SignalTools

A high-performance Swift library for digital signal processing (DSP) operations, built on top of Apple's Accelerate framework.

Features

Filtering

• IIR Filters: Cascading biquad infinite impulse response filters with support for:
  • Low-pass filters
  • High-pass filters
  • Custom filter parameters
• FIR Filters: Finite impulse response filters with:
  • Low-pass filter generation
  • Custom windowing functions (Hamming by default)
  • Zero-phase filtering with filtfilt
  • Support for both real and complex signals
• Filter Protocol: IIRFilter and FIRFilter conform to a shared protocol, Filter, allowing for abstraction.

Demodulation

• FM Demodulation: Multiple implementations for frequency modulation demodulation:
  • Fast vectorized implementation using vDSP
  • Conceptual slow implementation for educational purposes

Signal Processing Utilities

• Frequency Shifting:
  • Standard precision (Float) frequency shifting to baseband --> If you're having issues with this, try high precision.
  • High-precision (Double) frequency shifting for artifact-free processing
• Phase Operations:
  • Phase calculation and unwrapping
  • Radians to frequency conversion
• Sample/Time Conversion: Utilities for converting between sample indices and time

Array Extensions

• Float Array Extensions:
  • Statistical operations (mean, standard deviation)
  • Signal normalization
  • Optimized using vDSP functions
• Complex Signal Extensions:
  • Magnitude calculation for individual samples and arrays
  • Built on DSPComplex from Accelerate

Requirements

• Swift: 5.1 or later
• Platform: macOS 10.15 or later
• Dependencies: Apple Accelerate framework

Usage Examples

Basic Filtering

import SignalTools

// Create an IIR low-pass filter
let filter = IIRFilter()
    .addLowpassFilter(sampleRate: 44100, frequency: 1000.0, q: 0.707)

// Filter your signal
var signal: [Float] = // your audio data
filter.filteredSignal(&signal)

FM Demodulation

import SignalTools

// Demodulate FM signal
let complexSamples: [DSPComplex] = // your IQ data
let demodulated = demodulateFM(complexSamples)

Frequency Shifting

import SignalTools

var rawIQ: [DSPComplex] = // your IQ samples
var result = [DSPComplex](repeating: DSPComplex(real: 0, imag: 0), count: rawIQ.count)

shiftFrequencyToBaseband(
    rawIQ: rawIQ,
    result: &result,
    frequency: 1000.0,  // Hz
    sampleRate: 48000   // Hz
)

Signal Statistics

import SignalTools

let samples: [Float] = // your data
let mean = samples.average()
let stdDev = samples.standardDeviation()
let normalized = samples.normalize()

Architecture

The library is organized into several key components:

• Filter.swift: IIR and FIR filter implementations
• FM.swift: FM demodulation algorithms
• Utils.swift: Frequency shifting and phase utilities
• Extensions.swift: Convenience extensions for Float and DSPComplex arrays

Plans

'vDSP.' is only available in macOS 10.15 and above. At some point I'd like to replace this with C (vDSP_) functions to support older versions.