I don’t know how to explain this phenomenon. I was standing about 2 meters away from my monitor, eating a bowl of Honey Nut Cheerios. Whenever I was chewing, the monitor would appear to flicker as if doing a degauss. I couldn’t repeat this effect for any other object, just the monitor screen. How could chewing affect the way my eye perceives the monitor picture?
My refresh rate is 31-60 kHz horizontally, 56-75 Hz vertically. But the flicker was seen only in the vertical direction, so the 56-75 Hz figure is probably the relevant one. Those aren’t frequencies associated with vision or cranial vibration during chewing, are they? If they are, is it possible that I am witnessing resonance phenomena?
Come to think of it, the frequency of cranial vibrations probably depends on the size of the head in question, among other things. I can provide measurements upon request, if any poster wants to investigate that angle. Is there even an established method to estimate frequencies of cranial vibrations based on measurements of the head?
I was chomping on some Corn Nuts and looking at a monitor across the room. Almost same thing, but I’d call it a freeze, or pause or something. Like the vertical refresh stopped for a millisecond.
Yep, I’ve noticed it a lot at work (I’m an ice chewer, myself) but don’t have an explanation. It’s especially weird when you are looking at a whole roomful of monitors.
A similar phenomenon I’ve noticed is that if the refresh rate on the monitor is low enough, I can see a definite flicker in my peripheral vision, but not when I’m looking straight at the monitor. Don’t know if that is related at all or not…
Yep, Shoeless it is related. Your peripheral vision is better for detecting the refresh rate, although it is possible to detect the difference looking right at it when you use a computer a lot (like me). The reason the screen seems to wobble is because you are vibrating your head. When you are looking at the screen, the picture is made by a scanning electron beam. Your eyes are not fast enough to catch the beam as it makes its rounds, and so the scans are interpreted as a picture. By vibrating your head, your eyes are not quite in the same position during the scan, and so the picture seems distorted.
Clicking your teeth is the ultimate at showing this effect, and it’s most visible with peripheral vision.
Slow refresh rates (60 Hz) coupled with fast-fade phosphors show it badly. With refresh rates of 85 Hz or so, or with long persistance phosphor, it is very difficult to perceive.
Once at Comdex about a hundred years ago (when most monitors were monochrome refreshing in the 60 Hz range) I managed to convince a bunch of people that most all the monitors on display were really crummy… just stomp your foot on the floor and you can make them all flicker in unison! While those high-quality green monitors over there don’t flicker at all! (Monchrome green monitors usually had a LONG persistance.)
After I had them convinced, I suggested they look around and click their teeth gently… the puzzlement as they tried to rationalize how this could influence the monitors was great. Once they had become convinced they were making the monitors flicker they were very reluctant to discard the idea and accept it was their perception.
They didn’t seem to have a problem with the fact that everyone saw the monitors flicker when they personally stomped a foot, but not when anyone else did.
The effect is enhanced if you stand farther away from the monitor, too. My guess is that it’s because the farther away you are, the smaller angle of arc the picture occupies in your field of vision, so the greater the effect of a small head vibration.
Extra fun: put a vibrating pillow on your head and watch TV. It’s a similar effect, but the head vibrations are very regular, so the picture distortion is very regular as well, and it looks like the picture is waving like a flag.
Your eye doesn’t see continuously. It’s sort of like a movie camera but somewhat faster, frame by frame. You’ve probably seen wheels appear to move backward while the vehicle is going 30-40 mph. Each pulse of vision sees the wheel in a different position, so if the pulse comes slightly faster than the motion of the spokes of the wheel, it looks like the wheel is in reverse. If the pulse comes slightly behind the movement of the spoke, it will look like the wheel is moving forward, but very slowly in comparison to the vehicle.
It will happen with some LED clocks too. You’d think the AC would be chaged to DC to power the LEDs, but apparently some of the 60hz cycle is still present.
The consensus in this column by Cecil, taking into account the responses from the teeming millions, seems to be that the illusion of reverse rotation is caused by the periodicity of film reels or street lighting, not the periodicity of human vision. If you think it’s really due to the eye processing input discretely rather than continuously, perhaps you should start a thread in Comments on Cecil’s Columns.
Crazy! I was going to ask about this yesterday. I can see the monitor over my shoulder in the mirror and sometimes it flickers. I thought it must have to do with the mirror but now I guess it’s just that I’m seeing it out of the corner of my eye when it flickers.
Actually they turn the LEDs on and off so they can save themselves a few cents on the LED driver circuitry. You can trade off using fewer address lines for the 7 segment displays but part f the trade is that you lose the ability to drive multiple LEDs simultaneously. So instead you have the little microcontroller in the clock pulse quickly through the LEDs that you want on. If you do it quick enough then your eye can’t tell the difference (unless you are chewing something crunchy, which takes us back to the OP).
There’s another reason: An LED has a max continuous power it can be driven at. If you drive the LED at 10x the power with a 1/10th duty cycle it is driven at the same average power but it appears brighter to the eye. As you say, multiplexing the driving circuits also simplifies the design.
