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