fileName];
NSLog(#"%#", filePath);
NSURL *fileURL = [NSURL fileURLWithPath:filePath];
AudioStreamBasicDescription asbd;
memset(&asbd, 0, sizeof(asbd));
asbd.mSampleRate = SAMPLE_RATE;
asbd.mFormatID = kAudioFormatLinearPCM;
asbd.mFormatFlags = kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
asbd.mBitsPerChannel = 8;
asbd.mChannelsPerFrame = 2;
asbd.mFramesPerPacket = 1;
asbd.mBytesPerFrame = 2;
asbd.mBytesPerPacket = 2;
AudioFileID audioFile;
OSStatus audioErr = noErr;
audioErr = AudioFileCreateWithURL((CFURLRef) fileURL, kAudioFileAIFFType, &asbd, kAudioFileFlags_EraseFile, &audioFile);
assert(audioErr == noErr);
long maxSampleCount = SAMPLE_RATE * DURATION;
long sampleCount = 0;
UInt32 bytesToWrite = 2;
double wavelengthInSamples = SAMPLE_RATE / hz;
while (sampleCount < maxSampleCount) {
for (int i = 0; i < wavelengthInSamples; i++) {
SInt16 sample;
if (i < wavelengthInSamples /2) {
sample = CFSwapInt16HostToBig(SHRT_MAX);
}else{
sample = CFSwapInt16HostToBig(SHRT_MIN);
}
audioErr = AudioFileWriteBytes(audioFile, false, sampleCount*2, &bytesToWrite, &sample);
assert(audioErr == noErr);
sampleCount ++;
}
}
audioErr = AudioFileClose(audioFile);
assert(audioErr == noErr);
NSLog(#"wrote %ld samples", sampleCount);
}
return 0;
}

Answer 1

Just changing the ASBD from the book code doesn't magically fix everything. You haven't accounted for how you're writing the samples to the file. Also, 8-bit is going to sound like ass.
Go back to mBitsPerChannel = 16, and then account for the fact you're writing two channels per frame, meaning that mBytesPerFrame and mBytesPerPacket will now be 4 (they were 2 in the book). Think about why this is.
Then you should just be able to add a second call to AudioFileWriteBytes() -- or do a loop where you count over mChannelsPerFrame -- right after the first one. But you'll have to account for the different offsets in the file, since you're writing 4 bytes each pass instead of 2. I think this is right:
audioErr = AudioFileWriteBytes(audioFile, false, sampleCount*4, &bytesToWrite, &sample); // left
audioErr = AudioFileWriteBytes(audioFile, false, (sampleCount*4)+2, &bytesToWrite, &sample); // right
You need to figure out some of this stuff on your own in order for it to sink in.