Changing of alarm and control levels on analogue signals and controllers. 改变模拟信号和控制器的警报及控制水平。
Unlike analogue signals, which tend to degrade gradually when digital signals drop data, the picture can freeze abruptly. 模拟信号往往会逐渐劣化，数字信号不同，它丢失数据后，画面会突然静止。
