Realistic signals are made by summing sine waves of lots of frequencies, so that after some finite number of wavelengths they interfere destructively and you get a signal of finite width and duration.

• 现实中的信号由各种频率的正弦波组成，因此，在有限的波长数后，它们会干涉相消，你就得到一个有限宽度和有限延续的信号。