有没有人在iPhone/iOS上成功将32KHz的PCM转换为96Kbit的AAC？
我无法让这项工作在任何硬件设备上正确运行。我编写的代码只能在模拟器中正常工作。当在当前一代iPad/iPod/iPhone上运行时，我的代码会“跳过”大量音频。
生成的编码流包含一个重复的模式，即约640毫秒的“好”音频后跟约640毫秒的“坏”音频。
对16位线性和8.24固定点PCM进行编码产生了相同的结果。
以下是设置音频转换器以将MPEG4-AAC 96 kbits @ 32KHz编码的代码：
AudioStreamBasicDescription descPCMFormat;
descPCMFormat.mSampleRate = 32000;
descPCMFormat.mChannelsPerFrame = 1;
descPCMFormat.mBitsPerChannel = sizeof(AudioUnitSampleType) * 8;
descPCMFormat.mBytesPerPacket = sizeof(AudioUnitSampleType);
descPCMFormat.mFramesPerPacket = 1;
descPCMFormat.mBytesPerFrame = sizeof(AudioUnitSampleType);
descPCMFormat.mFormatID = kAudioFormatLinearPCM;
descPCMFormat.mFormatFlags = kAudioFormatFlagsAudioUnitCanonical;
AudioStreamBasicDescription descAACFormat;
descAACFormat.mSampleRate = 32000;
descAACFormat.mChannelsPerFrame = 1;
descAACFormat.mBitsPerChannel = 0;
descAACFormat.mBytesPerPacket = 0;
descAACFormat.mFramesPerPacket = 1024;
descAACFormat.mBytesPerFrame = 0;
descAACFormat.mFormatID = kAudioFormatMPEG4AAC;
descAACFormat.mFormatFlags = 0;
AudioConverterNew(& descPCMFormat, & descAACFormat, &m_hCodec);
UInt32 ulBitRate = 96000;
UInt32 ulSize = sizeof(ulBitRate);
AudioConverterSetProperty(m_hCodec, kAudioConverterEncodeBitRate, ulSize, & ulBitRate);
简单的转换例程。每32毫秒调用一次此例程，并使用1024个PCM样本块，期望384字节的编码AAC：
OSStatus CMyObj::Convert(
const AudioUnitSampleType * pSrc,
const size_t ulSrc,
uint8_t * pDst,
size_t & ulDst)
{
// error and sanity checking removed..
// assume caller is converting 1024 samples to at most 384 bytes
OSStatus osStatus;
m_pSrcPtr = (uint8_t*)pSrc;
m_ulSrcLen = ulSrc; // verified to be 1024*sizeof(AudioUnitSampleType);
AudioBufferList destBuffers;
destBuffers.mNumberBuffers = 1;
destBuffers.mBuffers[0].mNumberChannels = 1;
destBuffers.mBuffers[0].mDataByteSize = 384;
destBuffers.mBuffers[0].mData = pDst;
AudioStreamPacketDescription destDescription;
destDescription.mStartOffset = 0;
destDescription.mVariableFramesInPacket = 0;
destDescription.mDataByteSize = 384;
UInt32 ulDstPackets = 1;
osStatus = AudioConverterFillComplexBuffer(
m_hCodec,
InputDataProc,
this,
& ulDstPackets,
& destBuffers,
& destDescription);
ulDst = destBuffers.mBuffers[0].mDataByteSize;
return osStatus;
}
输入数据过程只需将1024个样本提供给编码器：
static OSStatus CMyObj::InputDataProc(
AudioConverterRef hCodec,
UInt32 *pulSrcPackets,
AudioBufferList *pSrcBuffers,
AudioStreamPacketDescription **ppPacketDescription,
void *pUserData)
{
// error and sanity checking removed
CMyObj *pThis = (CMyObj*)pUserData;
const UInt32 ulMaxSrcPackets = pThis->m_ulSrcLen / sizeof(AudioUnitSampleType);
const UInt32 ulRetSrcPackets = min(ulMaxSrcPackets, *pulSrcPackets);
if( ulRetSrcPackets )
{
UInt32 ulRetSrcBytes = ulRetSrcPackets * sizeof(AudioUnitSampleType);
*pulSrcPackets = ulRetSrcPackets;
pSrcBuffers->mBuffers[0].mData = pThis->m_pSrcPtr;
pSrcBuffers->mBuffers[0].mDataByteSize = ulRetSrcBytes;
pSrcBuffers->mBuffers[0].mNumberChannels = 1;
pThis->m_pSrcPtr += ulRetSrcBytes;
pThis-> m_ulSrcLen -= ulRetSrcBytes;
return noErr;
}
*pulSrcPackets = 0;
pSrcBuffers->mBuffers[0].mData = NULL;
pSrcBuffers->mBuffers[0].mDataByteSize = 0;
pSrcBuffers->mBuffers[0].mNumberChannels = 1;
return 500; // local error code to signal end-of-packet
}
在模拟器上运行时一切正常。
但是在设备上运行时，InputDataProc不一致地被调用。连续20次调用AudioConverterFillComplexBuffer会导致调用InputDataProc，并且一切看起来都很好。然后，在接下来的约21次调用AudioConverterFillComplexBuffer中，InputDataProc将不会被调用。这种模式会一直重复：
-> Convert
-> AudioConverterFillComplexBuffer
-> InputDataProc
-> results in 384 bytes of 'good' AAC
-> Convert
-> AudioConverterFillComplexBuffer
-> InputDataProc
-> results in 384 bytes of 'good' AAC
.. repeats up to 18 more times
-> Convert
-> AudioConverterFillComplexBuffer
-> results in 384 bytes of 'bad' AAC
-> Convert
-> AudioConverterFillComplexBuffer
-> results in 384 bytes of 'bad' AAC
.. repeats up to 18 more times
转换器从哪里获取输入数据以创建“坏”的AAC？它没有调用InputDataProc吗？
有人发现这种方法有什么明显的问题吗？
硬件编解码器需要进行特殊设置（MagicCookies或其他）吗？
硬件AAC编解码器是否支持32000采样率？