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ITA-5 → SITOR → ← ITA-2
ARQ alphabet
International Telegraph Alphabet No. 3 (ITA-3), also known as CCITT No. 3
or TOR No. 3, is a 7-bit error-detecting encoding scheme for digital telegraphy
(telex), invented in the 1940s — during WWII —
by Hendrik van Duuren in Delft (Netherlands), as an alternative
to International Telegraph Alphabet No. 2 (ITA-2, Baudot-Murray).
It is also known as Van Duuren code [1][2].
ITA-3 was first used with a transmission format known as
Automatic Repeat Request (ARQ) or, more precisely,
Automatic Repeat Request Multiplex (ARQ-M). It is also used with various other
ARQ schemes, including ARQ-28 and ARQ-56. It remaps the 32 characters
of the ITA-2 alphabet to 7-bit space and adds three special
ARQ characters: RQ, α and β.
This means that only 35 of the 128 possible codes are used.
93 codes remain unused.
One of the properties of ITA-3 is that the bits of each 7-bit character
have a 3:4 MARK/SPACE ratio. 1 Another property is that at least 2 bits are
different between two adjacent characters. This is known as Hemming
Distance ≥ 2.
ITA-3 should not be confused with SITOR, which is also an error-detecting
encoding scheme for telegraphy, with similar properties to ITA-3, but with
a different assignment of the codes. SITOR supports the same 32 telegraphy
characters as ITA-2 and ITA-3, but has 3 extra control codes.
➤ More about SITOR
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In this context, a MARK represents a logic '1', whilst SPACE is a logic '0'.
A 3:4 MARK/SPACE ratio means that each 7-bit character code consists of three
1s and 4 0s.
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The table below shows the assignment of the characters of the ITA-3 alphabet
to 7-bit values. Of the 128 characters that can be made with 7-bits (27),
only 35 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 three are the so-called ARQ characters
(RQ, α, β). In the table these are shown in green.
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| Hex | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |
| 0x | | | | | | | | I | | | | SP | | LF | E | |
| 1x | | | | R | | N | RQ | | | C | β | | D | | | |
| 2x | | | | L | | H | U | | | P | S | | A | | | |
| 3x | | O | FIG | | X | | | | LTR | | | | | | | |
| 4x | | | | G | | M | Z | | | V | α | | B | | | |
| 5x | | T | W | | Y | | | | Q | | | | | | | |
| 6x | | CR | J | | F | | | | K | | | | | | | |
| 7x | NUL | | | | | | | | | | | | | | | |
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Control character
Printable character
ARQ character
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The ITA-3 alphabet is used with the following data formats:
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The table below shows the 7-bit ITA-3 telegraphy alphabet (orange) in comparison
to the regular 5-bit ITA-2 alphabet and the 7-bit SITOR alphabet.
SITOR is similar to ITA-3 but its characters have a 4:3 MARK/SPACE
ratio. Furthermore, the character codes are assigned differently.
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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© Crypto Museum. Created: Saturday 08 February 2025. Last changed: Tuesday, 11 February 2025 - 10:37 CET.
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