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

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运行。在当前一代 iPad/iPod/iPhone 上运行时，我的代码会“跳过”大块音频。

生成的编码流包含约 640 毫秒的“好”音频后跟约 640 毫秒的“坏”音频的重复模式。

编码 16 位线性和 8.24 定点 PCM 产生相同的结果。

这是设置音频转换器以编码 MPEG4-AAC 96kbits @ 32KHz 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。此例程每 32 毫秒调用一次，包含 1024 个 PCM 样本 block ，并需要 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;
}

输入数据过程只是将 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
}

在模拟器上运行一切正常。

但是，当在设备上运行时，不会一致地调用 InputDataProc。连续多达 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，对于接下来的 ~ 21 次对 AudioConverterFillComplexBuffer 的调用，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

转换器从哪里获取输入数据来创建“坏”AAC，因为它没有调用 InputDataProc？

有没有人发现这种方法有什么明显的错误？

是否需要对硬件编解码器进行任何特殊设置(MagicCookies 或？)？

HW AAC编解码器是否支持32000采样率？

最佳答案

我发现:32KHz-input-PCM 的默认 outputBitRate 是 48000 位，44.1KHz-input-PCM 的默认 outputBitRate 是 64000 位。 当使用默认的 outputBitRate 时，32KHz 输入会产生巨大的噪音。 甚至使用 these codes from apple`s sample , 44.1KHz输入有一点噪音。

然后我将 outputBitRate 固定为 64kbs，32KHz 和 44.1KHz 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");
}

关于iPhone:使用 AudioConverterFillComplexBuffer 将 32KHz PCM 编码为 96Kbit AAC 时出现问题，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/6792620/