Question

I am doing some custom audio post-processing using audio units. I have two files that I am merging together (links below), but am coming up with some weird noise in the output. What am I doing wrong?

I have verified that before this step, the 2 files (workTrack1 and workTrack2) are in a proper state and sound good. No errors are hit in the process as well.

Buffer Processing code:

- (BOOL)mixBuffersWithBuffer1:(const int16_t *)buffer1 buffer2:(const int16_t *)buffer2 outBuffer:(int16_t *)mixbuffer outBufferNumSamples:(int)mixbufferNumSamples {
    BOOL clipping = NO;

    for (int i = 0 ; i < mixbufferNumSamples; i++) {
        int32_t s1 = buffer1[i];
        int32_t s2 = buffer2[i];
        int32_t mixed = s1 + s2;

        if ((mixed < -32768) || (mixed > 32767)) {
            clipping = YES; // don't break here because we dont want to lose data, only to warn the user
        }

        mixbuffer[i] = (int16_t) mixed;
    }
    return clipping;
}

Mixdown code:

////////////////////////////////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////      PHASE 4      ////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// In phase 4, open workTrack1 and workTrack2 for reading,
// mix together, and write out to outfile.

// open the outfile for writing -- this will erase the infile if they are the same, but its ok cause we are done with it
err = [self openExtAudioFileForWriting:outPath audioFileRefPtr:&outputAudioFileRef numChannels:numChannels];
if (err) { [self cleanupInBuffer1:inBuffer1 inBuffer2:inBuffer2 outBuffer:outBuffer err:err]; return NO; }

// setup vars
framesRead = 0;
totalFrames = [self totalFrames:mixAudioFile1Ref]; // the long one.
NSLog(@"Mix-down phase, %d frames (%0.2f secs)", totalFrames, totalFrames / RECORD_SAMPLES_PER_SECOND);

moreToProcess = YES;
while (moreToProcess) {

    conversionBuffer1.mBuffers[0].mDataByteSize = LOOPER_BUFFER_SIZE;
    conversionBuffer2.mBuffers[0].mDataByteSize = LOOPER_BUFFER_SIZE;

    UInt32 frameCount1 = framesInBuffer;
    UInt32 frameCount2 = framesInBuffer;

    // Read a buffer of input samples up to AND INCLUDING totalFrames
    int numFramesRemaining = totalFrames - framesRead; // Todo see if we are off by 1 here.  Might have to add 1
    if (numFramesRemaining == 0) {
        moreToProcess = NO; // If no frames are to be read, then this phase is finished

    } else {
        if (numFramesRemaining < frameCount1) { // see if we are near the end
            frameCount1 = numFramesRemaining;
            frameCount2 = numFramesRemaining;
            conversionBuffer1.mBuffers[0].mDataByteSize = (frameCount1 * bytesPerFrame);
            conversionBuffer2.mBuffers[0].mDataByteSize = (frameCount2 * bytesPerFrame);
        }

        NSbugLog(@"Attempting to read %d frames from mixAudioFile1Ref", (int)frameCount1);
        err = ExtAudioFileRead(mixAudioFile1Ref, &frameCount1, &conversionBuffer1);
        if (err) { [self cleanupInBuffer1:inBuffer1 inBuffer2:inBuffer2 outBuffer:outBuffer err:err]; return NO; }

        NSLog(@"Attempting to read %d frames from mixAudioFile2Ref", (int)frameCount2);
        err = ExtAudioFileRead(mixAudioFile2Ref, &frameCount2, &conversionBuffer2);
        if (err) { [self cleanupInBuffer1:inBuffer1 inBuffer2:inBuffer2 outBuffer:outBuffer err:err]; return NO; }

        NSLog(@"Read %d frames from mixAudioFile1Ref in mix-down phase", (int)frameCount1);
        NSLog(@"Read %d frames from mixAudioFile2Ref in mix-down phase", (int)frameCount2);

        // If no frames were returned, phase is finished
        if (frameCount1 == 0) {
            moreToProcess = NO;

        } else { // Process pcm data

            // if buffer2 was not filled, fill with zeros
            if (frameCount2 < frameCount1) {
                bzero(inBuffer2 + frameCount2, (frameCount1 - frameCount2));
                frameCount2 = frameCount1;
            }

            const int numSamples = (frameCount1 * bytesPerFrame) / sizeof(int16_t);

            if ([self mixBuffersWithBuffer1:(const int16_t *)inBuffer1
                                    buffer2:(const int16_t *)inBuffer2
                                  outBuffer:(int16_t *)outBuffer
                        outBufferNumSamples:numSamples]) {
                NSLog(@"Clipping");
            }
            // Write pcm data to the main output file
            conversionOutBuffer.mBuffers[0].mDataByteSize = (frameCount1 * bytesPerFrame);
            err = ExtAudioFileWrite(outputAudioFileRef, frameCount1, &conversionOutBuffer);

            framesRead += frameCount1;
        } // frame count
    } // else

    if (err) {
        moreToProcess = NO;
    }
} // while moreToProcess

// Check for errors
TTDASSERT(framesRead == totalFrames);
if (err) {
    if (error) *error = [NSError errorWithDomain:kUAAudioSelfCrossFaderErrorDomain
                                            code:UAAudioSelfCrossFaderErrorTypeMixDown
                                        userInfo:[NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:err],@"Underlying Error Code",[self commonExtAudioResultCode:err],@"Underlying Error Name",nil]];
    [self cleanupInBuffer1:inBuffer1 inBuffer2:inBuffer2 outBuffer:outBuffer err:err];
    return NO;
}
NSLog(@"Done with mix-down phase");

ASSUMPTIONS

• mixAudioFile1Ref is always longer than mixAudioFile2Ref
• After the mixAudioFile2Ref runs out of bytes, the outputAudioFileRef should sound exactly the same as mixAudioFile2Ref

The expected sound is supposed to be mixing a fade-in over a fade-out in the beginning to produce a self-crossfade when the track is looped. Please listen to the output, look at the code and let me know where I am going wrong.

Source tone sound: http://cl.ly/2g2F2A3k1r3S36210V23
Resulting tone sound: http://cl.ly/3q2w3S3Y0x0M3i2a1W3v

Answer 1

Turns out there were two problems here.

Buffer Processing Code

int32_t mixed = s1 + s2; was causing clipping. A better way is to divide by the number of channels mixed:int32_t mixed = (s1 + s2)/2; then normalize in another pass later.

Frames != bytes
When zeroing out the second track’s buffers when the sound ran out, I was incorrectly setting the offset and duration as frames not bytes. This produced garbage in the buffer and created the noise you hear periodically. Easy to fix:

if (frameCount2 < frameCount1) {
    bzero(inBuffer2 + (frameCount2 * bytesPerFrame), (frameCount1 - frameCount2) * bytesPerFrame);
    frameCount2 = frameCount1;
}

Now the sample is great: http://cl.ly/1E2q1L441s2b3e2X2z0J