中文说明：
利用双线性变换法，设计一个0.4dB截止频率为10kHz且在30kHz处有最小阻带衰减为50dB的数字巴特沃兹低通滤波器，其抽样率为100kHz。利用教材（4.35）式求解原型模拟滤波器的阶数，并利用Matlab的M文件buttap设计模拟原型滤波器。用M文件bilinear将模拟滤波器传输函数变换成期望的数字传输函数。请用Matlab画出其增益和相位响应，并给出设计的具体步骤。
English Description：
Using the bilinear transformation method, a digital Butterworth low-pass filter with a 0.4db cut-off frequency of 10kHz and a minimum stopband attenuation of 50dB at 30kHz is designed, and its sampling rate is 100kHz. The order of the prototype analog filter is solved by using the formula in the textbook (4.35), and the analog prototype filter is designed by using the m file buttap of MATLAB. The analog filter transfer function is transformed into the desired digital transfer function with m file bilinear. Please use matlab to draw its gain and phase response, and give the specific steps of the design< br/>