CW Transcription

Morse code decoding, conversation tracking, and auto-fill

CW Transcription decodes Morse code audio from your radio, helping you copy callsigns and exchange information. The decoder uses real-time audio processing with adaptive frequency and speed tracking.

Enabling Transcription

The CW tab is always available. It ships hidden in the tab bar by default — add it from More → Customize Tab Bar. Once visible, tap the CW tab and grant microphone permission when prompted.

The transcription panel fills the CW tab view.

How It Works

  1. Audio capture - Microphone picks up your radio’s audio
  2. Tone detection - Goertzel algorithm identifies CW tones at the target frequency
  3. Frequency tracking - Automatically locks onto the dominant tone and follows drift
  4. Speed estimation - Adaptive WPM tracking adjusts to the sender’s speed in real time
  5. Decoding - Morse patterns converted to text using timing analysis
  6. Display - Decoded text shown in real-time with conversation separation

Audio Processing

Goertzel Filter

The decoder uses the Goertzel algorithm for efficient single-frequency tone detection:

  • Targeted detection - Computes signal energy at a specific frequency rather than performing a full FFT
  • Low CPU overhead - Runs continuously in real time without significant battery impact
  • Configurable bandwidth - Detection window can be narrowed to reject adjacent signals
  • Sample-by-sample processing - Operates on the audio stream with minimal latency

The Goertzel filter is more efficient than FFT when monitoring a single tone, making it ideal for CW decoding on mobile devices.

Adaptive Thresholds

Background noise is analyzed to set:

  • Tone detection threshold (auto-adjusts to ambient noise floor)
  • Noise gate level
  • Signal-present confidence score

Audio Input Sources

  • Device microphone - Hold phone near speaker
  • Wired audio - Connect via audio cable for cleaner signal
  • WebSDR audio - When connected to a WebSDR receiver, CW transcription can decode the receiver’s audio stream directly

Frequency Tracking

Automatic

The decoder automatically finds and tracks the CW tone:

  • Detects dominant tone frequency in the audio passband
  • Adaptive tracking follows the tone as it drifts (common with analog receivers or temperature changes)
  • Works across typical CW pitch ranges (400-900 Hz)
  • Lock indicator shows when the decoder has acquired a stable tone

Manual

If automatic tracking struggles:

  1. Tap the frequency indicator
  2. Adjust the target frequency manually
  3. Tap to lock

Manual lock is useful in high-QRM conditions where multiple tones compete for the decoder’s attention.

WPM Estimation

Adaptive Speed Tracking

The decoder continuously estimates and adapts to the sender’s speed:

  • Initial detection - Analyzes the first few characters to estimate base speed
  • Running average - Maintains a weighted moving average of dot and dash lengths
  • Speed display - Current WPM shown in the transcription panel
  • Range - Tracks speeds from approximately 5 WPM to 40 WPM
  • Speed changes - Adapts within 2-3 characters when the sender’s speed changes

Farnsworth Timing

The decoder handles Farnsworth-spaced CW (where character speed is faster than word speed) by independently tracking:

  • Intra-character timing (dots and dashes within a character)
  • Inter-character spacing (gaps between characters)
  • Inter-word spacing (gaps between words)

Transcription Display

Live Transcript

Characters appear as they’re decoded. The display shows:

  • Decoded text in a scrolling view
  • Current character in progress (partial decode indicator)
  • Timing indicators for element detection

CW Conversation Tracker

The CW Conversation Tracker uses a state machine to separate and identify the two sides of a QSO:

State Machine:

  1. Listening - Waiting for initial transmission
  2. Station A transmitting - Receiving from the first station
  3. Pause detected - Gap between transmissions
  4. Station B transmitting - Receiving from the second station
  5. QSO in progress - Alternating between stations

Identification heuristics:

  • Callsign detection - When a callsign is decoded, the tracker associates it with the current transmission
  • Timing patterns - Long pauses indicate station transitions
  • Frequency shift - Slight frequency differences between stations (RIT offset) help distinguish them

Chat-Style Conversation View

For extended QSOs, the conversation view presents decoded text in a chat-bubble layout:

  • Left bubbles - Station A transmissions
  • Right bubbles - Station B transmissions (or your own, if identified)
  • Timestamps between exchanges
  • Callsign labels when identified
  • Color coding distinguishes the two stations

This view makes it easy to follow the flow of a QSO, especially at higher speeds where real-time copying is challenging.

Callsign Detection

The decoder identifies likely callsigns in the transcript:

  • Highlighted in the transcript with a distinct background color
  • Common patterns recognized (W/K/N/A prefix for US, VE for Canada, international prefixes)
  • Confidence indicator shows how likely the detected string is a valid callsign
  • Tap to auto-fill the Logger callsign field

Auto-Fill Integration

When a callsign is detected:

  1. Callsign appears highlighted in the transcript
  2. Tap to fill the Logger callsign field
  3. QRZ lookup triggers automatically
  4. Continue copying the QSO

WebSDR Integration

When connected to a WebSDR receiver, CW transcription integrates with the tuning system:

  • Auto-retune - If the CW tone drifts significantly, the decoder can request the WebSDR to retune for optimal reception
  • Frequency sync - The decoder’s target frequency stays in sync with the WebSDR’s passband center
  • Transcript sync with playback - During recording playback, the transcript scrolls in sync with the audio position

Tips for Best Results

Audio Setup

  • Hold phone near speaker, or use audio cable
  • Reduce background noise
  • Moderate volume (not too loud, not too quiet)
  • For best results, use the WebSDR audio path which bypasses the microphone entirely

Tuning

  • Center the signal in your passband
  • Use narrow filter if available
  • Reduce QRM by tuning away from interference

Limitations

CW transcription works best with:

  • Clean, well-spaced sending
  • Single signal (no QRM)
  • Standard timing

Challenging for:

  • Very fast sending (35+ WPM)
  • Poor sending (irregular timing)
  • Heavy QRM/QRN
  • Multiple overlapping signals

Accuracy

Machine CW decoding isn’t perfect. Use transcription as an aid, not a replacement for developing your own ear. When in doubt:

  • Ask for repeats
  • Verify callsigns via QRZ lookup
  • Use your own ears for critical info

See Also