ios - combine/concatenate multiple audio files

Question

I have a bunch of audio files (mp3, aac, whatever) and I want to concatenate them into 1 audio file. Has anyone done this before?

Answer 1

This is fairly simple using ExtAudioFile. In pseudocode what you want to do is:

1. Open the file you'll be using for output (ExtAudioFileOpen)
2. Set the client data format you'll use for ExtAudioFileWrite
3. For each input file you want to process:
    a. Open the file using ExtAudioFile
    b. Set the client data format used for reading to the same client format used in the output file for writing
    c. Loop through the input file using ExtAudioFileRead and save the data to the output file using ExtAudioFileWrite
    d. Close the input file
4. Close the output file

Answer 2

I have done this. To do the concatenation you first need to load the audio files into AVAssets. Specifically, you'll want to use a subclass of AVAsset called AVURLAssets, which can load up your URL: Loading AVAsset. You can then add each AVAsset into a AVMutableComposition, which is designed to contain multiple AVAssets. Once you've gotten it loaded into AVMutableComposition, you can use AVAssetExportSession to write the composition to a file.

Note that AVAssetExportSession doesn't give you much control over the file output (it'll export audio into a m4a file). If you need more control over the type of output, you'll need to use the AVAssetReader and AVAssetWriter classes to perform the export rather than the AVAssetExportSession. These classes are much more complex to use than AVAssetExportSession and it pays here to understand straight C.

I will also point out that there are no Apple-provided options to write out MP3 files. You can read them, but you can't write them. It's generally best to stick with a m4a/aac format.