我在nginx中遇到了一个奇怪的dash应用程序问题。如果我在下面的代码中不包含dash_variant行，一切都可以正常工作。我第二次取消注解它们，nginx配置失败，unknown directive "dash_variant"。我希望添加dash_variant指令，这样我就可以根据用户的带宽/连接质量来获得不同的比特率流。任何帮助将不胜感激！提前感谢！

application dash {
                                allow play all;
                                live on;
                                #record all;
                                record off;
                                record_path videos;
                                record_unique on;
                                #allow publish <your_sender_ip_here>;
                                #allow publish 127.0.0.1;
                                #deny publish all;
                                meta copy;
                                wait_key on;
                                wait_video on;
                                idle_streams off;

                                # Dash settings and pushing towards repeaters
                                dash on;
                                dash_nested on;
                                dash_path /tmp/dash;
                                dash_fragment 3;
                                dash_playlist_length 20;
                                dash_cleanup on;

                                #dash_clock_compensation http_head;
                                #dash_clock_helper_uri https://localhost/time;

                                dash_variant _low bandwidth="500000"  width="640"  height="360";
                                dash_variant _med bandwidth="1500000" width="1280"  height="720";
                                dash_variant _high bandwidth="5000000" width="1920" height="1080" max;

                                #push rtmp://localhost/hls/hls; // it goes to generate a HLS streaming
                        }

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