After arriving in Berlin in January of 2019, I treated myself to a Casio F-91W. It was my first (non-essential) purchase in Germany, and for less than 10 Euros, it was a steal. Plus, a colleague of mine wore it and it looked cool. Easy decision!

Little did I know that this watch had a cult following on the internet. People collected all versions and variants of it and wore it with pride. I got pulled into the hype surrounding the F-91W and eventually more of the Casio retro digital watches.

It was an easy “hobby” to pursue given it is basically just buying more stuff. Nevertheless, it isn’t all that bad given how cheap these retro watches are. It is all relative of course, but given I’m also into photography, acquiring Casio watches is easier on my wallet.

Today, I own a couple more and I’m low-key proud of my collection.

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Learning about Radio Clocks

Like with anything I get obsessed with, I spent a lot of time reading and watching YouTube videos about Casios. At some point I stumbled upon the Casio Wave-Ceptor series of watches. It struck me as something different: This fairly low tech wrist watch could set and correct its time automatically!

“But wait, it isn’t a smart watch” I thought. How does it do that?

That sent me down the rabbit hole which I’m still on my way down right now. Turns out, the watch can receive signals transmitted by a time signal transmitter and correct its local time.

In Germany, there’s Mainflingen longwave transmitter transmitting DCF77 time signal. The carrier signal has a 77.5 kHz frequency and the time signal is generated from the local atomic clock which syncs with the main atomic clock in Braunschweig. It is operated by the Physikalisch-Technische Bundesanstalt which also hosts four public NTP servers:

ptbtime1.ptb.de

ptbtime2.ptb.de

ptbtime3.ptb.de

ptbtime4.ptb.de

Querying the time using the commandline utility sntp and watching the network activity in Wireshark gives us some interesting data.

What’s next?

I’ve ordered a cheap radio receiver to play around with the radio time signal. On the watch side, I’ll look for cheap used radio Casio watches on the local Ebay here and try to acquire. I hope this new obsession ends up teaching me a thing or three about analog signals.

That is it for this side-quest post. Thank you for reading!