nb_samples(採樣數)轉換

可以參考：https://blog.csdn.net/Martin_chen2/article/details/100103655中的錯誤案例。
我的frame採樣數是2048，直接編碼aac格式會失敗，採用重採樣的方式，把一個frame拆分成兩個採樣數1024的frame，分兩次調用swr_convert接口返回採樣。

#define AAC_ENCODE_SIZE 1024

// 定義重採樣
SwrContext *swr = NULL;
swr = swr_alloc();
av_opt_set_int(swr, "in_channel_layout",  AV_CH_LAYOUT_MONO, 0);
av_opt_set_int(swr, "out_channel_layout", AV_CH_LAYOUT_MONO,  0);
av_opt_set_int(swr, "in_sample_rate",     44100, 0);
av_opt_set_int(swr, "out_sample_rate",    44100, 0);
av_opt_set_sample_fmt(swr, "in_sample_fmt",  AV_SAMPLE_FMT_S16, 0);
av_opt_set_sample_fmt(swr, "out_sample_fmt", AV_SAMPLE_FMT_S16,  0);
ret = swr_init(swr);
if (ret < 0){
    LOGD("swr_init error ret = %d, %s", ret, av_err2str(ret));
    return -1;
}

***
// 定義輸出
uint8_t *outs[2];
outs[0]=(uint8_t *)malloc(AAC_ENCODE_SIZE);
outs[1]=(uint8_t *)malloc(AAC_ENCODE_SIZE);
memset(outs[0], 0, AAC_ENCODE_SIZE);
memset(outs[1], 0, AAC_ENCODE_SIZE);

***

// nb_samples爲2014 重採樣爲1024
int count = swr_convert(swr, outs, AAC_ENCODE_SIZE, (const uint8_t **)pFrameOut->extended_data, pFrameOut->nb_samples);
if (count < 0){
   LOGD("swr_convert error 1 ret = %d, %s", count, av_err2str(count));
} else {
   LOGD("count = %d, outs[0] = %s, outs[1] = %s", count, NULL, NULL/*outs[0], outs[1]*/);
   pFrameOut->nb_samples = count;//aac 爲1024
   pFrameOut->data[0] = (uint8_t*)outs[0];
   pFrameOut->data[1] = (uint8_t*)outs[1];
}

// 進行編碼
encode:
      ret = avcodec_encode_audio2(pOutFormatCtx->streams[audioIndex]->codec, &enc_pkt, pFrameOut, &enc_got_frame);

***

//重採樣剩餘的1024個採樣，不需要輸入數據
count = swr_convert(swr, outs, AAC_ENCODE_SIZE, NULL, 0);
LOGD("cache count = %d, outs[0] = %s, outs[1] = %s", count, NULL, NULL/*outs[0], outs[1]*/);
if (count > 0){
   pFrameOut->nb_samples = count;
   pFrameOut->data[0] = (uint8_t*)outs[0];
   pFrameOut->data[1] = (uint8_t*)outs[1];
}

注意第二次重採樣不需要輸入，而是直接從緩存區把剩餘的AAC_ENCODE_SIZE個數據拿出來。

AAC對應的採樣數（nb_samples）和frame大小（frame_size）都是1024.
AAC對應的採樣數（nb_samples）和frame大小（frame_size）都是1152.