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?"

Comments

Not just 60 Hz

[Alternate Interior]

There are a number of refresh rates that I notice.

• 60 Hz is painful
• 72 and 75 are usable, but I notice
• 85 looks natural
• 90 is similar to 75
• 100 and 120 looks similar to 85

It doesn't make sense that 90 is worse than anything else at 85+, so perhaps it's just the particular monitor. And I won't claim I can identify everything. I can't differentiate 85/100/120, or 72/75/90. But amongst the three groups of refresh rates, I can identify the group.

Annoyed by flickr

[geoffspear]

God, please make the endless stream of bad cameraphone pictures go away!

Wait, what's this about refresh rates?

Everybody's different

[jgardn]

I'd like to think that I have the same ability because I have spent so much time in front of monitors. But I know a lot of people who have spent many many more years and who still can't see it.

I've marked this one up long ago to one of those weird attributes we're born with or we learn or that is a combination of both. I know that a lot of people thank me for raising their refresh rate, but there are some who can only work at about 60-65Hz and can't stand 75Hz or 80Hz, which is where I am comfortable.

I've also fallen in love with LCD screens. I can finally work without getting tired! Now if only my mind could keep up with my eyes.

I still use a CRT at work and that's led to the pile of notes and graphs and charts I've draqwn up and strewn about my office. I have to take a break from time to time or I can't look at the screen.

I didn't read TFA, but...

[xami]

..Toshiba, I think, made some tests in cinemas with 30fps and 70fps movies to see howmany people could distinguish between single frames.
Surprisingly, a lot of them couldn't even notice the difference but there was a significant number (15% afaik) who could even notice single frames in 70fps

Re:Lucky?

[Neil Blender]

I think he means it in the way that someone might say, "I'm one of those lucky people who have explosive bouts of diarrhea eight times a day."

the sound is unbearable

[St. Arbirix]

Who else can hear the sound a television or monitor makes? I can tell from another room when a TV has been turned off because the high-pitched squeal goes away. Those electronic mosquito repellers are equally annoying.

I've read in various places that women can hear higher pitches than men but I've yet to meet anybody male or female who could hear some of the crap I put up with. Walking into a computer lab is the same to my ears as diving to the bottom of a pool.

Re:the sound is unbearable

[bill_mcgonigle]

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.

Re:the sound is unbearable

[dpbsmith]

Hee, hee... when I was a kid I could hear the 17,750-cycle horizontal scanning frequency (as it was then... it became 17,734 at about the same time it became "Hz") clearly. I'd ask adults about it. And none of them knew what I was talking about. When I was about ten I finally insisted that someone turn a TV off and on, with the volume all the way down, in an adjacent room, so that I could _prove_ that I could tell whether the TV was on or off and wasn't just imagining things.

When I was in my twenties, I went to a dentist who used an "ultrasonic" cleaning device that was audible--and painfully loud--to me. I complained. They insisted that I couldn't possibly be hearing it because it was "ultrasonic." I said, "Oh? How do kids react to it?" They said, "Oh, we don't use it on kids, they jump out of the chair when we do." But they still didn't believe that it was because I, and the kids, heard it.

Also in my twenties, I visited the mineral and gem room of the American Museum of Natural History, which I had visited many times with pleasure, and this time I confronted with the loudest "ultrasonic" sound I've ever heard the displeasure of hearing. I'm pretty sure it was higher than 15,750 Hz. Heaven only knows how many DB it was. It didn't even sound like a high-pitched done; it sounded like someone was clamping my head in a vise. It gave me a splitting headache within about five minutes. There was obviously some kind of standing-wave effect because if I moved my head or walked around it would fluctuate; I probably could have determined the wavelength if I'd thought about it, but I didn't.

It got louder whenever I got near some little boxes mounted on the wall about ten feet. They were inconspicuous and painted black but in plain sight.

I went to the guard, and said that it was daytime and he was on duty, would he mind turning off the ultrasonic burglar alarm because it was giving me a headache. He sort of freaked out. He said that they didn't have any ultrasonic burglar alarm and what was I talking about? I said I was talking about the extremely high-pitched, extremely loud sound that was coming from those boxes on the wall, and pointed to them. He insisted that they were not part of a burglar alarm system and were not making any noise.

I don't know if he a) thought I was delusional, or b) was upset because the system was supposed to be top-secret, or whether c) I _was_ delusional... you figure it out.

Well time took its toll on the old hair cells, and I got the twentieth-anniversary special CD of Sergeant Pepper's Lonely Hearts Club Band in 2000 and listened to the band that supposedly has an ultrasonic tone at the end of it... on my headphones... with the volume turned pretty high... and you know what? I couldn't hear a thing.

Maybe I can visit the American Museum's gem room in comfort now.

Here's something...

[Otter]

I don't have time to research this thoroughly but figured I owed you something in return for your informing me of the existence of Google Scholar...

From Shane Sidebottom's Masters thesis:

One factor that is not often examined in the flicker research conducted in psychophysics is individual variability. Some people are more sensitive to flicker than others (Rogowitz 1986). To develop an accurate flicker standard for the general population, the nature of the distribution of flicker sensitivity in the population must be considered. Individual differences may be attributable to many factors, such as gender, age, or personality. Young people are more sensitive to flicker than older people (Kim and Mayer, 1994). Women are more sensitive to flicker than men are (Maxwell, 1992). Personality has been cited by psychologists as affecting flicker perception (Amir and Ali, 1989). The factors that lead to individual variation in flicker sensitivity may be of little real use to those designing displays for the average user; however, knowledge of the extent of individual variability is useful.

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.

variable refresh rate

[Froze]

Others have mentioned that they only get bothered when switching from one display to another. I think this is because your brain (optic whatever) can vary the uptake rate of information from your eye.

Test it out, try laying on the floor beneath a ceiling fan on low or medium. If you watch fan long enough you can stabilize the apparent interference rotation rate of the blades.

When you can vary the rotation rate at will, you have less chance of being annoyed by flicker since you adjust rapidly. If you can't see the rate change then your eyes can't compensate for the flicker. Hence they get tired more easily.

Nerves and the brain...

[tRenn]

Ok, I'd like to know if there's a fundamental frequency that the average human's optic nerve/brain refreshes that make us comfortable with viewing CRTs at various refresh rates?

Re:Audio corollary

[cniebla]

I can pick that noise too, but with an interesting twist: I actually can use it like some kind of sonar, really. Sometimes I'm on my laptop, giving my back to the door in the room, and to avoid a "comming-from-the-back-making-no-noise" joke, I just turn on the TV, with the sound mutted. It can give to me the actual position / movement from even a cat coming by, so no more jokes ;). By the way, where you found that freq.? (15750hz)

Re:Audio corollary

[Flying Purple Wombat]

You mean that most other people can't hear that noise? I thought that most everyone could, and I was the only one who was really annoyed by it.

Most people can't hear it. It used to drive me crazy, I could hear CRTs, ultrasonic motion detectors, and other things that most people can't. As with many things, this is an individual variation. My son inherited this ability, but my daughter did not.

Now that I'm 0x29 years old, I can't hear a CRT unless my left ear is within 6 inches of it (my right ear can't hear it at all). High frequency hearing loss is common as age increases. In my case, loud music and pyrotechnics accelerated the process.

My 2 cents...

[MrIcee]

While I am not a vision researcher I have spent countless years in digital imaging and understand some of the basics of what is going on.

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)

Re:LCD Response?

[PornMaster]

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.

Broaden you scope, Idealist

[Linux_Bastard]

In CRT based TV's the high voltage power supply vibrates.

Not to mention the macro item such as the speaker cone, buttons and knobs.

At the micro level you have lots and lots of e- and plain old heat.

Quick quiz,
How many moving parts in a solid state piezo tweeter?

The moment on the CRT's flyback is more than in that piezo tweeter.