iOS audio manipulation - play local .caf file backwards

Question

I'm wanting to load a local .caf audio file and reverse the audio (play it backwards). I've gathered that I basically need to flip an array of buffer data from posts like this

However, I'm not sure how to access this buffer data from a given audio file. I have a little experience playing sounds back with AVaudioPlayer and ObjectAL(an obj-c openAL library), but I don't know how to access something lower level like this buffer data array.

Could I please get an example of how I would go about getting access to that array?

Answer 1

Your problem reduces to the same problem described here, which was linked by P-i in the comment under your question. Kiran answered that question and re-posted his answer for you here. Kiran's answer is accurate, but you may need a few more details to be able to decide how to proceed because you're starting with a CAF file.

The simplest audio file format, linear pulse-code modulation (LPCM), is the easiest to read byte-for-byte or sample-for-sample. This means it's the easiest to reverse. Kiran's solution does just that.

The CAF format is a container/wrapper format, however. While your CAF file could contain a WAV file, it could also contain a compressed file format that cannot be manipulated in the same fashion.

You should consider first converting the CAF file to WAV, then reversing it as shown in the other solution. There are various libraries that will do this conversion for you, but a good place to start might be with the AudioToolbox framework, which includes Audio Converter Services. Alternately, if you can use the WAV file format from the start, you can prevent the need to convert to WAV.

You may need to know more if you find Kiran's sample code gives you an error (Core Audio is finicky). A great place to start is with the Core Audio 'Bible', "Learning Core Audio", written by Chris Adamson and Kevin Avila. That book builds your knowledge of digital sound using great samples. You should also check out anything written by Michael Tyson, who started the Amazing Audio Engine project on github, and wrote AudioBus.

Answer 2

I have worked on a sample app, which records what user says and plays them backwards. I have used CoreAudio to achieve this. Link to app code.

As each sample is 16-bits in size(2 bytes)(mono channel). You can load each sample at a time by copying it into a different buffer by starting at the end of the recording and reading backwards. When you get to the start of the data you have reversed the data and playing will be reversed.

// set up output file
AudioFileID outputAudioFile;

AudioStreamBasicDescription myPCMFormat;
myPCMFormat.mSampleRate = 16000.00;
myPCMFormat.mFormatID = kAudioFormatLinearPCM ;
myPCMFormat.mFormatFlags =  kAudioFormatFlagsCanonical; 
myPCMFormat.mChannelsPerFrame = 1;
myPCMFormat.mFramesPerPacket = 1;
myPCMFormat.mBitsPerChannel = 16;
myPCMFormat.mBytesPerPacket = 2;
myPCMFormat.mBytesPerFrame = 2;


AudioFileCreateWithURL((__bridge CFURLRef)self.flippedAudioUrl,
                   kAudioFileCAFType,
                   &myPCMFormat,
                   kAudioFileFlags_EraseFile,
                   &outputAudioFile);
// set up input file
AudioFileID inputAudioFile;
OSStatus theErr = noErr;
UInt64 fileDataSize = 0;

AudioStreamBasicDescription theFileFormat;
UInt32 thePropertySize = sizeof(theFileFormat);

theErr = AudioFileOpenURL((__bridge CFURLRef)self.recordedAudioUrl,kAudioFileReadPermission, 0, &inputAudioFile);

thePropertySize = sizeof(fileDataSize);
theErr = AudioFileGetProperty(inputAudioFile, kAudioFilePropertyAudioDataByteCount, &thePropertySize, &fileDataSize);

UInt32 dataSize = fileDataSize;
void* theData = malloc(dataSize);

//Read data into buffer
UInt32 readPoint  = dataSize;
UInt32 writePoint = 0;
while( readPoint > 0 )
{
   UInt32 bytesToRead = 2;

AudioFileReadBytes( inputAudioFile, false, readPoint, &bytesToRead, theData );
AudioFileWriteBytes( outputAudioFile, false, writePoint, &bytesToRead, theData );

writePoint += 2;
readPoint -= 2;
}

free(theData);
AudioFileClose(inputAudioFile);
AudioFileClose(outputAudioFile);

Answer 3

I think this sample code could help you.

Mixer Host Sample Code

It will load two caf files from the bundle and play it. It contains a function call readAudioFilesIntoMemory which is loading a caf file to a data array as you said.

The whole program is an example of core audio, I hope it can help you :)

Answer 4

Why not let CoreAudio's AudioConverter do it for you? See this post about "Getting PCM from MP3/AAC/ALAC File", and Apple's Core Audio Essentials

Answer 5

you can use libsox for iphone framework to apply audio effects easily.

it includes a sample project that shows how to do it.

libsox ios