G711编解码(G711与PCM类型互转)

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