Tempest For Eliza


Tempest for Eliza” is a program written by Erik Thiele to demonstrate the RF transmission properties of CRT monitors. The program is capable of transmitting sound over AM frequencies just by the unique way it drives the monitor. The electromagnetic emissions of CRTs can be a security problem as demonstrated by Van Eck phreaking, watching someone’s screen just by collecting RF transmissions. In the late ’90s Ross Anderson developed software to help reduce the RF transmissions of monitors. These specialized fonts combined with shielding can greatly reduce the risk of attack and is something the NSA has been researching for many years.

[thanks gsham]

27 thoughts on “Tempest For Eliza

  1. This is sooo old. I did this several years ago. As I recall, this works not only for CRT’s because its not actually the CRT its self that generates the RF noise, but the cable that carries the signal. I think it will work on LCD’s that have an analog connection. Of course if its digital, then no go.

  2. oo oldie-but-[very]-goodie. thanks eliot – i had forgotten about this one. i love hacks like this where the capabilities of a device are stretched far beyond their original scope. a crt as a transmitter? fantastic! its a wonderful thing to turn the (otherwise annoying) fcc rules, part 15b into something useful — or at the least, something amusing.

    regarding van eck phreaking: my thoughts immediately went to stephenson’s cryptonomicon. (if you havent heard of it and are reading this website, something is amiss. buy/steal/borrow a copy of this book immediately). given the design of a CRT, its understandably easier to detect residual radiation indicative of onscreen content than it is with an lcd. however, while many people scoff at the idea, LCDs do not really prevent the same effects from occuring. a substantial emf field is pumped out of every LCD too. luckily field dropoff 1/d^2 comes to our rescue. if you’re still worried, quickly don your tinfoil hat and into the faraday cage you go!

    for the doubting: if the eliza’s tempest blaring from your radio is unconvincing, find yourself a field strength meter and test it yourself. you might be surprised.

    ..and as an aside: oh man…you know you’ve got the bug when you’re excitedly typing out this stuff like i do. if you’ve read this far, many thanks for sharing the same sentiments.

  3. This reminds me of a computer show I was at back in ’78. Some of us smug Apple ][ owners went over to the Radio Shack booth to harrass their rep about how the TRS-80 didn’t have color or sound and the fact that we did. The rep showed us their new sound technology: he set an AM radio next to the computer and ran some software that created interference on the radio that barely passed as music.

  4. Haha we are not the only fools!
    In the eighties we had a Sinclair ZX-81, with an memory extension of 16k, and with some additional with which we could feed it machine code. My dad found out it caused our Philips b&w TV (connected to the ZX-81 as monitor) to make a sound at a certain action which i do not remember.
    He went totally fanatic on this, and made a machine code program to feed it with a tune in a string, such as A3C3G4 etc. And behold! It worked. On that particular TV set [add a lot of white noise here].

    We were a bit hurt about the lack of enthousiasm given to it at the ZX81 computer club…

  5. To post 11!… I remember my Dad typing in something from a magazine that did that on our zx81. You could press keys on the keyboard to create different notes.

    I can’t remember the make of the tv but it was white plastic with two black knobs, one for tuning to the channels and one for volume(on/off). The picture was b/w as well.

  6. That’s pretty cool.

    Did anyone get the MP3 playback to work, though? For me, it runs through the whole MP3 in like 5 seconds and doesn’t sound anything like it should. I looked at the code, and I don’t see anything that should slow it down. I’m using the proprietary NVidia drivers, and I tried both settings for VSYNC.

  7. Cool, i was wondering if this would work with a tv. because i have my computer hooked up to an lcd monitor and a tv. i have a crt monitor in the basement but im too lazy to go get it but i really want to try this.


  8. yeah, rock on with CRTs. More bang for your buck, and when you put it in a corner, the CRT’s extra size isn’t as much of a factor. Not that I don’t use LCDs whenever possible. Not that I don’t have them installed at all my clients’ offices. But I just bought a computer off of a friend, and Guild Wars looks amazing at 1600×1200 on max everything. Besides, after the cost of the computer, you use whatever you’ve got until the paycheck arrives. I can’t believe everyone’s offloading their CRTs! They make great server terminal consoles. I mean…yeah. Does anyone use an LCD as a server screen? I guess if space was limited…

    [/offtopic] I seem to remember doing something similar with a game system a while back, and something was picking up the sound. Guess that was just unshielded cabling, but anyone with a Nokia will recognize the buzzing noise emitted from nearby speakers when the cellular transmissions come in. This whole thing also reminds me of that hack with the DS a while back where images were being picked up on a nearby Watchman.

    …and people are surprised by hackers sniffing their “private” transmissions? lol. sorry for the musing.

  9. Do you remember that today is the anniversary of Sinclair ZX81 appeared to the market? It is thirty years now. Thus began the era of home computers. The heart of the computer was a Zilog Z80 microprocessor clocked at 3.25 Mhz. The basic version of the computer had one kilobyte of memory (two million times smaller than a modern laptop to 2GB of memory). Programs were loaded with cassette tapes connected via an ordinary tape recorder. The screen had a resolution of 64 to 44 pixels. It was produced for 3 years, until 1984. Sold over half a million copies. Great times don’t you think? :)

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