语音wav 8000采样,16位,单声道,PCM 转换成wav 8000采样 8位 单声道 CCITT A-Law格式

求助，语音wav 采样频率8000,采样位数16,单声道,PCM 怎么转换成 wav 8000采样 8位 单声道 CCITT A-Law格式，谁有源码或之前做过的？

我是反方向转换，，同求，

#include
#include
#include

/* wav音频头部格式 */
typedef struct _wave_pcm_hdr
{
char riff[4]; // = "RIFF"
int size_8; // = FileSize - 8
char wave[4]; // = "WAVE"
char fmt[4]; // = "fmt "
int fmt_size; // = 下一个结构体的大小 : 16
short int format_tag; // = PCM : 1
short int channels; // = 通道数 : 1
int samples_per_sec; // = 采样率 : 8000 | 6000 | 11025 | 16000
int avg_bytes_per_sec; // = 每秒字节数 : samples_per_sec * bits_per_sample / 8
short int block_align; // = 每采样点字节数 : wBitsPerSample / 8
short int bits_per_sample; // = 量化比特数: 8 | 16

char            data[4];                // = "data";
int             data_size;              // = 纯数据长度 : FileSize - 44

} wave_pcm_hdr;

int wave_16bit_to_8bit(char source_file,char *dest_file)
{
short *data_16 = NULL ;
char data_8 = 0 ;
unsigned char mdata_8 = 0 ;
unsigned char dstemp = (unsigned char*)malloc(4);
FILE *pInput = NULL , * pOutput = NULL ;

wave_pcm_hdr wave_header_16 = {0};
wave_pcm_hdr wave_header_8 = {0};

if(strcmp(source_file,dest_file)==0)
{
    printf("源文件名与目标文件名同名了;\n");
    return -2 ;
}

pInput = fopen(source_file,"rb");
pOutput = fopen(dest_file,"wb");
if(pInput == NULL || pOutput == NULL)
{
    printf("打开源文件或打开目标文件失败;\n");
    return -1 ;
}

fread(&wave_header_16,1,44,pInput);
printf("bits_per_sample:%d;\n",wave_header_16.bits_per_sample);//16
memcpy(&wave_header_8,&wave_header_16,sizeof(wave_pcm_hdr));
wave_header_8.size_8 = (wave_header_16.size_8+8)/2 + 14;
wave_header_8.avg_bytes_per_sec = 8000;
wave_header_8.block_align = 1 ;
wave_header_8.bits_per_sample = 8 ;//改为8 ，上面的值也需要随之改变
wave_header_8.data_size = wave_header_16.data_size/2 ;

fwrite(&wave_header_8,1,sizeof(wave_pcm_hdr),pOutput);

while(!feof(pInput))
{
    fread(dstemp,1,4,pInput);

    data_16 = (short*)dstemp;
    data_8 = (*data_16) >> 8;
    mdata_8 = data_8 + 128 ;
    fwrite(&mdata_8,1,1,pOutput);

    data_16 = (short*)(dstemp + 2);
    data_8 = (*data_16) >> 8;
    mdata_8 = data_8 + 128 ;
    fwrite(&mdata_8,1,1,pOutput);
}
fclose(pOutput);
fclose(pInput);
return 0 ;

}

int main(int argc , char **argv)
{
wave_16bit_to_8bit("tts_sample.wav","tts_sample.wav");
return 0 ;
}