Operating Modes

minimodem operates in one of two modes: transmit (TX) or receive (RX). The mode determines whether minimodem encodes data into audio or decodes audio into data.

Mode Selection

--tx

flag

Transmit mode: encode data to audioAliases: -t, --transmit, --write

--rx

flag

Receive mode: decode audio to data (default)Aliases: -r, --receive, --read

You cannot specify both --tx and --rx in the same command. Attempting to do so will result in an error.

Transmit Mode (—tx)

In transmit mode, minimodem reads data from standard input and generates FSK audio output.

Basic Operation

Command Line
Pipe Data
From File

minimodem --tx 1200

Type your message and press Ctrl+D (EOF) to complete transmission.

echo "Hello World" | minimodem --tx 1200

Pipe text or binary data directly to minimodem.

cat document.txt | minimodem --tx 1200
minimodem --tx 1200 < input.dat

Transmit contents of a file.

Audio Output

By default, transmit mode outputs audio to your system’s default audio playback device:

# Outputs to speakers/headphones
echo "test" | minimodem --tx 1200

Transmission Behavior

Transmit mode includes several automatic behaviors:

Leader Tone

A brief leader tone (mark frequency) is transmitted before data to help receivers acquire carrier lock. This is 2 bits in length by default.Disabled if: --startbits=0 is specified

Trailer Tone

A trailer tone (mark frequency) is transmitted after data ends to ensure complete transmission. This is 2 bits in length by default.

Interactive Mode

When transmitting to audio devices (not files), minimodem operates in interactive mode:

Monitors stdin for input
Starts transmitting when data arrives
Automatically stops after a brief timeout when input pauses
Can use --tx-carrier to maintain continuous carrier

Sync Bytes

For protocols like SAME that require synchronization, minimodem automatically transmits sync byte preambles:

minimodem --tx --sync-byte=0xAB same

This transmits 16 sync bytes before actual data.

TX-Specific Options

--volume

float

default:"1.0"

Output amplitude (0.0 to 1.0, or ‘E’ for FLT_EPSILON)Flag: -v

minimodem --tx --volume 0.5 1200

--lut

integer

default:"4096"

Sine wave lookup table length for TX tone generationLarger values provide higher quality but use more memory.

minimodem --tx --lut=8192 1200

--tx-carrier

flag

Transmit continuous carrier toneMaintains carrier signal even when no data is being sent (outputs idle mark tone).

minimodem --tx --tx-carrier 1200

--print-eot

flag

Print ”### EOT” message when transmission endsUseful for debugging or monitoring transmission completion.

Transmit Examples

echo "Testing" | minimodem --tx 1200

Receive Mode (—rx)

In receive mode, minimodem captures audio input, demodulates FSK signals, and outputs decoded data to standard output.

Basic Operation

Live Audio
Audio File
Save to File

minimodem --rx 1200

Captures audio from your default microphone/line-in device and decodes in real-time.

minimodem --rx --file recording.wav 1200

Decodes FSK data from a pre-recorded audio file.

minimodem --rx 1200 > decoded.txt

Saves decoded output to a file.

Carrier Detection

Receive mode automatically detects carrier signals:

Scanning: Searches for FSK carrier in the audio input
Acquisition: When carrier is detected, displays ### CARRIER message
Decoding: Demodulates and outputs data while carrier is present
Loss: When signal is lost, displays ### NOCARRIER with statistics

### CARRIER 1200 @ 1200.0 Hz ###
Hello World
### NOCARRIER ndata=11 confidence=2.145 ampl=0.850 bps=1200.00 ###

Carrier detection messages are printed to stderr, while decoded data goes to stdout. This allows you to separate the data from status messages:

minimodem --rx 1200 > data.txt 2> status.log

Auto-Carrier Detection

--auto-carrier

flag

Automatically detect carrier frequencyFlag: -aInstead of using fixed mark/space frequencies, minimodem will scan the audio spectrum to find the carrier.

minimodem --rx --auto-carrier 1200

This is useful when:

The exact carrier frequency is unknown
The transmitter frequency may drift
Multiple signals may be present

Auto-carrier detection may not work well with all protocols. It is not recommended for callerid mode.

Signal Quality

Receive mode monitors signal quality through confidence values:

--confidence

float

default:"1.5"

Minimum confidence threshold for signal detectionFlag: -cLower values accept weaker/noisier signals but may increase errors. Higher values require cleaner signals.

minimodem --rx --confidence 2.0 1200

--limit

float

default:"2.3"

Maximum confidence search limitFlag: -lControls performance vs. quality tradeoff:

Lower values: faster performance, may miss optimal frame positions
Higher values: better quality for noisy signals, slower
INFINITY: search exhaustively for best frame

minimodem --rx --limit 3.0 1200

RX-Specific Options

--rx-one

flag

Exit after receiving one complete transmissionUseful for scripting or batch processing of audio files:

minimodem --rx --rx-one --file recording.wav 1200

--bandwidth

float

RX filter bandwidth in HzFlag: -bNarrows or widens the receive filter:

Narrower: better noise rejection, requires accurate tuning
Wider: more tolerant of frequency errors, more noise

minimodem --rx --bandwidth 100 1200

Receive Examples

minimodem --rx 1200

Common Workflows

Two-Way Communication

Computer A (Transmit)
Computer B (Receive)

# Terminal 1: Send data
cat message.txt | minimodem --tx 1200

# Terminal 2: Receive data
minimodem --rx 1200 > received.txt

Test Loop

Test minimodem by looping audio output back to input:

# Terminal 1
echo "test message" | minimodem --tx 1200

# Terminal 2 (with audio looped back)
minimodem --rx 1200

On Linux, you can use PulseAudio or ALSA loopback devices to route audio internally without physical cables.

File Processing

Batch process multiple audio files:

#!/bin/bash
for file in *.wav; do
    echo "Processing $file..."
    minimodem --rx --rx-one --file "$file" 1200 > "${file%.wav}.txt"
done

Mode Comparison

Feature	Transmit (—tx)	Receive (—rx)
Input	stdin (data)	Audio device or file
Output	Audio device or file	stdout (data)
Sample Format	S16 or Float	Float only (for FFT)
Interactive	Yes (with timing)	Continuous
Status Messages	EOT (optional)	CARRIER/NOCARRIER
Auto-frequency	No	Yes (with —auto-carrier)
Signal Quality	N/A	Confidence metrics

Command Reference

Mode Selection

Transmit Mode (—tx)

Basic Operation

Audio Output

Transmission Behavior

TX-Specific Options

Transmit Examples

Receive Mode (—rx)

Basic Operation

Carrier Detection

Auto-Carrier Detection

Signal Quality

RX-Specific Options

Receive Examples

Common Workflows

Two-Way Communication

Test Loop

File Processing

Mode Comparison

Build docs developers (and LLMs) love

Command Reference

​Mode Selection

​Transmit Mode (—tx)

​Basic Operation

​Audio Output

​Transmission Behavior

​TX-Specific Options

​Transmit Examples

​Receive Mode (—rx)

​Basic Operation

​Carrier Detection

​Auto-Carrier Detection

​Signal Quality

​RX-Specific Options

​Receive Examples

​Common Workflows

​Two-Way Communication

​Test Loop

​File Processing

​Mode Comparison

Build docs developers (and LLMs) love

Mode Selection

Transmit Mode (—tx)

Basic Operation

Audio Output

Transmission Behavior

TX-Specific Options

Transmit Examples

Receive Mode (—rx)

Basic Operation

Carrier Detection

Auto-Carrier Detection

Signal Quality

RX-Specific Options

Receive Examples

Common Workflows

Two-Way Communication

Test Loop

File Processing

Mode Comparison