Reading audio samples via AVAssetReader

Question

How do you read audio samples via AVAssetReader? I\'ve found examples of duplicating or mixing using AVAssetReader, but those loops are always controlled by the AVAssetWrite

Answer 1

The answers here are not generic. The call to CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer might fail when the AudioBufferList needs to be sized differently. When having non-intererleaved samples as example.

The correct way is to call CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer twice. The first call queries the size needed for the AudioBufferList and second one actually fills the AudioBufferList.

size_t bufferSize = 0;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
    sampleBuffer,
    &bufferSize,
    NULL,
    0,
    NULL,
    NULL,
    0,
    NULL
);

AudioBufferList *bufferList = malloc(bufferSize);
CMBlockBufferRef blockBuffer = NULL;
CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
    sampleBuffer,
    NULL,
    bufferList,
    bufferSize,
    NULL,
    NULL,
    kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
    &blockBuffer
);

// handle audio here

free(bufferList);
CFRelease(blockBuffer);

In a real world example you must perform error handling and also you should not malloc every frame, instead cache the AudioBufferList.