说明：  “包络” 谱图的术语不是对信号处理过程的确切描述，但仍是我们为了简化时所用的术语。 这个ppt描述包络的基本知识
("Envelope" of the spectrum, the term is not precise description of signal processing, but still is when we have to simplify the terminology used. This ppt describes the basics of the envelope)