What

Add support for CW key input via MIDI devices (USB MIDI), as an alternative to the current serial-port-based paddle input.

In this mode, AetherSDR would:

• Open a MIDI input device
• Listen for MIDI events
• Map:
  • Note On → key down
  • Note Off → key up
• Use these events to key CW transmission

This allows using external hardware keyers that already implement iambic logic and timing.

Example of such a device (tested concept):

• Teensy-based MIDI keyer firmware:
  https://github.com/DL3LSM/MidiIambicKeyer

Why

The current serial-based approach (CTS/DSR/DCD polling) works, but has some limitations:

• Polling-based → susceptible to latency and jitter
• Depends on USB-serial adapter behavior
• Requires handling:
  • debounce
  • iambic timing
• Electrical differences between adapters can cause inconsistencies

Using MIDI instead:

• Event-driven (no polling)
• Lower and more stable latency
• On macOS, MIDI runs with high scheduling priority (CoreMIDI)
• Enables use of external keyers that:
  • implement iambic A/B
  • generate precise timing in hardware

This effectively moves timing-critical CW generation out of the client, simplifying the software side.

How Other Clients Do It

• SmartSDR for Mac uses serial control lines (RTS/CTS/DCD), but in a loopback configuration:

  • avoids GND dependency
  • improves signal reliability

• MIDI-based input is not common in SDR clients, but widely used in:

  • DAWs
  • real-time audio systems

These systems rely on:

• CoreMIDI (macOS)
• ALSA MIDI (Linux)

Both provide:

• event-driven input
• optional timestamping
• lower jitter than generic serial polling

Suggested Behavior

Settings

CW Input Source:
Serial (default)
MIDI

MIDI mode

When selected:

1. List available MIDI input devices
2. User selects one device
3. AetherSDR listens to that device

Event mapping

MIDI Event	Action
Note On (velocity > 0)	Key down
Note Off	Key up
Note On (velocity = 0)	Key up

Interaction with CW logic

• In MIDI mode:
  • Treat input as straight key (on/off)
  • Disable internal paddle/iambic processing
• External device is responsible for:
  • iambic mode
  • timing
  • debouncing

Optional (future)

• Filter by:
  • MIDI channel
  • note number
• Separate MIDI note for PTT

Protocol Hints

No changes expected on FlexLib side.

Existing TX/CW control should be reused:

• trigger TX on key down
• release TX on key up

So this is purely an input layer change.

Implementation Notes

Architecture

class CWInput {
public:
virtual void start() = 0;
std::function<void(bool)> onKey;
};

Implementations:

• SerialCWInput (existing)
• MidiCWInput (new)

macOS

Use CoreMIDI:

• MIDIClientCreate
• MIDIInputPortCreate
• callback with MIDIPacketList

Linux

Options:

• ALSA sequencer (snd_seq_*)
• or RtMidi (simpler cross-platform abstraction)

Summary

• Adds a modern, low-latency CW input path
• Keeps full backward compatibility
• Enables use of external hardware keyers
• Simplifies timing handling inside AetherSDR