Crate idsp

Embedded DSP algorithms

This crate contains some tuned DSP algorithms for general and especially embedded use. Many of the algorithms are implemented on integer (fixed point) datatypes.

One comprehensive user for these algorithms is Stabilizer.

Fixed point

Cosine/Sine

cossin() uses a small (128 element or 512 byte) LUT, smart octant (un)mapping, linear interpolation and comprehensive analysis of corner cases to achieve a very clean signal (4e-6 RMS error, 9e-6 max error, 108 dB SNR typ), low spurs, and no bias with about 40 cortex-m instruction per call. It computes both cosine and sine (i.e. the complex signal) at once given a phase input.

Two-argument arcus-tangens

atan2() returns a phase given a complex signal (a pair of in-phase/x/cosine and quadrature/y/sine). The RMS phase error is less than 5e-6 rad, max error is less than 1.2e-5 rad, i.e. 20.5 bit RMS, 19.1 bit max accuracy. The bias is minimal.

PLL, RPLL

PLL, RPLL: High accuracy, zero-assumption, fully robust, forward and reciprocal PLLs with dynamically adjustable time constant and arbitrary (in the Nyquist sampling sense) capture range, and noise shaping.

Unwrapper, Accu, saturating_scale()

Unwrapper, Accu, saturating_scale(): Tools to handle, track, and unwrap phase signals or generate them.

Float and Fixed point

IIR/Biquad

iir::Biquad are fixed point (i8, i16, i32, i64) and floating point (f32, f64) biquad IIR filters. Robust and clean clipping and offset (anti-windup, no derivative kick, dynamically adjustable gains and gain limits) suitable for PID controller applications. Three kinds of filter actions: Direct Form 1, Direct Form 2 Transposed, and Direct Form 1 with noise shaping supported. Coefficient sharing for multiple channels.

Comparison

This is a rough feature comparison of several available biquad crates, with no claim for completeness, accuracy, or even fairness. TL;DR: idsp is slower but offers more features.

Feature\Crate	biquad-rs	fixed-filters	idsp::iir
Floating point f32/f64	✅	❌	✅
Fixed point i32	❌	✅	✅
Parametric fixed point i32	❌	✅	❌
Fixed point i8/i16/i64/i128	❌	❌	✅
DF2T	✅	❌	✅
Limiting/Clamping	❌	✅	✅
Fixed point accumulator guard bits	❌	❌	✅
Summing junction offset	❌	❌	✅
Fixed point noise shaping	❌	❌	✅
Configuration/state decoupling/multi-channel	❌	❌	✅
f32 parameter audio filter builder	✅	✅	✅
f64 parameter audio filter builder	✅	❌	✅
Additional filters (I/HO)	❌	❌	✅
f32 PI builder	❌	✅	✅
f32/f64 PI²D² builder	❌	❌	✅
PI²D² builder limits	❌	❌	✅
Support for fixed point a1=-2	❌	❌	✅

Three crates have been compared when processing 4x1M samples (4 channels) with a biquad lowpass. Hardware was thumbv7em-none-eabihf, cortex-m7, code in ITCM, data in DTCM, caches enabled.

Crate	Type, features	Cycles per sample
biquad-rs	f32	11.4
idsp::iir	f32, limits, offset	15.5
fixed-filters	i32, limits	20.3
idsp::iir	i32, limits, offset	23.5
idsp::iir	i32, limits, offset, noise shaping	30.0

State variable filter

svf is a simple IIR state variable filter simultaneously providing highpass, lowpass, bandpass, and notch filtering of a signal.

Lowpass, Lockin

Lowpass, Lockin are fast, infinitely cascadable, first- and second-order lowpass and the corresponding integration into a lockin amplifier algorithm.

FIR filters

hbf::HbfDec, hbf::HbfInt, hbf::HbfDecCascade, hbf::HbfIntCascade: Fast f32 symmetric FIR filters, optimized half-band filters, half-band filter decimators and integators and cascades. These are used in stabilizer-stream for online PSD calculation on log frequency scale for arbitrarily large amounts of data.

Delta Sigma modulator/noise shaper

Dsm is a delta sigma modulator/noise shaper in MASH-(1)^K architecture.

Modules

• hbf
  Half-band filters and cascades
• iir
  IIR filters, coefficients and applications
• svf
  State variable filter

Structs

• Accu
  Wrapping Accumulator
• Cascade
  Combine two different filters in cascade
• Complex
  A complex number in Cartesian form.
• Dsm
  Delta-sigma modulator
• Lockin
  Lockin filter
• Lowpass
  Arbitrary order, high dynamic range, wide coefficient range, lowpass filter implementation. DC gain is 1.
• Nyquist
  Nyquist zero
• PLL
  Type-II, sampled phase, discrete time PLL
• RPLL
  Reciprocal PLL.
• Repeat
  Repeat another filter
• Unwrapper
  Overflow unwrapper.

Traits

• Coefficient
  Helper trait unifying fixed point and floating point coefficients/samples
• ComplexExt
  Complex extension trait offering DSP (fast, good accuracy) functionality.
• Filter
  Single inpout single output i32 filter
• MulScaled
  Full scale fixed point multiplication.

Functions

• atan2
  2-argument arctangent function.
• cossin
  Compute the cosine and sine of an angle. This is ported from the MiSoC cossin core. https://github.com/m-labs/misoc/blob/master/misoc/cores/cossin.py
• overflowing_sub
  Subtract y - x with signed overflow.
• saturating_scale
  Combine high and low i32 into a single downscaled i32, saturating monotonically.

Type Aliases

• Lowpass1
  First order lowpass
• Lowpass2
  Second order lowpass