The flyback sonar

[Image: Close-up photo of a PCB with various components, including three potentiometers and a black bulky device labeled 'MONITRONICS INC TAIWAN'.]

One of the sounds of my childhood was the 15.6 kHz noise of the vibrating television flyback transformer, caused by a phenomenon known as magnetostriction. It has since disappeared – not because I grew old enough not to hear it (phew, not yet), but because cathode ray tubes have been replaced by modern display technologies that don't use magnetic deflection coils.

The piercing, ubiquitous sound would easily reveal if someone had a TV on in the house. It would also change a little when channels were changed. But due to its short wavelength, it would also reveal when someone was moving around in the room the TV was in. This was apparent as a modulation that could even be heard through a closed door and in another floor. Listening to it was much like using a passive sonar.

Now, inspired by some people at the University of Washington and Microsoft Research, I decided to investigate what the modulation was actually about.

Faking it a little

To get a stable tone at the flyback transformer, the TV apparently has to be tuned to a channel. This poses a problem: all analogue TV broadcasts in this country were discontinued in 2007. I can use my RF modulator to generate a TV channel, which stabilizes the sound. But it's still quite weak for scientific purposes, coming from such a tiny portable TV.

So I ended up sampling the sound and then using a digitally generated version, after confirming that it is indeed a pure sinusoid. I'll play the sound through the speakers and use the laptop's microphone to record the soundscape in the room. Of course, this is different from the setup where the moving person is between the sound source and the listener; but nevertheless, it should give us some insight.

It's a Doppler shift!

Blocking the sound source obviously modulates the signal amplitude to some extent. But turns out even a slight movement anywhere in the room causes noticeable frequency modulation in the echoed tone. This is obviously due to Doppler shift, since the sign of the shift correlates with the direction of the movement in respect to the laptop.

In this video, the zero shift frequency has been filtered out of the spectrogram.

Another childhood mystery settled.


  1. Hey Oona, you mentioned that you used baudline to measure and view signals. I gave it a try, but I'm getting an "all input devices disabled" error and when I run baudline with aoss, I get other errors.

    Did you do anything special to your Ubuntu install to get baudline working?

    1. I run it under aoss. I experimented with different -fragsizes to get it right. Currently it works with "aoss baudline -fragsize 12" using sampling frequencies from 44.1k up to 96k. FAQ: I get a "/dev/audio requested fragsize ignored" error message when I try to record.

  2. Nice work and explanations :) I wonder, how accurate representation of persons movements you could get with these kind of techniques? And should one try to visualize it somohow + train the operator (analog style) or should one aim to maximize bits and degrees of freedoms (digital style). Hmm, actually we'd approach a indoor close range sonar/radar here.

    Reminds me also of that one Batman movie, where Morgan Freeman built the thing to capture persons movements indoors. I thought it was Hollywood functional BS, but Oona made it live :)

  3. A couple of high frequency emitters (20kHz+) around the room, and some high quality mikes -- would that be enough to triangulate where there's movement?

  4. Careful Oona, that's "University of Washington" and not "Washington University". This matters because there's a notable university in St. Louis, Missouri called "Washington University in St. Louis" (WUSTL). ^_^

  5. Love your site! This reminded me of an idea. A passive radar artificial vision system using ambient radio signals. It could be used for urban warfare during the night and replace night vision goggles.. My idea is a helmet with two or three antennas, a plurality of SDR receiver chips and software to glean Doppler shift from the signals emanating from cellular and broadcast transmissions. Inside the helmet would be a heads up display to overlay objects in view of the system. RFI-EMI-GUY USA


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