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

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 <stdio.h>
#include <memory.h>
#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;
} <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/2
	conf->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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