使用FAAD库解码AAC实例及 及 faad解码后的通道数不正确的问题

/** * faaddec.c * use faad library to decode AAC, only can decode frame with ADTS head  */#include <stdio.h>#include <memory.h>#include "faad.h"#define FRAME_MAX_LEN 1024*5 #define BUFFER_MAX_LEN 1024*1024void 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?}#endifhDecoder->channelConfiguration = *channels;#ifdef SBR_DEC/* implicit signalling */if (*samplerate <= 24000 && (hDecoder->config.dontUpSampleImplicitSBR == 0)){    *samplerate *= 2; // samplerate 变为32000    hDecoder->forceUpSampling = 1;} <pre name="code" class="cpp">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)) 

我们看看在neaacdec.h 文件中该结构体的定义

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/2conf->outputFormat = FAAD_FMT_16BIT ; //conf->dontUpSampleImplicitSBR = 1;<p>NeAACDecSetConfiguration(decoder, conf);</p><p></p>

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

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