210: iir_int: add optimized integer iir implementation r=jordens a=jordens



Co-authored-by: Robert Jördens <rj@quartiq.de>
This commit is contained in:
bors[bot] 2020-12-22 15:51:39 +00:00 committed by GitHub
commit 1bf71f335b
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2 changed files with 59 additions and 0 deletions

58
dsp/src/iir_int.rs Normal file
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@ -0,0 +1,58 @@
use serde::{Deserialize, Serialize};
pub type IIRState = [i32; 5];
fn macc(y0: i32, x: &[i32], a: &[i32], shift: u32) -> i32 {
// Rounding bias, half up
let y0 = ((y0 as i64) << shift) + (1 << (shift - 1));
let y = x
.iter()
.zip(a)
.map(|(x, a)| *x as i64 * *a as i64)
.fold(y0, |y, xa| y + xa);
(y >> shift) as i32
}
/// Integer biquad IIR
///
/// See `dsp::iir::IIR` for general implementation details.
/// Offset and limiting disabled to suit lowpass applications.
/// Coefficient scaling fixed and optimized.
#[derive(Copy, Clone, Deserialize, Serialize)]
pub struct IIR {
pub ba: IIRState,
// pub y_offset: i32,
// pub y_min: i32,
// pub y_max: i32,
}
impl IIR {
/// Coefficient fixed point: signed Q2.30.
/// Tailored to low-passes PI, II etc.
const SHIFT: u32 = 30;
/// Feed a new input value into the filter, update the filter state, and
/// return the new output. Only the state `xy` is modified.
///
/// # Arguments
/// * `xy` - Current filter state.
/// * `x0` - New input.
pub fn update(&self, xy: &mut IIRState, x0: i32) -> i32 {
let n = self.ba.len();
debug_assert!(xy.len() == n);
// `xy` contains x0 x1 y0 y1 y2
// Increment time x1 x2 y1 y2 y3
// Shift x1 x1 x2 y1 y2
// This unrolls better than xy.rotate_right(1)
xy.copy_within(0..n - 1, 1);
// Store x0 x0 x1 x2 y1 y2
xy[0] = x0;
// Compute y0 by multiply-accumulate
let y0 = macc(0, xy, &self.ba, IIR::SHIFT);
// Limit y0
// let y0 = y0.max(self.y_min).min(self.y_max);
// Store y0 x0 x1 y0 y1 y2
xy[n / 2] = y0;
y0
}
}

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@ -98,6 +98,7 @@ where
}
pub mod iir;
pub mod iir_int;
pub mod lockin;
pub mod pll;
pub mod trig;