36 lines
1.0 KiB
Rust
36 lines
1.0 KiB
Rust
use generic_array::{ArrayLength, GenericArray};
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/// 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, Default)]
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pub struct Lowpass<N: ArrayLength<i32>> {
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// IIR state storage
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y: GenericArray<i32, N>,
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}
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impl<N: ArrayLength<i32>> 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 `k` bits headroom.
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/// * `k`: Log2 time constant, 0..31.
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///
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/// # Return
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/// Filtered output y, with gain of `1 << k`.
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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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// 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 DF-II and the zeros at Nyquist.
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let mut x = x << k;
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for y in self.y.iter_mut() {
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let dy = (x - *y + (1 << (k - 1))) >> k;
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*y += dy;
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x = *y - (dy >> 1);
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}
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x
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}
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}
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