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SITOR Philips
FSK modem with ARQ and FEC
- this page is a stub
STB-75 is a Frequency-Shift Keying (FSK) radio modem for
Teletype-Over-Radio (TOR), developed around 1971 by
Philips Telecommunicatie Industrie (PTI)
in Hilversum (Netherlands).
Intended for use on wireless telegraphy networks –
such as ship-ship and ship-shore communication – it provided (near) error-free
messaging. It was mainly intended as a replacement for
morse code.
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The device provides simplex data transmission with Automatic Repeat Request
(ARQ), which is based on SITOR-A.
It also provides a broadcast mode with Forward Error Correction (FEC),
which is based on SITOR-B (also known as SITOR-FEC,
SIFEC or SITOR-broadcast).
It also has a so-called direct mode
in which the signal from the teleprinter (telex) is passed through unaltered.
The device was entrirely built with discrete logic integrated circuits (ICs),
spread over no less than 16 removable printed circuit boards (PCBs), as shown
in the brochure image on the right [A].
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The STB-75 became a popular device in the maritime world, not only because it
replaced morse code, but also because it made TOR
much more reliable by adding advanced features like ARQ and FEC.
Around 1977, the STB-75 was succeeded by the microprocessor-based STB-750,
also made by Philips [5]. It was about half the size of the STB-75 and could be
upgraded more easily.
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- Replacement for morse
- About four times faster than morse
- Error correction
- Low error rate
- Selective calling
- Can be patched by coastal stations
- Printed copies of received messages
- Optional on-line encryption
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The diagram below shows the signal flow between two simplex stations
in ARQ mode. The flow is identical to that of SITOR-A.
At the left is the Information Sending Station (ISS), which in this case
is a ship. The Information Receiving Station (IRS) is the costal station
at the right.
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Communication in ARQ mode (one-on-one)
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Data is sent in a 7-bit synchronous format using the
SITOR alphabet, which is basically a 7-bit remapping of the
5-bit characters of the ITA-2 alphabet.
It is similar to the ITA-3 alphabet —
commonly used with ARQ — but with a different assignment of the characters.
Depending on which station initiated the traffic, one station is the Master
and the other one is the Slave. Data is sent by the ISS in short bursts
of 210 ms, which is equivalent to 21 bits or 3 characters.
The IRS replies with a 70 ms answer, which is either a CS1 or a CS2 character.
If all goes well, the IRS alternates between CS1 and CS2 on successive
replies. When the ISS receives two identical successive replies, it
resends the last 3 characters, preceeded by the repeat character (RQ).
The ARQ-mode is also known as SITOR-A, SITOR-ARQ or just SITOR.
It was developed in the 1960s by the Dutch Post Office (PTT).
In the Amateur Radio world it is known as AMTOR-A.
➤ More about SITOR-A
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The diagram below shows the data flow in broadcast mode. In this mode,
the ISS sends an uninterrupted stream of characters that are
interleaved with data from previous characters, with a distance of 350 ms
(5 characters). This method is also known as Forward Error Correction (FEC).
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Communication in FEC mode (one-to-many)
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The FEC- or broadcast-mode is also known as SITOR-B, SITOR broadcast,
SITOR-FEC or SIFEC. Like SITOR-A, it was developed in the mid-1960s by
the Dutch Post Office (PTT) for long-range telegraphic
traffic over HF radio channels.
In the Amateur Radio world it is known as AMTOR-B.
➤ More about SITOR-B
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The table below shows the assignment of the characters of the SITOR alphabet
to 7-bit values. Of the 128 characters that can be made with 7-bits (27),
only 38 are used. 32 of these characters are the same as the 32 characters of
the ITA-2 alphabet (Baudot), remapped to 7-bit space.
The remaining six are special ARQ/SITOR characters
(RQ, α, β, CS1, CS2, CS3), shown below in green.
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| Hex | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |
| 0x | | | | | | | | | | | | | | | | α |
| 1x | | | | | | | | J | | | | F | | C | K | |
| 2x | | | | | | | | W | | | | Y | | P | Q | |
| 3x | | | | β | | G | FIG | | | M | X | | V | | | |
| 4x | | | | | | | | A | | | | S | | I | U | |
| 5x | | | | D | | R | E | | | N | LTR | | SP | | | |
| 6x | | | | Z | | L | RQ | | | H | NUL | | LF | | | |
| 7x | | O | B | | T | | | | CR | | | | | | | |
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Control character
Printable character
ARQ/SITOR character
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The table below shows the 7-bit SITOR telegraphy alphabet (orange) in
comparison to the regular 5-bit ITA-2 alphabet and the
7-bit ITA-3 ARQ alphabet.
SITOR is very similar to ITA-3
but uses a different mapping of the telegraph characters and has
a 4:3 MARK/SPACE radio of the seven bits.
In the table below,
all binary values are shown with the least significant bit (lsb) at the
right. This is the regular notation in computer software. The dot (·) shows the
position of the sprocket hole in punched paper tape. Note that
5-channel paper tape (ITA-2)
has the same layout as the bit order shown here (lsb at the right).
With 7-channel paper tape (ITA-3, SITOR), the lsb is at the left.
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Device Telex Over Radio and Forward Error Correction system Purpose Error-free ship-ship and ship-shore communication Model STB-75 Manufacturer Philips Telecommunication Industry (PTI) Year 1974 Country Netherlands Successor STB-750 Standard SITOR-A (ARQ), SITOR-B (FEC) Modes see below Power (int) +5V, +12C, -12V, +80V (200 mA) Mains 110, 127, 220 or 240V AC (+10%/-15%), 50-60 Hz (±10%) Consumption 90W Humidity 75% (95%) Temperature 0°C to +45°C (storage +70°C) Enclosure 19" rack mount, or free standing Dimensions 285 × 440 × 300 mm Weight 36 kg
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Speed 50 baud Alphabet ITA-2 Format Asynchronous 7 or 7½ units (1 start-, 5 data-, 1.5 stop-bit) Interface Serial single-current loop (80V, 60mA)
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Speed 100 baud Alphabet SITOR Format Synchronous 7 units, with 4:3 MARK/SPACE ratio Modes A (ARQ), B (FEC) Interface Single current (80V, 40mA), FSK 600Ω FSK 70 Hz shift (F1 = 1415 Hz, F2 = 1585 Hz) 1
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Optional: F1 = 1615 Hz, F2 = 1785 Hz.
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- ARQ, automatic repeat request, half-duplex
- FEC, error detection and correction, simplex
- Direct (uncorrected), receive only
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Remote Control Unit (RCU)
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Model CBX-75 Dimensions 240 × 215 × 145 mm Weight 4 kg
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- STB-75 brochure
PTI, February 1974.
- H. da Silva, E. Goldstern & J.A. Kok, Simplex TOR STB-75
Philips Technical Review 30, No. 1, 1971. pp. 1-14. WANTED
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- Wikipedia, SITOR
Visited 15 February 2025.
- H. da Silva, E. Goldstern & J.A. Kok, Simplex TOR STB-75
Philips Technical Review 30, No. 1, 1971. pp. 1-14. WANTED
- Hannover Messe Vorschau, Short description of STB-75
FUNK-TECHNIK 1971 No. 9.
- Philips STB-750 advert
Maritime Reporter, 15 October 1977. p. 48.
- R. Azimullah & R. Deraemaeker,
Simplex TOR STB-750 for world-wide radio telegraph communication
Philips Telecommunication Review 35, No. 4, 1977. pp. 186-199.
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© Crypto Museum. Created: Saturday 05 October 2019. Last changed: Tuesday, 18 February 2025 - 10:10 CET.
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![Philips STB-75 interior. Photograph extracted from the original brochure [A].](img/stb75_int.jpg)
