CW Decoder

Status

Status: Supported · Applies to: FlexRadio, Hermes-Lite 2 (experimental), Networked Icom (early; IC-7300MK2 supported)

AetherSDR includes a built-in CW (Morse code) decoder powered by ggmorse (MIT license). It automatically detects the CW tone pitch and keying speed, and displays decoded text in real time in a panel below the waterfall. It can decode what you receive, what you send, or both.

The decoder works on every radio family that has a CW mode: FlexRadio, the Hermes-Lite 2, and Networked Icom radios (it opens when an Icom slice is in CW).

Requirements

• The selected slice is in CW or CWL.
• On a FlexRadio, a DAX RX channel (1–8) assigned to the selected slice is recommended, so the decoder hears that slice alone. See Which audio is decoded.

Setup

Decoding is controlled in Settings → Radio Setup... → Phone & CW, next to Decode:

Toggle	Default	What it decodes
RX	On	The received CW on the selected slice
TX	Off	Your own keying, from AetherSDR's sidetone — useful for checking paddle or bug timing

Both appear in the same panel. Your own sending is shown in cyan, so it stands apart from the confidence-coloured received text.

Using the CW decoder

When the selected slice is in CW or CWL, the decode panel appears below the waterfall, and it hides again when you switch to another mode.

Drag the grip on the panel's edge to resize it; the height is remembered. Right-click the text for font size and Clear.

How it works

The decoder processes the received audio on a separate worker thread. It detects the dominant tone frequency, then uses a Goertzel filter and timing analysis to decode Morse characters.

• Auto pitch detection within the pitch search range
• Auto speed detection within the speed search range
• Cost function: measures how well the detected on/off timing matches ideal Morse ratios; this drives the confidence colours

Retuning resets the decoder state; pitch and speed locks survive a retune.

Which audio is decoded

The decoder follows the selected slice, independently of speaker volume or mute:

• On a FlexRadio, assign a DAX RX channel to the selected slice (1–8). The decoder then reads that slice's audio alone, regardless of speaker gain, mute, or other audible slices.
• Without a DAX channel, it uses the shared receive audio, which mixes every audible slice and follows speaker gain and mute. The panel shows RX: shared audio; hover it for the explanation.

Reading the confidence colours

Received characters are colour-coded by decode confidence (see Confidence colours). Green text can be trusted. Yellow is usually correct. Orange and red should be read sceptically — they often appear during QSB, QRM, or with operators who have irregular spacing.

Callsign lookup

When the decoder copies a station identifying itself (DE <call>), and QRZ.com lookups are enabled, a contact card for that station appears beside the decoded text. Click the callsign to open its QRZ.com page. See Callsign Lookup.

MQTT

Decoded text is published to the MQTT topic aethersdr/cw/decode, one JSON message per character, with "rx": false marking text from the TX decoder. See MQTT Station Automation.

Tips for best results

• Tune accurately: Centre the CW signal in your filter passband. The decoder auto-detects the pitch, but a clean signal helps.
• Use narrow filters: CW filter widths of 200–500 Hz reduce noise and competing signals that confuse the decoder.
• APF helps: The Audio Peaking Filter (APF) narrows the audio around the CW pitch, giving the decoder a cleaner signal.
• Give it a DAX channel: On a FlexRadio, a DAX RX channel on the selected slice keeps other slices and your speaker settings out of the decode.
• Narrow the search ranges or lock: If the decoder wanders between signals, narrow the Pitch and WPM ranges, or lock pitch and speed once it has the right station.
• Strong signals decode best: Weak signals near the noise floor produce mostly red/orange text.
• Client-side noise reduction is voice-only and turns itself off in CW, so it does not affect the decoder.

Reference

Panel controls

Control	What it does
Stats	Detected pitch (Hz) and speed (WPM)
Sens	0–100 (default 30). Hides low-confidence characters: 0 shows everything, higher values show only confident decodes.
🔒P	Lock the decoder's pitch at the current value
🔒S	Lock the decoder's speed at the current WPM
Pitch	Pitch search range, 300–1200 Hz (default 500–700 Hz)
WPM	Speed search range, 5–60 WPM (default 15–40 WPM)
CPY ALL / CPY VIS	Copy all decoded text, or just the visible text, to the clipboard
A− / A+	Decoded-text size (8–32 px), remembered
CLR	Clear the decoded text
✕	Close the decoder panel

Confidence colours

Colours are based on ggmorse's cost function:

Colour	Confidence	Cost range	Meaning
Green	High	< 0.15	Clean, well-timed CW
Yellow	Medium	0.15–0.35	Slightly imperfect timing
Orange	Questionable	0.35–0.60	May be incorrect
Red	Low	0.60–1.00	Likely noise or bad decode

Technical details

• Library: ggmorse by Georgi Gerganov (MIT license)
• Bundled directly — no external dependency
• Runs on a dedicated worker thread — does not block audio playback
• CPU usage: negligible

Known issues

• The decoder shows no text on some FLEX-8600 setups (#6158).
• The pitch search range is not kept when a slice is reopened, even with both range locks engaged (#5921).
• There is no setting to stop the automatic QRZ contact card from appearing for decoded callsigns (#5454).
• The squelch threshold line can appear on the panadapter in CW, although squelch is unavailable there (#6276).

Troubleshooting

The decode panel doesn't appear

The panel follows the mode of the selected slice.

1. Select the slice you want to decode.
2. Switch it to CW or CWL.
3. Check that RX is on next to Decode: in Settings → Radio Setup... → Phone & CW.

The decode changes with speaker volume, mute or other slices

The decoder has no DAX channel on the selected slice, so it uses the shared receive audio. The panel shows RX: shared audio.

1. On a FlexRadio, assign a DAX RX channel (1–8) to the selected slice.
2. Check that RX: shared audio is no longer shown.

The decoder jumps between stations

Another signal inside the pitch or speed search range is pulling the decoder away.

1. Narrow the filter to 200–500 Hz, or turn on APF.
2. Narrow the Pitch and WPM search ranges around the station you want.
3. Once it copies the right station, lock pitch (🔒P) and speed (🔒S).

Text is mostly orange or red

The signal is weak, poorly tuned or irregularly sent.

1. Centre the signal in the filter passband.
2. Narrow the filter or turn on APF.
3. Raise Sens to hide low-confidence characters.

See also

• CWX Panel — sending CW from the keyboard
• RTTY Operation — the built-in RTTY decoder works the same way
• DAX Virtual Audio
• Callsign Lookup
• ggmorse