使用FAAD库解码AAC实例

使用FAAD解码AAC音频为PCM数据流程可以参考下面的文章

http://blog.csdn.net/gavinr/article/details/6959198#reply

 /**
  * faaddec.c
  * use faad library to decode AAC, only can decode frame with ADTS head
  */
 #include
 #include
 #include "faad.h"

 #define FRAME_MAX_LEN 1024*5
 #define BUFFER_MAX_LEN 1024*1024

 void show_usage()
 {
     printf("usage\nfaaddec src_file dst_file");
 }

 /**
  * fetch one ADTS frame
  */
 int get_one_ADTS_frame(unsigned char* buffer, size_t buf_size, unsigned char* data ,size_t* data_size)
 {
     size_t size = 0;

     if(!buffer || !data || !data_size )
     {
         return -1;
     }

     while(1)
     {
         if(buf_size  < 7 )
         {
             return -1;
         }

         if((buffer[0] == 0xff) && ((buffer[1] & 0xf0) == 0xf0) )
         {
             size |= ((buffer[3] & 0x03) <<11);     //high 2 bit
             size |= buffer[4]<<3;                //middle 8 bit
             size |= ((buffer[5] & 0xe0)>>5);        //low 3bit
             break;
         }
         --buf_size;
         ++buffer;
     }

     if(buf_size < size)
     {
         return -1;
     }

     memcpy(data, buffer, size);
     *data_size = size;

     return 0;
 }

 int main(int argc, char* argv[])
 {
     static unsigned char frame[FRAME_MAX_LEN];
     static unsigned char buffer[BUFFER_MAX_LEN] = {0};

     char src_file[128] = {0};
     char dst_file[128] = {0};
     FILE* ifile = NULL;
     FILE* ofile = NULL;

     unsigned long samplerate;
     unsigned char channels;
     NeAACDecHandle decoder = 0;

     size_t data_size = 0;
     size_t size = 0;

     NeAACDecFrameInfo frame_info;
     unsigned char* input_data = buffer;
     unsigned char* pcm_data = NULL;

     //analyse parameter
     if(argc < 3)
     {
         show_usage();
         return -1;
     }
     sscanf(argv[1], "%s", src_file);
     sscanf(argv[2], "%s", dst_file);


     ifile = fopen(src_file, "rb");
     ofile = fopen(dst_file, "wb");
     if(!ifile || !ofile)
     {
         printf("source or destination file");
         return -1;
     }

      data_size = fread(buffer, 1, BUFFER_MAX_LEN, ifile);

      //open decoder
     decoder = NeAACDecOpen();
     if(get_one_ADTS_frame(buffer, data_size, frame, &size) < 0)
     {
         return -1;
     }

     //initialize decoder
     NeAACDecInit(decoder, frame, size, &samplerate, &channels);
     printf("samplerate %d, channels %d\n", samplerate, channels);

     while(get_one_ADTS_frame(input_data, data_size, frame, &size) == 0)
     {
        // printf("frame size %d\n", size);

         //decode ADTS frame
         pcm_data = (unsigned char*)NeAACDecDecode(decoder, &frame_info, frame, size);

         if(frame_info.error > 0)
         {
             printf("%s\n",NeAACDecGetErrorMessage(frame_info.error));

         }
         else if(pcm_data && frame_info.samples > 0)
         {
             printf("frame info: bytesconsumed %d, channels %d, header_type %d\
                 object_type %d, samples %d, samplerate %d\n",
                 frame_info.bytesconsumed,
                 frame_info.channels, frame_info.header_type,
                 frame_info.object_type, frame_info.samples,
                 frame_info.samplerate);

             fwrite(pcm_data, 1, frame_info.samples * frame_info.channels, ofile);      //2个通道
             fflush(ofile);
         }
         data_size -= size;
         input_data += size;
     }

     NeAACDecClose(decoder);

     fclose(ifile);
     fclose(ofile);

     return 0;
 }

     之前用FAAC编码了一段PCM数据（源数据是16000采样率，单通道，16位取样），编码时设置的参数也是16000采样率，单通道，16位取样。。。然后用FAAD解码时，在NeAACDecInit的时候，是先在之前编码好的aac数据（ADTS头封装的）上往buffer中写入一帧的含ADTS头的数据，然后传入到NeAACDecInit（）中初始化解码器，但不知道为什么，返回的采样率总是32000，通道数总是2，我已经查看过编码后的数据，其中与采样率、通道数相应的位，表示的就是16000采样率和1通道。。 

后来跟踪源码才发现 FAAD的NeAACDecInit() 源代码，问题出现在这里

 long NEAACDECAPI NeAACDecInit(NeAACDecHandle hpDecoder,
                               unsignedchar *buffer,
                               unsignedlong buffer_size,
                               unsignedlong *samplerate,
                               unsignedchar *channels)
 {

 #if (defined(PS_DEC) || defined(DRM_PS))
 /* check if we have a mono file */
 if (*channels == 1)
 {
      /* upMatrix to 2 channels for implicit signalling of PS */
      *channels = 2;  // 这里channels改变为2,why?
 }
 #endif

 hDecoder->channelConfiguration = *channels;

 #ifdef SBR_DEC
 /* implicit signalling */
 if (*samplerate <= 24000 && (hDecoder->config.dontUpSampleImplicitSBR == 0))
 {
     *samplerate *= 2;    // samplerate 变为32000
     hDecoder->forceUpSampling = 1;
 }

 else if (*samplerate > 24000 && (hDecoder->config.dontUpSampleImplicitSBR == 0)) {
      hDecoder->downSampledSBR = 1;
 }
 #endif
 }

  上面那些宏,在源代码中貌似已经定死了,不是通过条件编译生成的,

也就是 PS_DEC 和 SBR_DEC是define过的...也不是通过./configure 的时候生成的...

再仔细看上面代码的if语句 

if (*samplerate <= 24000 && (hDecoder->config.dontUpSampleImplicitSBR == 0))

只要让 hDecoder->config.dontUpSampleImplicitSBR 不为0 不就OK吗？

我们看看在neaacdec.h 文件中该结构体NeAACDecConfiguration 的定义

现在解决这个问题就简单了，初始化前把参数dontUpSampleImplicitSBR设置为1即可。

NeAACDecConfigurationPtr conf = NeAACDecGetCurrentConfiguration(decoder);

 conf->defObjectType = LC;

 conf->defSampleRate = 8000; //real samplerate/2

 conf->outputFormat = FAAD_FMT_16BIT ; //

 conf->dontUpSampleImplicitSBR = 1;

 NeAACDecSetConfiguration(decoder, conf);

最后吐槽一下。在从官网下载的faad 说明文档 对NeAACDecConfiguration 这个结构体说明，

居然没有dontUpSampleImplicitSBR这个成员，太坑爹了