Why do 7 segment LCD displays break apart when I brush my teeth?

Question

This question might sound silly, but it is very serious (although geeky, I must admit). When I use my electric toothbrush in front of my alarm clock (one of those with a big red LED display), the numbers seem to break apart. Why?

With my current alarm clock the first 3 segments (a,b,c) make one group and the other 4 make another one. The striking thing is that both groups seem to slowly oscillate, in anti-phase. I cannot affirm that the grouping is the same for every display of the kind -- and I cannot check now because I only have this one LED display at home -- but I've seen this illusion on many different displays in the past.

I think this illusion is fascinating (told you I'm a geek) and I would like to understand why it is...

I believe that it has something to do with 1) the vibrations of the toothbrush making my eyes oscillate in their orbits so the image seems to move (a bit like when you touch your eye on the side with your finger and the whole image seems to move), 2) with the periodic refresh of the segments. I suspect that the two groups I'm seeing actually correspond to two groups of bars blinking in sync, so there really are two groups, (but why do they dislocate like that?) 3) with our periodic perceptual "refresh rate". Something similar to what makes us see car wheels as stationary when they rotate at a certain speed.

I must say my competence in electronics is close to zero, so my questions might be trivial to you guys. How does the refresh of the line segments in a 7 segments display happen? Is it cycling (a,b,c,d,e,f,g,a,b,c...)? How come I see two groups, then? At what frequency are the LEDs blinking? I understand some of these questions depend on the specifications of my alarm clock, but since I've seen this illusion on basically every type of LED display I passed by with my toothbrush (microwave, VHS video recorder (yes the story started long ago. I bought the toothbrush I'm now using just for understanding this silly illusion, but it was actually as a teenager that I noticed it)...), I guess there is something constant there...

Answer 1

Yes, it sounds like you are seeing artifacts of toothbrush frequency vibrating your head and therefore your eyes, and that beating against the LED refresh frequency.

This is a similar effect to eating potato chips (actually anything crunchy) while watching the LED display. In that case the head vibrations are more random, so parts of the LED display will appear to jump around randomly. Some segments will be displayed during a head-high part of a vibration, and others during head-low. These will appear in different locations.

LEDs are refreshed all kinds of ways. A lot has to do with how clever or competent the engineer was that wrote the firmware. I've seen naive refresh algorithms that simply do each digit in order. Those have the most apparent flicker for any one refresh rate. Better means interleave digits, sortof like interlacing of old TV scans lines. The whole display is still refreshed at the same rate, but the apparent flicker is at least in part related to the interlace rate. There are fancy schemes which interleave both digits and segments, but these are often not possible with common displays where whole digits are already partially wired together.

Answer 2

The power line noise could be beating against the LCD display refresh rate (assuming an underfiltered or undershielded power supply and a scanned matrix display output). The motor in the toothbrush can create an oscillating EM field which can couple to power lines. This is just like playing 2 slightly differently out-of-tune notes and hearing a "beat", except visually.

Answer 3

So this doesn't happen when the brush is merely running but not in your mouth; you have to be actually brushing your teeth? If that's true, then I think you nailed it, that it's a stroboscopic effect between the brush interacting with your eyes, and the refresh rate of the clock's display.

I see something related when I'm driving on the highway at night and approaching a car with LED tail lights, if I sweep my eyes across the road quickly while scanning traffic. Those tail lights look like a dotted red line, while the incandescent ones look like a single red streak.

Answer 4

@Max <<

It is called the "aliasing effect" or the intermodulation (IM) of the LED refresh rate for row or column matrix scanning and the vibration of the tooth brush.

Like the wagonwheel spokes it mixes and any harmonic of the Shannon sampling rate of the LED display and the fundamental of the vibration. So with all the IM Products and only 3 are shown below for brevity, ,

One of these frequency differences is at baseband or seismic seizure frequencies. For more details on IM or aliasing effects.

Both are synonyms and occur in mechanical, electrical, magnetic, optical and nuclear interferences of harmonic mixing. There are an infinite number of by-products limited only by the bandwidth of the channel.

You may also look up interference patterns for extra points and if you are really keen, you may find examples for brain wave interference patterns in chemistry. For Alpha, Beta and gamma waves in certain non-linear channels.

Glad you were inspired to ask.