1. All Things Digital

2. Terms Simplex Telex Over Radio – SITOR Radio Teletype - RTTY
Narrow Band Direct Printing - NBDB
Frequency Shift Keying – FSK
Error Correction
SITOR A - Automatic Repeat Request (point to point)
SITOR B – Forward Error Correction (broadcast)

3. Terms
The Characters
SITOR sends 7-bit characters as a bit stream at 100 baud (which, in this case, is 100 bits per second, 10 milliseconds per bit, or 70 milliseconds per character).
The bitstream is FSK modulated with a 170 Hz frequency shift. The high frequency is a mark, and the low frequency is a
space.
Wikipedia.org

4. ASCII Code American Standard Code for Information Interchange
Wikipedia.org

5. The Code
RTTY uses ITA2 is a five bit code with 32 possible code points.
Four code points are used for null (BLANK), space (SPACE), carriage return (CR), and line feed (LF).
Two code points are used for a letter shift (LTRS) or a figure shift (FIGS).
The remaining 26 code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 52 (2*26) additional characters.
Wikipedia.org

6. The Code
CCIR Extends ITA2 from 5 to 7 bits, using the extra 2 bits as check digits.
Each 7-bit character in CCIR 476 has 4 marks (ones) and 3 spaces (zeros).
Each valid character code has a Hamming distance of at least 2 from every other character.
The number of valid characters in CCIR 476 is the number of ways to choose 4 marks for 7 bit positions, and the number can be calculated using the binomial coefficient:
CCIR 476 has 3 additional code points over ITA2. SITOR uses the additional code points for idle, phasing, and repeat requests. In addition, some of the ordinary characters are reused as control signals.
Wikipedia.org

7. Error Correction Error Correction
SITOR A - Automatic Repeat Request (point to point)
Transmission in synchronous frames of 450ms.
Three characters are transmitted by Information Sending Station (ISS).
Takes 210 milliseconds.
The ISS then waits 240 ms for a response.
Information Receiving Station (IRS) receives the three characters and checks that they each have 4 marks (1) and 3 spaces (0).
If they do, then the IRS transmits an acknowledgement. If they don't, then the IRS requests retransmission.
At the beginning of the next frame, the ISS either retransmits the last 3 characters or the next three characters.

8. Error Correction Error Correction SITOR B - Broadcast
Time Diversity Error Correction
Each character is sent twice DX and the RX.
The first transmission (DX) of a specific character is followed by the transmission of four other characters, after which the retransmission (RX) of the first character takes place, allowing for time-diversity reception of 280 ms.

9. Digital Selective Calling

10. Digital Selective Calling
The system is a synchronous system using characters composed from a ten-bit error-detecting code

11. Digital Selective Calling
Apart from the phasing characters, each character is transmitted twice (time diversity aka FEC) in a time-spread mode; the first transmission (DX) of a specific character is followed by the transmission of four other characters before the re-transmission (RX) of that specific character takes place, allowing for a time-diversity reception interval of:
400 ms for HF and MF channels
331/3 ms for VHF radio-telephone channels
The classes of emission, frequency shifts and modulation rates are as follows:
F1B or J2B 170 Hz and 100 Bd for use on HF and MF channels
G2B for use on VHF at 1200 Bd

12. Digital Selective Calling
Dot Pattern and Phasing
The phasing sequence provides information to the receiver to permit correct bit phasing and unambiguous determination of the positions of the characters within a call sequence.
To provide appropriate conditions for earlier bit synchronization and to allow for scanning methods to monitor several HF and MF frequencies by ship stations, the phasing sequence should be preceded by a dot pattern (i.e. alternating B-Y or Y-B sequence bit synchronization signals) with duration of:
200 bits
At HF and MF, for "distress", "distress acknowledgement", "distress relay" and "distress relay acknowledgement" calls and for all calling sequences to ship stations.
20 bits
At HF and MF, for all acknowledgement sequences (except distress acknowledgements and distress relay acknowledgements – see § and Note 1) and for all calling sequences to coast stations (except distress relay calls – see § 3.4.1). At VHF for all calls.

13. NAVTEX