Simple AudioQueue sine wave—why the distortion?

问题

As a learning exercise, I'm using an AudioQueue to generate and play a 300 Hz sine wave. (I understand there are a variety of tools to generate and play audio, but yes, this is just to build up my Core Audio chops and this task is all about the AudioQueue.)

The wave plays, but with distortion. Recording and plotting the sound shows that there is some distortion at the boundary between buffers (every half second), in addition to other short bursts of distortion here and there. I've included my code below. If anyone could shine some light on the problem, that would be amazing—thanks for reading!

EDIT: Found the problem. It should read bufferByteSize=numPacketsForTime*asbd.mBytesPerPacket;

static void MyAQOutputCallback(void *inUserData,
                               AudioQueueRef inAQ,
                               AudioQueueBufferRef inCompleteAQBuffer){

    int i;
    MyWave *inData=(MyWave*)inUserData;

    // synth params
    int phaseL =inData->sampleCount;
    float FL = (2.0 * 3.14159265 * 300.0) / 44100.0;
    float amp = 0.5;
    int frameCount=22050;

    // Get the info struct and a pointer to our output data
    short *coreAudioBuffer = (short*) inCompleteAQBuffer->mAudioData;

    // Need to set this
    inCompleteAQBuffer->mAudioDataByteSize = 2*frameCount; // two shorts per frame, one frame per packet
    // For each frame/packet (the same in our example)
    for(i=0;i<frameCount;i++) {
        // Render the sine waves - signed interleaved shorts (-32767 -> 32767), 16 bit stereo
        float sampleL = (amp * sin(FL * (float)phaseL));
        short sampleIL = (int)(sampleL * 32767.0);
        coreAudioBuffer[i ] =   sampleIL;
        phaseL++;
    }
    // "Enqueue" the buffer
    AudioQueueEnqueueBuffer(inAQ, inCompleteAQBuffer, 0, NULL);
    inData->sampleCount=phaseL;


}

int main(int argc, const char * argv[])
{

    // Open an audio file
    MyWave thisWave={0};
    // Set up format
    AudioStreamBasicDescription asbd;
    memset(&asbd,0,sizeof(asbd));
    asbd.mSampleRate=SAMPLE_RATE;
    asbd.mFormatID=kAudioFormatLinearPCM;
    asbd.mFormatFlags=kLinearPCMFormatFlagIsSignedInteger|kAudioFormatFlagIsPacked;
    asbd.mBitsPerChannel=16;
    asbd.mChannelsPerFrame=1;
    asbd.mFramesPerPacket=1;
    asbd.mBytesPerFrame=2;
    asbd.mBytesPerPacket=2;

    // Set up queue
    AudioQueueRef queue;
    CheckError(AudioQueueNewOutput(&asbd,
                                   MyAQOutputCallback,
                                   &thisWave,
                                   NULL,
                                   NULL,
                                   0,
                                   &queue),
               "AudioQueueNewOutput failed");

    UInt32 bufferByteSize;
    Float64 numPacketsForTime=asbd.mSampleRate/asbd.mFramesPerPacket*0.5;
    bufferByteSize=numPacketsForTime;
    AudioQueueBufferRef buffers[kNumberPlaybackBuffers];
    int i;
    for (i=0;i<kNumberPlaybackBuffers;++i){
        CheckError(AudioQueueAllocateBuffer(queue,
                                            bufferByteSize,
                                            &buffers[i]),
                   "AudioQueueAllocateBuffer failed");
        MyAQOutputCallback(&thisWave, queue, buffers[i]);
    }


    // Start queue
    CheckError(AudioQueueStart(queue,
                               NULL),
               "AudioQueueStart failed");
    printf("Playing...\n");
    do
    {
        CFRunLoopRunInMode(kCFRunLoopDefaultMode,
                           0.25,
                           false);
    }while (1==1);
    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 2, false);

    // Clean up queue when finished
    CheckError(AudioQueueStop(queue,
                              TRUE),
               "AudioQueueStop failed");
    AudioQueueDispose(queue, TRUE);
    return 0;
}

回答1:

Problem found, it should read:

 bufferByteSize = numPacketsForTime*asbd.mBytesPerPacket;

I'll leave this up here, as somebody may find the code useful!

来源：https://stackoverflow.com/questions/24551881/simple-audioqueue-sine-wave-why-the-distortion

标签

ios

core-audio

audioqueue