generate a square wave using Core Audio

Question

I'm trying to generate a square wave using Core Audio with the code below. The AIFF-file generates nothing whatever I pass any arguments.The sample is from Learning Core Audio by Kevin Avila http://www.amazon.com/Learning-Core-Audio-Hands-On-Programming/dp/0321636848

Any Ideas? Please help!

The main.m looks as follows;

#import <Foundation/Foundation.h>
#import <AudioToolbox/AudioToolbox.h>
#define SAMPLE_RATE 44100
#define DURATION 50.0
#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]; NSURL *fileURL = [NSURL fileURLWithPath: filePath];
    // Prepare the format
    AudioStreamBasicDescription asbd;
    memset(&asbd, 0, sizeof(asbd));
    asbd.mSampleRate = SAMPLE_RATE;
    asbd.mFormatID = kAudioFormatLinearPCM;
    asbd.mFormatFlags = kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    asbd.mBitsPerChannel = 16;
    asbd.mChannelsPerFrame = 1;
    asbd.mFramesPerPacket = 1;
    asbd.mBytesPerFrame = 2; asbd.mBytesPerPacket = 2;
    // Set up the file
    AudioFileID audioFile;
    OSStatus audioErr = noErr;
    audioErr = AudioFileCreateWithURL((__bridge CFURLRef)fileURL,
                                      kAudioFileAIFFType,
                                      &asbd,
                                      kAudioFileFlags_EraseFile,
                                      &audioFile);
    assert (audioErr == noErr);
    // Start writing samples
    long maxSampleCount = SAMPLE_RATE;
    long sampleCount = 0;
    UInt32 bytesToWrite = 2;
    double wavelengthInSamples = SAMPLE_RATE / hz;
    while (sampleCount < maxSampleCount) {
        for (int i=0; i<wavelengthInSamples; i++) {
            // Square wave
            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

Pretty sure your AudioFileClose() should not be inside the for-loop. That should go outside the loop, after you've written all the samples.

Please check against the downloadable code at the book's home page. I just downloaded and ran it, and it works as described in the book (although, since it was built for Lion, you will have to update the target SDK in the project settings if you're on Mountain Lion or Mavericks)

--Chris (invalidname)