int_iir: use taylor for lowpass
-use super::cossin;
+use core::f64::consts::PI;
use serde::{Deserialize, Serialize};
/// Generic vector for integer IIR filter.
@@ -13,16 +13,19 @@ impl IIRState {
///
/// # Args
/// * `f` - Corner frequency, or 3dB cutoff frequency (in units of sample rate).
+ /// This is only accurate for low corner frequencies less than ~0.01.
/// * `q` - Quality factor (1/sqrt(2) for critical).
/// * `k` - DC gain.
///
/// # Returns
/// 2nd-order IIR filter coefficients in the form [b0,b1,b2,a1,a2]. a0 is set to -1.
+ ///
+ /// # Note
pub fn lowpass(f: f64, q: f64, k: f64) -> IIRState {
- let scale = (1i64 << 32) as f64;
- let fcossin = cossin((f * scale) as u32 as i32);
- let fcos = fcossin.0 as f64 / scale;
- let fsin = fcossin.1 as f64 / scale;
+ // 3rd order Taylor approximation of sin and cos.
+ let f = f * 2. * PI;
+ let fsin = f - f * f * f / 6.;
+ let fcos = 1. - f * f / 2.;
let alpha = fsin / (2. * q);
// IIR uses Q2.30 fixed point
let a0 = (1. + alpha) / (1 << IIR::SHIFT) as f64;
@@ -88,3 +91,14 @@ impl IIR {
y0
}
}
+
+#[cfg(test)]
+mod test {
+ use super::IIRState;
+
+ #[test]
+ fn lowpass_gen() {
+ let ba = IIRState::lowpass(1e-3, 1. / 2f64.sqrt(), 2.);
+ println!("{:?}", ba.0);
+ }
+}
