Struct SymFir

pub struct SymFir<'a, T, const M: usize, const N: usize> { /* private fields */ }

Symmetric FIR filter prototype.

Generics

• M: number of taps, one-sided. The filter has effectively 2*M DSP taps
• N: state size: N = 2*M - 1 + {input/output}.len()

Half band decimation/interpolation filters

Half-band filters (rate change of 2) and cascades of HBFs are implemented in HbfDec and HbfInt etc. The half-band filter has unique properties that make it preferrable in many cases:

• only needs M multiplications (fused multiply accumulate) for 4*M taps
• HBF decimator stores less state than a generic FIR filter
• as a FIR filter has linear phase/flat group delay
• very small passband ripple and excellent stopband attenuation
• as a cascade of decimation/interpolation filters, the higher-rate filters need successively fewer taps, allowing the filtering to be dominated by only the highest rate filter with the fewest taps
• In a cascade of HBF the overall latency, group delay, and impulse response length are dominated by the lowest-rate filter which, due to its manageable transition band width (compared to single-stage filters) can be smaller, shorter, and faster.
• high dynamic range and inherent stability compared with an IIR filter
• can be combined with a CIC filter for non-power-of-two or even higher rate changes

The implementations here are all no_std and no-alloc. They support (but don’t require) in-place filtering to reduce memory usage. They unroll and optimmize extremely well targetting current architectures, e.g. requiring less than 4 instructions per input item for the full HbfDecCascade on Skylake. The filters are optimized for decent block sizes and perform best (i.e. with negligible overhead) for blocks of 32 high-rate items or more, depending very much on architecture.

Implementations

impl<'a, T: Copy + Zero + Add + Mul<Output = T> + Sum, const M: usize, const N: usize> SymFir<'a, T, M, N>

pub fn new(taps: &'a [T; M]) -> Self

Create a new SymFir.

Args

• taps: one-sided FIR coefficients, expluding center tap, oldest to one-before-center

pub fn buf_mut(&mut self) -> &mut [T]

Obtain a mutable reference to the input items buffer space.

pub fn get(&self) -> impl Iterator<Item = T> + '_

Perform the FIR convolution and yield results iteratively.

pub fn keep_state(&mut self, offset: usize)

Move items as new filter state.

Args

• offset: Keep the 2*M-1 items at offset as the new filter state.

Trait Implementations

impl<'a, T: Clone, const M: usize, const N: usize> Clone for SymFir<'a, T, M, N>

fn clone(&self) -> SymFir<'a, T, M, N>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'a, T: Debug, const M: usize, const N: usize> Debug for SymFir<'a, T, M, N>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'a, T: Copy, const M: usize, const N: usize> Copy for SymFir<'a, T, M, N>