How do I add ADTS header when reading m4a raw data from iPod library?

Question

Objective

Reading m4a file bought from iTunes Store via AVAssetReader. Stream via HTTP and consumed by MobileVLCKit.

What I've tried

As far as I know, AVAssetReader only generates audio raw data, so I guess I should add ADTS header in front of every sample.

NSError *error = nil;
AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
if (error != nil) {
    NSLog(@"%@", [error localizedDescription]);
    return -1;
}

AVAssetTrack* track = [asset.tracks objectAtIndex:0];
AVAssetReaderTrackOutput *readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
                                                                                        outputSettings:nil];
[reader addOutput:readerOutput];
    [reader startReading];
    while (reader.status == AVAssetReaderStatusReading){
        AVAssetReaderTrackOutput * trackOutput = (AVAssetReaderTrackOutput *)[reader.outputs objectAtIndex:0];
        CMSampleBufferRef sampleBufferRef;
        @synchronized(self) {
            sampleBufferRef = [trackOutput copyNextSampleBuffer];
        }
        CMItemCount = CMSampleBufferGetNumSamples(sampleBufferRef);
        ...
    }

So, my question is, how do I loop every sample and add ADTS header?

What do you want from the m4a file? LPCM samples or AAC packets? — Rhythmic Fistman, Apr 23 '15 at 06:35
I want MobileVLCKit can recognize the codec, generate LPCM needs extra decoding effort and time, so I prefer AAC packet if possible. — vampirewalk, Apr 23 '15 at 06:49
AVFoundation can decide to LPCM using hardware... Why use MobileVLCKit at all? — Rhythmic Fistman, Apr 23 '15 at 07:43
This is a part of my implementation of UPnP Digital Media Server (DMS), I have to make sure it can be played by other players, including my local one. — vampirewalk, Apr 23 '15 at 08:26

score 1 · Accepted Answer · edited May 23 '17 at 11:58

First, you don't need trackOutput, it's the same as readerOutput that you already have.

UPDATE
My mistake, you're absolutely right. I thought the usual 0xFFF sync words were part of AAC, instead they're ADTS headers. So you must add an ADTS header to each of your AAC packets to stream them as ADTS or "aac". I think you have two choices:

Use AudioFileInitializeWithCallbacks + kAudioFileAAC_ADTSType to get the AudioFile API to add the headers for you. You write AAC packets to the AudioFileID and it will call your write callback from where you can stream AAC in ADTS.
Add the headers to the packets yourself. They're only 7 fiddly bytes (9 with checksums, but who uses them?). Some readable implementations here and here

Either way you need to call either CMSampleBufferGetAudioStreamPacketDescriptions or CMSampleBufferCallBlockForEachSample to get the individual AAC packets from a CMSampleBufferRef.

I have tried streaming raw aac data to VLC, but it doesn't work. According to http://wiki.multimedia.cx/?title=Understanding_AAC , it seems I have to use ADTS or ADIF header to package AAC data. — vampirewalk, Apr 23 '15 at 11:04
Thank you for your help. Although I achieved my goal yesterday in another way(http://stackoverflow.com/a/11600811/1060971), but the api you post here make me write cleaner code. — vampirewalk, Apr 24 '15 at 06:48

How do I add ADTS header when reading m4a raw data from iPod library?

1 Answers1