Firmware CW Decoder

The CW decoder in LISTEN mode is a lightweight, state-machine implementation that needs no external libraries. It lives in the system-monitoring section of AegisBeacon.ino and is driven by cwDecodeSample().

Sampling loop

runListenMode() reads radio.getRSSI() every CW_SAMPLE_MS (5 ms). A sample counts as a mark when the RSSI is at or above cfg.rssiThreshold; anything below is a gap. The sample cadence is fixed, so the decoder measures time by counting samples: each tick adds CW_SAMPLE_MS to the current mark or gap duration.

Timing model

The decoder assumes the transmitter uses the PARIS standard, which the beacon itself does (see Morse Timing Reference):

Element	Duration
Unit	1200 / WPM ms
Dot	1 unit
Dash	3 units
Intra-character gap	1 unit
Character gap	3 units
Word gap	7 units

The configured cfg.wpm drives unit; the decoder clamps it to at least 20 ms so a very high WPM cannot produce degenerate timings.

State machine

1. Mark starts (gap to mark edge): the accumulated gap is compared:
   • gap >= 7 units: flush the pending symbol, then insert a space;
   • gap >= 3 units: flush the pending symbol;
   • otherwise: same character continues.
2. Mark ends (mark to gap edge): the mark duration is classified:
   • mark <= 2 units: append a dot (.);
   • mark > 2 units: append a dash (-).
3. The symbol buffer is capped at CW_MAX_SYMBOLS (40); overflow resets the buffer.

Character decode

When a symbol is flushed, cwCharFromSymbols() matches it against the full ITU table:

• Letters A-Z, digits 0-9;
• Punctuation: . , ? - " / ! + = @ $ ( ).

Unknown symbols (for example a prosign not in the table) decode to nothing and the buffer resets, so the text window never shows garbage characters.

Output

• cwAppendChar() inserts the character into the 96-character text buffer (CW_TEXT_LEN) shown on the OLED. When full, the buffer scrolls left by one word.
• Every non-space character increments the decoded counter, blinks the blue LED and prints AEGIS:CW:<char> over serial.
• Consecutive spaces are collapsed.

Tuning advice

Symptom	Likely cause	Fix
Random characters in noise	Threshold too low	Raise rssiThreshold
Dots become dashes	WPM too high for the sender	Set WPM to the sender’s speed
Text lags or misses characters	Signal fading below threshold	Lower threshold or improve antenna

The decoder is a timing-based detector: it does not shape the signal, so a clean, stable carrier (as produced by the beacon’s own transmitDirect() keying) decodes far better than an overdriven or distorted transmission.