Two tones.
Three ways to carry text.

01 / The physical channel

The same shift does not mean the same code

Both defaults use two audio tones 170 Hz apart. RTTY runs at 45.45 baud, with mark above space in the USB audio convention. SITOR-B runs at 100 baud: the higher tone is B and the lower is Y. In the seven-unit table B is stored as zero and Y as one; this differs from RTTY’s mark-as-one convention. A frequency shift is the distance between tones, not the signal’s occupied bandwidth.

02 / Asynchronous characters

A start, five code units, and a stop

The default RTTY character has one space start unit, five ITA2 code units in order 1 through 5, and 1.5 mark stop units. That is 7.5 unit times: about 165.02 ms at 45.45 Bd, or 6.06 codewords per second before shift characters and other controls. There is no character checksum or retransmission in this framing. An incorrect but valid-looking character can print without a warning.

03 / One alphabet, two cases

Shifts give five bits a larger vocabulary

Five bits have 32 combinations. Letter shift (LTRS, 11111 in transmitted unit order) selects letters; figure shift (FIGS, 11011) selects numbers and punctuation. The same 10101 code prints Y in letters and 6 in figures. A lost shift can affect a run of text, not just one character. The console defaults to US-TTY figures and unshift on space (USOS); --no-rtty-usos keeps figure state across a space.

US-TTY and international ITA2 differ in eight figures entries; their letter codes agree. SITOR retains the shift idea but maps characters into its own seven-unit alphabet. See the full code tables for controls and both figures variants.

04 / Time diversity

SITOR-B separates two copies of each character

A SITOR word has seven units and exactly four B tones plus three Y tones. That ratio detects any single-unit inversion; it cannot detect every multiple-unit error. Seven units at 100 Bd occupy 70 ms. Direct (DX) and repeated (RX) slots alternate. Four other 70 ms slots lie between a word and its copy: a 280 ms clear interval, or 350 ms from one start to the next.

The receiver checks both copies. A valid copy can replace an invalid one; matching valid copies agree. Two different valid copies leave the character unresolved. hfmodem also uses soft tone evidence when both hard decisions are damaged; confidence thresholds and shift-state handling belong to its receiver implementation. This is time-diversity forward error correction, without a return-path acknowledgement.

The waveform starts with alternating phasing signals to establish slot alignment. The library default is 16 pairs, consistent with M.625’s minimum, and ends with idle-alpha signals in the DX stream. These defaults are not a complete NAVTEX transmitting schedule: broadcast-specific phasing and identification rules apply beyond basic waveform generation. Selective SITOR-B addressing and SITOR-A’s acknowledged 450 ms cycles are outside these console monitors.

05 / From text stream to bulletin

NAVTEX adds identity and message boundaries

NAVTEX uses SITOR-B as its broadcast transport. A bulletin starts with ZCZC, then a space and four identifier characters: B1 identifies the transmitter, B2 identifies the subject category, and B3B4 form the two-digit serial. NNNN marks the end. A decoder can print SITOR text while a NAVTEX parser has not yet received a complete bulletin.

hfmodem keeps partial headers and unterminated text across receive windows. It emits complete bulletin records, omits garbled identifiers instead of inventing a station or serial, and permits garbles in the body. Parsing a bulletin does not establish its authority, currency or operational accuracy. The service’s message selection and duplicate-rejection rules are broader than this text parser.

NAVTEX service frequencies include 518 kHz, 490 kHz and 4209.5 kHz. Those are assigned signal centres, not USB VFO settings. With the 1700 Hz SITOR audio centre, a 518 kHz signal corresponds to 516.3 kHz USB dial. A KiwiSDR’s displayed centre and the console’s local USB dial option must not be treated as interchangeable.

06 / At the operating desk

Choose the text decoder and keep listening

hfmodem console rtty monitor
hfmodem console sitor-b monitor
hfmodem console navtex monitor
hfmodem console rtty monitor --no-rtty-usos

These use the rig’s current dial and leave sideband and receive-filter choices to the operator. Optional --freq KHZ tunes once. You may retune the radio itself while the monitor runs; tone acquisition can search audio up to 4 kHz. Read the monitor guide for audio-source selection, recordings, status output and KiwiSDR handling.

Tune both tones inside the receive passband. Incorrect rate, tone polarity or alphabet state can turn plausible signal energy into wrong text. RTTY does not supply delivery confirmation. SITOR-B may leave unresolved characters, and NAVTEX may need a complete later bulletin before it produces output. The library’s RTTY and collective SITOR-B transmit functions do not make these passive console entries a two-way terminal.

07 / Implement it yourself

Tables, timing and receiver decisions

The figures show wire order, tone centres and default timing. The files below contain complete tables and the implementation-specific acquisition and decision rules. The diagrams are explanatory schematics; the linked code remains the source for exact behavior.