按键式电话如下图所示，当按下某一个键时会产生如图所示的相应的频率模拟信号，并通过对该信号频率的检测，识别出对应的按键。试通过编写一个数字信号分析程序，模拟完成上面的功能。-Touch-tone phone as shown below, when pressing a key will be generated as shown in the corresponding frequency analog signals and, through detection of the signal f