G711编解码(G711与PCM类型互转)【转】

来自：http://blog.csdn.net/qq_24551315/article/details/51134689

总结网上的G711编解码, 将其一直到andorid系统上去, 通过jni的来进行G711和PCM 的互转,  由于对于这两种格式没有深入的了解,, 所以如果想深入的了解他们可以百度, 在这里只是实现这样的功能罢了.

g711.c 包含编码和解码的方法

 //http://www.easydarwin.org/article/Streaming/38.html               参考文章
 //https://github.com/EasyDarwin/EasyAACEncoder/blob/master/g711.cpp
 //http://www.oschina.net/code/snippet_1173523_38946
 //http://blog.csdn.net/rightorwrong/article/details/4209467  PCM 2 G711  G711 2 PCM

 unsigned char encode(short pcm);
 short decode(unsigned char alaw);

 /** 个人理解
  * bitsize 应该为16, pBuffer(pcm数据) 两个char 合成一个 short ,长度自然就是原来的一半(nBufferSize/2),
  * 通过编码后short类型的数据变为char类型,让后复制给pCodecBits
  */
 int g711_encode(unsigned char* pCodecBits, const char* pBuffer, int nBufferSize)
 {
     short* buffer = (short*)pBuffer;
     int i;
     for(i=0; i<nBufferSize/2; i++)
     {
         pCodecBits[i] = encode(buffer[i]);
     }

     return nBufferSize/2;
 }

 /** 个人理解
  * bitsize 应该为16, pcm 数组的宽度变为原来两倍(short *out_data = (short*)pRawData;),
  * 通过对pBuffer(g711数据)中char解码转为两个字节的short ,后复制给out_data数组, 在使用的时候又转为char类型, 则 解码后的数据就是原来的两倍(nBufferSize*2)
  */
 int g711_decode(char* pRawData, const unsigned char* pBuffer, int nBufferSize)
 {
     short *out_data = (short*)pRawData;
     int i;
     for(i=0; i<nBufferSize; i++)
     {
         out_data[i] = decode(pBuffer[i]);
     }

     return nBufferSize*2;
 }

 #define MAX 32635
 unsigned char encode(short pcm)
 {
     int sign = (pcm & 0x8000) >> 8;
     if (sign != 0)
         pcm = -pcm;
     if (pcm > MAX) pcm = MAX;
     int exponent = 7;
     int expMask;
     for (expMask = 0x4000; (pcm & expMask) == 0
         && exponent>0; exponent--, expMask >>= 1) { }
     int mantissa = (pcm >> ((exponent == 0) ? 4 : (exponent + 3))) & 0x0f;
     unsigned char alaw = (unsigned char)(sign | exponent << 4 | mantissa);
     return (unsigned char)(alaw^0xD5);
 }

 short decode(unsigned char alaw)
 {
     alaw ^= 0xD5;
     int sign = alaw & 0x80;
     int exponent = (alaw & 0x70) >> 4;
     int data = alaw & 0x0f;
     data <<= 4;
     data += 8;
     if (exponent != 0)
         data += 0x100;
     if (exponent > 1)
         data <<= (exponent - 1);

     return (short)(sign == 0 ? data : -data);
 }

jni中实现编解码,只贴出主要代码

G711 -> PCM

 JNIEXPORT void JNICALL Java_com_ff_aacdemo_jni_G711Coder_g711Topcm
   (JNIEnv *env, jobject obj){
     FILE* fpOut = fopen("/storage/emulated/0/t/pcm_to_g711.pcm", "wb");
     FILE* fpIn = fopen("/storage/emulated/0/t/pcm_to_g711.g711", "rb");//g711格式文件在最后会给出连接

     int g711_BufferSize = 1024;
     char g711_Buffer[g711_BufferSize];

     int len;
     while((len = fread(g711_Buffer, 1, g711_BufferSize, fpIn)) > 0){
         LOGD("g711topcm length = %d", len);
         char pcmBuffer[len];
         int pcmbufsize = g711_decode(pcmBuffer, g711_Buffer, len);
         fwrite(pcmBuffer, 1, pcmbufsize, fpOut);
     }
     LOGD("g711topcm end");
     fclose(fpIn);
     fclose(fpOut);
 }

PCM -> G711

 JNIEXPORT void JNICALL Java_com_ff_aacdemo_jni_G711Coder_pcmTog711
   (JNIEnv *env, jobject obj){
     FILE* fpIn = fopen("/storage/emulated/0/t/pcm_to_g711.pcm", "rb");
     FILE* fpOut = fopen("/storage/emulated/0/t/pcm_to_g711.g711", "wb");

     LOGD("pcmTog711 1");

     int pcm_BufferSize = 1024;
     char pcm_Buffer[pcm_BufferSize];

     int len;
     while((len = fread(pcm_Buffer, 1, pcm_BufferSize, fpIn)) > 0){
         LOGD("pcmTog711 length = %d", len);
         char g711Buffer[len];
         int  g711BufSize = g711_encode(g711Buffer, pcm_Buffer, len);
         fwrite(g711Buffer, 1, g711BufSize, fpOut);
     }
     LOGD("pcmTog711 end");
     fclose(fpIn);
     fclose(fpOut);
 }

虽然能够实现他们的相互转换, 可是,不怎么好找到播放器播放, 效果始终没有验证, 在下一章中, 将总结将 G711 转换为 AAC , AAC这种格式将能够使用常规的播放器进行播放

g711文件 http://download.csdn.net/detail/yuanzhenhai/2693878