pounder_test/dsp/src/lowpass.rs

40 lines
1.1 KiB
Rust

/// Arbitrary order, high dynamic range, wide coefficient range,
/// lowpass filter implementation. DC gain is 1.
///
/// Type argument N is the filter order.
#[derive(Clone)]
pub struct Lowpass<const N: usize> {
// IIR state storage
y: [i32; N],
}
impl<const N: usize> Default for Lowpass<N> {
fn default() -> Self {
Lowpass { y: [0i32; N] }
}
}
impl<const N: usize> Lowpass<N> {
/// Update the filter with a new sample.
///
/// # Args
/// * `x`: Input data. Needs 1 bit headroom but will saturate cleanly beyond that.
/// * `k`: Log2 time constant, 1..=31.
///
/// # Return
/// Filtered output y.
pub fn update(&mut self, x: i32, k: u8) -> i32 {
debug_assert!(k & 31 == k);
// This is an unrolled and optimized first-order IIR loop
// that works for all possible time constants.
// Note T-DF-I and the zeros at Nyquist.
let mut x = x;
for y in self.y.iter_mut() {
let dy = x.saturating_sub(*y) >> k;
*y += dy;
x = *y - (dy >> 1);
}
x.saturating_add((self.y.len() as i32) << (k - 1).max(0))
}
}