20 巴特沃斯滤波器的设计

发布于 2021-10-25 15:26:17 浏览：1181 订阅该版

myLLgf 2021-10-26

这家伙很懒，什么也没写！

给你个二阶低通巴特沃斯的代码，记得采纳

```c
typedef struct lowpassFilter2p
{
    // 历史采样数据
    float _delay_element1;
    float _delay_element2;

// 权重系数
    float _a1;
    float _a2;
    float _b0;
    float _b1;
    float _b2;

// 截至频率
    float _cutoff_freq;

// 采样频率
    float _sample_freq;
} lowpassFilter2p_t;

void lp2p_init(lowpassFilter2p_t *lp, float sample_freq, float cutoff_freq)
{
    lp2p_set_cutoff_frequency(lp, sample_freq, cutoff_freq);
}

float lp2p_reset(lowpassFilter2p_t *lp, float sample)
{
    const float input = isfinite(sample) ? sample : 0.0f;

if (fabsf(1 + lp->_a1 + lp->_a2) > FLT_EPSILON)
    {
        lp->_delay_element1 = lp->_delay_element2 = input / (1 + lp->_a1 + lp->_a2);

if (!isfinite(lp->_delay_element1) || !isfinite(lp->_delay_element2))
        {
            lp->_delay_element1 = lp->_delay_element2 = input;
        }
    }
    else
    {
        lp->_delay_element1 = lp->_delay_element2 = input;
    }

return lp2p_apply(lp, input);
}

void lp2p_disable(lowpassFilter2p_t *lp)
{
    lp->_sample_freq = 0.f;
    lp->_cutoff_freq = 0.f;

lp->_delay_element1 = 0.0f;
    lp->_delay_element2 = 0.0f;

lp->_b0 = 1.f;
    lp->_b1 = 0.f;
    lp->_b2 = 0.f;

lp->_a1 = 0.f;
    lp->_a2 = 0.f;
}

void lp2p_set_cutoff_frequency(lowpassFilter2p_t *lp, float sample_freq,
                               float cutoff_freq)
{
    if ((sample_freq <= 0.f) || (cutoff_freq <= 0.f) ||
        (cutoff_freq >= sample_freq / 2) || !isfinite(sample_freq) ||
        !isfinite(cutoff_freq))
    {

lp2p_disable(lp);
        return;
    }

// reset delay elements on filter change
    lp->_delay_element1 = 0;
    lp->_delay_element2 = 0;

lp->_cutoff_freq = constrain_float(
        cutoff_freq, 1.f,
        sample_freq / 2); // TODO: min based on actual numerical limit
    lp->_sample_freq = sample_freq;

const float fr = lp->_sample_freq / lp->_cutoff_freq;
    const float ohm = tanf(M_PI_F / fr);
    const float c = 1.f + 2.f * cosf(M_PI_F / 4.f) * ohm + ohm * ohm;

lp->_b0 = ohm * ohm / c;
    lp->_b1 = 2.f * lp->_b0;
    lp->_b2 = lp->_b0;

lp->_a1 = 2.f * (ohm * ohm - 1.f) / c;
    lp->_a2 = (1.f - 2.f * cosf(M_PI_F / 4.f) * ohm + ohm * ohm) / c;

if (!isfinite(lp->_b0) || !isfinite(lp->_b1) || !isfinite(lp->_b2) ||
        !isfinite(lp->_a1) || !isfinite(lp->_a2))
    {
        lp2p_disable(lp);
    }
}

float lp2p_apply(lowpassFilter2p_t *lp, float sample)
{
    float delay_element_0 =
        sample - lp->_delay_element1 * lp->_a1 - lp->_delay_element2 * lp->_a2;

const float output = delay_element_0 * lp->_b0 +
                         lp->_delay_element1 * lp->_b1 +
                         lp->_delay_element2 * lp->_b2;
    lp->_delay_element2 = lp->_delay_element1;
    lp->_delay_element1 = delay_element_0;
    return output;
}

```