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Minimodem is a versatile tool with applications ranging from data transfer to amateur radio, debugging, and emergency communications. This guide explores practical use cases and detailed scenarios.

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Data Transfer

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Local Computer-to-Computer Transfer

Audio Cable Data Transfer

Transfer data between nearby computers using a 3.5mm audio cable.Setup:
  • Connect audio output of Computer A to audio input of Computer B
  • Use a standard 3.5mm stereo cable
  • Adjust audio levels to avoid distortion
Computer A (Transmitter):
minimodem --tx 1200 < data.txt
Computer B (Receiver):
minimodem --rx 1200 > received.txt
Advantages:
  • No network infrastructure required
  • Works between different operating systems
  • Air-gapped security (for isolated systems)
  • High reliability with direct cable connection
Typical applications:
  • Transferring small configuration files
  • Debugging isolated systems
  • Security-critical environments requiring air-gap
  • Backup communication channel

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Acoustic Coupling (Air-Gap Transfer)

Wireless Audio Transfer

Transfer data through the air using speakers and microphones - no cables required!Setup:
  • Place computers close together (1-3 feet)
  • Ensure quiet environment
  • Use lower baud rates for reliability
# Transmit
echo "Secret message" | minimodem --tx 100

# Receive
minimodem --rx --confidence 1.0 100
Use cases:
  • Emergency communication when cables unavailable
  • Demonstrations and education
  • Temporary data sharing
  • IoT device configuration without network
Use 100 bps or lower for best results through air. Higher rates like 300 bps work in ideal conditions.

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File Transfer with Error Checking

# Transmitter (with MD5 hash)
cat document.txt | tee >(md5sum >&2) | minimodem --tx 1200

# Receiver (verify with MD5)
minimodem --rx 1200 | tee received.txt | md5sum
Compare the MD5 hashes to verify successful transfer.

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Amateur Radio Applications

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RTTY (Radioteletype)

Decoding Amateur RTTY

Monitor and decode RTTY transmissions on amateur radio bands.Typical frequencies:
  • 20m: 14.085 MHz
  • 40m: 7.040 MHz
  • 80m: 3.580 MHz
Setup:
  1. Connect radio audio output to computer audio input
  2. Tune to RTTY frequency
  3. Use USB or LSB mode as appropriate
Receiving:
minimodem --rx --auto-carrier rtty
Transmitting:
echo "CQ CQ CQ DE YOURCALL YOURCALL YOURCALL K" | minimodem --tx rtty
Tips:
  • Use --auto-carrier to automatically detect transmitter frequencies
  • RTTY uses 170 Hz shift (45.45 baud)
  • May need --inverted depending on sideband (USB/LSB)
  • Adjust --confidence 1.0 for weak signals
Ensure you have proper amateur radio licensing before transmitting.

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Packet Radio Integration

# Use minimodem as part of a packet radio setup
# with tools like direwolf or soundmodem

# Bell202 AFSK (common for APRS)
minimodem --rx 1200
Minimodem can be used as the modem component in larger packet radio systems.

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Weak Signal Experimentation

# Very low baud rate for noisy conditions
minimodem --tx 0.5

# Or receive weak signals
minimodem --rx --confidence 1.0 --limit 5.0 0.5
Lower baud rates (0.5-45 bps) work better under poor propagation conditions or high noise.

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Weather and Emergency Services

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NOAA Weather Radio (SAME Protocol)

Weather Alert Monitoring

Decode NOAA Specific Area Message Encoding (SAME) weather alerts.NOAA Weather Radio frequencies:
  • 162.400 MHz
  • 162.425 MHz
  • 162.450 MHz
  • 162.475 MHz
  • 162.500 MHz
  • 162.525 MHz
  • 162.550 MHz
Setup:
# Connect weather radio output to computer
minimodem --rx same
Message format: SAME messages begin with ZCZC preamble followed by:
  • Event code
  • Location codes
  • Valid time period
  • Timestamp
Example alert:
ZCZC-WXR-RWT-012345-567890+0030-1234567-NOCALL/NWS
Applications:
  • Automated weather alert logging
  • Emergency notification systems
  • Integration with home automation
  • Scientific weather monitoring
SAME uses 520.83 bps with special framing (0 start bits, 0 stop bits, sync byte 0xAB).

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Telecommunications

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Caller-ID Decoding

Telephone Caller-ID

Decode caller identification information from telephone lines.Setup:
  • Use phone line to audio adapter
  • Connect between phone line and computer audio input
  • Ensure proper isolation
Receiving Caller-ID:
minimodem --rx callerid
Output example:
MDMF
DATE: 1209
TIME: 1430
NMBR: 5551234567
NAME: John Smith
Applications:
  • Call logging systems
  • Automated call routing
  • Security systems
  • Vintage telephony restoration
Formats supported:
  • MDMF (Multiple Data Message Format) - includes name and number
  • SDMF (Single Data Message Format) - number only
Use proper phone line isolation equipment. Direct connection between phone lines and computer audio can damage equipment.

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TTY/TDD Communication

Telecommunications for the Deaf

Decode TTY/TDD (Telecommunications Device for the Deaf) transmissions.
minimodem --rx tdd
# or
minimodem --tx --baudot --stopbits 2.0 45.45
Characteristics:
  • 45.45 baud Baudot encoding
  • 2.0 stop bits
  • Compatible with TTY/TDD devices
Use cases:
  • Testing TTY equipment
  • Archival of TTY communications
  • Educational demonstrations
  • Accessibility research

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System Administration & Debugging

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Suspend/Resume Debugging

System State Monitoring

One of minimodem’s first practical applications was debugging suspend/resume issues.Concept:
  • Send continuous data during system suspend
  • Monitor for data corruption or timing issues
  • Detect when system actually enters/exits suspend
Setup:
# Transmitter (monitored system)
while true; do 
  echo "System active: $(date)"
  sleep 1
done | minimodem --tx 300

# Receiver (monitoring system)
minimodem --rx 300 | tee suspend-test.log
Analysis:
  • Gaps in received data indicate suspend periods
  • Corrupted data suggests timing issues
  • Can identify which components fail during suspend
This technique was used by Colin Ian King to debug Linux kernel suspend/resume issues. Read more

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Serial Port Alternative

# Replace broken serial connection with audio
# Useful when serial ports are unavailable or broken

# Terminal 1 (acts as serial tx)
minimodem --tx 300

# Terminal 2 (acts as serial rx)  
minimodem --rx 300
Applications:
  • Debugging embedded systems
  • Router console access (OpenWRT)
  • Vintage computer interfacing
  • IoT device recovery

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Educational and Experimental

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Teaching Signal Processing

Demonstrate Modulation

Visualize FSK modulation:
echo "HELLO" | minimodem --tx --file demo.wav 300
# Open demo.wav in Audacity or similar
Students can see:
  • Mark vs. space frequencies
  • Start and stop bits
  • Baudot vs. ASCII encoding
  • Effect of baud rate on bandwidth

Hands-on Communications

Build a communication system:
  • Connect two Raspberry Pis via audio
  • Implement custom protocols
  • Add error detection
  • Experiment with different modulation parameters
Great for:
  • University coursework
  • Ham radio clubs
  • Maker workshops
  • Science fairs

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Acoustic Data Transfer Experiments

Underwater Communication Research

Experimental acoustic data transmission:
# Ultra-low baud rate for challenging acoustic environments
minimodem --tx 0.5
Research areas:
  • Underwater communication
  • Through-wall data transfer
  • Covert acoustic channels
  • Speaker-to-microphone channels in various media
Parameters to experiment with:
  • Baud rate (0.5 to 12000+)
  • Frequency selection (--mark / --space)
  • Confidence thresholds
  • Bandwidth settings

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Protocol Reverse Engineering

# Capture and analyze unknown FSK protocols
minimodem --rx --auto-carrier --binary-raw 10 300
Use for:
  • Understanding proprietary protocols
  • Vintage computer data recovery
  • Industrial equipment interfacing
  • Security research

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Specialized Applications

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Train Communication (UIC-751-3)

Railway Communications

European train-to-ground communication protocol.Train to ground:
minimodem --rx uic-train
Ground to train:
minimodem --rx uic-ground
  • 600 bps data rate
  • Railway-specific protocol
  • Safety-critical communications

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Data Exfiltration Testing

Security Research

Testing acoustic covert channels:
# Simulated covert channel
echo "data" | minimodem --tx --volume 0.1 12000
Research areas:
  • Air-gap bridging detection
  • Ultrasonic data transmission
  • Covert channel analysis
  • Security policy testing
Only test on systems you own or have explicit permission to test. Covert channels may violate security policies.

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Vintage Computing

Retro Computer Interfaces

Load programs into vintage computers:
# Load BASIC program into vintage computer
minimodem --tx 300 < program.bas
Applications:
  • Cassette tape replacement
  • Data recovery from old media
  • Retro gaming and computing
  • Computer history preservation
Compatible with:
  • Kansas City Standard tapes
  • Early home computers
  • Vintage modems
  • Historical equipment

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Creative Applications

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Audio Steganography

# Hide data in audio files
echo "hidden message" | minimodem --tx --file music.wav --volume 0.05 12000

# Extract hidden data
minimodem --rx --file music.wav --confidence 1.0 12000

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Interactive Art Installations

  • Visitors send messages between exhibits via audio
  • Sound-based interaction without wireless infrastructure
  • Data transmission as part of artistic expression
  • Educational exhibits about communication technology

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IoT and Home Automation

# Configure IoT device without network
echo "config:wifi:ssid:password" | minimodem --tx 100

# Device listens for configuration
minimodem --rx --rx-one 100 > /tmp/config
Advantages:
  • No network required for initial setup
  • Works through speaker and microphone
  • Simple fallback configuration method
  • No Bluetooth or WiFi needed

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Performance Benchmarking

# Test different configurations
for rate in 100 300 1200 9600; do
  echo "Testing $rate bps"
  echo "benchmark" | minimodem --tx --file test.wav $rate
  time minimodem --rx --file test.wav $rate > /dev/null
done

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Best Practices by Use Case

  • Use lower baud rates (100-300 bps)
  • Physical cable connection
  • Add error detection at application layer
  • Increase confidence threshold
  • Use appropriate sample rate
  • High baud rates (1200-12000 bps)
  • Direct cable connection
  • High sample rate (48000+ Hz)
  • Clean signal path
  • Integer sample format for lower CPU
  • Lower baud rates (< 100 bps)
  • Reduce confidence threshold (1.0-1.5)
  • Increase search limit
  • Use auto-carrier detection
  • Consider frequency selection carefully
  • Lower confidence search limit
  • Reduce sample rate if possible
  • Use ALSA instead of PulseAudio
  • Disable benchmarks at compile time
  • Consider integer-only processing

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Getting Started

Pick a use case that interests you and start experimenting! Begin with:
1

Test locally

Use file-based audio to verify minimodem works:
echo "test" | minimodem --tx --file test.wav 1200
minimodem --rx --file test.wav 1200
2

Try acoustic coupling

Place two computers near each other:
minimodem --tx 100  # Computer 1
minimodem --rx 100  # Computer 2
3

Expand to your use case

Adapt the techniques to your specific application, adjusting parameters as needed.
Start with lower baud rates and increase once you have reliable communication.

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