Data Transfer - minimodem

Overview

This guide provides complete, practical examples of transferring various types of data using minimodem. Whether you’re sending text over a radio link, transferring files via telephone, or creating an acoustic coupler, these workflows demonstrate real-world data transfer scenarios.

Text Transfer

Simple Text Message

The most basic use case - sending and receiving text messages:

Transmit
Receive
Testing

# Send a simple message
echo "Hello World" | minimodem --tx 1200

# Send multi-line text
cat message.txt | minimodem --tx 1200

# Interactive typing
minimodem --tx 1200
# Type your message and press Ctrl+D when done

# Receive and display
minimodem --rx 1200

# Receive and save to file
minimodem --rx 1200 > received.txt

# Receive quietly (no status messages)
minimodem --rx 1200 --quiet 2>/dev/null > received.txt

# Create audio file
echo "Test Message" | minimodem --tx 1200 -f test.wav

# Decode audio file
minimodem --rx 1200 -f test.wav

# Verify output
minimodem --rx 1200 -f test.wav --quiet 2>/dev/null
# Should output: Test Message

Reliable Text Transfer

Add error detection and correction:

Prepare message with checksum

# Create message with MD5 checksum
MESSAGE="Important data here"
CHECKSUM=$(echo -n "$MESSAGE" | md5sum | cut -d' ' -f1)
echo "${MESSAGE}|${CHECKSUM}"

Transmit with verification

# Send message with checksum
echo "${MESSAGE}|${CHECKSUM}" | minimodem --tx 1200

Receive and verify

# Receive message
RECEIVED=$(minimodem --rx 1200 --quiet 2>/dev/null)

# Split message and checksum
DATA=$(echo "$RECEIVED" | cut -d'|' -f1)
RECV_CHECKSUM=$(echo "$RECEIVED" | cut -d'|' -f2)

# Verify
CALC_CHECKSUM=$(echo -n "$DATA" | md5sum | cut -d' ' -f1)
if [ "$RECV_CHECKSUM" = "$CALC_CHECKSUM" ]; then
    echo "Verified: $DATA"
else
    echo "ERROR: Checksum mismatch"
fi

File Transfer

Binary File Transfer

Transfer any file using Base64 encoding:

Transmit File
Receive File
Verify Transfer

# Encode and transmit
base64 document.pdf | minimodem --tx 1200

# With progress indication (requires pv)
cat document.pdf | base64 | pv | minimodem --tx 1200

# Save transmission to audio file
base64 document.pdf | minimodem --tx 1200 -f transfer.wav

# Receive and decode
minimodem --rx 1200 --quiet 2>/dev/null | base64 -d > received.pdf

# From audio file
minimodem --rx 1200 -f transfer.wav --quiet 2>/dev/null | \
    base64 -d > received.pdf

# Compare checksums
md5sum document.pdf
md5sum received.pdf

# Or use sha256
sha256sum document.pdf received.pdf

# Verify files are identical
if cmp -s document.pdf received.pdf; then
    echo "Transfer successful!"
else
    echo "Transfer failed - files differ"
fi

Compressed File Transfer

Reduce transfer time with compression:

# Transmit compressed
gzip -c largefile.txt | base64 | minimodem --tx 1200

# Receive and decompress
minimodem --rx 1200 --quiet 2>/dev/null | base64 -d | gunzip > largefile.txt

# Calculate transfer time
FILE_SIZE=$(wc -c < largefile.txt)
BAUD_RATE=1200
TRANSFER_TIME=$((FILE_SIZE * 10 / BAUD_RATE))  # 10 bits per byte
echo "Estimated transfer time: ${TRANSFER_TIME} seconds"

Chunked Transfer

Transfer large files in chunks with verification:

#!/bin/bash
# chunked-transfer.sh - Transfer file in verified chunks

FILE="$1"
CHUNK_SIZE=1024  # bytes
BAUD=1200

# Split file
split -b $CHUNK_SIZE "$FILE" chunk_

# Transmit each chunk
for chunk in chunk_*; do
    echo "Sending $chunk..."
    (
        echo "CHUNK:$(basename $chunk)"
        cat "$chunk" | base64
        echo "MD5:$(md5sum $chunk | cut -d' ' -f1)"
        echo "END"
    ) | minimodem --tx $BAUD
    
    # Wait for acknowledgment (implement as needed)
    sleep 1
done

echo "Transfer complete"
rm chunk_*

Receiver script:

#!/bin/bash
# receive-chunks.sh - Receive and verify chunks

BAUD=1200
OUTPUT="received_file"

while true; do
    # Receive chunk
    DATA=$(minimodem --rx $BAUD --rx-one --quiet 2>/dev/null)
    
    # Extract chunk info
    CHUNK_NAME=$(echo "$DATA" | grep "^CHUNK:" | cut -d: -f2)
    CHUNK_DATA=$(echo "$DATA" | sed -n '/^CHUNK:/,/^MD5:/p' | grep -v "^CHUNK:" | grep -v "^MD5:")
    CHUNK_MD5=$(echo "$DATA" | grep "^MD5:" | cut -d: -f2)
    
    # Decode and verify
    echo "$CHUNK_DATA" | base64 -d > "$CHUNK_NAME"
    CALC_MD5=$(md5sum "$CHUNK_NAME" | cut -d' ' -f1)
    
    if [ "$CHUNK_MD5" = "$CALC_MD5" ]; then
        echo "$CHUNK_NAME verified OK"
        cat "$CHUNK_NAME" >> "$OUTPUT"
        rm "$CHUNK_NAME"
    else
        echo "$CHUNK_NAME FAILED verification"
        # Request retransmit
    fi
    
    # Check for end
    echo "$DATA" | grep -q "^END$" && break
done

echo "File received: $OUTPUT"

Two-Way Communication

Half-Duplex Communication

Simple back-and-forth communication:

Station A
Station B
Automated Script

# Send message
echo "This is Station A, over" | minimodem --tx 300

# Wait for reply
echo "Listening..."
minimodem --rx 300 --rx-one

# Receive message
echo "Listening..."
minimodem --rx 300 --rx-one

# Send reply
echo "Station A, this is Station B, over" | minimodem --tx 300

#!/bin/bash
# half-duplex-chat.sh

while true; do
    echo -n "You: "
    read message
    echo "$message" | minimodem --tx 300
    
    echo "Them: "
    minimodem --rx 300 --rx-one
done

Full-Duplex Simulation

Simulate full-duplex using different frequencies:

# Terminal 1: Transmit on 1200 Hz, receive on 2200 Hz
(
    minimodem --rx 300 --mark 2270 --space 2070 &
    minimodem --tx 300 --mark 1270 --space 1070
)

# Terminal 2: Transmit on 2200 Hz, receive on 1200 Hz  
(
    minimodem --rx 300 --mark 1270 --space 1070 &
    minimodem --tx 300 --mark 2270 --space 2070
)

Radio Communication

HF Radio Transfer

Optimized for shortwave radio conditions:

# Transmit at 300 baud (more reliable for HF)
echo "CQ CQ CQ DE W1ABC" | minimodem --tx 300

# Receive with auto-carrier (handles frequency drift)
minimodem --rx 300 --auto-carrier --confidence 1.0

# RTTY mode for amateur radio
echo "DE W1ABC" | minimodem --tx rtty
minimodem --rx rtty --auto-carrier

VHF/UHF Radio Transfer

Higher speed for VHF/UHF FM:

# Transmit at 1200 baud (APRS/Packet radio speeds)
echo "Data packet" | minimodem --tx 1200

# With continuous carrier (important for FM radio)
cat data.txt | minimodem --tx 1200 --tx-carrier

# Receive with tight bandwidth
minimodem --rx 1200 --bandwidth 200

Weather Radio Reception

Decode NOAA SAME alerts:

# Receive SAME headers
minimodem --rx same --auto-carrier > alerts.txt

# From recorded audio
minimodem --rx same -f weather-recording.wav > decoded-alerts.txt

# Parse SAME message
# Format: ZCZC-ORG-EEE-PSSCCC+TTTT-JJJHHMM-LLLLLLLL-

Telephone/Modem Transfer

Classic Modem Emulation

Emulate a classic dial-up modem:

# Bell 103 (300 baud) - Original modem standard
echo "Data" | minimodem --tx 300
minimodem --rx 300

# Bell 202 (1200 baud) - Faster modem standard
echo "Data" | minimodem --tx 1200  
minimodem --rx 1200

# V.21 (300 baud) - International standard
echo "Data" | minimodem --tx 300 --mark 980 --space 1180
minimodem --rx 300 --mark 980 --space 1180

Caller ID Decoding

Decode telephone caller ID:

# Record caller ID from phone line
minimodem --rx callerid -f phone-recording.wav

# Real-time caller ID (if you have phone line connected)
minimodem --rx callerid

Acoustic Coupling

Speaker-to-Microphone Transfer

Transfer data acoustically between devices:

Position devices

Place speaker of transmitting device near microphone of receiving device

Calibrate volume

# Transmitter: Test tone
echo "TEST" | minimodem --tx 300 --volume 0.7

# Receiver: Monitor levels
minimodem --rx 300
# Adjust transmit volume until carrier is detected reliably

Transfer data

# Transmitter
cat data.txt | minimodem --tx 300 --volume 0.7

# Receiver
minimodem --rx 300 > received.txt

Acoustic Coupling Tips:

Use 300 baud for better reliability
Minimize background noise
Use medium volume (0.5-0.7) to avoid distortion
Keep devices stable and close together

Air-Gapped Transfer

Transfer data between isolated systems:

# Isolated system (transmit)
cat secret-data.txt | base64 | minimodem --tx 300 -f transfer.wav

# Physically move audio file (USB stick, etc.)

# Connected system (receive)
minimodem --rx 300 -f transfer.wav --quiet 2>/dev/null | base64 -d > secret-data.txt

Specialized Protocols

RTTY for Amateur Radio

# Transmit RTTY (45.45 baud, 1.5 stop bits)
echo "CQ CQ CQ DE W1ABC W1ABC K" | minimodem --tx rtty

# Receive RTTY with custom shift
minimodem --rx rtty --mark 2125 --space 2295

# TDD mode (45.45 baud, 2.0 stop bits)
echo "GA" | minimodem --tx tdd
minimodem --rx tdd

UIC Train Communication

Decode European train-to-ground communication:

# Train-to-ground decoder
minimodem --rx uic-train -f train-recording.wav

# Ground-to-train decoder  
minimodem --rx uic-ground -f ground-recording.wav

Custom Protocols

Create custom framing:

# Custom start/stop bits
echo "DATA" | minimodem --tx 1200 \
    --startbits 2 \
    --stopbits 2.0

minimodem --rx 1200 \
    --startbits 2 \
    --stopbits 2.0

# Custom frequencies
echo "DATA" | minimodem --tx 1200 \
    --mark 1000 \
    --space 2000

minimodem --rx 1200 \
    --mark 1000 \
    --space 2000

# Raw binary (no framing)
echo "DATA" | minimodem --tx 1200 \
    --startbits 0 \
    --stopbits 0 \
    --binary-raw 8

Performance Optimization

Speed vs Reliability

Maximum Reliability
Balanced
Maximum Speed

# 300 baud, narrow bandwidth, high threshold
echo "Critical data" | minimodem --tx 300
minimodem --rx 300 --bandwidth 50 --confidence 2.0

# Use for:
# - Noisy environments
# - Long-distance HF radio
# - Acoustic coupling
# - Low signal strength

# 1200 baud, standard settings
echo "Regular data" | minimodem --tx 1200
minimodem --rx 1200

# Use for:
# - General purpose
# - VHF/UHF FM radio
# - Good audio connections
# - Moderate distances

# Custom high baud rate
echo "Fast data" | minimodem --tx 4800
minimodem --rx 4800 --bandwidth 300 --confidence 1.2

# Use for:
# - Clean audio paths
# - Short distances
# - Wired connections
# - Large file transfers

Transfer Time Estimation

#!/bin/bash
# estimate-transfer-time.sh

FILE="$1"
BAUD="${2:-1200}"

# Calculate file size
FILE_SIZE=$(wc -c < "$FILE")

# Account for Base64 encoding (33% overhead)
ENCODED_SIZE=$((FILE_SIZE * 4 / 3))

# Account for framing (10 bits per 8 data bits)
TOTAL_BITS=$((ENCODED_SIZE * 10))

# Calculate transfer time
TRANSFER_SECONDS=$((TOTAL_BITS / BAUD))
MINUTES=$((TRANSFER_SECONDS / 60))
SECONDS=$((TRANSFER_SECONDS % 60))

echo "File: $FILE"
echo "Size: $FILE_SIZE bytes"
echo "Encoded size: $ENCODED_SIZE bytes"
echo "Baud rate: $BAUD"
echo "Estimated transfer time: ${MINUTES}m ${SECONDS}s"

Usage:

./estimate-transfer-time.sh document.pdf 1200
# File: document.pdf
# Size: 102400 bytes
# Encoded size: 136533 bytes
# Baud rate: 1200
# Estimated transfer time: 18m 58s

Error Handling

Retry on Failure

#!/bin/bash
# retry-transfer.sh - Retry transmission until successful

FILE="$1"
MAX_RETRIES=3
BAUD=1200

# Calculate checksum
CHECKSUM=$(md5sum "$FILE" | cut -d' ' -f1)

for ((i=1; i<=MAX_RETRIES; i++)); do
    echo "Attempt $i of $MAX_RETRIES..."
    
    # Transmit file with checksum
    (
        base64 "$FILE"
        echo "CHECKSUM:$CHECKSUM"
    ) | minimodem --tx $BAUD
    
    # Wait for acknowledgment
    echo "Waiting for acknowledgment..."
    ACK=$(timeout 30 minimodem --rx $BAUD --rx-one --quiet 2>/dev/null)
    
    if echo "$ACK" | grep -q "^OK:$CHECKSUM$"; then
        echo "Transfer successful!"
        exit 0
    else
        echo "Transfer failed, retrying..."
        sleep 2
    fi
done

echo "Transfer failed after $MAX_RETRIES attempts"
exit 1

Automatic Error Correction

# Use forward error correction (FEC) with par2
par2create -r10 file.bin

# Transmit file and parity
cat file.bin | base64 | minimodem --tx 1200
cat file.bin.par2 | base64 | minimodem --tx 1200

# Receive and repair
minimodem --rx 1200 --quiet 2>/dev/null | base64 -d > received.bin
minimodem --rx 1200 --quiet 2>/dev/null | base64 -d > received.bin.par2
par2repair received.bin

Monitoring and Debugging

Transfer Monitoring

# Monitor transfer with statistics
minimodem --rx 1200 | tee received.txt

# Log status messages
minimodem --rx 1200 > received.txt 2> transfer.log

# Parse logs for statistics
grep "NOCARRIER" transfer.log | awk '{print $4, $5, $6}'

Audio Debugging

# Record transmitted audio for analysis
echo "TEST" | minimodem --tx 1200 -f test.wav

# Analyze with spectral view
ffmpeg -i test.wav -lavfi showspectrumpic=s=1024x512 spectrum.png

# Or use sonic-visualizer, audacity, etc.
audacity test.wav

Get Started

Core Concepts

Usage Guides

Protocols

Advanced

​Overview

​Text Transfer

​Simple Text Message

​Reliable Text Transfer

​File Transfer

​Binary File Transfer

​Compressed File Transfer

​Chunked Transfer

​Two-Way Communication

​Half-Duplex Communication

​Full-Duplex Simulation

​Radio Communication

​HF Radio Transfer

​VHF/UHF Radio Transfer

​Weather Radio Reception

​Telephone/Modem Transfer

​Classic Modem Emulation

​Caller ID Decoding

​Acoustic Coupling

​Speaker-to-Microphone Transfer

​Air-Gapped Transfer

​Specialized Protocols

​RTTY for Amateur Radio

​UIC Train Communication

​Custom Protocols

​Performance Optimization

​Speed vs Reliability

​Transfer Time Estimation

​Error Handling

​Retry on Failure

​Automatic Error Correction

​Monitoring and Debugging

​Transfer Monitoring

​Audio Debugging

​See Also

Build docs developers (and LLMs) love

Overview

Text Transfer

Simple Text Message

Reliable Text Transfer

File Transfer

Binary File Transfer

Compressed File Transfer

Chunked Transfer

Two-Way Communication

Half-Duplex Communication

Full-Duplex Simulation

Radio Communication

HF Radio Transfer

VHF/UHF Radio Transfer

Weather Radio Reception

Telephone/Modem Transfer

Classic Modem Emulation

Caller ID Decoding

Acoustic Coupling

Speaker-to-Microphone Transfer

Air-Gapped Transfer

Specialized Protocols

RTTY for Amateur Radio

UIC Train Communication

Custom Protocols

Performance Optimization

Speed vs Reliability

Transfer Time Estimation

Error Handling

Retry on Failure

Automatic Error Correction

Monitoring and Debugging

Transfer Monitoring

Audio Debugging

See Also