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Flickering Curiosity?
N8F8 writes "Why do some people see monitor flicker more readily than others? I happen to be one of the lucky folks who can spot a 60Hz monitor from across the room. Most people seem oblivious to this flicker. Other people can only see it in their peripheral vision. I tried researching an answer and I stumbled on plenty of information about something called 'Critical Fusion Frequency'. There even appears to be quite a bit of research into this phenomena but I couldn't find much information on why flicker perception varies so greatly. Can anyone shed some (flicker-free) light on this?"
You will tend to notice it more out of the corner of your eye then head on. (Go ahead try it now), that is because the rods are more sensetive to motion.
Also some people don't notice it because it is all they have ever really used. A lot of people just assume their monitor sucks. They notice it but have no idea what causes it.
I'd do something interesting, but my server can't handle a slashdotting.
Nah, some people will complain that their eyes are tired after hours on a computer. A refresh fix eliminates that in many cases.
From Shane Sidebottom's Masters thesis:
There's more (plus a graph!), and the references should keep you occupied for a while. As to *why* -- well, *why* is a question that can keep researchers busy for quite a while.
What I'm listening to now on Pandora...
The DLP rainbow effect is something else many people don't notice, but it really bothers me for the first 15 minutes when watching something on a DLP projector. Even after that, I see it from time to time when my eyes move quickly.
Some department stores have security systems that cause the same effect. For me, anyway, the sound of a blank TV sounds different than a typical high-pitched sound. Does it for you?
One source of very-high-pitched sound perception (~26-28KHz) is sympathetic vibrations of the ear bones. Normally the sound waves vibrate the tympanic membrane and the ear bones pick it up and transmit it to the cochlea which fires the nerve. But sometimes the bones can be vibrated directly which results in the perception of a high-pitched sound, higher than the tympanic membrane can respond to. This is one of the chief complaints of the 44.1KHz limit on CD's - they can't represent frequencies over 22050Hz, and people can perceive higher pitches even if they can't 'hear' them with the eardrum.
Perhaps that's somehow involved here. Just a guess.
My God, it's Full of Source!
OUTSIDE_IP=$(dig +short my.ip @outsideip.net)
A TV/CRT has no moving parts. The movement of the beam is controlled by magnetic fields.
The 60 hz frame rate (or actually, 2 fields at 30 hz) and the vertical retrace period can be viewed by just about any person - regardless of gifts....
Think you know someone who can't see the flicker? Have them stand about 15 feet away from a monitor and chew a stick of gum - they will be amazed to see the jitter (or just pretend to chew a stick of gum or bob or weave VERY slightly on your toes).
Not everyone is wired exactly the same and some of us have eye sensitivity - what we can actually consider a 'sample rate' that is higher, or lower, than the norm. In effect, what you are seeing when you see the jitter can be likened to aliasing effects - every now and then your sample rate syncs up (or desyncs slightly) and you catch the vertical blanking interval and/or scan itself.
It is well known that dogs and cats in general don't see TV like we do - because their vision system is at a faster rate than ours - to them the screen looks like a lot of weird angled lines. However, everyone knows about that ONE cat or ONE dog that DOES watch TV - most people think they are gifted but instead, they are 'slower' in their vision system.
Consider as well, car tires. As you go down the road they spin but you are very familiar with the effect of them 'apparently' spinning backwards - again, the sample rate of your eyes is just slightly out of sync with the rotation speed (or harmonic of) causing the hubcap to appear to turn backwords. And if your right in sync with the speed (or harmonic of) then the hubcap appears to be standing still (e.g., in it's rotational axis).
If you were to consider two people, looking at the same tire rotation, they would undoubtedly report slight differences in what they see. The same effect is happening on the screen - the refresh rate is similar to the rotating hubcap and you are just slightly faster or slower than it is.
(btw, yes, I've been able to see it for years and years and year)
SPATIAL DOMAIN:
Yes. Distance is critical. Because of interleaving, you have neighboring rasters 180 out of phase...and that is intended to reduce percieved flicker. If you happen to view a monitor from such a distance that the space between even numbered [or harmonics ] rasters approximately projects to the retina at the spacing be individual receptors [rods? cones? idunno] you might undo some of the interleaving effect. This won't be a strong phenomenon because receptor cells are not layed out in a grid.Less than 6 feet and imaged raster separation may exceed receptor spacing, [you begin to be able to actually pick out rasters on a low quality screen but you don't see flicker], greater than [what would it be ? 10, 12, 20] feet projected raster spacing is well less than recpetor spacing so each receptor is summing [ie averaging out] interleaved raster pairs and you can't see flicker.
TIME DOMAIN:
ever noticed flicker [ in periperal vision in my case] if you are looking off to one side of a monitor across the lab WHILE SOMETHING BUMPS YOUR HEAD? I had to stop the experiments I was doing to get to the bottom of this phenomenon before anyone caught me just standing there looking into space and banging my head with my hand. [sometimes life is just strange but would it add or subtract from your comment to have it modded "Strange"?]
SLASHDOT: news for people who can't concentrate on work or have no life at all and got tired of yelling back at the TV.
LCD pixels don't go dark between refreshes of what's on the screen. CRTs only light up a pixel when the electrons hit the phosphor, then dim until hit again. The LCD pixel stays lit between changes.
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The issue at hand is constructive interference.
90 Hz will interact very strongly with all the things around you that do pulse at 60 Hz. Fluorescent lights are the main culprit here, but almost anything may, since the A/C is being provided at 60 Hz.
Two out of every three refreshes on the 90 Hz monitor will not coincide with the lighting flickering, but one will. That results in a weaker 30 Hz flicker on top of the 90 Hz.
You might also want to look up "Critical Flicker Frequency" I am fairly sure this is either the same thing or a related thing. It describes the effect that allows for movies and other moving images based on rapidly shifting images.
This link will explain it all
Required reading for internet skeptics
There isn't really a fixed frequency at which nerve cells operate/communicate at. The first thing to understand is that the rate at which nerves fire generally varies according to the intensity of the stimulus.
Furthermore, there are many different types of nerve cells and receptors in your eyes. Some only respond to certain features based on the shape of the cell. Some only are activated with a certain intensity of stimulus. Some are only activated when a stimulus changes. Some are only activated when certain combinations of neighboring cells are stimulated.
So the short answer is there is no fixed "refresh rate" of the brain. There is so much we don't know about the nervous system it is amazing. Over 50 neurotrasmitter substances have been identified, and there are more out there! We dont know exactly what half of them even do. Research into these types of questions is really on the cutting edge of science so there are a lot of unanswered questions out there...
The sending of this message pretty much inconveniences everyone involved.
Ah, but the light flashes at 120hz. When the current is near 0 A, the light goes out, and that happens twice for every cycle.
Your post is curious.
Parent poster is referring to the whine that a TV makes, caused by mechanical vibrations in the windings of the flyback transformer. The whine is at the line retrace frequency, or about 15 kHz. A similar noise can be found in a computer monitor if it's a CRT type, but not in an LCD or plasma.
You mentioned Johnson-Nyquist noise. Johnson noise is broadband 'white' noise, caused by random thermal motion of charges in a resistance. It can be amplified in an audio system and be reproduced by the speakers, expecially if the gainstaging is incorrect ("the TV volume is on full blast but the reciever volume is low"), but if people around you say they can't hear it - dude, they're fucking with you. White noise has the same power at all frequencies, and the only way to not hear it is to be stone deaf. They may have gotten used to it and be willing to ignore it, but that's not the same as being unable to hear a 15 kHz tone.
Now, if you hear white noise in "everything", you may in fact be nuts, or have some serious hearing issues that need to be checked out. A computer monitor with no speaker, e.g., should emit essentially ZERO amplified Johnson noise. Interestingly, a case fan or CPU fan should emit fairly 'white' noise with some relatively constant tones mixed in.
In any case, parent poster is NOT referring to Johnson noise, he is referring to flyback whine.
Oh, and you misspelled 'reciever'.
Ce n'est pas un vrai mouvement de robot!
To test yourself / your computer's sound system, try in_tone.dll for winamp. Just drop it in your plugins directory, and copy the below to a .m3u file. I can hear 20k, but just barely. Hard to say if it's the actual tone or some weird subharmonics thing going on.
n e://35,101 0: //80,101 0o ne://440,10/ 1200,105 00,100 0,100 00,106 000,301 9000,30
--- begin tones.m3u ---
tone://20,10
tone://25,10
tone://30,10
to
tone://40,10
tone://42,10
tone://45,
tone://50,10
tone://55,10
tone://60,10
tone
tone://77,10
tone://110,10
tone://120,
tone://156,10
tone://220,10
tone://311,10
t
tone://622,10
tone://880,10
tone:/
tone://1800,10
tone://2600,10
tone://3
tone://5000,10
tone://7000,10
tone://100
tone://11000,10
tone://12000,10
tone://13
tone://14000,30
tone://15000,30
tone://1
tone://17000,30
tone://18000,30
tone://
tone://20000,30
Vehicle wheels only appear to be going backwards in 2 cases: 1) they're on film/tv where aliasing happens due to the "sampling" with each frame. 2) for it to happen live you need a strobe light - i.e. driving at night under bright non-incandecent lights (quite common). There is no "frame rate" for the eye, so this normally doesn't happen with real observed objects. You may also notice some intereting things when parts of the wheel shadow other parts - the large lugnuts on big trucks sometimes do this in the right light.