中文说明：
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。
English Description：
The push-button telephone is shown in the figure below. When a key is pressed, the corresponding frequency analog signal as shown in the figure will be generated, and the corresponding key will be identified through the detection of the signal frequency. Try to complete the above functions by writing a digital signal analysis program