OpenFreqAudio

Physics-based radio communication simulation for flight simulators, primarily targeting Falcon BMS. Models radio propagation over terrain and AM radio demodulation.

Features

RF Propagation Physics

Radio frequency propagation modeling based on ITU-R standards:

• Knife-edge diffraction (ITU-R P.526) for terrain obstruction
• Two-ray ground reflection model with curved-Earth geometry
  • Sea surface reflections with Debye-Kirchhoff roughness
• Fresnel zone clearance for path obstruction
• Atmospheric refraction using altitude-dependent refractivity (SAND2012-10690)
• Doppler shift from transmitter and receiver velocities
• Multipath fading: fast flutter (dropout rate) and slow deep fades
• Weather attenuation (0.02 dB/km)

Audio Processing

Baseband simulation of AM demodulation, with realistic:

• Radio bandwidth and filtering
• Automatic gain control and squelch based on received signal power
• Interference from multiple simultaneous transmitters, with "stepped-on" tones from mistuned or Doppler-shifted carriers

Demo: https://youtu.be/Sxe9OoFJ44g

Background Noise

Frequency-appropriate ambient noise based on radio type:

Radio Type-Specific Noise

• VHF AM: Crackling static with atmospheric pops
  • Pink noise base with crackles (10-20 events/second)
  • Exponentially decaying envelopes (1-4ms)
• UHF AM: Lighter crackling, reduced atmospheric interference

Sender-Specific Ambient

• Cockpit: Engine rumble, avionics hum, airflow
• Ground vehicle: Engine and movement sounds
• Stationary station: Clean radio room ambiance

Architecture

Projects

OpenFreqAudio

Core audio engine library providing:

• RF propagation simulation (FastPathAudioSim)
• Audio effects processing (RadioEffect)
• Real-time mixing and playback (RadioPlayback, RadioMixer)
• Background noise generation (BackgroundNoiseGenerator)
• DEM (Digital Elevation Model) terrain integration

BMSAudioSim

Demo application for testing and visualization:

• Avalonia UI cross-platform interface
• 2 transmitters + 1 receiver simulation
• BMS heightmap integration for terrain-based propagation
• 3D position simulation with velocity controls
• Heterodyne testing: adjustable PPM offset

Allows testing of Doppler effects, terrain fading, multipath interference, and distance-dependent propagation.

Technology Stack

• .NET 10.0
• BASS Audio Library (ManagedBass, ManagedBass.Mix)
• Avalonia 11.3

Building

Prerequisites

• .NET 10.0 SDK
• BASS v2.4 audio library (native DLLs/SOs included in Assets/)

Windows

dotnet build OpenFreqAudio.sln -c Release

Linux

dotnet build OpenFreqAudio.sln -c Release

Native BASS libraries are automatically copied from Assets/windows/ or Assets/linux/ depending on the target platform.

Running BMSAudioSim

# Windows
.\BMSAudioSim\bin\Release\net10.0\win-x64\BMSAudioSim.exe

# Linux
./BMSAudioSim/bin/Release/net10.0/linux-x64/BMSAudioSim

Demo Features

• Load BMS theater heightmap (.DEM format)
• Position two transmitters and one receiver
• Adjust frequency, power, and PPM offsets
• Test heterodyne effects with same-frequency transmissions
• Visualize terrain profile along propagation path

Physics References

• ITU-R P.526: Propagation by diffraction
• SAND2012-10690: Radar horizon and target visibility calculations (atmospheric refraction)
• Debye-Kirchhoff roughness model: Sea surface reflection attenuation
• Two-ray propagation model: Ground reflection interference
• Fresnel zones: Obstacle clearance and diffraction analysis

License

Mozilla Public License Version 2.0 (MPLv2)