/// read stream
    ///
    AVPacket pkt;
    AVFrame frame;
    av_frame_alloc(&frame);
    int gotFrame;
    int gotPacket;
    while (av_read_frame(pFormatCtx, &pkt) >= 0) {
        avcodec_decode_audio4(&pDecoderCtx, &frame, &gotFrame, &pkt);
        avcodec_encode_audio2(&pEncoderCtx, &pkt, &frame, &gotPacket);
    }