说明：  正余弦信号的谱分析 数字信号处理方法的一个重要用途是在离散时间域中确定一个连续时间信号的频谱，通常称为频谱分析，
(Cosine signals are digital signal processing method of spectrum analysis is an important use is in the discrete time domain to determine a continuous-time signal spectrum, commonly referred to as spectrum analysis,)