Baud Modes

Available baudmode keywords and protocols in minimodem

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)

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

Medium Speed (100-399 baud)

Bell 103 style frequencies

• Mark frequency: 1270 Hz
• Space frequency: 1070 Hz
• Shift: 200 Hz
• Bandwidth: 50 Hz

Low Speed (< 100 baud)

RTTY style frequencies

• Mark frequency: 1585 Hz (default)
• Space frequency: 1415 Hz
• Shift: -170 Hz
• Bandwidth: 10 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: 1.5

Classic 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: 2.0

Telecommunications 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 standard

Decodes 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 bits

European 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 bits

European 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 Hz

International 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 baud

Best for:

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

For Reliability

Use 300 baud

Best for:

• Noisy channels
• Long distances
• Radio transmission
• Difficult conditions

For Amateur Radio

Use rtty

Best for:

• Ham radio operations
• HF/shortwave bands
• International contacts
• Traditional RTTY

For Compatibility

Use standard modes

Best 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

Computer-to-Computer

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