Shmoocon 2017: Software Defined Radio For Terahertz Frequencies

Before Bluetooth, before the Internet of Things, and before network-connected everything, infrared was king. In the 90s, personal organizers, keyboards, Furbys, and critical infrastructure was built on infrared. Some of these devices are still around, hiding in plain sight. This means there’s a lot of opportunities for some very fun exploits. This was the focus of [Mike Ossmann] and [Dominic Spill]’s talk at this year’s Shmoocon, Exploring The Infrared World. What’s the hook? Using software-defined radio with terahertz frequencies.

[Dominic]’s infrared detector
Infrared communication hasn’t improved since the days of IrDA ports on laptops, and this means the hardware required to talk to these devices is exceptionally simple. The only thing you need is an IR phototransistor and a 4.7k resistor. This is enough to read signals, but overkill is the name of the game here leading to the development of the Gladiolus GreatFET neighbor. This add-on board for the GreatFET is effectively a software defined IR transceiver capable of playing with IrDA, 20 to 60 kHz IR remote control systems, and other less wholesome applications.

Demos are a necessity, but the world seems to have passed over IR in the last decade. That doesn’t mean there still aren’t interesting targets. A week before Shmoocon, [Mike Ossmann] put out the call on Twitter for a traffic light and the associated hardware. Yes, police cars and ambulances use infrared signaling to turn traffic lights green. You shouldn’t. You can, but you shouldn’t.

What was the takeaway from this talk? IR still exists, apparently. Yes, you can use it to send documents directly from your PalmPilot to a laser printer without any wires whatsoever. One of the more interesting applications for IR is an in-car wireless headphone unit that sends something almost, but not quite, like pulse coded audio over infrared. The demo that drew the most applause was an infrared device that changed traffic lights to green. The information to do that is freely available on the web, but you seriously don’t want to attempt that in the wild.

29 thoughts on “Shmoocon 2017: Software Defined Radio For Terahertz Frequencies

  1. There are rumours that using xenon headlights minicab drivers can cause the “blues & toos” sensor in some traffic lights to trigger and start a cycle to change.

    Some drivers claim it’s possible, others faint ignorance.

    All I know is there was a large section of magazines dedicated to this and special registration plate masking paint.

    But this at least seems a credible way to achieve the same effect.

    1. I’ve seen it work in the past but they changed it from merely being a flashing light causing the trigger in the 1980’s and 90’s to a bit more of a special sequence which is much harder to do. Was fun while it lasted though.

      1. the old xenon-strobe triggered versions still exist, if you know where…

        im not sure of the exact flash pattern, but the beat of most electro-house techno works okay… just need to turn off strobing in order for the crosswalk to let you cross, otherwise you’ll have to wait til your song is no longer strobing for “walk” light.

        and if there are traffic cameras you’d better’d hope it “looked” like an accidental triggering as i assume the triggers get red-flagged…

        as for lightning?
        ive never hung-around tall metal posts in a lightning-storm long enough to notice any effects, and i do not reccomend you do either.

  2. 4Mbaud IRDA receivers are really handy for all sorts of IR work, and performance is better than anything you can make easily from discretes.
    A big advantage of IR over radio for some applications is you can make it very directional, and control the range, and not worry about it going through walls, so good for things like indoor location beacons.

    1. Nothing more than a sound card in a pc and appropriate audio transformer in between the card and the it led/diode. There’s PC software that can modulate QAM64 over a single mono audio channel to send any file/data. I have used a program called JQam to send a 370kb/s QAM64 signal over an LED/photo transistor arrangement. The program can even send streaming video and audio over either a QAM 64 or 16 signal. It has a theoretical limit of about 400kb/s per second.

      There’s a demo available to see if you like the features, though it’s limited to just QAM 16 at 17kbps.

      1. Also it costs $25. But that gets you 2 licenses so you can set up two PC’s for send and receive. Though to be honest the guy has put in zero DRM of any kind so there’s really no limit to how many PC’s you install it too once you have a full copy.

        1. I see what you’re saying, and maybe that’s what Joslyn meant as well. But its simplicity makes it a great place to start, even if it’s not exactly true QAM 64 modulation on its own.

          I was thinking of another way to directly modulate QAM64 over a light source as I typed the above response. You could use 2 LEDs, each a different wavelength, and each handle one of the two axis of the QAM signal. Then all you would need to do is very the intensity of each to produce the QAM signal without having to deal with phase. If you ran this off a sound card capable of producing a 192khz signal, you could potentially output 96,000 symbols per second, that’s 567kbps. Not too shabby for a sound card.

  3. I didn’t read TFA, but it might be useful to say that with 4.7 K resistor high speeds might be problematical. Lower resistance will allow higher speeds, but with lower sensitivity. Higher resistances will give low speeds, but higher sensitivity.

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