CODEWITHSUNDEEP
macoscore-audiolossless-compressionflac
Matthew Leon
Matthew Leon

Reputation: 319

What is the appropriate value for the ExtAudioFileWrite inNumberFrames parameter?

I'm working on a FLAC-to-ALAC transcoder and trying to use ExtAudioFile to write to ALAC. I am using the FLAC library's callback-based system to read in the FLAC file, which means that every frame in the FLAC file results in a function call. Within that call, I set up my buffers and call the ExtAudioFileWrite code as follows:

AudioBufferList * fillBufList;
fillBufList = malloc(sizeof *fillBufList + (frame->header.channels - 1) * sizeof fillBufList->mBuffers[0]);
fillBufList->mNumberBuffers = frame->header.channels;

for (int i = 0; i < fillBufList->mNumberBuffers; i++) {
    fillBufList->mBuffers[i].mNumberChannels = 1; // non-interleaved
    fillBufList->mBuffers[i].mDataByteSize = frame->header.blocksize * frame->header.bits_per_sample / 8;
    fillBufList->mBuffers[i].mData = (void *)(buffer[i]);
    NSLog(@"%i", fillBufList->mBuffers[i].mDataByteSize);
}

OSStatus err = ExtAudioFileWrite (outFile, 1, fillBufList);

Now, the number 1 in the final line is something of a magic number that I've chosen because I figured that one frame in the FLAC file would probably correspond to one frame in the corresponding ALAC file, but this seems not to be the case. Every call to ExtAudioFileWrite returns an error value of -50 (error in user parameter list). The obvious culprit is the value I'm providing for the frame parameter.

So I ask, then, what value should I be providing?

Or am I barking up the wrong tree?

(Side note: I suspected that, despite the param-related error value, the true problem could be the buffer setup, so I tried mallocing a zeroed-out dummy buffer just to see what would happen. Same error.)

Upvotes: 1

Views: 762

Answers (1)

lucius
lucius

Reputation: 8685

For ExtAudioFileWrite, the number of frames is equal to the number of samples you want to write. If you're working with 32-bit float interleaved data, it would be the mDataByteSize / (sizeof(Float32) / mNumberChannels). It shouldn't be 1 unless you meant to write only one sample, and if you're writing a compressed format, it expects a certain number of samples, I think. It's also possible that the -50 error is another issue.

One thing to check is that ExtAudioFile expects only one buffer. So your fillBufList->mNumberBuffers should always equal 1, and if you need to do stereo, you need to interleave the audio data, so that mBuffers[0].mNumberChannels is equal to 2 for stereo.

Upvotes: 2

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