说明：  小波变换已应用到工程的各个方面，但一般只是应用MATLAB的小波工具包来编程，有时需要自己编程实现，故这里给出了DB4小波变换的C实现
(Wavelet transform has been applied to all aspects of the project, but generally only apply MATLAB wavelet toolkit for programming, sometimes need their own programming, so here is a DB4 wavelet transform C implementation)