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POAVAssetWriter How to write down-sampled/compressed m4a/mp3 files
 

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  1. POAVAssetWriter How to write down-sampled/compressed m4a/mp3 files
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    <p>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). </p> <p>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).</p> <p>Then, following several examples here on SO, I tried using AVAssetReader/AVAssetWriter to preform this 'export'. </p> <p>I create my reader/writer as such:</p> <pre><code>NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:@"out.m4a"]; NSURL *exportURL = [NSURL fileURLWithPath:outPath]; // reader NSError *readerError = nil; AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:&amp;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:&amp;writerError]; AudioChannelLayout channelLayout; memset(&amp;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:&amp;channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey, nil]; AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio outputSettings:outputSettings]; [writerInput setExpectsMediaDataInRealTime:NO]; [writer addInput:writerInput]; </code></pre> <p>And then I start writing...</p> <pre><code>[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 &amp;&amp; (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; } } }]; </code></pre> <p>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?</p> <p><strong>Update</strong> The writer status after calling <code>appendSampleBuffer:</code> is AVAssetWriterStatusFailed, though the readers status is successful, and seems to read through the entire file. </p>
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    1. CODoes it ever attempt to write anything? What is the reader status on the first time it tries to append the samplebuffer?
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    2. COAh ha, it appears at every step when `[writerInput appendSampleBuffer:]` is called, the writer status is 3, which by looking at the enum, looks like AVAssetWriterStatusFailed. It looks like the reader is copying all of the sample buffers from my asset though - appendSampleBuffer is called *many* times
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