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POHow can I append to a recorded MPEG4 AAC file?
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  1. POHow can I append to a recorded MPEG4 AAC file?
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    copied!<p>I'm recording audio on an iPhone, using an <code>AVAudioRecorder</code> with the following settings:</p> <pre><code>NSMutableDictionary *recordSettings = [[NSDictionary alloc] initWithObjectsAndKeys: [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey, [NSNumber numberWithFloat:44100.0], AVSampleRateKey, [NSNumber numberWithInt:1], AVNumberOfChannelsKey, [NSNumber numberWithInt:12800], AVEncoderBitRateKey, [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey, [NSNumber numberWithInt: AVAudioQualityHigh], AVEncoderAudioQualityKey, nil]; </code></pre> <p>(I can be flexible on most of these settings, but I have to use MPEG4 AAC.)</p> <p>I save the audio to a file.</p> <p>The user needs to be able to come back at a later date and continue recording to the same file. There doesn't seem to be an option to do this directly with <code>AVAudioRecorder</code>, so instead I'm recording to a new file and concatenating them.</p> <p>At the moment I'm appending the files using an <code>AVMutableComposition</code> and an <code>AVMutableCompositionTrack</code> as <a href="https://stackoverflow.com/questions/4191569/how-to-append-two-audio-files">here</a>, but it's really slow for longer recordings so this isn't really feasible.</p> <p>I'm thinking it would be much quicker if I could strip the header from the second file, append the audio data to the first file, then alter the header of the combined file to reflect the new duration. As I know both files were created with exactly the same settings, I figure the other details in the headers should be identical.</p> <p>Unfortunately I can't find any information about what format the headers are in, or if it's possible to combine files in this way.</p> <p>So my questions are:</p> <ul> <li>What is the format of the MPEG-4 AAC file header, when created on an iPhone?</li> <li>Can I combine two audio files by messing with the headers like this?</li> <li>Is there a better way of appending two MPEG-4 AAC audio files almost instantaneously?</li> </ul>
 

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