SITOR
SITOR is a system for transmitting text messages. Although it uses the same frequency-shift keying modulation used by regular radioteletype, SITOR uses error detection, redundancy, and/or retransmission to improve reliability.
There are two SITOR modes:
- SITOR-A is used for point to point links. SITOR-A uses automatic repeat request to gain reliability. If the receiver detects an error, it requests a retransmission.
- SITOR-B is used for broadcast links. SITOR-B transmits each character in a message twice to gain reliability. If the receiver detects an error in the first character, it uses the copy. If both characters are garbled, the receiver won't know what was sent.
- SITOR-B by definition uses forward error correction, versus ARQ for SITOR-A.
The bitstream is FSK modulated with a 170 Hz frequency shift. The high frequency is a mark, and the low frequency is a space.
Applications
SITOR is used commonly on shortwave bands, where it is used to transmit maritime-related information such as weather forecasts and storm warnings.SITOR-B is used for narrow-band direct printing.
NAVTEX marine weather and safety messages are broadcast using SITOR-B. The NAVTEX messages have a specific format that is interpreted by NAVTEX receivers.
Amateur radio uses SITOR but calls it AMTOR, AMateur Teleprinting Over Radio. AMTOR-A is SITOR-A. AMTOR-B is SITOR-B. In 1991, an AMTOR extension was described that includes lower case and other printable ASCII characters.
Technical Details
Character set
RTTY uses the ITA2 character code. ITA2 is a five bit code with 32 possible code points. Four code points are used for null, space, carriage return, and line feed. Two code points are used for a letter shift or a figure shift. The remaining 26 code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 52 additional characters.SITOR recasts ITA2 into a 7 bit code called CCIR 476. Each 7-bit character in CCIR 476 has 4 marks and 3 spaces. Each valid character code has a Hamming distance of at least 2 from every other character. A one bit error will disrupt the balance of marks and spaces; a second bit error may bring the count back to 4 marks and 3 spaces. Consequently, the CCIR 476 alphabet is guaranteed to detect all single bit errors within a 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:. Thus 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.
| Code | Mode A | Mode B |
| 1100101 | Control signal 1 | |
| 1101010 | Control signal 2 | |
| 1011001 | Control signal 3 | |
| 0110101 | Control signal 4 | |
| 1101001 | Control signal 5 | |
| 0110011 | Idle signal β | Idle signal β |
| 0001111 | Idle signal α | Phasing signal 1 Idle signal α |
| 1100110 | Signal repetition | Phasing signal 2 |