Some people suggested to read the audio data from end to start and create a copy written from start to end, and then simply play that reversed audio data.
Are there existing examples for iOS how this is done?
I found an example project called MixerHost, which at some point uses an
AudioUnitSampleType holding the audio data that has been read from file, and assigning it to a buffer.
This is defined as:
typedef SInt32 AudioUnitSampleType;
#define kAudioUnitSampleFractionBits 24
And according to Apple:
The canonical audio sample type for audio units and other audio processing in iPhone OS is noninterleaved linear PCM with 8.24-bit fixed-point samples.
So in other words it holds noninterleaved linear PCM audio data.
But I can't figure out where this data is beeing read in, and where it is stored. Here's the code that loads the audio data and buffers it:
- (void) readAudioFilesIntoMemory {
for (int audioFile = 0; audioFile < NUM_FILES; ++audioFile) {
NSLog (@"readAudioFilesIntoMemory - file %i", audioFile);
// Instantiate an extended audio file object.
ExtAudioFileRef audioFileObject = 0;
// Open an audio file and associate it with the extended audio file object.
OSStatus result = ExtAudioFileOpenURL (sourceURLArray[audioFile], &audioFileObject);
if (noErr != result || NULL == audioFileObject) {[self printErrorMessage: @"ExtAudioFileOpenURL" withStatus: result]; return;}
// Get the audio file's length in frames.
UInt64 totalFramesInFile = 0;
UInt32 frameLengthPropertySize = sizeof (totalFramesInFile);
result = ExtAudioFileGetProperty (
audioFileObject,
kExtAudioFileProperty_FileLengthFrames,
&frameLengthPropertySize,
&totalFramesInFile
);
if (noErr != result) {[self printErrorMessage: @"ExtAudioFileGetProperty (audio file length in frames)" withStatus: result]; return;}
// Assign the frame count to the soundStructArray instance variable
soundStructArray[audioFile].frameCount = totalFramesInFile;
// Get the audio file's number of channels.
AudioStreamBasicDescription fileAudioFormat = {0};
UInt32 formatPropertySize = sizeof (fileAudioFormat);
result = ExtAudioFileGetProperty (
audioFileObject,
kExtAudioFileProperty_FileDataFormat,
&formatPropertySize,
&fileAudioFormat
);
if (noErr != result) {[self printErrorMessage: @"ExtAudioFileGetProperty (file audio format)" withStatus: result]; return;}
UInt32 channelCount = fileAudioFormat.mChannelsPerFrame;
// Allocate memory in the soundStructArray instance variable to hold the left channel,
// or mono, audio data
soundStructArray[audioFile].audioDataLeft =
(AudioUnitSampleType *) calloc (totalFramesInFile, sizeof (AudioUnitSampleType));
AudioStreamBasicDescription importFormat = {0};
if (2 == channelCount) {
soundStructArray[audioFile].isStereo = YES;
// Sound is stereo, so allocate memory in the soundStructArray instance variable to
// hold the right channel audio data
soundStructArray[audioFile].audioDataRight =
(AudioUnitSampleType *) calloc (totalFramesInFile, sizeof (AudioUnitSampleType));
importFormat = stereoStreamFormat;
} else if (1 == channelCount) {
soundStructArray[audioFile].isStereo = NO;
importFormat = monoStreamFormat;
} else {
NSLog (@"*** WARNING: File format not supported - wrong number of channels");
ExtAudioFileDispose (audioFileObject);
return;
}
// Assign the appropriate mixer input bus stream data format to the extended audio
// file object. This is the format used for the audio data placed into the audio
// buffer in the SoundStruct data structure, which is in turn used in the
// inputRenderCallback callback function.
result = ExtAudioFileSetProperty (
audioFileObject,
kExtAudioFileProperty_ClientDataFormat,
sizeof (importFormat),
&importFormat
);
if (noErr != result) {[self printErrorMessage: @"ExtAudioFileSetProperty (client data format)" withStatus: result]; return;}
// Set up an AudioBufferList struct, which has two roles:
//
// 1. It gives the ExtAudioFileRead function the configuration it
// needs to correctly provide the data to the buffer.
//
// 2. It points to the soundStructArray[audioFile].audioDataLeft buffer, so
// that audio data obtained from disk using the ExtAudioFileRead function
// goes to that buffer
// Allocate memory for the buffer list struct according to the number of
// channels it represents.
AudioBufferList *bufferList;
bufferList = (AudioBufferList *) malloc (
sizeof (AudioBufferList) + sizeof (AudioBuffer) * (channelCount - 1)
);
if (NULL == bufferList) {NSLog (@"*** malloc failure for allocating bufferList memory"); return;}
// initialize the mNumberBuffers member
bufferList->mNumberBuffers = channelCount;
// initialize the mBuffers member to 0
AudioBuffer emptyBuffer = {0};
size_t arrayIndex;
for (arrayIndex = 0; arrayIndex < channelCount; arrayIndex++) {
bufferList->mBuffers[arrayIndex] = emptyBuffer;
}
// set up the AudioBuffer structs in the buffer list
bufferList->mBuffers[0].mNumberChannels = 1;
bufferList->mBuffers[0].mDataByteSize = totalFramesInFile * sizeof (AudioUnitSampleType);
bufferList->mBuffers[0].mData = soundStructArray[audioFile].audioDataLeft;
if (2 == channelCount) {
bufferList->mBuffers[1].mNumberChannels = 1;
bufferList->mBuffers[1].mDataByteSize = totalFramesInFile * sizeof (AudioUnitSampleType);
bufferList->mBuffers[1].mData = soundStructArray[audioFile].audioDataRight;
}
// Perform a synchronous, sequential read of the audio data out of the file and
// into the soundStructArray[audioFile].audioDataLeft and (if stereo) .audioDataRight members.
UInt32 numberOfPacketsToRead = (UInt32) totalFramesInFile;
result = ExtAudioFileRead (
audioFileObject,
&numberOfPacketsToRead,
bufferList
);
free (bufferList);
if (noErr != result) {
[self printErrorMessage: @"ExtAudioFileRead failure - " withStatus: result];
// If reading from the file failed, then free the memory for the sound buffer.
free (soundStructArray[audioFile].audioDataLeft);
soundStructArray[audioFile].audioDataLeft = 0;
if (2 == channelCount) {
free (soundStructArray[audioFile].audioDataRight);
soundStructArray[audioFile].audioDataRight = 0;
}
ExtAudioFileDispose (audioFileObject);
return;
}
NSLog (@"Finished reading file %i into memory", audioFile);
// Set the sample index to zero, so that playback starts at the
// beginning of the sound.
soundStructArray[audioFile].sampleNumber = 0;
// Dispose of the extended audio file object, which also
// closes the associated file.
ExtAudioFileDispose (audioFileObject);
}
}
Which part contains the array of audio samples which have to be reversed? Is it the AudioUnitSampleType?
bufferList->mBuffers[0].mData = soundStructArray[audioFile].audioDataLeft;
Note: audioDataLeft is defined as an AudioUnitSampleType, which is an SInt32 but not an array.
I found a clue in a Core Audio Mailing list:
Well, nothing to do with iPh*n* as far as I know (unless some audio API has been omitted -- I am not a member of that program). AFAIR, AudioFile.h and ExtendedAudioFile.h should provide you with what you need to read or write a caf and access its streams/channels. Basically, you want to read each channel/stream backwards, so, if you don't need properties of the audio file it is pretty straightforward once you have a handle on that channel's data, assuming it is not in a compressed format. Considering the number of formats a caf can represent, this could take a few more lines of code than you're thinking. Once you have a handle on uncompressed data, it should be about as easy as reversing a string. Then you would of course replace the file's data with the reversed data, or you could just feed the audio output (or wherever you're sending the reversed signal) reading whatever stream you have backwards.
This is what I tried, but when I assign my reversed buffer to the mData of both channels, I hear nothing:
AudioUnitSampleType *leftData = soundStructArray[audioFile].audioDataLeft;
AudioUnitSampleType *reversedData = (AudioUnitSampleType *) calloc (totalFramesInFile, sizeof (AudioUnitSampleType));
UInt64 j = 0;
for (UInt64 i = (totalFramesInFile - 1); i > -1; i--) {
reversedData[j] = leftData[i];
j++;
}
