FAAD2解码步骤举例

AAC有两种格式，ADIF和ADTS，以下示例仅限ADTS。

#include <stdio.h>
#include <stdlib.h>
#include "faad.h"

// 读取aac文件内容到buffer
static unsigned char* getAacData(char *name, int *dataSize){
    FILE *fp = fopen(name, "rb");
    if(fp == NULL) return NULL;
    fseek(fp, 0, SEEK_END);
    int size = ftell(fp);
    fseek(fp, 0, SEEK_SET);
    unsigned char *buffer = malloc(size+16);
    fread(buffer, size, 1, fp);
    *dataSize = size;
    fclose(fp);
    return buffer;
}

int main(int argc, char* argv[]){
    if(argc < 2) return 0;
    int aacDataSize = 0;
    unsigned char *aacData = getAacData(argv[1], &aacDataSize);
    printf("aacDataSize = %d\n", aacDataSize);
    
    // 打开faad2解码器
    NeAACDecHandle faadHandle = NeAACDecOpen();
    if(faadHandle == NULL) return -1;
    
    // 解码之前需要初始化
    unsigned char channels = 0;
    unsigned long samplerate = 0;
    long ret = NeAACDecInit(faadHandle, aacData, aacDataSize, &samplerate, &channels);
    printf("ret = %ld, channels = %u, samplerate = %lu\n", ret, channels, samplerate);
    
    // 循环解码
    int aacOffset = 0;
    void *sampleData = NULL;
    NeAACDecFrameInfo frameInfo;
    while(aacOffset < aacDataSize){
        
        // faad2 解码
        sampleData = NeAACDecDecode(faadHandle, &frameInfo, aacData+aacOffset, aacDataSize-aacOffset);
        if((sampleData == NULL) || (frameInfo.error)) break;
        
        // sampleData是解码出来的sample，注意不要free(sampleData)。
        // frameInfo.bytesconsumed 是本次被解码掉的字节数
        // frameInfo.samples       是sample个数
        // frameInfo.samplerate    是频率
        // frameInfo.channels      是通道数
        printf("bytesconsumed = %lu, samples = %lu, samplerate = %lu, channels = %u\n",
            frameInfo.bytesconsumed,
            frameInfo.samples,
            frameInfo.samplerate,
            frameInfo.channels
        );
        
        aacOffset += frameInfo.bytesconsumed;
    }
    
    // 关闭解码器
    NeAACDecClose(faadHandle);
    free(aacData);
    return 0;
}

遇到过5.1声道的情况，经过探索，解码出来的5.1声道PCM内存布局应该是这样的：
[正中声道, 左声道, 右声道, 左环绕, 右环绕, 低频增强]。
5.1声道转2声道采取以下算法效果较好：
左 = 正中声道 + 左声道 + 左环绕 + 低频增强;
右 = 正中声道 + 右声道 + 右环绕 + 低频增强;