Morse code video decoder

How to use it

1. Press Choose a video and pick a video from your phone or computer, or drop it on the box.
2. Wait while it plays through at double speed. It looks for a light sending morse and listens to the sound at the same time.
3. Check the box round the light it read. If it picked the wrong one, press Tap the light and tap the right one.
4. Switch between Light and Sound to see both readings, fix the transcript if you need to, and press Copy text.

Videos longer than 2 minutes are read up to the 2-minute mark. Play the video once it is read to see each letter light up as it is sent.

What it reads

• A flashing light: a signal lamp, a flashlight, an LED or a screen flashing on and off, anywhere in the picture, even small.
• Beeps in the sound track, the tone used on radio and in recordings, read as the audio decoder reads them.
• Any video your browser can play, such as MP4 from a phone, or WebM.

It reads one light at a time. It doesn’t read morse written in the picture; for that, take a screenshot and use the image decoder.

How it reads a light

1. It measures every frame. Each frame is cut into a grid 64 squares wide, and the brightness of every square is noted, frame by frame.
2. It finds the light. A light sending morse jumps between two levels, dark and bright, and stays at each for a while. Squares that do that are candidates; squares beside them that brighten at the same moments are the same light. It reads the five best candidates and keeps the one that reads most clearly, so a flickering screen or a steady street light loses to the lamp.
3. It follows the camera. The dark level is measured again every tenth of a second, so a phone that changes its exposure, or a scene that brightens, doesn’t confuse it.
4. It times the flashes. From there it works like the audio decoder: it learns the sender’s dit (the short signal) from the flashes, names each flash a dit or a dah (the long one), and sorts the dark gaps into three sizes.

Those sizes are set by ITU-R M.1677-1. A dah is three dits long; the gap inside a letter is one dit, between letters three, and between words seven.

How accurate it is

We test it on generated videos of a lamp sending a short sentence with four digits in it, three times each. These are made by our test code, not filmed, so a real video can read worse.

• No characters wrong at 24, 30 and 60 frames a second, at every speed from 5 to 20 words per minute (WPM), with the speed read to within 1%.
• No characters wrong with heavy camera grain, the exposure drifting by 30%, one frame in ten dropped, a light smaller than a two-hundredth of the picture’s width, a bright background, a flickering screen beside it, a brighter street light in the picture, or the scene brightening halfway. The same with grain, drift, dropped frames and both decoy lights at once.
• Faster sending is untested. At 20 WPM a dit lasts 60 milliseconds, just under two frames at 30 frames a second. Much faster, and a dit can fall between frames.

When it doesn’t read

It boxes the wrong light

Press Tap the light and tap the one sending morse.

It finds no light

Film closer, so the light is bigger in the picture, and hold the phone still or rest it on something. A light that is always partly lit, like a lamp glowing between flashes, still works if the flashes are clearly brighter.

Dits go missing

The sending is too fast for the video’s frame rate. Film at 60 frames a second if your phone can, or ask the sender to slow down.

Letters run together or split apart

The dark gaps are too alike. A sender should leave three dits of dark between letters and seven between words.

More morse tools

• Morse code audio decoder

  Hold your microphone to morse, or choose a recording, to read it.

• Morse code image decoder

  Choose a photo or screenshot of morse to turn it into text.

• Morse code translator

  Type text to hear it in morse, or paste morse to read it.