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copied!<p>Though we ask the <code>AVAudioRecorder</code> to record in MPEG4-AAC format, it always produces a .caf (Core Audio Format) file. This is just a wrapper format, however, and the actual audio data it contains is in AAC format.</p> <p>In the end, appending files came down to manipulating the .caf files byte-by-byte. The spec for Core Audio Format files is <a href="http://developer.apple.com/library/mac/#documentation/MusicAudio/Reference/CAFSpec/CAF_spec/CAF_spec.html" rel="nofollow">here</a>. Digesting this document and processing the files accordingly was a little off-putting at first, but it turns out the spec is very clear and complete, so it wasn't too onerous.</p> <p>As the spec explains, .caf files consist of chunks with four-byte names at the beginning. For AAC files, there's always a <code>desc</code> chunk and a <code>kuki</code> chunk. As we know our two original files are in the same format, we can copy these chunks unchanged to the output file.</p> <p>There's also a <code>pakt</code> chunk and a <code>data</code> chunk. We can't guarantee which order these will be in within the input files. There may or may not be a <code>free</code> chunk - but this just contains padding 0x00's, so we needn't copy this to the output file.</p> <p>To combine the <code>pakt</code> chunks, we need to examine the chunk headers and produce a new <code>pakt</code> chunk whose <code>mNumberPackets</code> and <code>mNumberValidFrames</code> fields are the sums of those in the input files. The <code>mPrimingFrames</code> and <code>mRemainderFrames</code> are always zero - these are only relevant for streaming media. The bulk of the <code>pakt</code> chunks (ie. the actual packet table data) can just be concatenated.</p> <p>Similarly for the <code>data</code> chunks: the <code>mChunkSize</code> fields need to be summed and then the bulk of the data can be concatenated.</p> <p>Be careful when reading data from all the binary numeric fields within these files: the files are big-endian but the iPhone is little-endian.</p> <p>For extra credit, you might also like to consider deleting segments of audio from within a file, or inserting one audio file into the middle of another. This is a little trickier as you have to parse the contents of the <code>pakt</code> chunk. Again it's a case of following the spec: there's a good description of how the packet sizes are stored in variable-length integers, so you'll have to parse these to find how many bytes each packet takes up in the <code>data</code> chunk, and calculate their positions accordingly.</p> <p>All in all this is rather more hassle than I was hoping for. Maybe there's an open source library that will do all this for you, but I couldn't find one.</p> <p>However, handling raw files like this is blinding fast compared to using <code>AVMutableComposition</code> and <code>AVMutableCompositionTrack</code> as in the original question - inserting an hour-long recording into another of the same length takes about two seconds.</p> <p>Good luck!</p>

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