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Overview

Transmit mode (--tx) converts data from standard input into FSK (Frequency-Shift Keying) audio tones. minimodem reads data byte-by-byte and modulates it into audio signals that can be played through speakers, written to audio files, or transmitted over radio.

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Basic Transmit Usage

1

Specify transmit mode

Use the --tx flag to enable transmit mode:
minimodem --tx 1200
2

Provide input data

Data is read from standard input. You can pipe data or type interactively:
echo "Hello World" | minimodem --tx 1200
3

Audio output

By default, audio is played through your system’s default audio device in real-time.

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Command-Line Options

The transmit mode is activated using any of these equivalent flags: 
minimodem --transmit 1200

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Interactive Transmission

When transmitting to a real-time audio device (not a file), minimodem operates in interactive mode: 
minimodem --tx 300
# Type your message and press Enter
# Each keystroke is transmitted in real-time

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Interactive Mode Features

Leader Tone

Transmits 2 bits of mark tone before data to establish carrier

Trailer Tone

Transmits 2 bits of mark tone after data to complete transmission

Idle Carrier

Emits mark tone during input pauses (1/25 second timeout)

Auto Flush

Adds 0.5 seconds of silence to flush audio buffers

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Transmit Workflow

Here’s how minimodem processes data in transmit mode (from src/minimodem.c:114-250):
1

Read input

Reads data byte-by-byte from stdin using read() system call
2

Encode data bits

Converts each byte to data bits using the configured encoder (ASCII, Baudot, etc.)
3

Add framing

Wraps data bits with start bits and stop bits according to the protocol
4

Generate tones

  • Mark frequency (logical 1): Higher frequency tone
  • Space frequency (logical 0): Lower frequency tone
  • Each bit is transmitted for exactly 1/baud_rate seconds
5

Output audio

Writes audio samples to the audio backend (speakers or file)

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Practical Examples

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Text Transmission

echo "HELLO" | minimodem --tx 1200
Transmits at 1200 baud using Bell 202 encoding:
  • Mark: 1200 Hz
  • Space: 2200 Hz
  • 8 data bits + 1 start + 1 stop

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Binary Data Transmission

Transmit binary files by piping them directly: 
# Transmit a small binary file
cat data.bin | minimodem --tx 1200

# Compress and transmit
gzip -c file.txt | minimodem --tx 1200

# Transmit with Base64 encoding for reliability
base64 file.bin | minimodem --tx 1200

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Custom Frequencies

Override the default mark and space frequencies: 
# Custom frequencies for 1200 baud
echo "TEST" | minimodem --tx 1200 \
  --mark 1200 \
  --space 2400

# Inverted frequencies (space > mark)
echo "TEST" | minimodem --tx 1200 --inverted

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Volume Control

Adjust transmission amplitude: 
# Half volume
echo "HELLO" | minimodem --tx 1200 --volume 0.5

# Quarter volume
echo "HELLO" | minimodem --tx 1200 -v 0.25

# Very quiet (useful for testing)
echo "HELLO" | minimodem --tx 1200 -v 0.1
Volume Range: The --volume parameter accepts values from 0.0 to 1.0, where 1.0 is full amplitude. Values greater than 1.0 may cause clipping and distortion.

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Advanced Transmit Options

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Custom Frame Format

Modify the data frame structure: 
# No start or stop bits (NOAA SAME format)
echo "TEST" | minimodem --tx 1200 \
  --startbits 0 \
  --stopbits 0

# Two stop bits (TDD/TTY format)
echo "TEST" | minimodem --tx tdd

# 1.5 stop bits (RTTY format)
echo "TEST" | minimodem --tx rtty

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Sync Bytes

Add synchronization bytes at the start of transmission: 
# NOAA SAME format with sync byte 0xAB
echo "ZCZC-WXR-RWT-012345+0015-0321234-STATION-" | \
  minimodem --tx same

# Custom sync byte (16 sync bytes transmitted)
echo "DATA" | minimodem --tx 1200 --sync-byte 0x55

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Continuous Carrier

Keep the carrier tone active even when idle: 
# Useful for radio transmission
minimodem --tx 300 --tx-carrier < input.txt
The --tx-carrier option prevents the carrier from dropping during input pauses, maintaining a constant presence for radio applications where rapid carrier on/off switching is undesirable.

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Sample Rate Control

Change the audio sample rate: 
# 48 kHz (default)
echo "TEST" | minimodem --tx 1200 --samplerate 48000

# 44.1 kHz (CD quality)
echo "TEST" | minimodem --tx 1200 -R 44100

# 8 kHz (narrow bandwidth)
echo "TEST" | minimodem --tx 1200 -R 8000

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Technical Details

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Tone Generation

minimodem uses a sine wave lookup table (LUT) for efficient tone generation (src/minimodem.c:979): 
// Initialize with 4096-entry sine table at full amplitude
simpleaudio_tone_init(tx_sin_table_len, tx_amplitude);
Performance: The default LUT size of 4096 entries provides excellent quality with minimal CPU usage. Larger tables use more memory but don’t significantly improve quality.

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Frame Transmission

Each data byte is transmitted as a frame with this structure:
1

Start bits

Transmitted as space frequency (or mark if --invert-start-stop)
  • Default: 1 start bit
  • Duration: 1/baud_rate seconds per bit
2

Data bits

Transmitted LSB-first (unless --msb-first specified)
  • 5 bits for Baudot
  • 7 or 8 bits for ASCII
  • Custom bit counts supported
3

Stop bits

Transmitted as mark frequency (or space if --invert-start-stop)
  • Default: 1 stop bit
  • Can be fractional (e.g., 1.5 or 2.0)

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Timing Accuracy

Bit timing is calculated as (src/minimodem.c:132): 
size_t bit_nsamples = sample_rate / data_rate + 0.5f;
For 1200 baud at 48000 Hz:
  • Samples per bit: 48000 / 1200 = 40 samples
  • Bit duration: 40 / 48000 = 0.833 ms

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Troubleshooting

  • Check volume: minimodem --tx 1200 --volume 1.0
  • Verify audio device: minimodem --tx 1200 --alsa=hw:0,0
  • Test with file output: minimodem --tx 1200 --file test.wav
  • Reduce volume: --volume 0.5
  • Check sample rate compatibility
  • Ensure CPU is not overloaded (shouldn’t be an issue with minimodem)
  • Verify stdin has data: echo "TEST" | minimodem --tx 1200
  • Check for EOT marker: minimodem --tx 1200 --print-eot
  • Enable quiet mode to reduce stderr noise: minimodem --tx 1200 --quiet
  • Explicitly set frequencies: --mark 1200 --space 2200
  • Verify baud mode: minimodem --tx 1200 not minimodem --tx 300
  • Check for frequency inversion: remove --inverted if present

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See Also

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