ios - Convert video's audio to AAC - ios

Question

I'm trying to encode any audio format to AAC format, with 44100Hz sample rate.
So basically : input (mp3, aac? etc, any sample rate) -> AAC (44100Hz)
The source audio comes from a video (mp4), but I can extract it to m4a (AAC). The thing is I also want to change the sample rate to 44100Hz.
I'm trying to achieve this with AVAssetReader and AVAssetWriter, but not sure if its possible or if it's the best solution. Any other solution would be very much appreciated !
Here's my code so far :
// Input video audio (.mp4)
AVAsset *videoAsset = <mp4 video asset>;
NSArray<AVAssetTrack *> *videoAudioTracks = [videoAsset tracksWithMediaType:AVMediaTypeAudio];
AVAssetTrack *videoAudioTrack = [videoAudioTracks objectAtIndex:0];
// Output audio (.m4a AAC)
NSURL *exportUrl = <m4a, aac output file URL>;
// ASSET READER
NSError *error;
AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:videoAsset
error:&error];
if(error) {
NSLog(#"error:%#",error);
return;
}
// Asset reader output
AVAssetReaderOutput *assetReaderOutput =[AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoAudioTrack
outputSettings:nil];
if(![assetReader canAddOutput:assetReaderOutput]) {
NSLog(#"Can't add output!");
return;
}
[assetReader addOutput:assetReaderOutput];
// ASSET WRITER
AVAssetWriter *assetWriter = [AVAssetWriter assetWriterWithURL:exportUrl
fileType:AVFileTypeAppleM4A
error:&error];
if(error) {
NSLog(#"error:%#",error);
return;
}
AudioChannelLayout channelLayout;
memset(&channelLayout, 0, sizeof(AudioChannelLayout));
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
NSDictionary *outputSettings = #{AVFormatIDKey: #(kAudioFormatMPEG4AAC),
AVNumberOfChannelsKey: #2,
AVSampleRateKey: #44100.0F,
AVChannelLayoutKey: [NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)],
AVEncoderBitRateKey: #64000};
/*NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
[NSNumber numberWithFloat:44100.f], AVSampleRateKey,
[NSNumber numberWithInt:2], AVNumberOfChannelsKey,
[NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
[NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
[NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
[NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
[NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
nil];*/
// Asset writer input
AVAssetWriterInput *assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio
outputSettings:outputSettings];
if ([assetWriter canAddInput:assetWriterInput])
[assetWriter addInput:assetWriterInput];
else {
NSLog(#"can't add asset writer input... die!");
return;
}
assetWriterInput.expectsMediaDataInRealTime = NO;
[assetWriter startWriting];
[assetReader startReading];
CMTime startTime = CMTimeMake (0, videoAudioTrack.naturalTimeScale);
[assetWriter startSessionAtSourceTime: startTime];
__block UInt64 convertedByteCount = 0;
dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
[assetWriterInput requestMediaDataWhenReadyOnQueue:mediaInputQueue
usingBlock: ^
{
while (assetWriterInput.readyForMoreMediaData)
{
CMSampleBufferRef nextBuffer = [assetReaderOutput copyNextSampleBuffer];
if (nextBuffer)
{
// append buffer
[assetWriterInput appendSampleBuffer: nextBuffer];
convertedByteCount += CMSampleBufferGetTotalSampleSize (nextBuffer);
CMSampleBufferInvalidate(nextBuffer);
CFRelease(nextBuffer);
nextBuffer = NULL;
}
else
{
[assetWriterInput markAsFinished];
// [assetWriter finishWriting];
[assetReader cancelReading];
break;
}
}
}];
And here is the error I get with a video that contains an mp3 audio track :
Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '*** -[AVAssetWriterInput
appendSampleBuffer:] Cannot append sample buffer: Input buffer must
be in an uncompressed format when outputSettings is not nil'
Any help would be much appreciated, thanks !

Answer 1

You should be able to achieve this by configuring your AVAssetReaderOutput output settings:
NSDictionary *readerOutputSettings = #{ AVSampleRateKey: #44100, AVFormatIDKey: #(kAudioFormatLinearPCM) };
AVAssetReaderOutput *assetReaderOutput =[AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoAudioTrack
outputSettings:readerOutputSettings];

Answer 2

I'm not native to Obj-C and I had to google around to figure out the accepted answer in Swift.
Here is the Swift version:
let audioSettings: [String : Any] = [
AVFormatIDKey: kAudioFormatLinearPCM,
AVSampleRateKey: 44100
]
let assetReaderAudioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: audioSettings)