1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
//! Short-term synthesis filter.
use crate::fixed::{acc, hi, mul, sat, shift};
use crate::{LPC_ORDER as ORDER, SUBFRAME};
/// Longest block any caller filters in one go, which is one subframe.
const BLOCK: usize = SUBFRAME;
/// Accumulate the prediction from the ten outputs immediately before a sample.
fn prediction(a: &[i16; ORDER + 1], history: &[i16]) -> i64 {
let mut sum = 0i64;
for i in 1..=ORDER {
sum = acc(sum - mul(a[i], history[ORDER - i]));
}
sum
}
/// Run `1/A(z)` over `input`, writing `output` and carrying `mem` forward.
///
/// `a` is the direct-form predictor in Q12 (`a[0]` is unused here); the signal
/// is Q15, so the accumulated prediction is shifted left by three to line the
/// two formats up. The reference has two entry points into it — the same loop, called
/// with different lengths.
pub fn synthesis(a: &[i16; ORDER + 1], input: &[i16], output: &mut [i16], mem: &mut [i16; ORDER]) {
// A sliding window holding the ten previous outputs followed by the new
// ones, which is how the filter memory is addressed. No caller runs
// the filter over more than one subframe at a time.
debug_assert!(input.len() <= BLOCK);
let mut storage = [0i16; ORDER + BLOCK];
let window = &mut storage[..ORDER + input.len()];
window[..ORDER].copy_from_slice(mem);
for n in 0..input.len() {
let sum = prediction(a, &window[n..n + ORDER]);
let y = hi(sat(acc(shift(sum, 3)
+ ((input[n] as i64) << 16)
+ (1 << 15))));
window[ORDER + n] = y;
output[n] = y;
}
mem.copy_from_slice(&window[window.len() - ORDER..]);
}