Audio Codec介绍-3

1.    AGC(Automatic Gain Control)

即自动增益控制; 对一个输入信号进行放大，为了方便处理，应该保证输出有一定的幅度同时又不会饱和，但是由于输入信号的幅度通常变化很大，所以不能采用一个简单的单一放大倍数，AGC就是根据输入信号调整放大倍数，是输出信号幅度一致。

2.    音频数据的格式WAVE_FORMAT

音频格式包括Sample rate，Mono/Stereo等内容，具体的类型包括下面的几种：

#define WAVE_FORMAT_1M08       0x00000001       /* 11.025 kHz, Mono,   8-bit  */ #define WAVE_FORMAT_1S08       0x00000002       /* 11.025 kHz, Stereo, 8-bit  */ #define WAVE_FORMAT_1M16       0x00000004       /* 11.025 kHz, Mono,   16-bit */ #define WAVE_FORMAT_1S16       0x00000008       /* 11.025 kHz, Stereo, 16-bit */ #define WAVE_FORMAT_2M08       0x00000010       /* 22.05  kHz, Mono,   8-bit  */ #define WAVE_FORMAT_2S08       0x00000020       /* 22.05  kHz, Stereo, 8-bit  */ #define WAVE_FORMAT_2M16       0x00000040       /* 22.05  kHz, Mono,   16-bit */ #define WAVE_FORMAT_2S16       0x00000080       /* 22.05  kHz, Stereo, 16-bit */ #define WAVE_FORMAT_4M08       0x00000100       /* 44.1   kHz, Mono,   8-bit  */ #define WAVE_FORMAT_4S08       0x00000200       /* 44.1   kHz, Stereo, 8-bit  */ #define WAVE_FORMAT_4M16       0x00000400       /* 44.1   kHz, Mono,   16-bit */ #define WAVE_FORMAT_4S16       0x00000800       /* 44.1   kHz, Stereo, 16-bit */

通过Wave API waveInGetDevCaps()可以获取Audio驱动所支持的音频格式，如下所示：

mmRtn=waveInGetDevCaps(g_dwDeviceNum-1,&woc,sizeof(woc)); CheckMMRESULT(mmRtn,"ERROR:  Failed to get device caps.  waveInGetDevCaps",TPR_FAIL,"Driver responded incorrectly");

WinCE下面标准的Audio驱动中采样率为48000（48K），软件上在通过插值运算来支持各个各样的采样率，这部分实现可以在类StreamContext的继承类诸如OutputStreamContext1M8/OutputStreamContext1M16等中的方法Rend2中实现。

         当我们使用Wave API waveInOpen来创建音频Stream的时候，需要通过实参向驱动传送结构体WAVEFORMATEX以告知驱动待创建的Stream的种类，如下：

WAVEFORMATEX wfx; WAVEHDR      wh; ZeroMemory( &wfx, sizeof( wfx ) );   wfx.wFormatTag           = WAVE_FORMAT_PCM; wfx.nChannels            = 1; wfx.nSamplesPerSec       = 11025; wfx.wBitsPerSample       = 8; wfx.nBlockAlign          = wfx.nChannels * wfx.wBitsPerSample / 8; wfx.nAvgBytesPerSec      = wfx.nSamplesPerSec * wfx.nBlockAlign; wfx.cbSize               = 0;   //---- Open the Wave Input Stream mmRtn = waveInOpen( &hwi, g_dwDeviceNum-1, &wfx, NULL, 0, NULL ); if( MMSYSERR_NOERROR != mmRtn ) {     LOG( TEXT( "FAIL in %s @ line %u:" ), TEXT( __FILE__ ), __LINE__ );     LOG( TEXT( "/twaveInOpen returned error code, #%d." ), mmRtn );     return TPR_FAIL; }

         驱动中会依次调用WAV_IOControlà HandleWaveMessageà DeviceContext::OpenStream()来创建音频流。在函数DeviceContext::OpenStream的实现过程中，首先会去调用DeviceContext::IsSupportedFormat()来判断是否为支持的音频格式流，如果不支持的话则创建失败，这部分代码如下：

BOOL DeviceContext::IsSupportedFormat(LPWAVEFORMATEX lpFormat) {     if (lpFormat->wFormatTag != WAVE_FORMAT_PCM)         return FALSE;       if (  (lpFormat->nChannels!=1) && (lpFormat->nChannels!=2) )         return FALSE;       if (  (lpFormat->wBitsPerSample!=8) && (lpFormat->wBitsPerSample!=16) )         return FALSE;       if (lpFormat->nSamplesPerSec < 100 || lpFormat->nSamplesPerSec > 192000)         return FALSE;       return TRUE; }

         对于支持的音频流格式，则会调用虚函数CreateStream()来创建对应的流，具体的实现将在类DeviceContext的继承中找到，以继承类OutputDeviceContext的实现为例，代码如下：

StreamContext *OutputDeviceContext::CreateStream(LPWAVEOPENDESC lpWOD) {     LPWAVEFORMATEX lpFormat = lpWOD->lpFormat;   #if ENABLE_MIDI       if (lpFormat->wFormatTag == WAVE_FORMAT_MIDI)     {         return new CMidiStream;     }   #endif       // 应用层可以通过api的参数来告诉driver有几个channel     // 下面会根据具体的通道数和采样来决定创建什么样的output device stream 类实例     switch (lpFormat->nChannels)     {         case 1:               switch (g_pHWContext->m_dwWavOutChnum)             {                 case 1:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext1M8 :                            (StreamContext *)new OutputStreamContext1M16;                   case 2:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext2M8 :                            (StreamContext *)new OutputStreamContext2M16;                 case 6:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext6M8 :                            (StreamContext *)new OutputStreamContext6M16;                 default:                     break;             }               break;           case 2:               switch (g_pHWContext->m_dwWavOutChnum)             {                 case 1:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext1S8 :                            (StreamContext *)new OutputStreamContext1S16;                   case 2:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext2S8 :                            (StreamContext *)new OutputStreamContext2S16;                 case 6:                     return (lpFormat->wBitsPerSample == 8) ?                            (StreamContext *)new OutputStreamContext6S8 :                            (StreamContext *)new OutputStreamContext6S16;                 default:                     break;             }               break;           default:               switch (g_pHWContext->m_dwWavOutChnum)             {                 case 6:                     return new OutputStreamContext6X16;             }               break;     }       return NULL; }

         其实，驱动中可以选择去支持那些格式，没有必要去支持每一种音频Stream格式。