All Things Digital
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)
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
ASCII Code
American Standard Code for Information Interchange
Wikipedia.org
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
The Code
• CCIR 476 - 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
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.
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.
Digital Selective Calling
Digital Selective Calling
• The system is a
synchronous system
using characters
composed from a
ten-bit errordetecting code
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
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 § 3.4.1
and Note 1) and for all calling sequences to coast stations (except distress
relay calls – see § 3.4.1). At VHF for all calls.
NAVTEX