Practical examples covering common minimodem use cases.
Basic Communication Simple Text Transfer Transfer text between two computers using audio:
echo "Hello from computer A" | minimodem --tx 1200
Output: ### CARRIER 1200 @ 1200.0 Hz ### Hello from computer A ### NOCARRIER ndata=22 confidence=2.145 ampl=0.850 bps=1200.00 ###
Interactive Chat Type messages in real-time:
# Start transmitter (type and press Enter for each message) minimodem --tx 1200 # On another computer/terminal minimodem --rx 1200
Press Ctrl+D to end transmission.
File Transfer Send a Text File # Transmit cat document.txt | minimodem --tx 1200 # Receive minimodem --rx 1200 > received_document.txt
Send Binary Data Transfer any file type:
cat image.jpg | minimodem --tx --binary-output 1200
For binary files, verify data integrity with checksums: md5sum original.jpg received.jpg
Large File with Compression Compress before transmission:
# Transmit gzip -c largefile.dat | minimodem --tx 1200 # Receive minimodem --rx 1200 | gunzip > largefile.dat
Audio File Processing Record and Decode Later Record audio first, decode later:
# Record transmission to file (using arecord, sox, or similar) arecord -f S16_LE -r 48000 recording.wav
minimodem --rx --file recording.wav 1200 > decoded.txt
Create Audio Files Generate audio files from data:
# Create WAV file with encoded message echo "This is a test" | minimodem --tx --file message.wav 1200 # Play it back later minimodem --rx --file message.wav 1200
Batch Process Multiple Files Decode multiple recordings:
#!/bin/bash for file in recordings/*.wav ; do echo "Processing $file ..." minimodem --rx --rx-one --file " $file " 1200 > "${ file % . wav }.txt" done
Radio Communication Amateur Radio RTTY Classic radioteletype:
echo "CQ CQ CQ DE W1ABC W1ABC K" | minimodem --tx rtty
HF Radio with Custom Frequencies Use specific mark/space tones for radio:
# USB mode, 1000/1200 Hz shift minimodem --tx -M 1000 -S 1200 300 # Receive with matching settings minimodem --rx -M 1000 -S 1200 300
NOAA Weather Radio Decode SAME alerts from weather radio:
# From radio connected to sound card minimodem --rx same # From recording minimodem --rx --file weather_alert.wav same
Example output:
ZCZC-WXR-012345-678901+0123-1234567-NNNN-
Telephone/Modem Emulation Caller ID Decoding Decode phone caller ID:
# Record caller ID signal from phone line # (requires appropriate audio interface) minimodem --rx callerid
Bell 103 Modem Emulate classic 300 baud modem:
# Transmit at 300 baud (Bell 103) cat data.txt | minimodem --tx 300 # Receive minimodem --rx 300 > received.txt
Advanced Configurations Custom Protocol Implement a custom FSK protocol:
# 7-bit data, 2 start bits, 1.5 stop bits, custom frequencies minimodem --tx -7 --startbits=2 --stopbits=1.5 \ -M 2200 -S 1200 600 # Matching receiver minimodem --rx -7 --startbits=2 --stopbits=1.5 \ -M 2200 -S 1200 600
High-Speed Transfer Push to higher baud rates:
# Try 2400 baud (requires good audio path) minimodem --tx 2400 minimodem --rx 2400 # Or even 4800 baud minimodem --tx 4800 minimodem --rx --bandwidth 300 4800
Low-Speed RTTY Variants Custom RTTY-like modes:
# 50 baud Baudot minimodem --tx --baudot 50 # 75 baud with custom shift minimodem --tx --baudot -M 1445 -S 1275 75
Testing and Debugging Loopback Test Test with audio loopback:
# Terminal 1 echo "test" | minimodem --tx 1200 # Terminal 2 (with audio looped back) minimodem --rx 1200
On Linux, use PulseAudio module-loopback: pactl load-module module-loopback latency_msec= 1
Frequency Verification Check actual frequencies in use:
# Generate test tone and inspect with spectrum analyzer minimodem --tx --tx-carrier 1200 # Use sox or other tool to visualize rec -r 48000 -c 1 test.wav & minimodem --tx --tx-carrier 1200 # ... stop after a few seconds sox test.wav -n spectrogram
Signal Quality Testing Test different confidence thresholds:
# Very sensitive (may have false detections) minimodem --rx --confidence 1.0 1200 # Normal minimodem --rx --confidence 1.5 1200 # Strict (clean signals only) minimodem --rx --confidence 3.0 1200
Noisy Channel Simulation Test robustness:
# Add background noise to recording sox recording.wav -p --norm | \ sox -m - noise.wav noisy_recording.wav # Try to decode minimodem --rx --file noisy_recording.wav \ --confidence 2.0 --limit 3.5 1200
Scripting Examples Automated Data Logger Continuously log received data:
#!/bin/bash LOGDIR = "logs" mkdir -p " $LOGDIR " while true ; do TIMESTAMP = $( date +%Y%m%d_%H%M%S ) LOGFILE = " $LOGDIR /data_ $TIMESTAMP .txt" echo "Listening... (log: $LOGFILE )" minimodem --rx --rx-one 1200 > " $LOGFILE " 2>&1 # Add small delay before next listen sleep 1 done
Error Correction Wrapper Add simple error detection:
#!/bin/bash # tx_with_checksum.sh data = " $1 " checksum = $( echo -n " $data " | md5sum | cut -d ' ' -f1 ) echo "${ data }|${ checksum }" | minimodem --tx 1200
#!/bin/bash # rx_with_verify.sh received = $( minimodem --rx --rx-one --quiet 1200 ) data = $( echo " $received " | cut -d '|' -f1 ) rcvd_sum = $( echo " $received " | cut -d '|' -f2 ) calc_sum = $( echo -n " $data " | md5sum | cut -d ' ' -f1 ) if [ " $rcvd_sum " = " $calc_sum " ]; then echo "OK: $data " else echo "ERROR: Checksum mismatch" exit 1 fi
Retry on Failure Automatically retry failed receptions:
#!/bin/bash MAX_RETRIES = 3 for i in $( seq 1 $MAX_RETRIES ); do echo "Attempt $i of $MAX_RETRIES ..." if timeout 30 minimodem --rx --rx-one 1200 > output.txt 2>&1 ; then if [ -s output.txt ]; then echo "Success!" exit 0 fi fi echo "Failed, retrying..." sleep 2 done echo "All attempts failed" exit 1
Integration Examples With GNU Radio Pipe to/from GNU Radio:
# Receive from GNU Radio flowgraph # (flowgraph outputs audio to stdout) ./gr_flowgraph.py | minimodem --rx --file /dev/stdin 1200
With netcat Network to audio bridge:
# Receive from network, transmit as audio nc -l 12345 | minimodem --tx 1200 # Receive audio, send to network minimodem --rx 1200 | nc remote_host 12345
With socat Serial port bridge:
# Serial to audio socat /dev/ttyUSB0,raw,echo=0,b9600 - | minimodem --tx 1200 # Audio to serial minimodem --rx 1200 | socat - /dev/ttyUSB0,raw,echo=0,b9600
Real-World Scenarios Emergency Communication Air-gapped data transfer:
# Computer A (no network, has speakers) cat emergency_message.txt | minimodem --tx 300 # Computer B (no network, has microphone) minimodem --rx 300 > received_message.txt
Use 300 baud for maximum reliability in difficult conditions.
Data Over Intercom Send data through building intercom system:
# Transmit (into intercom) echo "Door code: 1234" | minimodem --tx --volume 0.6 300 # Receive (from intercom speaker) minimodem --rx --confidence 2.0 300
Acoustic Data Transfer Air-gapped systems or underwater communication:
# High-volume transmission minimodem --tx --volume 1.0 300 # Sensitive reception minimodem --rx --confidence 1.8 --limit 3.0 300
Radio Beacon Continuous identification beacon:
#!/bin/bash while true ; do echo "BEACON ID:W1ABC GRID:FN42 $( date -u +%H%M)Z" | \ minimodem --tx rtty sleep 300 # Every 5 minutes done
Low-Latency Mode Minimize decode latency:
minimodem --rx --limit 1.5 --confidence 1.2 \ --samplerate 22050 1200
High-Quality Recording Maximum quality for file output:
cat data.txt | minimodem --tx --file output.flac \ --volume 0.8 --lut 8192 --float-samples \ --samplerate 96000 1200
Low-Resource System Minimize CPU/memory usage:
minimodem --rx --samplerate 22050 --limit 2.0 1200
Troubleshooting Examples Too Many Errors Increase robustness:
# Reduce baud rate minimodem --rx 300 # instead of 1200 # Adjust confidence minimodem --rx --confidence 2.5 --limit 3.5 1200 # Narrow bandwidth minimodem --rx --bandwidth 100 1200
No Carrier Detection Diagnose frequency issues:
# Try auto-detect minimodem --rx --auto-carrier 1200 # Check inverted minimodem --rx --inverted 1200 # Reduce confidence threshold minimodem --rx --confidence 1.0 1200
Audio Level Issues # Increase TX volume minimodem --tx --volume 1.0 1200 # Check audio levels arecord -D default -f S16_LE -r 48000 -c 1 test.wav aplay test.wav # Listen for proper level
See Also
Overview CLI basics and syntax
Modes Transmit and receive modes
Options Complete options reference