Baud Modes

The baudmode parameter specifies the data rate and protocol. minimodem supports several preset protocols and custom baud rates.

Syntax

minimodem [options] {baudmode}

The baudmode is always the last argument on the command line.

Standard Baud Rates

Bell 202 (1200 baud)

High-speed mode using Bell 202 standard.

1200

baudmode

Protocol: Bell 202
Data Rate: 1200 bps
Encoding: ASCII 8-N-1
Mark Frequency: 1200 Hz
Space Frequency: 2200 Hz
Bandwidth: 200 Hz

minimodem --tx 1200
minimodem --rx 1200

This is the most common mode for computer-to-computer communication over audio.

Bell 103 (300 baud)

Low-speed mode using Bell 103 standard.

300

baudmode

Protocol: Bell 103
Data Rate: 300 bps
Encoding: ASCII 8-N-1
Mark Frequency: 1270 Hz
Space Frequency: 1070 Hz
Bandwidth: 50 Hz

minimodem --tx 300
minimodem --rx 300

Bell 103 is more robust for noisy channels and longer distance transmission.

Custom Baud Rates

any_number_N

baudmode

Protocol: Bell-like
Data Rate: N bps
Encoding: ASCII 8-N-1 (default)You can specify any numeric baud rate, and minimodem will automatically configure appropriate frequencies:

minimodem --tx 600
minimodem --rx 2400
minimodem --tx 150

Frequency Assignment Rules

minimodem automatically assigns mark and space frequencies based on baud rate:

High Speed (≥ 400 baud)
Medium Speed (100-399 baud)
Low Speed (< 100 baud)

Bell 202 style frequencies

Mark frequency: (baud_rate / 2) + 600 Hz
Space frequency: mark - (baud_rate × 5/6) Hz
Bandwidth: 200 Hz

Example at 1200 baud:

Mark: 1200 Hz
Space: 2200 Hz
Shift: -1000 Hz

You can override automatic frequency assignment with --mark and --space options.

Baudot Modes

Baudot encoding uses 5-bit characters instead of ASCII.

RTTY

rtty

baudmode

Protocol: RTTY (Radioteletype)
Data Rate: 45.45 bps
Encoding: Baudot 5-N-1.5
Mark/Space: Auto-assigned (typically 1585/1415 Hz)
Stop Bits: 1.5Classic amateur radio teletype mode.

minimodem --tx rtty
minimodem --rx rtty

RTTY uses Baudot encoding, where letters and numbers share the same codes. Special shift characters switch between LTRS (letters) and FIGS (figures/numbers) mode.

TDD (TTY)

tdd

baudmode

Protocol: TDD/TTY (Telecommunications Device for the Deaf)
Data Rate: 45.45 bps
Encoding: Baudot 5-N-2
Mark Frequency: 1400 Hz
Space Frequency: 1800 Hz
Stop Bits: 2.0Telecommunications device for deaf/hearing-impaired users.

minimodem --tx tdd
minimodem --rx tdd

The TDD mode differs from RTTY in stop bits and specific frequencies.

Special Protocols

SAME (NOAA Weather Radio)

same

baudmode

Protocol: NOAA SAME (Specific Area Message Encoding)
Data Rate: 520.83 bps (520 + 5/6 bps exactly)
Encoding: ASCII 8-bit, no start/stop bits
Mark Frequency: 2083.33 Hz (2083 + 1/3)
Space Frequency: 1562.5 Hz
Sync Byte: 0xAB (automatically inserted)Used by NOAA Weather Radio for emergency alerts.

minimodem --rx same

SAME protocol is receive-only in minimodem.

SAME messages have a specific format:

ZCZC-WXR-012345-678901+0123-1234567-NNNN-

The protocol automatically:

Adds 16 sync bytes (0xAB) in transmit
Removes sync bytes in receive
Uses no start/stop bits
Sets bandwidth equal to data rate

Caller ID

callerid

baudmode

Protocol: Bell 202 Caller ID
Data Rate: 1200 bps
Encoding: Special Caller ID format
Frequencies: Bell 202 standardDecodes telephone Caller ID information.

minimodem --rx callerid

Caller ID mode is receive-only.

Caller ID uses a special frame format with checksums. The databits_decode_callerid decoder handles the protocol-specific details.

Using --auto-carrier with callerid is not recommended, as the timing of Caller ID signals is critical.

UIC-751-3 (European Train Control)

uic-train

baudmode

Protocol: UIC-751-3 Train-to-Ground
Data Rate: 600 bps
Encoding: 39 data bits, 8 start bits, 0 stop bits
Mark Frequency: 1300 Hz
Space Frequency: 1700 Hz
Frame Format: 11110010 + 39 data bitsEuropean train control system communication protocol (train side).

minimodem --rx uic-train

UIC modes are receive-only.

uic-ground

baudmode

Protocol: UIC-751-3 Ground-to-Train
Data Rate: 600 bps
Encoding: 39 data bits, 8 start bits, 0 stop bits
Mark Frequency: 1300 Hz
Space Frequency: 1700 Hz
Frame Format: 11110010 + 39 data bitsEuropean train control system communication protocol (ground side).

minimodem --rx uic-ground

Both UIC modes use the same frequencies and baud rate but different decoders for protocol-specific message formats. Reference: EU Railway Interoperability TSI Annex

V.21 (International Standard)

V.21

baudmode

Protocol: ITU-T V.21
Data Rate: 300 bps
Encoding: ASCII 8-N-1
Mark Frequency: 980 Hz
Space Frequency: 1180 HzInternational standard for 300 baud FSK modem.

minimodem --tx V.21
minimodem --rx V.21

Baudmode Comparison Table

Baudmode	Baud Rate	Encoding	Start	Stop	Use Case
1200	1200	ASCII 8-bit	1	1.0	Fast data transfer
300	300	ASCII 8-bit	1	1.0	Robust, long distance
rtty	45.45	Baudot 5-bit	1	1.5	Amateur radio
tdd	45.45	Baudot 5-bit	1	2.0	TTY/accessibility
same	520.83	ASCII 8-bit	0	0	Weather alerts
callerid	1200	Special	1	1.0	Phone caller ID
uic-train	600	39-bit	8	0	Train control
uic-ground	600	39-bit	8	0	Train control
V.21	300	ASCII 8-bit	1	1.0	International std

Choosing a Baud Mode

For Speed

Use 1200 baudBest for:

Short distances
Clean audio channels
Maximum throughput
Computer-to-computer

For Reliability

Use 300 baudBest for:

Noisy channels
Long distances
Radio transmission
Difficult conditions

For Amateur Radio

Use rttyBest for:

Ham radio operations
HF/shortwave bands
International contacts
Traditional RTTY

For Compatibility

Use standard modesBest for:

Interoperating with existing equipment
Standard protocols (SAME, Caller ID, etc.)
Regulatory compliance

Modifying Baud Modes

You can customize any baudmode with additional options:

# Change frequencies
minimodem --rx -M 2200 -S 1200 1200

# Adjust frame format
minimodem --rx --startbits=2 --stopbits=1.5 300

# Change encoding
minimodem --tx --ascii 45.45      # ASCII instead of Baudot
minimodem --tx --baudot rtty      # Explicit Baudot
minimodem --tx -5 45.45           # 5-bit mode

# Invert frequencies
minimodem --rx --inverted 1200

See the Options Reference for complete details.

Examples by Use Case

# Fast, clean connection
minimodem --tx 1200
minimodem --rx 1200

Command Reference

Syntax

Standard Baud Rates

Bell 202 (1200 baud)

Bell 103 (300 baud)

Custom Baud Rates

Frequency Assignment Rules

Baudot Modes

RTTY

TDD (TTY)

Special Protocols

SAME (NOAA Weather Radio)

Caller ID

UIC-751-3 (European Train Control)

V.21 (International Standard)

Baudmode Comparison Table

Choosing a Baud Mode

For Speed

For Reliability

For Amateur Radio

For Compatibility

Modifying Baud Modes

Examples by Use Case

Build docs developers (and LLMs) love

Command Reference

​Syntax

​Standard Baud Rates

​Bell 202 (1200 baud)

​Bell 103 (300 baud)

​Custom Baud Rates

​Frequency Assignment Rules

​Baudot Modes

​RTTY

​TDD (TTY)

​Special Protocols

​SAME (NOAA Weather Radio)

​Caller ID

​UIC-751-3 (European Train Control)

​V.21 (International Standard)

​Baudmode Comparison Table

​Choosing a Baud Mode

For Speed

For Reliability

For Amateur Radio

For Compatibility

​Modifying Baud Modes

​Examples by Use Case

Build docs developers (and LLMs) love

Syntax

Standard Baud Rates

Bell 202 (1200 baud)

Bell 103 (300 baud)

Custom Baud Rates

Frequency Assignment Rules

Baudot Modes

RTTY

TDD (TTY)

Special Protocols

SAME (NOAA Weather Radio)

Caller ID

UIC-751-3 (European Train Control)

V.21 (International Standard)

Baudmode Comparison Table

Choosing a Baud Mode

Modifying Baud Modes

Examples by Use Case