c – 使用AudioFileServices在iOS 6中进行粒度合成

我有一个关于我正在研究的声音合成应用程序的问题.我试图读取一个音频文件,使用 granular synthesis techniques创建随机’粒子’,将它们放入输出缓冲区,然后能够使用OpenAL将其播放回用户.出于测试目的,我只是将输出缓冲区写入一个文件然后我可以回听.

从我的结果来看,我在正确的轨道上,但我得到一些混叠问题和播放声音似乎不太正确.输出文件中间通常有一个相当响亮的弹出,音量水平有时很大.

以下是我为获得所需结果而采取的步骤,但我对一些事情感到困惑,即我为AudioStreamBasicDescription指定的格式.

>从我的mainBundle读取一个音频文件,这是一个.aiff格式的单声道文件：

ExtAudioFileRef extAudioFile;
CheckError(ExtAudioFileOpenURL(loopFileURL,&extAudioFile),"couldn't open extaudiofile for reading");
memset(&player->dataFormat,sizeof(player->dataFormat));

player->dataFormat.mFormatID = kAudioFormatLinearPCM;
player->dataFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
player->dataFormat.mSampleRate = S_RATE;
player->dataFormat.mChannelsPerFrame = 1;
player->dataFormat.mFramesPerPacket = 1;
player->dataFormat.mBitsPerChannel = 16;
player->dataFormat.mBytesPerFrame = 2;
player->dataFormat.mBytesPerPacket = 2;

// tell extaudiofile about our format
CheckError(ExtAudioFileSetProperty(extAudioFile,kExtAudioFileProperty_ClientDataFormat,sizeof(AudioStreamBasicDescription),&player->dataFormat),"couldnt set client format on extaudiofile");

SInt64 fileLengthFrames;
UInt32 propSize = sizeof(fileLengthFrames);
ExtAudioFileGetProperty(extAudioFile,kExtAudioFileProperty_FileLengthFrames,&propSize,&fileLengthFrames);

player->bufferSizeBytes = fileLengthFrames * player->dataFormat.mBytesPerFrame;

>接下来,我声明我的AudioBufferList并设置更多属性

AudioBufferList *buffers;
UInt32 ablSize = offsetof(AudioBufferList,mBuffers[0]) + (sizeof(AudioBuffer) * 1);
buffers = (AudioBufferList *)malloc(ablSize);

player->sampleBuffer = (SInt16 *)malloc(sizeof(SInt16) * player->bufferSizeBytes);

buffers->mNumberBuffers = 1;
buffers->mBuffers[0].mNumberChannels = 1;
buffers->mBuffers[0].mDataByteSize = player->bufferSizeBytes;
buffers->mBuffers[0].mData = player->sampleBuffer;

>我的理解是.mData将是formatFlags中指定的任何内容(在本例中,键入SInt16).由于它是类型(void *),我想将其转换为浮点数据,这对于音频操作来说是显而易见的.在我设置一个for循环之前,它只迭代缓冲区并将每个样本转换为float *.这似乎是不必要的,所以现在我将.mData缓冲区传递给我创建的函数,然后对音频进行粒化：

float *theOutBuffer = [self granularizeWithData:(float *)buffers->mBuffers[0].mData with:framesRead];

>在这个函数中,我动态分配一些缓冲区,创建随机大小的粒子,在使用汉明窗口窗口化后将它们放在我的输出缓冲区中并返回该缓冲区(这是浮点数据).到目前为止,一切都很酷.
>接下来我设置了所有输出文件ASBD等：

AudioStreamBasicDescription outputFileFormat;

bzero(audioFormatPtr,sizeof(AudioStreamBasicDescription));

outputFileFormat->mFormatID = kAudioFormatLinearPCM;
outputFileFormat->mSampleRate = 44100.0;
outputFileFormat->mChannelsPerFrame = numChannels;
outputFileFormat->mBytesPerPacket = 2 * numChannels;
outputFileFormat->mFramesPerPacket = 1;
outputFileFormat->mBytesPerFrame = 2 * numChannels;
outputFileFormat->mBitsPerChannel = 16;
outputFileFormat->mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsPacked;

UInt32 flags = kAudioFileFlags_EraseFile;
ExtAudioFileRef outputAudioFileRef = NULL;
NSString *tmpDir = NSTemporaryDirectory();
NSString *outFilename = @"Decomp.caf";
NSString *outPath = [tmpDir stringByAppendingPathComponent:outFilename];
NSURL *outURL = [NSURL fileURLWithPath:outPath];


AudioBufferList *outBuff;
UInt32 abSize = offsetof(AudioBufferList,mBuffers[0]) + (sizeof(AudioBuffer) * 1);
outBuff = (AudioBufferList *)malloc(abSize);

outBuff->mNumberBuffers = 1;
outBuff->mBuffers[0].mNumberChannels = 1;
outBuff->mBuffers[0].mDataByteSize = abSize;
outBuff->mBuffers[0].mData = theOutBuffer;

CheckError(ExtAudioFileCreateWithURL((__bridge CFURLRef)outURL,kAudioFileCAFType,&outputFileFormat,NULL,flags,&outputAudioFileRef),"ErrorCreatingURL_For_EXTAUDIOFILE");

CheckError(ExtAudioFileSetProperty(outputAudioFileRef,sizeof(outputFileFormat),&outputFileFormat),"ErrorSettingProperty_For_EXTAUDIOFILE");

CheckError(ExtAudioFileWrite(outputAudioFileRef,framesRead,outBuff),"ErrorWritingFile");

该文件以CAF格式正确写入.我的问题是：我是否正确处理.mData缓冲区,因为我正在将样本转换为浮动数据,操纵(粒化)各种窗口大小,然后使用ExtAudioFileWrite(CAF格式)将其写入文件？有没有更优雅的方法来执行此操作,例如将我的ASBD formatFlag声明为kAudioFlagIsFloat？我的输出CAF文件中有一些点击,当我在Logic中打开它时,看起来有很多别名.这是有道理的,如果我试图发送它浮动数据,但有一种转换发生,我不知道.

提前感谢您对此事的任何建议！我一直是网上几乎所有源材料的狂热读者,包括核心音频书,各种博客,教程等.我的应用程序的最终目标是通过耳机实时向用户播放粒化音频,以便写入文件的东西目前只是用于测试.谢谢！

short data[4] = {-27158,16825,23024,15};
void *pData = data;

float *pfData = (float *)pData;
printf("%d == %fn",data[0],pfData[0]);

int k;
float *pfBuf = (float *)malloc(n_data * sizeof(float));
short *psiBuf = (short *)buffers->mBuffers[0].mData[k];
for (k = 0; k < n_data; k ++)
{
    pfBuf[k] = psiBuf[k];
}
[self granularizeWithData:pfBuf with:framesRead];
for (k = 0; k < n_data; k ++)
{
    psiBuf[k] = pfBuf[k];
}
free(pfBuf);