iPhone：使用音频转换器FillComplexBuffer将32KHz PCM编码为96Kbit AAC时出现问题

有人在iPhone/iOS上成功地将32KHz PCM转换为96Kbit AAC吗

我无法让它在任何硬件设备上正常工作。我写的代码只能在模拟器中正常工作。在当前一代iPad/iPod/iPhone上运行时，我的代码会“跳过”大量音频

生成的编码流包含约640ms的“好”音频和约640ms的“坏”音频的重复模式

对16位线性和8.24定点PCM进行编码得到了相同的结果

以下是设置音频转换器以对MPEG4-AAC 96kbits@32KHz进行编码的代码：

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);

简单的转换程序。此例程每32毫秒调用一次，块为1024个PCM样本，需要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;
}

输入数据过程仅向编码器提供1024个样本：

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，不会一致地调用InputDataProc。连续20次，对AudioConverterFillComplexBuffer的调用会引发对InputDataProc的调用，一切看起来都很好。然后，对于对AudioConverterFillComplexBuffer的下21次调用，将不会调用InputDataProc。这种模式永远重复：

-> 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

转换器在哪里获取输入数据以创建“坏”AAC，因为它没有调用InputDataProc

有人认为这种方法有什么明显的错误吗

是否需要在硬件编解码器（MagicCookies或？）上进行任何特殊设置

---

HW AAC编解码器是否支持32000采样率？

我发现：32KHz输入PCM的默认输出比特率为48000位，44.1KHz-input-PCM的默认输出比特率为64000位。 当使用默认输出比特率时，32KHz输入会产生巨大的噪声。 即使使用，44.1KHz的输入也有一点噪声

然后我将输出比特率固定为64kbs、32KHz和44.1KHz，这两种情况都很好

UInt32 outputBitRate = 64000; // 64kbs
UInt32 propSize = sizeof(outputBitRate);
if (AudioConverterSetProperty(m_converter, kAudioConverterEncodeBitRate, propSize, &outputBitRate) != noErr) {
} else {
    NSLog(@"upyun.com uplivesdk  UPAACEncoder error 102");
}

---

我可以通过强制使用AudioConverterNewSpecific并指定mManufacturer KappleSoftwareAudoCodecManufacturer来实现与模拟器相同的硬件行为。了解如何使用硬件编解码器仍然很好！罗托瓦，你能对我的问题发表意见吗。我看你在这方面有经验。