Overview
TDD (Telecommunications Device for the Deaf), also known as TTY (TeleTYpewriter), is a telecommunications protocol specifically designed for deaf and hard-of-hearing users to communicate over telephone lines. Like RTTY, it uses 5-bit Baudot encoding at 45.45 baud but with different audio frequencies optimized for telephone circuits.Technical Specifications
Frequencies and Timing
Baud Rate: 45.45 baud
Mark Frequency (1): 1400 Hz
Space Frequency (0): 1800 Hz
Data Bits: 5 (Baudot code)
Stop Bits: 2.0
Frequency Shift: 400 Hz
Mark Frequency (1): 1400 Hz
Space Frequency (0): 1800 Hz
Data Bits: 5 (Baudot code)
Stop Bits: 2.0
Frequency Shift: 400 Hz
- Data rate: 45.45 baud (identical to RTTY)
- Character encoding: 5-bit Baudot (same as RTTY)
- Stop bits: 2.0 (more than RTTY’s 1.5)
- Mark frequency: 1400 Hz
- Space frequency: 1800 Hz
Framing Parameters
- Start bit: 1 (space condition)
- Data bits: 5 (LSB first, Baudot encoding)
- Stop bits: 2.0 (mark condition)
- Total character time: ~154 ms per character
Key Differences from RTTY
While TDD and RTTY both use 45.45 baud Baudot encoding, they differ in:
| Parameter | RTTY | TDD |
|---|---|---|
| Mark frequency | 1585 Hz (default) | 1400 Hz |
| Space frequency | 1415 Hz (default) | 1800 Hz |
| Frequency shift | 170 Hz (standard) | 400 Hz |
| Stop bits | 1.5 | 2.0 |
| Primary use | Radio/maritime | Telephone |
Common Applications
Telephone Communication
TDD devices connect directly to telephone lines, allowing deaf and hard-of-hearing users to:- Make and receive phone calls
- Access emergency services (911)
- Communicate with hearing users via relay services
Relay Services
Telecommunications relay services (TRS) connect TDD users with voice telephone users through an operator who reads typed messages and types responses.Emergency Access
- 711 nationwide relay access
- Direct 911 TDD/TTY access
- Emergency alert systems
Legacy Systems
Many older telephone systems still support TDD:- Hotel room phones
- Public phones
- Business phone systems
Usage Examples
Receiving TDD Signals
Transmitting TDD Signals
Important: minimodem supports TDD reception (
--rx tdd) but does not support transmission mode for TDD. From minimodem.c:850-853, TDD --tx mode is not implemented.Advanced Reception
With Carrier Detection
With Custom Stop Bits
Quiet Mode (Less Output)
TDD Communication Conventions
Special Abbreviations
TDD users employ special abbreviations to facilitate communication:- GA (Go Ahead) - Your turn to type
- SK (Stop Keying) - End of conversation
- Q - Question mark substitute
- GA to SK - Ready to end conversation
- HD (Hold) - Please wait
- NBR (Number) - The following is a number
- PLS (Please) - Common courtesy
- THX (Thanks) - Common courtesy
Conversation Etiquette
Typical TDD conversation flow:
- Caller types: “[name] HERE GA”
- Receiver responds: “HI [name] GA”
- Conversation continues with “GA” after each turn
- End with: “GA TO SK” then “SK SK”
Technical Details
Frequency Choice
The 1400/1800 Hz frequency pair was chosen because:- Falls within telephone passband (300-3400 Hz)
- Avoids common noise frequencies
- 400 Hz shift provides good discrimination
- Well-separated from voice frequencies
Stop Bit Duration
TDD uses 2.0 stop bits (vs RTTY’s 1.5) to:- Provide extra synchronization time
- Accommodate older electromechanical devices
- Improve reliability on noisy phone lines
- Allow for timing variations in different devices
Baudot Character Set
TDD uses the same 5-bit Baudot (ITA-2) encoding as RTTY: Letters Mode: A-Z (uppercase only)Figures Mode: 0-9 and limited punctuation Shift Characters:
- LTRS (0x1F): Switch to letters mode
- FIGS (0x1B): Switch to figures mode
Signal Requirements
Audio Quality
- Frequency response: 1000-2500 Hz minimum
- SNR: > 10 dB recommended for telephone use
- Sampling rate: 8 kHz (telephone quality) sufficient
- Levels: -10 dBm typical on telephone circuits
Timing Tolerance
- Baud rate tolerance: ±1% maximum
- Stop bit timing: Must be at least 1.5 bit periods
- Frequency accuracy: ±20 Hz acceptable
Hardware Compatibility
TDD Devices
- Ultratec Superprint series
- Ameriphone TTY devices
- CapTel captioned telephones
- Modern smartphones with TTY adapters
Phone Line Interface
Connecting to phone lines requires:
- Audio coupling (direct or acoustic)
- Proper impedance matching (600Ω for telephone)
- Ring detection for incoming calls
- DTMF tone avoidance
Advantages of TDD
Accessibility
- Enables phone access for deaf/hard-of-hearing
- Works on standard telephone infrastructure
- No special carrier required
- Wide device compatibility
Reliability
- Simple, robust protocol
- Narrow bandwidth
- Works on poor quality lines
- Error-tolerant
Legal Requirements
In the United States:- ADA mandates TDD access for public services
- FCC requires relay services
- Emergency services must accept TDD calls
Modern Alternatives
Real-Time Text (RTT)
- Successor to TDD on modern networks
- Character-by-character transmission
- Better integration with VoIP and wireless
Video Relay Service (VRS)
- Sign language interpretation via video
- Higher bandwidth but more natural for native signers
Text Messaging (SMS/IM)
- Asynchronous alternative
- More convenient for short messages
- Lacks real-time nature of TDD
Troubleshooting
No Signal Detected
- Verify 1400/1800 Hz frequencies
- Check audio input levels
- Ensure phone line connection
- Test with known TDD device
Garbled Characters
- Check audio quality and noise
- Verify baud rate synchronization
- Adjust stop bit settings if needed
- Check for LTRS/FIGS mode errors
Cannot Transmit
- Remember: minimodem does not support TDD
--txmode - Use receive-only for monitoring
- Use RTTY mode with custom frequencies as workaround:
Comparison with Related Protocols
*RTTY frequencies are configurable; shown are minimodem defaults
Related Protocols
- RTTY - Radio teletype using similar encoding
- Bell 103 - Low-speed ASCII modem
- Caller ID - Another telephone signaling protocol