On occasion, like now, I sit in front (as opposed to behind) my computers’ monitor listening to the radio. Humming along to Metallica (Insomnia-nia-nia-nia), I notice the picture jumping around a bit when I hit a lower (E-ish) note. Is my skull not put together correctly? My wife can’t do it, mebbe because her voice isn’t deep enough. Is there a frequency at which the average male can match that of a computer monitor thereby creating some sort of disruption in the convergence of eyeball and screen? Or am I in need of a straightjacket?
man i remember my first hum job…oh wrong thread!
i don’t know what causes this but it happens to me when blowing razzberies (i couldn’t figure out a non-sexual way to say that) it works with digital clocks too (instead of your monitor) you know, the ones with red glowing numbers.
eggo
G,day.
Isn’t it to do with the the fact that the picture is shown to you at about 24 frames per second. Look at the screen with your peripheral vision or wave your finger in front of the monitor and you will see what I mean.
I thought you meant something else…leaving now.
“Screw you guys…I’m goin’ home!”
I experience a similar thing when I brush my teeth with an electric toothbrush and watch the TV. It must vibrate my skull just out of synch with the CRT’s frequency.
This was covered by UC in the second and third section of this column…
http://www.straightdope.com/classics/a3_270.html
Bert wrote:
Not really. It doesn’t explain why the vibration has this effect. It all has to do with stroboscopic effects and the eye’s inherent “frequency of operation”.
First a bit of a setup:
We’ve all seen the stroboscopic ‘freeze frame’ effect. Someone who is fluidly moving in a dark room with a stobe light flashing at some interval will appear to freeze at each flash. We are all also aware that the human eye cannot perceive image changes faster than about 30 ‘frames’ per second. This fact is why movies and television seem to show ‘fluid’ motion, when we know that they are just a sequence of still images.
Other similar effects to the chopsticks phenomenon:
It has already been mentioned that a vibrator and raspberries will also produce this effect, but so will humming at the right frequency and volume - which is what sly noted. I used to do this all the time as a kid and got pretty good at it. I could also use this technique to ‘slow’ down the blades of ceiling fans. Likewise, if you put a turning fan in front of a TV, the blades will appear to slow or sometimes spin backwards. Hold your hand up in front of your computer monitor, fingers spread wide. Now wave it in a sweeping motion in front of the CRT and focus on your fingers. You should see a freeze frame effect with your fingers. Same principle.
So what’s going on?
All of these effects, whether induced by vibration or motion, are essentially the same. Two, fairly low frequency visual phenomena, at slightly different frequencies will create a beat frequency cancellation effect that your brain will perceive as non fluid motion (similar to the stroboscopic effect mentioned earlier). At higher frequencies, where the beat frequency (or cancellations) are faster than 30fps, we longer notice the distortions.
Oops! Meant to add. You don’t notice the effect at lower beat frequencies either, which is actually where a lot of action takes place in real life.
Thanks, bud. I notice this does not occur with my laptop (LCD) screen. Different frequency or something?
In a CRT (regular monitor), each pixel turns ‘on’ only when hit by the electron beam; it’s ‘off’ while waiting for the beam to hit it again when the next frame is drawn. This waiting time is what causes the flicker.
An LCD screen is completely different, because all the pixels are ‘on’ at once. Instead of just flickering very fast, they actually stay lit the whole time, which is why you can’t get that neat strobe effect from a laptop.
(Disclaimer: Though I’m fairly sure this is correct, I am a programmer, i.e. a software specialist. By definition, anything I say about hardware must be suspect.)
I’m not a warlock. I’m a witch with a Y chromosome.
Just wanted to nitpick. That 30fps limit is a myth. I know I can certainly tell the difference between 60fps and 40fps when I play computer games.
TV and Movies appear fluid at such a low rate because they are relatively low resolution and blur the frames together. (I think)
AuraSeer wrote:
Not precisely. First there’s two kinds of LCDs active matrix and passive. Passive matrix is the older technology and it used much slower refresh rates, but they weren’t generally noticable for several reasons. The cells did not emit light, so contrast (or lack of it) makes it difficult to see. Also, in passive matrix LCDs the physics of the cell is such that there is persistence (i.e. the pixels tend to stay on even after the voltage is gone). This is why older displays used to streak and smear when things were moving on the screen. The newer technology is called active matrix, which basically just means the cells are ‘pulled’ on and ‘pulled’ off, so the response mechanism is much faster. Still, the entire display has a fresh cycle, but it’s just very fast (probably in the MHz range).
If you were in a room with a special light source that strobed at a frequency very near to that of your active matrix LCD (differing in about 15Hz), I’m pretty sure you’d be able to see flicker.
[disclaimer]
I’ve not tried this and there may be other factors like persistence that I’m not aware of… so I could be wrong about this.
TheNerd wrote:
True, I started to qualify this a bit, but decided it was unnecessary… clearly I was wrong. [wink]
The 30fps is meant to be kind of a subjective average. Unless you’re specifically looking for it, your brain tends to blur the discontinuities and perceives fluid motion. It’s also dependent on what is going on on the screen. In typical TV shows, the camera action is specifically tailored to try and prevent two different objects in motion at radically different speeds from being in the viewers’ field of vision - otherwise, the 30fps would be more obvious. In games, it’s not uncommon for some objects to move relatively slow while others race across the screen quite fast. Most computer monitors have refresh rates between 60Hz and 85Hz to minimize flicker (and squeeze more pixels on the screen).
quote
So what’s going on?
All of these effects, whether induced by vibration or motion, are essentially the same. Two, fairly low frequency visual phenomena, at slightly different frequencies will create a beat frequency cancellation effect that your brain will perceive as non fluid motion (similar to the stroboscopic effect mentioned earlier). At higher frequencies, where the beat frequency (or cancellations) are faster than 30fps, we longer notice the distortions.
IMHO, the vibration that humming induces on your skull causes a slight angular disruption of the photons as they move through the eye to your retina. Unlike the other methods mentioned, humming doesn’t interrupt the incoming light, but rather alters its path.
Persistance of vision relies on an individual rod or cone, receiving the same information from refresh to refresh. When your eyes are being vibrated, the information is being shifted to new parts of the retina during each ‘beat’ and the result is a wavy, or shifting image.
For those who can’t sing low enough to experience this phenomenon. Try ‘growling’ (with your mouth closed) a low pitch, as if imitating an Aboriginal digeridoo [sp?] while watching your computer screen. When you get the pitch right, the screen suddenly appears somewhat fluid as if the vertical hold is screwing up. Sometimes you can get the same sort of effect looking at an LED clock while chewing on crunchy food. The surrounding image seems solid, while the numbers appear to be mounted on tiny springs.
Stephen
http://stephen.fathom.org
Satellite Hunting 1.1.0 visible satellite pass prediction shareware available for download at
http://stephen.fathom.org/sathunt.html
Steven wrote:
I wasn’t trying to define the nature of the distortion, only that the phenomenon is a function of some regular, low frequency visual distortion.
I’m not sure about this. It seems to me that the effect I see more closely resembles a strobe. In fact, as a kid, I once mentioned to a teacher that I could freeze the image of blades of a ceiling fan in my eye. He didn’t believe me and devised an experiment where he taped a piece of paper onto one of the blades of a ceiling fan. On this paper was drawn a letter with a felt tip marker and the ceiling fan was turned on. Just looking at it normally, the white paper was a circular streak, but when I started my hum, I could read the letter. If it was just a shifting of which photons were hitting which cones, I would expect a shimmy not a strobe.
Sly, Excellent! Another Metallica fan!
But, if it is a strobe effect, that would mean you are going blind between beats, which I seriously doubt. I would imagine, in the case of the ceiling fan, you were in a sense ‘discarding’ the information between beats by shifting it off to random parts of the retina, while ‘reinforcing’ the image of the letter by consistently returning it to the same rods and cones. This would act similar to a strobe, but you are (in a sense) averting your gaze instead of blocking the light.
Stephen
stephmon@aol.com
http://stephen.fathom.org
Satellite Hunting 1.1.0 visible satellite pass prediction shareware available for download at
http://stephen.fathom.org/sathunt.html