dsp实现无限冲激响应滤波器(IIR)算法，低通巴特沃斯滤波器在其通带边缘1kHz 处的增益为-3dB，12kHz 处的阻带衰减为30dB，采样频率25kHz-dsp Infinite Impulse Response Filter (IIR) algorithm, low-pass Butterworth filter gain in its passband edge at 1kHz-3dB 12kHz at the stop-band attenuation of 30dB, the sa