How to decode Morse code from an audio recording
To decode Morse code from an audio recording, slow the recording down, loop a few seconds at a time, and write what you hear as dots and dashes on paper, with a space at each letter gap and a slash at each word gap. Then translate the written Morse with a chart or a Morse to text translator. If the tones are too fast or too faint to hear cleanly, open the file in an audio editor and read them off the waveform instead: short blocks are dits, long blocks are dahs.
It is slower than copying by ear in real time, but it works on almost any recording, and you need no skill beyond patience and a chart.
What you need
- The recording, as a file you can open in an audio editor. If the Morse is in a video, export or record the sound to an audio file first.
- An audio editor that can slow playback, loop a selection and show the waveform. Many free ones can.
- Headphones. They make faint dits and short gaps far easier to hear than speakers do.
- Paper and a pencil, or a plain text file.
- A Morse chart, or the Morse tree if you find it faster.
Step 1: listen once for the shape
Play the whole recording through once without writing anything. You are listening for its shape: where the Morse starts and stops, roughly how fast it is, whether it is one even, machine-sent rhythm or a human hand that speeds up and slows down, and whether anything repeats.
Repeats are a gift. A recording of an amateur radio call may begin with CQ, -.-. --.-, calling any station, and a callsign sent two or three times. DE, -.. ., means “from” and is usually followed by a callsign. The end of a transmission is often K, -.-, for “over”, or AR, .-.-., for end of message. Spotting one of these early tells you where words begin and gives you a few letters for free.
Step 2: slow it down and loop it
Set the playback to half speed with the pitch kept the same; audio editors often call this changing the tempo, or time stretching. Slowing down stretches every dit, dah and gap by the same amount, so the proportions that make Morse readable stay intact. If your editor can only slow down by lowering the pitch, that works too, as long as the tone stays clear.
Then select a few seconds, a word or two, and loop it. Each pass, write down a little more. When that stretch is done, move the selection on and overlap it slightly with the last one, so you do not lose a letter at the join.
If there is hiss, music or other signals under the Morse, an equalizer or filter set to keep a narrow band around the pitch of the tone can make it stand out. Find the pitch by ear first, or from the spectrogram view described below.
Step 3: write the rhythm, not the letters
Write each dit as a period and each dah as a hyphen, a space at each letter gap and a slash at each word gap. Do not translate while you write. The moment you stop to think “is that F?”, you miss what comes next. Capture the rhythm first; the letters come later, in one calm pass.
Judge the gaps against each other, not against a clock. A gap inside a letter is the length of a dit. A gap between letters is about the length of a dah, three dits. A gap between words is longer again, about seven dits. In practice recordings sent with Farnsworth spacing, the gaps between letters and words are stretched beyond that, but a word gap still stays clearly longer than a letter gap.
Hand-sent Morse is the hard case. A real operator’s dahs are not all the same length, and their gaps wander. Decide each one by comparing it with its neighbors: if one gap is clearly longer than those around it, it is probably a letter gap.
Reading Morse off the waveform
When your ears give up, use your eyes. In an audio editor’s waveform view, each tone is a solid block and each silence a flat line. Widen the view until a single word fills the screen, and the Morse is drawn in front of you:
| On the waveform | What it is | Length |
|---|---|---|
| Short block | dit | 1 unit |
| Long block | dah | 3 units |
| Short flat line | gap inside a letter | 1 unit |
| Medium flat line | gap between letters | 3 units |
| Long flat line | gap between words | 7 units |
Some audio editors also have a spectrogram view, which shows pitch from bottom to top. There the Morse appears as a single horizontal line broken into short and long dashes, and a second signal or a burst of noise sits at its own height, so it is far easier to tell apart.
The waveform also tells you the speed. Select one dit in the original recording, not a slowed copy, read its length in milliseconds, and divide 1200 by it to get words per minute. A dit of 60 ms means 20 WPM; a dit of 100 ms means 12 WPM. Once you know the dit length, you can measure any doubtful element or gap instead of guessing. How long is a dit? explains where the 1200 comes from.
Step 4: translate and check
With the whole recording on paper, translate it group by group. A group that matches nothing is almost always a spacing problem: two letters you heard as one, or one letter you split in two. Try splitting or joining groups until a real word appears; Morse code to text walks through how. A few other things that look like mistakes but are not:
- Eight dits,
........, is the error signal: the sender made a mistake, and what follows usually replaces the last word. - Run-together letters at the start or end of a message may be a prosign, such as AR,
.-.-., or SK,...-.-. - Contest recordings often send the signal report 599 as 5NN,
..... -. -., where N stands for 9. - Abbreviations such as TU (thank you), 73 (best regards) and Q codes such as QTH (location) are normal radio shorthand, not garbled words.
On a Mac, Dot & Dash can help with the last step only. It does not listen: it cannot decode Morse from a recording, the microphone, a radio or a video, so the decoding is still yours, by ear or from the waveform. Once you have written the dots and dashes down, you can type them into Transcribe’s Morse to Text, and the message appears as you go.
Getting faster at it
Decoding a recording with a slow-down and a pencil works from your very first try. Copying Morse by ear in real time takes longer, but each recording you work through by hand trains your ear a little, especially if you replay the finished stretch at full speed while reading along. How to start copying Morse code by ear is the next step if you want to stop needing the editor.
Questions
Can Dot & Dash decode Morse code from audio?
No. Dot & Dash does not listen to recordings, the microphone, radios or videos. It translates Morse you type or paste, so you decode the sound by ear or from the waveform first, then type the dots and dashes into Transcribe.
How can I tell how fast the Morse in a recording is?
Measure one dit in the original recording in an audio editor and divide 1200 by its length in milliseconds. A 60 ms dit is 20 words per minute, a 120 ms dit is 10.
What if two letters sound like one?
Then you probably missed a letter gap. Write the group as you heard it, and when you translate, try splitting it into two real letters and keep the split that makes a word.
Does slowing a recording down change the Morse?
No. Slowing it stretches every element and gap by the same amount, so the 1, 3 and 7 proportions stay the same and the code reads exactly as it did at full speed.