AVAssetWriter How to write down-sampled/compressed m4a/mp3 files - ios

Question

I'm trying to take a local m4a or mp3 file and compress/down-sample this file (for the purposes of making a smaller file).
Originally, I was using the AVAssetExportSession to export an AVAsset to a temp directory, but I didn't have any control over compression/down-sampling (you can only use presets, which of them, only .wav file formats support quality degradation).
Then, following several examples here on SO, I tried using AVAssetReader/AVAssetWriter to preform this 'export'.
I create my reader/writer as such:
NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:#"out.m4a"];
NSURL *exportURL = [NSURL fileURLWithPath:outPath];
// reader
NSError *readerError = nil;
AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset
error:&readerError];
AVAssetTrack *track = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
AVAssetReaderTrackOutput *readerOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:track
outputSettings:nil];
[reader addOutput:readerOutput];
// writer
NSError *writerError = nil;
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:exportURL
fileType:AVFileTypeAppleM4A
error:&writerError];
AudioChannelLayout channelLayout;
memset(&channelLayout, 0, sizeof(AudioChannelLayout));
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
// use different values to affect the downsampling/compression
NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
[NSNumber numberWithFloat:44100.0], AVSampleRateKey,
[NSNumber numberWithInt:2], AVNumberOfChannelsKey,
[NSNumber numberWithInt:128000], AVEncoderBitRateKey,
[NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
nil];
AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
outputSettings:outputSettings];
[writerInput setExpectsMediaDataInRealTime:NO];
[writer addInput:writerInput];
And then I start writing...
[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];
[reader startReading];
dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
[writerInput requestMediaDataWhenReadyOnQueue:mediaInputQueue usingBlock:^{
NSLog(#"Asset Writer ready : %d", writerInput.readyForMoreMediaData);
while (writerInput.readyForMoreMediaData) {
CMSampleBufferRef nextBuffer;
if ([reader status] == AVAssetReaderStatusReading && (nextBuffer = [readerOutput copyNextSampleBuffer])) {
if (nextBuffer) {
NSLog(#"Adding buffer");
[writerInput appendSampleBuffer:nextBuffer];
}
} else {
[writerInput markAsFinished];
switch ([reader status]) {
case AVAssetReaderStatusReading:
break;
case AVAssetReaderStatusFailed:
[writer cancelWriting];
break;
case AVAssetReaderStatusCompleted:
NSLog(#"Writer completed");
[writer endSessionAtSourceTime:asset.duration];
[writer finishWriting];
NSData *data = [NSData dataWithContentsOfFile:exportPath];
NSLog(#"Data: %#", data);
break;
}
break;
}
}
}];
When I'm done writing, the data i've supposedly written to the exportURL is null, and the writer reports a successful completion. Any ideas what might be going wrong?
Update The writer status after calling appendSampleBuffer: is AVAssetWriterStatusFailed, though the readers status is successful, and seems to read through the entire file.

Answer 1

I came across the solution:
Using NSHomeDirectory() in NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:#"out.m4a"] was causing the writer to not be able to create the file. Not exactly sure why, or what I would need to do to allow this to work, but changing NSHomeDirectiory() to NSTemporaryDirectory() has solved my problems in the meantime.