Put a pin in it
Click any square to push a pin into the drum, and click again to pull it out. Turn the drum and the pins pluck the comb as they go past.
There is a tune in there already. Take the pins out and put your own in.
The third way to encode anything
A pin does not sound like a note, and nobody had to agree what it means. Where it sits is the whole message: how far round the drum is when the note happens, and which tooth of the comb it plucks is which note. Time along one way, pitch along the other.
That is a graph you can hold, and it is exactly what sheet music is: higher up the page is higher in pitch, further right is later in time. It is also a piano roll, a punched card, and the Jacquard loom that wove patterns from holes and gave Charles Babbage the idea for a computer.
And the drum turning is the clock. A code sent through time only works if both sides agree how fast. Send morse too quickly and it stops being morse. Turn the drum too slowly and the tune is unrecognisable, even though every pin is in the right place.
Codes that resemble Codes by agreement Back to signals