7.1ban

代码说明：

  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 frequency to identify the corresponding button. Test through the preparation of a digital signal analysis program to simulate the function of the completion of the above.)