RTTY

The RTTY cockpit is a Baudot/ITA2 teleprinter station — 45.45 baud and a 170 Hz shift out of the box, the HF standard. It gives you a live decoder with per-character confidence, a mark/space waterfall you click to net onto a signal, four F-key macros and a type-and-send bar, two keying back-ends (soundcard AFSK or true FSK on a serial line), and an optional QSO auto-sequencer that never transmits until you start it. It is deliberately not a contest station: no serials, no dupe check, no log strip — a contact you work by hand goes in the Logbook yourself.

RTTY is an opt-in section. Turn it on in the first-run wizard’s “which modes?” step or in Settings ▸ Appearance ▸ Features.

The tour

The header. Left to right: a mode badge reading RTTY 45.45 · 170 Hz — your live baud and shift, so you can see at a glance what the decoder and the transmitter are set to — then a keying-backend pill reading AFSK or FSK, then a TX ▲ pill that appears only while an over is on the air or queued behind one. The badge explains itself on hover: “RTTY — Baudot/ITA2 at the configured baud + shift (45.45 / 170 Hz is the HF standard; change it in Settings → RTTY).”

The big frequency readout is editable — type a dial and press Enter. A frequency outside the band plan is rejected with a toast rather than sent to the rig. Beside it, a compact band picker serves the built-in RTTY watering holes: ten HF channels from 160 m (1.838) through 10 m (28.083), each with a note on what shares the window — 40 m carries both the US 7.080 and the EU/DX 7.045 entries, and 20 m parks at 14.083, above the FT4 cluster at 14.080. The list is filtered by license class per band, so a band where your class holds no digital privileges never appears.

On the right: ⊞ Panels, the TX On / TX Off latch, Stop TX and the CAT pill. The latch is this cockpit’s Enable-Tx — the top bar’s TX cluster is hidden behind the digital chrome here, so without it a send would sit at “TX is off” with no way to arm from this screen. There is no power slider and no Tune button in this cockpit: drive is Tx Power in Settings ▸ Radio ▸ Audio, set so ALC reads just zero.

The waterfall is the same instrument the FT cockpit uses, run at the live-band cadence: a new row every 50 ms rather than the 120 ms the slot-synchronous FT surfaces use, so a signal drifting across the passband moves smoothly instead of in steps. Two named cursors replace the usual RX/TX markers — a green M at the mark tone and an orange S at the space tone, drawn from exactly the tone pair the demodulator built its filters from, so what you see and what you decode can never disagree. The hint above the canvas reads click nets the decoder: any click moves the decode centre to that audio frequency (clamped 300–3700 Hz) and drops the demodulator for a clean re-acquire. Palette, view span, the G/Z contrast knobs, the 3D stacked-spectrum toggle and pause-with-scrollback all work as they do elsewhere.

The scope is a fixed share of the window — 22% of the effective viewport height, with a floor that yields on a short or heavily zoomed display so it can never eat the transcript. RTTY is the one scope in Nexus with no operator splitter: there is no grip to drag, so the scope/transcript split is what the window gives you. On a short window the whole cockpit scrolls rather than clipping anything.

Decoded text is the one content pane, and the only entry in this cockpit’s ⊞ menu. Its head carries, in order:

• Arm RX — “start decoding RTTY from the receive audio (RX only, never keys the rig).” Arming is session state and is never persisted, so the app never launches with a decoder running. Once armed the button reads RX armed and the transcript fills; disarmed, the pane reads “Arm RX to decode RTTY from the receive audio.”
• Auto — arms the QSO sequencer. “It runs the QSO after you click CQ (run) or answer a heard CQ (search & pounce); it never transmits on its own.” Turning it off is itself a stop: the queue is cleared and the rig unkeyed.
• The AFC pill, shown while armed: a signed offset such as +12 Hz, gaining a 🔒 once the correction has acquired and frozen. It freezes on purpose — a locked decoder that keeps tracking walks onto a stronger neighbour mid-QSO.
• Re-tune, shown while armed: “drop and rebuild the demodulator (use when it froze on the wrong signal).”
• Clear wipes the transcript and re-pins it to the bottom, so an emptied pane follows new copy even if you had scrolled up.

The transcript itself prints every character the demodulator produced, and fades the ones it was unsure about. Confidence is the ATC slicer’s own margin, carried out per character: above 75 prints solid, then progressively fainter at 50 and 25, and the weakest copy at 30% opacity. Nothing is ever hidden — faint means “read this twice”, not “discarded”. The ring holds the last 4000 characters and follows new copy while you are at the bottom; scroll up and it stays where you put it, so you can re-read a callsign mid-over.

Underneath the print, the decoder is a Rust port of fldigi’s receive path. Mark and space go through baseband mixers into 1024-point overlap-add FFT filters — one matched filter per tone, so what reaches the bit decision is your tone pair rather than the rest of the passband. The W7AY SNR-optimized ATC slicer leans each decision on whichever of the two tones currently has the better signal-to-noise, which is what holds copy together through the selective fading that takes one tone of a pair down. Straddle-point bit-clock recovery judges every bit at the middle of its window and re-centres on each character’s start bit, so a sender running a little off 45.45 baud does not slide into garbage part-way down a line. Phase-difference AFC is clamped so it can never cross onto the neighbouring tone (±45% of your shift — about ±76 Hz at 170). A signal-presence squelch gates the printed output, so an armed decoder on a quiet band stays silent instead of streaming garbage. It runs in its own thread on a 100 ms drain and keeps decoding while you are on another section; the cockpit polls it twice a second while RTTY is the visible view, so the first tick after you come back catches the display up.

The TX dock — macros, compose, and the sequencer row when Auto is on — is pinned below the pane and cannot be scrolled out of reach or hidden. The macro row holds a Their call… field feeding the {CALL} token, four macros, and Esc / Stop:

Key	Label	Sends
F1	CQ	CQ CQ CQ DE {MYCALL} {MYCALL} K
F2	Answer	{CALL} DE {MYCALL} {MYCALL} K
F3	Exchange	{CALL} DE {MYCALL} UR 599 599 K
F4	73	{CALL} DE {MYCALL} TU 73 SK

Hovering a macro shows it fully expanded with your call and theirs, so you can read what will key before you click it. {MYCALL} comes from Settings ▸ Station; {CALL} from the Their-call field. Both tokens also expand in the compose bar, where Enter sends.

Every send is checked by the engine before anything is queued, and a refusal tells you why: not in the RTTY section, TX not armed, the frequency outside your license privileges, another mode holding the transmitter, or — on the FSK backend — the data line and a serial PTT line configured onto the same physical line. Sending while an over is already going out queues behind it (type-ahead); Stop TX or Esc / Stop drops the whole queue and unkeys.

Errors surface as a banner above the pane: an FSK port that would not open, a rig that refused PTT, or a sequencer whose gate closed mid-QSO.

How this differs from the FT8 cockpit

RTTY is free-running. There is no slot clock, so nothing here is timed against UTC, the clock-offset discipline FT8 lives or dies by does not bear on your copy, and there are no transmit periods to pick — you key when you key. The sequencer, when you use one, is driven by pattern-matching the decoded text rather than by a slot boundary, which is why it needs timeouts where the FT8 sequencer needs none. The decoder prints one running transcript instead of a decode list, a band activity table and a call roster, because a teleprinter signal is text and there is nothing to tabulate. One tone pair decodes at a time: whatever the M and S cursors sit on, not the whole passband. And you start every transmission, directly or by starting a sequencer run — there is no double-click-a-decode-to-call path. RTTY and FT8/FT4 are mutually exclusive by construction: the transmit gate demands the RTTY section owns the rig, so the two sequencers can never key together.

Core workflows

Copy a station

1. Pick a band from the header’s channel list, or type a dial into the readout. Entering the cockpit commands the rig’s mode for you — a DATA submode on the LSB side for AFSK, the rig’s own RTTY mode for FSK — and re-homes the frequency when you have genuinely changed mode, not when you are returning to a section you were already in.
2. Click Arm RX. On a quiet frequency the pane reads “listening…” — the squelch is holding the print closed until a real signal opens it.
3. Click the signal on the waterfall. The M and S cursors jump there and the demodulator re-acquires; the AFC walks onto the tone and freezes after eight consecutive clean frames, which on a diddle preamble is a handful of characters. The pill shows the offset it settled on and gains its 🔒.
4. If the copy is upside down — the classic all-garble against a strong signal — turn on Reverse (swap mark/space) in Settings ▸ Modes ▸ RTTY. There is no reverse button in the cockpit; the M and S cursors trading places on the waterfall are the only on-screen sign of which sense you are running.
5. If the AFC locked onto the wrong signal, press Re-tune to drop and rebuild the demodulator rather than fighting it with the VFO.

Send an over by hand

1. Arm transmit with the header’s TX Off latch, so it reads TX On.
2. Type the other station’s call into Their call… — it feeds {CALL} and nothing else.
3. Click a macro, or type in the compose bar and press Enter. Text is uppercased and filtered to the ITA2 character set before it is queued, so what is queued is exactly what keys — a line with nothing encodable in it is refused with “Nothing to send — RTTY carries A–Z, 0–9 and basic punctuation.”
4. Esc / Stop is live exactly while an over is on the air. It aborts the transmission, drops anything queued behind it, and unkeys. Stop TX in the header does the same and is never disabled.

Run a QSO with the auto-sequencer

1. Turn on Auto. This builds the state machine and nothing else — no transmission happens on the toggle.
2. To run: click CQ · Auto call. To pounce: when a CQ … DE <CALL> appears in the transcript the Answer button lights up with that callsign; click it. Those two clicks are the only doors in, which is what keeps a contact operator-initiated (ARRL Field Day rule 6.4). Answer offers the newest CQ still in the 4000-character transcript, so it can be several minutes old — Clear drops a stale one. Until there is one the button sits disabled showing a dash, explaining itself on hover: “No CQ heard yet — Answer lights up when the decoder surfaces one.” Nexus only looks for CQs at all while Auto is on.
3. The row then shows the live state — Calling CQ, Answering, Exchange sent, Confirmed, Done — plus the station being worked and their exchange as you copy it. Callsigns are matched with one character of fuzz, forgiven only where the demodulator itself was unsure; 599 garbled to TOO by a lost FIGS shift and the 5NN cut convention both normalize.
4. 30 seconds of silence does whatever fits where the machine is: an unanswered CQ goes out again, a call the runner never returned is repeated, and no-copy mid-exchange asks AGN. Three fruitless cycles inside a QSO end the session — a runner falls back to calling CQ, a pouncer returns to idle. A CQ run itself never ends on its own.
5. Esc · Abort kills the session, drops the queue and unkeys.

The exchange is table-driven: casual RST/name/QTH, taking name and QTH from your Settings ▸ Station operator name and state, or Field Day class and section when the Field Day master switch is on. Once both exchanges validate, the contact is logged — if Auto-log QSOs is on — with mode RTTY, 599 sent, their copied report received, and any class/section riding the comment field so nothing you copied is lost.

Choose a keying backend

Set this in Settings ▸ Modes ▸ RTTY; the cockpit’s pill shows which one is live.

• AFSK (default) plays a two-tone waveform through the same TX audio path FT8 uses, with the rig in a DATA submode on the LSB side (DATA-L / LSB-D / PKTLSB) — soundcard-clocked, so the bit timing is jitter-free. The DATA submode is what routes the USB codec to the modulator: plain LSB takes TX audio from the mic on most Icom and default Yaesu setups, which is the red-light-no-output failure. Mark and space NCOs run continuously and each bit edge is a raised-cosine cross-fade, which is what keeps the keying sidebands narrow. This is the robust path: if FT8 transmits on your station, RTTY will.
• True FSK bit-bangs DTR or RTS into the rig’s FSK input with the rig in its own RTTY mode, which unlocks its narrow RTTY filters. Bit edges come from OS thread scheduling, so a loaded machine can jitter individual edges by a few ms — casual and Field-Day grade rather than contest grade. Edges are scheduled against absolute deadlines, so jitter never accumulates into baud drift. PTT must ride CAT or the other control line; Nexus refuses the send outright if you point PTT at the same line as the data.

Either way the bit stream is identical — the same framed Baudot, 1 start bit, 5 data bits, 1.5 stop bits, at true 45.45 baud never rounded to 45.

Land here from the Needed board

Click an RTTY row on the Needed board and Nexus QSYs to the spot’s exact frequency and opens this cockpit. Unlike CW and Phone it does not pre-fill a callsign — type it into Their call… yourself. If the RTTY section is switched off, the click still QSYs the rig to the spot so you can work it from wherever you are.

Honest limits

• The F-key labels are labels. Nothing in this cockpit is bound to a keyboard key except Enter in the compose bar — F1–F4 and Esc name the buttons, and you click them. (The CW cockpit does bind its keys; this one does not.)
• Nothing you work by hand is logged. There is no log strip here. Only the auto-sequencer writes a QSO, and only when Auto-log QSOs is on in Settings ▸ Modes ▸ Digital — with auto-log off, even a completed auto-run contact is not written anywhere.
• An auto run works one station and stops. After the contact is logged and your closing goes out, the sequencer reaches Done and stays there. Press Esc · Abort to return it to idle before you can call CQ again — it will not chain into the next QSO.
• An unanswered auto CQ repeats indefinitely — every 30 seconds, by design (“the operator owns stopping a run”). A bare repeat deliberately does not reset the transmit watchdog, so the Tx Watchdog in Settings ▸ Modes ▸ Digital (6 minutes by default) is the backstop that eventually disarms TX. Abort and Stop TX are the immediate ones.
• The report is always 599. The F3 macro sends it literally and the sequencer substitutes it; there is no field to change it and no signal-report control anywhere in the cockpit.
• The sequencer’s messages are not editable. The CQ, answer, exchange, AGN and sign-off templates ship as fixed text — there is no Settings surface and no in-cockpit editor for them. No shipped exchange sends a contest serial either; casual and Field Day are the two schemas.
• Netting the waterfall moves the receiver only. Your transmitted tones are fixed — AFSK always keys 2125 Hz mark with space at mark plus your shift — so clicking a station 400 Hz away lets you copy them but does not move your transmission onto them. Use the VFO for that. There is no TX marker on this waterfall at all, only M and S.
• One signal at a time. A single demodulator sits on a single tone pair, so two stations in the passband do not decode into two columns and the second one does not decode at all. This is a receiver, not a skimmer.
• Send-and-done: no diddle. The transmitter unkeys the moment the final stop bit ends. Nexus does not hold an idle mark between overs, which some stations’ AGC and some contest software expect.
• The character set is ITA2 and nothing else — A–Z, 0–9 and the classic punctuation. Everything is uppercased, and anything with no Baudot code (+, =, @, accented letters) is dropped silently before it reaches the rig. One over is capped at 1000 characters, about 2¾ minutes at 45.45 baud.
• Unshift-on-space and AFC are always on. Both are decoder defaults with no control in the cockpit or in Settings — you cannot hold the figures shift across spaces for contest-style 599 599 copy, and you cannot pin the AFC off.
• The fade is approximate on badly broken copy. Above 200 quality changes in the transcript the per-character fading regroups into equal blocks scored by their mean confidence, which can average a short bad burst back into a good block. Every decoded character still prints in order; only the fade is smoothed, and only on copy already too broken for per-character fading to mean much.
• Baud and shift are picked from lists, not typed: 45.45 or 75 baud; 170, 425 or 850 Hz shift. The band plan is HF only — 160 m through 10 m, with no VHF RTTY channel.
• The scope has no splitter and no pop-out. It takes its share of the window and that is that.

Hiding Decoded Text in ⊞ Panels takes the Auto toggle away with it, since that toggle lives in the pane head. Stop TX, the Esc/Stop macro and the TX-enable latch all render outside the pane, so no layout you save can leave you unable to stop a transmission.

Related guides

• CW
• SSTV — the other free-running mode in the Digital group
• Phone (SSB)
• Operate — FT8/FT4 digital
• Needed — DX that’s on the air now
• Logbook & QSL
• Contesting & POTA/SOTA
• Settings reference