VoiceEngine Codec数据结构

1、VoiceEngine Codec数据结构

       WebRTC中,用一个结构体struct CodecInst表示特定的音频编解码器对象:

 

struct CodecInst
{
    int pltype;      //payload type负载类型
    char plname[32]; //payload name负载名称,32个字符表示
    int plfreq;      //payload frequence负载频率
    int pacsize;     //packet size包大小
    int channels;    //声道
    int rate;        //速率或自适应
};

参数详细说明:


1、 pltype范围在1~126之间才是有效值;

        pltype的值是否有效可以通过调用下面ValidPayloadType(int payload_type)方法来判断,在...\src\modules\audio_coding\main\source\acm_codec_database.cc定义

 

// Checks if the payload type is in the valid range.
bool ACMCodecDB::ValidPayloadType(int payload_type) {
   if ((payload_type < 0) || (payload_type > 127)) {
      return false;
   }
   return true;
}

 

2、 plname是编解码器的名称,可能的值在CreateCodecInstance已定义,如WebRTC默认的"ISAC"

        VoiceEngine支持多个音频编解码器,具体支持的编解码器在CreateCodecInstance(const CodecInst* codec_inst)定义,比如ISAC\PCMU\PCMA\ILBC\AMR等等,在...\src\modules\audio_coding\main\source\acm_codec_database.cc定义


 

ACMGenericCodec* ACMCodecDB::CreateCodecInstance(const CodecInst* codec_inst) {
  // All we have support for right now.
  if (!STR_CASE_CMP(codec_inst->plname, "ISAC")) {
#if (defined(WEBRTC_CODEC_ISAC) || defined(WEBRTC_CODEC_ISACFX))
    return new ACMISAC(kISAC);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "PCMU")) {
    return new ACMPCMU(kPCMU);
  } else if (!STR_CASE_CMP(codec_inst->plname, "PCMA")) {
    return new ACMPCMA(kPCMA);
  } else if (!STR_CASE_CMP(codec_inst->plname, "ILBC")) {
#ifdef WEBRTC_CODEC_ILBC
    return new ACMILBC(kILBC);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "AMR")) {
#ifdef WEBRTC_CODEC_AMR
    return new ACMAMR(kGSMAMR);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "AMR-WB")) {
#ifdef WEBRTC_CODEC_AMRWB
    return new ACMAMRwb(kGSMAMRWB);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "G722")) {
#ifdef WEBRTC_CODEC_G722
    return new ACMG722(kG722);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "G7221")) {
    switch (codec_inst->plfreq) {
      case 16000: {
#ifdef WEBRTC_CODEC_G722_1
        int codec_id;
        switch (codec_inst->rate) {
          case 16000 : {
            codec_id = kG722_1_16;
            break;
          }
          case 24000 : {
            codec_id = kG722_1_24;
            break;
          }
          case 32000 : {
            codec_id = kG722_1_32;
            break;
          }
          default: {
            return NULL;
          }
          return new ACMG722_1(codec_id);
        }
#endif
      }
      case 32000: {
#ifdef WEBRTC_CODEC_G722_1C
        int codec_id;
        switch (codec_inst->rate) {
          case 24000 : {
            codec_id = kG722_1C_24;
            break;
          }
          case 32000 : {
            codec_id = kG722_1C_32;
            break;
          }
          case 48000 : {
            codec_id = kG722_1C_48;
            break;
          }
          default: {
            return NULL;
          }
          return new ACMG722_1C(codec_id);
        }
#endif
      }
    }
  } else if (!STR_CASE_CMP(codec_inst->plname, "CN")) {
    // For CN we need to check sampling frequency to know what codec to create.
    int codec_id;
    switch (codec_inst->plfreq) {
      case 8000: {
        codec_id = kCNNB;
        break;
      }
      case 16000: {
        codec_id = kCNWB;
        break;
      }
      case 32000: {
        codec_id = kCNSWB;
        break;
      }
      default: {
        return NULL;
      }
    }
    return new ACMCNG(codec_id);
  } else if (!STR_CASE_CMP(codec_inst->plname, "G729")) {
#ifdef WEBRTC_CODEC_G729
    return new ACMG729(kG729);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "G7291")) {
#ifdef WEBRTC_CODEC_G729_1
    return new ACMG729_1(kG729_1);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "speex")) {
#ifdef WEBRTC_CODEC_SPEEX
    int codec_id;
    switch (codec_inst->plfreq) {
      case 8000: {
        codec_id = kSPEEX8;
        break;
      }
      case 16000: {
        codec_id = kSPEEX16;
        break;
      }
      default: {
        return NULL;
      }
    }
    return new ACMSPEEX(codec_id);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "CN")) {
    // For CN we need to check sampling frequency to know what codec to create.
    int codec_id;
    switch (codec_inst->plfreq) {
      case 8000: {
        codec_id = kCNNB;
        break;
      }
      case 16000: {
        codec_id = kCNWB;
        break;
      }
      case 32000: {
        codec_id = kCNSWB;
        break;
      }
      default: {
        return NULL;
      }
    }
    return new ACMCNG(codec_id);
  } else if (!STR_CASE_CMP(codec_inst->plname, "L16")) {
#ifdef WEBRTC_CODEC_PCM16
    // For L16 we need to check sampling frequency to know what codec to create.
    int codec_id;
    switch (codec_inst->plfreq) {
      case 8000: {
        codec_id = kPCM16B;
        break;
      }
      case 16000: {
        codec_id =kPCM16Bwb;
        break;
      }
      case 32000: {
        codec_id = kPCM16Bswb32kHz;
        break;
      }
      default: {
        return NULL;
      }
    }
    return new ACMPCM16B(codec_id);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "telephone-event")) {
#ifdef WEBRTC_CODEC_AVT
    return new ACMDTMFPlayout(kAVT);
#endif
  } else if (!STR_CASE_CMP(codec_inst->plname, "red")) {
#ifdef WEBRTC_CODEC_RED
    return new ACMRED(kRED);
#endif
  }
  return NULL;
}

 



3、 plfreq一般取如下值(在common_types.h定义);

//负载频率值
enum PayloadFrequencies
{
      kFreq8000Hz   = 8000,
      kFreq16000Hz = 16000,
      kFreq32000Hz = 32000
};

4、 pacsize取值是与plfreq有关系的,单位为kbps,下面是计算公式

计算公式如下:

         如果:plfreq = 16000(单位为hz)

         如果我需要30ms(毫秒)的packet size

         那么pacsize = (plfreq *30) /1000 = 480kbps;

也即是:要得到k ms的packet size,则可计算出

                 pacsize =( plfreq * k) / 1000

而如果plfreq = 32000;20ms的packet size,则pacsize  = 640;


5、 channels取值

        channels = 1 表示单声道

        channels = 2 表示立体声道

        注意:channels  = -1时,表示此时只支持单声道模式


6、 rate取值,单位是bps

        一般取rate = 16000,32000,48000这些16000整数倍的值,即16kbps,32kbps,48kpbs

        注意:当rate = -1时,表示此时启动自适应信道速率


 

2、查看VoiceEngine支持的所有Codec信息示例代码

//列出(获得)引擎支持的所有编解码器信息
//支持平台:Windows, Mac OS X, Linux

#include "voe_base.h"
#include "voe_codec.h"

VoiceEngine* ve = VoiceEngine::Create();
VoECodec* codec = VoECodec::GetInterface(ve);

for (int = 0; i < codec->NumOfCodecs(); i++)

{

     CodecInst cinst;

     codec->GetCodec(i, cinst);

     DISPLAY_CODEC_INFO(i, cinst);

}

   // 释放sub-API
   codec->Release();

   //删除引擎
   VoiceEngine::Delete(ve);

3、初始化VoiceEngine Codec示例代码

//初始化VoiceEngine Codec示例代码
//支持平台:Windows, Mac OS X, Linux

#include "voe_codec.h"

CodecInst cinst;

//初始化iSAC编解码器参数
strcpy(cinst.plname, "ISAC");

cinst.plfreq    = 16000; // iSAC宽带模式取样频率
cinst.pltype    = 103; 
cinst.pacsize   = 480;    //使用30ms packet size,480kbps
cinst.channels = 1;       // 单声道
cinst.rate       = -1;      // 信道自适应模式
//初始化完成

//在ID为0的channel激活已初始化的iSAC
codec->SetSendCodec(0, cinst);

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