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FT8/FT4 Receive & Decode List

The panadapter includes on-device FT8 and FT4 decoders with real-time spectrum waterfall and a live list of heard stations.

1. FT8 & FT4 View

Swipe → from the left edge to toggle to FT8/FT4 view. The same decode list and waterfall work for both modes — switch modes via the Preset dropdown in the left pane (top).

FT4 notes: - Decodes refresh roughly twice as fast as FT8 because slots are 7.5 seconds - Slot countdown shows 7.5 s instead of 15 s - All filtering, priority, and auto-reply features work identically in FT4 - Time-sync from decoded signal timing works in both FT4 and FT8 (the bottom-bar clock shows UTC(FT4) when synced from an FT4 decode) - v0.19.4 made FT4 reliable: a memory-placement fault previously killed decoding on every other slot (FT8's longer slots mostly hid it), the EVEN/ODD slot markers were on the wrong 15-second grid, and the slot clock could visibly jump between slots — all fixed; if FT4 seemed deaf or erratic on an earlier version, update the firmware

You'll see:

  • Decode list (left pane) — every station heard, ordered so the ones that matter to you are at the top (see Decode List below)
  • Waterfall (right pane) — same real-time spectrum as panadapter mode
  • Call CQ button — start a CQ (requires QMX on the air)
  • Options button — include/exclude stations, prioritise by SNR/distance, show only CQ callers

2. Decode List

The list shows every decoded FT8 message:

Column Meaning
SL Slot parity: E (blue) or O (amber)
CALL Their callsign
MESSAGE The full decoded message text
CTY Country as a 3-letter code (from the callsign prefix)
SNR Signal-to-noise estimate, colour-banded by strength
DT Slot-timing offset in seconds, relative to the band — an on-time station reads ~0.0 (v1.3.1)
HZ The station's audio tone within the FT8 passband (v1.3.1)
KM / MI Great-circle distance from your grid
BRG Bearing from your grid
HRD Times decoded since last appearance

Own call highlight — your callsign is shown in inverted colours (red fill, white text) so you spot replies to you instantly.

How the list is ordered. Top to bottom:

  1. The station you are working (asked for by Don WB0LQW) — during an exchange there are several transmissions each way, and the partner's messages used to sort down-screen where you had to hunt for them. Now everything from them stays at the top: their reply to you, their CQ while you are mid-exchange, and a message they send to a third station — which is exactly when you most want to see it. It covers a hand-typed reply as well as an automatic exchange, and matches on their callsign, so a third station merely mentioning them is not promoted. It releases the moment the QSO completes (their closing 73 contains your callsign, so rule 2 keeps it up there anyway).
  2. Messages addressed to you — anything containing your callsign.
  3. CQ calls.
  4. Everything else, strongest signal first.

Stale entries — the list is a live picture of who's on frequency now; stations not heard again within 2 minutes drop off automatically.

During your own CQ run — stations transmitting on your transmit slot (which you can't hear while transmitting) are hidden from the list and return when the run ends; other stations' CQ rows are hidden during a run as before.

Nonstandard callsigns — special-event and compound calls now resolve to the full callsign instead of showing <...>, once the station has been heard in full.

3. FT8 Options

Tap the Options button to open the Options modal:

  • Include callsigns — only show these calls (space or comma-separated)
  • Include callsigns containing — show any call with these substrings (e.g., /P for portable)
  • Exclude callsigns — hide these calls
  • Exclude if contains — hide any call matching these substrings
  • Exclude worked before — skip calls you've already logged QSOs with
  • Show only CQ callers — hide replies, show only CQ messages
  • Skip TX1 — when you pounce a station, open with your signal report straight away instead of the grid exchange, for a quicker QSO (falls back to the normal grid exchange if the station has dropped out of the decode list)
  • Allow grey-listing — off by default. A station that times out two of your pounces in a row is set aside: the robot and Auto-work-pileup skip it, its decode row turns violet, and tapping it offers to clear it from the grey-list instead of opening the TX dialog. Handy on a busy band where one station simply never comes back. The list is held in memory only and forgotten at power-off
  • Auto-reply priority — Strongest SNR, Weakest SNR, Most distant grid
  • Robot mode — auto-answer CQ (disabled by default; see FT8 Transmit)

All filters persist across sessions. Toggle any filter on/off to enable or disable it without erasing the criteria.

4. Decoding Performance

On a reasonably busy band, expect roughly 15–20 distinct messages per slot in either mode. That is messages, not contacts — one QSO produces several.

Decoding uses both processor cores: the decode task works one half of the candidate list on core 1 while a helper takes the other half on core 0, and both run against a waterfall that was built during the capture rather than after it. That is what makes the results appear a second or two into the following slot instead of most of the way through it.

What limits it:

  • LDPC error correction is heavy, and most candidates the search finds are false alarms that use the full iteration budget before being discarded
  • The spectrum and waterfall are being drawn from the same audio at the same time
  • USB audio must be serviced continuously — a gap there costs decodes far more than a slow decoder does

FT4 advantage: because FT4 slots are half as long (7.5 s vs 15 s), you get refresh feedback roughly twice as fast, which feels snappier on a crowded band even though the per-slot decode count is the same.

The panadapter prioritises receive latency (decode every 15 s, not accumulate) over volume.

5. Real-Time Waterfall

Unlike WSJT-X (which buffers a whole 15-second capture), the Tab5 builds the waterfall on-the-fly as audio arrives. You see CW and other traffic in real time, and FT8 stations appear instantly as their symbol blocks are decoded.

The waterfall also shows non-FT8 activity: - CW (carrier lines) - SSB (broader vertical smears) - RTTY (FSK sidebands) - Interference (wideband noise)

6. Tap to Reply

Tap any FT8 row in the decode list to prepare a reply:

  1. A confirmation modal pops up showing your message
  2. You can review the target callsign, grid, and SNR
  3. Tap Transmit to send, or Cancel to abort

The reply always follows FT8 protocol (correct parity, proper message sequence) — you can't accidentally send a garbled or out-of-order message.

Pile-up list. If a station answers you while you're busy with another contact, they used to disappear from the list once they stopped transmitting. They're now remembered in a Pileup list — the ADIF-log button on the FT8 screen turns into a Pileup button (a different colour) whenever callers are waiting. Tap it to see who's called you, tap a station to work them, or tap the ✕ to dismiss one. See FT8 Transmit for the full workflow.

7. Auto-Reply (Robot Mode)

⚠️ Experimental — enabled via a checkbox in the Options modal.

When robot mode is on, the Tab5 automatically replies to CQ without waiting for you to tap. It scans each FT8 slot for CQ callers matching your filters, picks the highest-priority station, and sends a full QSO exchange (TX1 → wait for report → TX2 → wait for RR73 → TX3). Everything is logged to ADIF.

Important: Robot mode keys the QMX for real — your signal goes on the air. Never leave it unattended unless you're confident in your filters and your station is in a safe state.

8. Search & Window

The decoder uses an FFT-based search window to find candidate tone blocks, at whichever tone spacing the active mode uses — 6.25 Hz for FT8's 8 tones, 20.83 Hz for FT4's 4.

Frequency stability: The QMX's USB audio clock isn't bit-exact 48 kHz, so decodes slide slowly over time. The panadapter re-locks to UTC boundaries every 15 seconds (the slot boundary), preventing long-term drift.

9. Time Sync for FT8

FT8 requires UTC time accurate to ±1 second for slot synchronisation. The panadapter gets time from:

  1. WiFi + SNTP (if available) — most accurate, syncs every ~hour
  2. Tab5 RTC (if set) — accurate to ±1 min, no internet needed (POTA)
  3. QMX GPS (if QMX has GPS) — re-syncs the RTC every 5 minutes
  4. Manual — you can set time via the settings drawer

See Time Sync for details.

Next: Learn about FT8 Transmit or go back to Panadapter.