用窗函数法设计线性相位FIR低通滤波器。要求通带截止频率ωc=π/4，单位脉冲响应h(n)的长度N=23。绘出h(n)及其幅频响应特性曲线-Window function with linear phase FIR design a low pass filter. Required pass-band cutoff frequency ωc = π/4, unit impulse response h (n) of length N = 23. Plot h (n) and frequency