用一个基准频率去同步一个未知频率,并通过一个可视监控器,查看接近零频时的音频间隙,从而获得一个零差频。图1就是这种可视差频监控器,它通过监控两盏灯而获得一个零差频。L1点亮时,未知频率太高;L1关闭时,未知频率太低。在L1改变状态的时点, A reference frequency is used to synchronize an unknown frequency, and a visual monitor is used to see the audio gap near zero to obtain a zero difference frequency. Figure 1 shows the visual difference frequency monitor, which obtains a zero difference frequency by monitoring two lamps. When L1 is lit, the unknown frequency is too high; when L1 is off, the unknown frequency is too low. When L1 changes state,