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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 640 毫秒的“好”音频重复模式，然后是约 640 毫秒的“坏”音频。

对 16 位线性和 8.24 定点 PCM 进行编码产生了相同的结果。

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);

简单的转换程序。该例程每 32 毫秒调用一次，带有 1024 个 PCM 样本块，并且需要 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
}

在模拟器上运行时一切正常。

但是，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 AudioConverterFillComplexBuffer 调用，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？

有没有人认为这种方法有什么明显的错误？

是否需要对硬件编解码器（MagicCookies 或？）进行任何特殊设置？

硬件 AAC 编解码器是否支持 32000 采样率？

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1 回答 1

0

I find that: the default outputBitRate for 32KHz-input-PCM is 48000 bit, the default outputBitRate for 44.1KHz-input-PCM is 64000 bit. When use the the default outputBitRate, 32KHz input makes huge noise. Even use these codes from apple`s sample , 44.1KHz input have a little noise.

Then i fix the outputBitRate to 64kbs, 32KHz & 44.1KHz both works well。</p> 

UInt32 outputBitRate = 64000; // 64kbs
UInt32 propSize = sizeof(outputBitRate);
if (AudioConverterSetProperty(m_converter, kAudioConverterEncodeBitRate, propSize, &outputBitRate) != noErr) {
} else {
    NSLog(@"upyun.com uplivesdk  UPAACEncoder error 102");
}
于 2016-08-22T10:14:43.053 回答