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Overview

Receive mode (--rx) is the default operating mode of minimodem. It captures audio input (from a microphone, line-in, or audio file), analyzes the FSK (Frequency-Shift Keying) tones, and decodes them back into the original data.

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Basic Receive Usage

1

Start minimodem

Simply specify the baud rate (—rx is the default):
minimodem 1200
Or explicitly use the receive flag:
minimodem --rx 1200
2

Present audio signal

Play the FSK audio through your speakers or provide an audio file:
minimodem --rx 1200 --file recording.wav
3

View decoded output

Decoded data appears on stdout, status messages on stderr:
### CARRIER 1200 @ 1200.0 Hz ###
HELLO WORLD
### NOCARRIER ndata=11 confidence=2.456 ampl=0.892 bps=1199.98 (rate perfect) ###

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Command-Line Options

Receive mode is the default, but can be explicitly specified: 
minimodem 1200

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Carrier Detection

minimodem automatically detects when FSK carrier signals are present in the audio stream.

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Carrier Acquisition

When a valid signal is detected, minimodem displays (src/minimodem.c:1336-1348): 
### CARRIER 1200 @ 1200.0 Hz ###
This indicates:
  • Baud rate: 1200 bps
  • Mark frequency: 1200.0 Hz (the detected carrier frequency)

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Carrier Loss

When the signal ends or becomes too weak, minimodem reports (src/minimodem.c:1469-1473): 
### NOCARRIER ndata=45 confidence=2.234 ampl=0.756 bps=1199.92 (rate perfect) ###
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ndata
integer
Number of data frames successfully decoded
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confidence
float
Average signal-to-noise confidence level (higher is better)
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ampl
float
Average signal amplitude (0.0 to 1.0 range)
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bps
float
Actual measured data rate in bits per second
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rate status
string
  • (rate perfect): Transmitter clock matches exactly
  • (0.5% fast): Transmitter clock is slightly fast
  • (1.2% slow): Transmitter clock is slightly slow

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Auto-Carrier Detection

By default, minimodem uses fixed frequencies for the mark and space tones. Use --auto-carrier to automatically detect the carrier frequency: 
# Automatically find the carrier frequency
minimodem --rx 1200 --auto-carrier

# Short form
minimodem --rx 1200 -a
When to use auto-carrier: Useful when the transmitter uses non-standard frequencies or when the signal is off-frequency due to radio tuning, Doppler shift, or transmitter drift.
Auto-carrier detection increases CPU usage and may take longer to acquire the signal. It’s not recommended for callerid mode.

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Receive Processing Algorithm

minimodem uses a sophisticated FSK demodulation algorithm (src/minimodem.c:1014-1463):
1

Sample buffering

Reads audio samples into a circular buffer. Buffer size is automatically calculated based on frame length:
size_t samplebuf_size = ceilf(nsamples_per_bit) * (nbits+1) * 2;
2

FFT analysis

Uses Fast Fourier Transform to analyze frequency content and measure mark/space tone amplitudes at expected frequencies.
3

Frame detection

Scans for valid data frames by correlating the expected bit pattern (start bits, data bits, stop bits) with the received signal.
4

Confidence scoring

Each detected frame receives a confidence score based on signal-to-noise ratio:
  • Score > confidence_threshold (default 1.5): Valid frame
  • Score < confidence_threshold: Ignored as noise
5

Frame refinement

When carrier is first acquired or confidence drops, performs a fine-grained scan with 8 steps instead of 3 to find the optimal frame position.
6

Data extraction

Extracts data bits from the validated frame and passes them to the decoder (ASCII, Baudot, binary, etc.).

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Confidence Threshold

The confidence threshold controls the receive sensitivity and noise rejection: 
# Default threshold (1.5)
minimodem --rx 1200

# More sensitive (lower threshold) - accepts weaker signals
minimodem --rx 1200 --confidence 1.0

# Less sensitive (higher threshold) - rejects more noise
minimodem --rx 1200 -c 2.0

Low Threshold (< 1.5)

Pros: Decodes weaker signalsCons: More false decodes from noiseUse case: Marginal signals, distant stations

High Threshold (> 1.5)

Pros: Fewer false decodesCons: May miss valid weak signalsUse case: Strong signals, noisy environments

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Confidence Search Limit

Controls the performance vs. quality tradeoff: 
# Default limit (2.3)
minimodem --rx 1200

# Higher quality (slower, searches for best frame)
minimodem --rx 1200 --limit 5.0

# Higher performance (faster, accepts first good frame)
minimodem --rx 1200 -l 1.8
Performance Impact: The search limit dramatically affects CPU usage. Higher values search more thoroughly for the best frame position, improving quality on difficult signals at the cost of performance.

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Practical Examples

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Basic Reception

# Receive from microphone
minimodem 1200

# Save to file
minimodem 1200 > received.txt

# Quiet mode (no status messages)
minimodem 1200 --quiet > received.txt 2>/dev/null

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Binary Data Reception

Receive and decode binary data: 
# Receive binary data
minimodem --rx 1200 --binary-output > received.bin

# Receive Base64 encoded data and decode
minimodem 1200 | base64 -d > file.bin

# Receive and decompress
minimodem 1200 | gunzip > file.txt

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Custom Frequencies

# Specify exact mark and space frequencies
minimodem --rx 1200 --mark 1200 --space 2200

# Inverted frequencies (for LSB/USB mode switching)
minimodem --rx 1200 --inverted

# Custom frequencies for non-standard modems
minimodem --rx 1200 --mark 1000 --space 2000

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Bandwidth Control

Adjust the receive bandwidth for better performance: 
# Narrow bandwidth (better noise rejection)
minimodem --rx 1200 --bandwidth 100

# Wide bandwidth (better for frequency-unstable signals)
minimodem --rx 1200 -b 300

# Very narrow (RTTY default is 10 Hz)
minimodem rtty --bandwidth 10
Bandwidth Selection: Narrower bandwidths provide better noise rejection but require more stable transmitter frequency. Wider bandwidths are more forgiving of frequency drift but allow more noise.

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Advanced Receive Options

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Decode Only One Carrier

Exit after decoding a single carrier: 
# Receive one transmission and exit
minimodem --rx 1200 --rx-one
Useful in scripts where you want to capture a single transmission: 
#!/bin/bash
data=$(timeout 30 minimodem --rx 1200 --rx-one --quiet 2>/dev/null)
if [ $? -eq 0 ]; then
    echo "Received: $data"
fi

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Custom Data Formats

# 7-bit ASCII (rare)
minimodem --rx 1200 -7

# 5-bit Baudot
minimodem --rx 45.45 --baudot
minimodem --rx 45.45 -5

# Raw binary (custom bit count)
minimodem --rx 1200 --binary-raw 12

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Filter Non-Printable Characters

Replace non-printable characters with periods: 
minimodem --rx 1200 --print-filter

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Sync Byte Detection

For protocols that use sync bytes: 
# NOAA SAME format (weather radio)
minimodem --rx same

# Custom sync byte
minimodem --rx 1200 --sync-byte 0x55

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Sample Rate Configuration

Higher sample rates provide better frequency resolution: 
# Default 48 kHz
minimodem --rx 1200

# 96 kHz (better for weak signals)
minimodem --rx 1200 --samplerate 96000

# 8 kHz (minimum, faster processing)
minimodem --rx 1200 -R 8000
Minimum Sample Rate: The sample rate must be at least 2× the highest frequency component (Nyquist theorem). For Bell 202 (2200 Hz space tone), use at least 4400 Hz, but 48000 Hz is recommended.

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Technical Details

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Frame Scanning

minimodem scans the audio buffer looking for valid frames (src/minimodem.c:1236-1274): 
// Scan with 3 steps across the try_max_nsamples range
#define FSK_ANALYZE_NSTEPS 3
unsigned int try_step_nsamples = try_max_nsamples / FSK_ANALYZE_NSTEPS;

// Fine-grained refinement with 8 steps
#define FSK_ANALYZE_NSTEPS_FINE 8

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Amplitude Tracking

minimodem tracks signal amplitude with hysteresis (src/minimodem.c:1286-1288): 
// No-confidence if amplitude drops to < 25% of tracked amplitude
if ( amplitude < track_amplitude * 0.25f ) {
    confidence = 0;
}

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No-Confidence Counter

Carrier is declared lost after 20 consecutive frames with insufficient confidence (src/minimodem.c:1290-1313): 
#define FSK_MAX_NOCONFIDENCE_BITS 20

if ( confidence <= fsk_confidence_threshold ) {
    if ( ++noconfidence > FSK_MAX_NOCONFIDENCE_BITS ) {
        // Carrier lost
    }
}

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Output Format

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Standard Output (stdout)

Decoded data bytes are written to stdout: 
minimodem 1200 > output.txt

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Standard Error (stderr)

Status messages appear on stderr: 
# Capture only decoded data
minimodem 1200 2>/dev/null > data.txt

# Capture only status messages
minimodem 1200 > data.txt 2> status.log

# Suppress all status messages
minimodem 1200 --quiet > data.txt 2>/dev/null

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Troubleshooting

Symptoms: No ### CARRIER ### message appearsSolutions:
  • Increase volume: Check input mixer levels
  • Lower confidence threshold: --confidence 1.0
  • Enable auto-carrier: --auto-carrier
  • Check frequencies: --mark 1200 --space 2200
  • Verify audio input: Test with audio file first
Symptoms: Random characters or binary garbageSolutions:
  • Verify baud rate matches transmitter
  • Check for frequency inversion: try --inverted
  • Reduce noise: --confidence 2.0
  • Adjust bandwidth: --bandwidth 100
  • Check sample rate: Use 48000 Hz
Symptoms: Carrier detected but data drops outSolutions:
  • Lower confidence threshold: --confidence 1.2
  • Increase search limit: --limit 3.0
  • Check audio buffer: CPU may be overloaded
  • Reduce noise at source
  • Use narrower bandwidth
Symptoms: NOCARRIER shows “2% fast” or “slow”Solutions:
  • Small deviations (< 2%) are normal and handled automatically
  • Large deviations indicate clock mismatch between transmitter/receiver
  • Verify both use same sample rate
  • Check for audio device timing issues
Symptoms: Process uses excessive CPUSolutions:
  • Reduce search limit: --limit 2.0
  • Use lower sample rate: --samplerate 48000
  • Disable auto-carrier if enabled
  • Use faster baudrate if possible

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See Also

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