Adding metadata to AAC M4A via AVAssetExportSession

Question

I am creating and storing an AAC-encoded .m4a file using AVAudioRecorder. This produces a playable .m4a file just fine. I want to then use AVAssetExportSession to process the file in order to add metadata to the file. The below code is producing a .m4a file of a similar size (1 KB less than source), but when it plays back, there is just silence.

NSURL* url = [NSURL fileURLWithPath:self.m4aPath];
AVURLAsset* asset = [AVAsset assetWithURL:url];
AVMutableMetadataItem* t = [AVMutableMetadataItem metadataItem];
t.key = AVMetadataCommonKeyTitle;
t.keySpace = AVMetadataKeySpaceCommon;
t.value = @"Unit Test";
NSArray* metadata = [NSArray arrayWithObject:t];

AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetAppleM4A];

exportSession.outputURL = [NSURL fileURLWithPath:[[NSFileManager  rawRecordingsDirectory] stringByAppendingPathComponent:@"test.m4a"]];
exportSession.outputFileType = AVFileTypeAppleM4A;
exportSession.metadata = metadata;

[exportSession exportAsynchronouslyWithCompletionHandler:^{....}];

One more piece of info: When I look at the source and exported file in the Finder, the source file has the black iTunes icon, while the exported file has the white iTunes icon. Not sure what this means in practice, but hoping it might be helpful. Moreover, double-clicking source adds it to iTunes and starts playback, while double-clicking the exported opens iTunes but does nothing.

Answer 1

I had a similar issue where my output m4a file had the white icon (instead of black) and wouldn't play. Though that was when I was creating the original source file from raw sample data, not when adding metadata to it.

My issue was that I wasn't closing the exported file in my code (I was just terminating the app before calling the close function). Once I called the close function, it started working. You might want to check that.

Also, I found "open with->Quicktime" useful as that gives an error when the file is corrupt, and plays it fine when it isn't. More useful than iTunes silently ignoring the error.