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Overview

Bell 202 is a frequency-shift keying (FSK) modem standard operating at 1200 bits per second. Originally developed by AT&T Bell Labs, it became widely used for data communication over telephone lines and is still used today for applications like caller ID and packet radio.

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Technical Specifications

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Frequencies and Timing

Baud Rate: 1200 bps
Mark Frequency (1): 1200 Hz
Space Frequency (0): 2200 Hz
Frequency Shift: 1000 Hz
From minimodem.c:900-908, Bell 202 uses the following formula:
  • Mark frequency: baud_rate / 2 + 600 = 1200 Hz
  • Space frequency: mark + (baud_rate * 5/6) = 2200 Hz
  • Bandwidth: 200 Hz

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Framing Parameters

  • Data bits: 8 (default)
  • Start bits: 1 (standard)
  • Stop bits: 1 (default)
  • Parity: None (configurable)

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Common Applications

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Caller ID

Bell 202 is the physical layer for North American caller ID systems, transmitting at 1200 bps between the first and second ring.

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Packet Radio

Amateur radio APRS (Automatic Packet Reporting System) uses Bell 202 for 1200 baud VHF packet transmission.

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Industrial Control

Many legacy industrial systems and PLCs use Bell 202 for serial communication.

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Usage Examples

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Receiving Bell 202 Data

minimodem --rx 1200 -f audio.wav

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Transmitting Bell 202 Data

echo "Hello World" | minimodem --tx 1200

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Advanced Configuration

Custom Framing
# 7 data bits, even parity, 2 stop bits (7E2)
minimodem --rx 1200 --databits 7 --parity e --stopbits 2
Carrier Detection
# Auto-detect carrier with threshold
minimodem --rx 1200 --auto-carrier 2.5

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Technical Details

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Mark and Space

In FSK modulation:
  • Mark (1) is represented by 1200 Hz
  • Space (0) is represented by 2200 Hz
The lower frequency represents binary 1, and the higher frequency represents binary 0.

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Signal Bandwidth

Bell 202 uses a detection bandwidth of approximately 200 Hz, which provides good noise immunity while allowing accurate frequency discrimination.

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Compatibility

Bell 202 is compatible with:
  • Standard telephone lines (300-3400 Hz bandwidth)
  • Audio tape recording
  • VHF/UHF FM radio (with appropriate deviation)
  • Sound card input/output (typical 8kHz+ sample rate)

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Performance Characteristics

  • Typical Range: Works well over telephone lines and FM radio links
  • Noise Immunity: Good for SNR > 10 dB
  • Error Rate: Low with proper signal levels
  • Latency: Minimal (< 10ms processing delay)

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See Also

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