Why do monitors flicker on TV?
 

..

Refresh rate. Your used to seeing a 60 Hz refresh rate (actually two
alternating frames at 30 Hz) on your home TV.

If you go to Europe, you go nuts for the first few days getting used to the
flicker as they use a 50Hz refresh rate.


"hnmm" wrote in message
...
.

hnmm wrote:
Human eyes can detect the refresh rate flicker up to a certain
frequency. My peripheral vision can detect the 120 Hz flicker
of florescent lights as well as flicker in computer monitors.
Drives me crazy.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

hnmm wrote:
Human eyes can detect the refresh rate flicker up to a certain
frequency. My peripheral vision can detect the 120 Hz flicker
of florescent lights as well as flicker in computer monitors.
Drives me crazy.

The scan rate of most computer monitors is closely related to multiples
of the scan rate of a NTSC television signal scan rate. This leads to a
signal that is often twice the rate of NTSC scan. But since the monitors
are not synchronized, the computer monitor will show lighter and darker
areas depending on it's scan rate. Computer monitors will vary in their
"look" on television, from a whole screen flicker to a darker colored
bar that works it's way down the screen. This is all due to the scan
rate frequencies.

- Mike KB3EIA -

On Sat, 03 Jan 2004 20:09:14 GMT, Mike Coslo
wrote:

Cecil Moore wrote:

hnmm wrote:
Human eyes can detect the refresh rate flicker up to a certain
frequency. My peripheral vision can detect the 120 Hz flicker
of florescent lights as well as flicker in computer monitors.
Drives me crazy.

The scan rate of most computer monitors is closely related to multiples
of the scan rate of a NTSC television signal scan rate. This leads to a
signal that is often twice the rate of NTSC scan. But since the monitors
are not synchronized, the computer monitor will show lighter and darker
areas depending on it's scan rate. Computer monitors will vary in their
"look" on television, from a whole screen flicker to a darker colored
bar that works it's way down the screen. This is all due to the scan
rate frequencies.

Yes, the exception would be if the monitors and the TV camera are
synchronized, as might be done with the news set.

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom

On Sat, 03 Jan 2004 10:38:53 -0800, hnmm wrote:


I think the question was a bit different than the answers I've been
seeing.

IF I understand the question, he wants to know why when a monitor is
shown on TV it flickers, but not when actually looking at the monitor.

When video taping, TV sets AND computer monitors will flicker. It's
due to the differences in scan rates on the monitors compared to the
camera.

Even when they are running the same frequency there will still be the
appearance of either a rolling image, or a moving bar as the scan will
not be perfectly in sync between the camera and the monitor. If
they are the same frequency the dark line on the screen will not move.
It's rare to see one where the monitor and camera sync pulses are ...
well...in sync. When that happens the TV screen looks normal.

The image on a computer monitor and a TV screen consist of a bunch of
almost horizontal lines. With a TV set the image starts at the top
and is drawn every other line, one at a time. When the line reaches
the bottom it has completed one frame. It returns to the top and draws
in the lines that were skipped in the first frame. This noticeably
reduces the perceived flicker.

Computer screens may use the same method or they may draw the entire
image on one pass. Their much higher sweep rate may allow them to do
so without a noticeable flicker.

As to regular monitor flicker, I only notice it occasionally in my
peripheral vision, but not when viewed directly. Part of that is due
to the persistence of the phosphor, but with TV sets and monitors the
persistence is very short. OTOH viewing a TV screen under
fluorescent lighting can really accentuate the flicker.

You should see a moving image on a longer persistence phosphor. They
smear....


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

.

"w4jle" W4JLE(remove this to wrote in
:

Refresh rate. Your used to seeing a 60 Hz refresh rate (actually two
alternating frames at 30 Hz) on your home TV.

If you go to Europe, you go nuts for the first few days getting used
to the flicker as they use a 50Hz refresh rate.

What a half correct and half junk science answer!

Yes here in the UK we have a refresh rate of 50Hz on our PAL standard
sets compared to 60Hz on NTSC standard TVs but I can't think why it would
drive you nuts? If that's the case, you should be going mad constantly
when looking at PC monitors, EPOS displays and ATM machines as most have
different refresh rates from 47Hz upwards!

The reason is correct as at the top - refresh rate or more a sync problem
- when a computer monitor is 'filmed' but the camera used is not
'synced' to monitor's refresh rate, you see the monitor at one rate being
filmed at another - hence a mismatch.

Really the camera operator should lock the scan rate of the camera to the
refresh rate of the monitor, then both are at the same rate and you get a
steady image with no flicker!

Simple as that and now pretty much a feature on most professional video
cameras.

Brian.

On 8 Jan 2004 18:44:20 GMT, Brian Potter wrote:
- when a computer monitor is 'filmed' but the camera used is not
'synced' to monitor's refresh rate, you see the monitor at one rate being
filmed at another - hence a mismatch.

Actually, the correct term is "beat."

73's
Richard Clark, KB7QHC

It drives me nuts, because at the 25 Hz frame rate I can see the flicker.
After a few days in Merry Ole', I get used to it and don't notice it.

Every trip back to England requires a day or so to get my brain to ignore
the flicker. The other monitors you refereed to do indeed have 47 Hz refresh
rates, but they are non-interlaced video.


"Brian Potter" wrote in message
...
"w4jle" W4JLE(remove this to wrote in
:


Yes here in the UK we have a refresh rate of 50Hz on our PAL standard
sets compared to 60Hz on NTSC standard TVs but I can't think why it would
drive you nuts? If that's the case, you should be going mad constantly
when looking at PC monitors, EPOS displays and ATM machines as most have
different refresh rates from 47Hz upwards!

and the persistence (?) of the tube will play a part in perceived flicker !!

Nick

"Mike Coslo" wrote in message
t...
Cecil Moore wrote:

The scan rate of most computer monitors is closely related to multiples
of the scan rate of a NTSC television signal scan rate.

How do you work that out? NTSC is 60Hz (30Hz Interlaced)
So Most CRT monitors run at 70Hz, 75Hz, 85Hz and maybe 100Hz for a high end
monitor at 1024*768

Where is the link between the vertical refresh rates?

Have a google search for Vesa refresh rates for more info.

Robin.. ( I used to work for NEC building computer monitors before NEC
pulled out of manufacturing them in the UK)

Robin,

I *haven't* worked in this area, but I still knew this comment was misplaced
since I've seen all kinds of scan freqs over the years.

--
Steve N, K,9;d, c. i My email has no u's.

"Robin" wrote in message
...
"Mike Coslo" wrote in message
t...
Cecil Moore wrote:

The scan rate of most computer monitors is closely related to multiples
of the scan rate of a NTSC television signal scan rate.

How do you work that out? NTSC is 60Hz (30Hz Interlaced)
So Most CRT monitors run at 70Hz, 75Hz, 85Hz and maybe 100Hz for a high
end
monitor at 1024*768

Where is the link between the vertical refresh rates?

Have a google search for Vesa refresh rates for more info.

Robin.. ( I used to work for NEC building computer monitors before NEC
pulled out of manufacturing them in the UK)

"Bill Turner" wrote in message
...
Keep in mind the difference in phosphor persistence. TV CRTs are
designed with enough persistence to make 60 Hz flicker invisible.
Computer CRTs have no such limitation.

--
Bill, W6WRT

Computer CRT's also have phosphor persistence, this is also defined by the
physical size and depth of each layer of phosphor as in a TV CRT, the
difference is that the phosphor pixels are smaller, so can convert less
electron beam energy to light, and for a shorter period of time.
Robin

Robin wrote:

"Bill Turner" wrote in message
...

Keep in mind the difference in phosphor persistence. TV CRTs are
designed with enough persistence to make 60 Hz flicker invisible.
Computer CRTs have no such limitation.

--
Bill, W6WRT


Computer CRT's also have phosphor persistence, this is also defined by the
physical size and depth of each layer of phosphor as in a TV CRT, the
difference is that the phosphor pixels are smaller, so can convert less
electron beam energy to light, and for a shorter period of time.
Robin


Hey Robin! *You* tell us why computer screens flicker on television.
Then explain why many of them have horizontal bars running vertically
through the pictures. Then explain why the bars are of different sizes
and timing from appearance from top to bottom of the screen. Then
explain why some don't.

- Mike KB3EIA -

w4jle wrote:

If you go to Europe, you go nuts for the first few days getting used to the
flicker as they use a 50Hz refresh rate.

If a european ever goes to the US or far east he'll posibly go nuts over
one of the main features of the NTSC color system, AKA as "Not Twice
Same Colors"

;-)

73,

Volker

In message , Volker Kerkhoff
writes
w4jle wrote:

If you go to Europe, you go nuts for the first few days getting used to the
flicker as they use a 50Hz refresh rate.

If a european ever goes to the US or far east he'll posibly go nuts
over one of the main features of the NTSC color system, AKA as "Not
Twice Same Colors"

;-)

73,

Volker



Not really true, surely?

There is 'something' about NTSC compared with PAL. However, I've never
seen bad colo(u)r on NTSC sets which was not mainly due to poor signal
quality anyway, and where PAL would probably been the same.

These days, don't they transmit the correct colours as a reference in
the vertical interval, and the set uses (or can use) this to correct the
phase errors?

Ian.

--

Ian Jackson wrote:
In message , Volker Kerkhoff
writes

w4jle wrote:

If you go to Europe, you go nuts for the first few days getting used
to the
flicker as they use a 50Hz refresh rate.


If a european ever goes to the US or far east he'll posibly go nuts
over one of the main features of the NTSC color system, AKA as "Not
Twice Same Colors"

;-)

73,

Volker



Not really true, surely?

There is 'something' about NTSC compared with PAL. However, I've never
seen bad colo(u)r on NTSC sets which was not mainly due to poor signal
quality anyway, and where PAL would probably been the same.

These days, don't they transmit the correct colours as a reference in
the vertical interval, and the set uses (or can use) this to correct the
phase errors?

It is a source of great amusement to listen to people dissing one or
the other - NTSC or PAL, or even SECAM.

They all suck very much. Trying to decide which one sucks the most is a
waste of time.

- Mike KB3EIA -

I would agree with that. Once I am use to the flicker, I marvel at the
cnsistancy of color from camera to camera.

"Volker Kerkhoff" wrote in message
...
w4jle wrote:

If you go to Europe, you go nuts for the first few days getting used to
the
flicker as they use a 50Hz refresh rate.

If a european ever goes to the US or far east he'll posibly go nuts over
one of the main features of the NTSC color system, AKA as "Not Twice
Same Colors"

;-)

73,

Volker

rite this down, "sync rate/refresh rate/scan rate/any other combinations
of words that means the same thing". Now stop this nonscense. Lets
start a new thread, "did I spell non-scense rite ?" New thread idea,
" do the quote marks come inside or outside the question mark"?"

Butch KF5DE

Bill Turner wrote:
On Sat, 17 Jan 2004 19:47:49 -0000, "Robin" wrote:


Computer CRT's also have phosphor persistence


__________________________________________________ _______

I never said computer CRT's don't *have* persistence, I said that they
are not limited by the 60Hz requirement of TVs.

All CRT phosphors, whether for TV, computer or oscilloscope, have *some*
degree of persistence. Hopefully the manufacturer has matched the
persistence to the use.

--
Bill, W6WRT

On Mon, 19 Jan 2004 03:37:46 GMT, Mike Coslo
wrote:

It is a source of great amusement to listen to people dissing one or
the other - NTSC or PAL, or even SECAM.

They all suck very much. Trying to decide which one sucks the most is a
waste of time.

Most probably correct. And what disturbs me most is that nobody has
come up with something better available to the consumer "in general" I
mean, maybe I'm expecting too much from an industry that has not been
able to control "ghosting" effects in the end credits (white lettering
on black background) on a reasonably new TV set when fed with S-Video
from a digital source.

OTOH, SECAM is a bit more complex than NTSC or PAL. To the best of my
understanding, they actually send the chroma portion of the signal
*after* the luminance, so sets that are fit for SECAM gotta have a
delay line somewhere to brinc Chroma and Luminance back together,
IIRC?

And what about and adaption of "our" SSTV standards to "Fast Scan"
television? Would that work within reasonable parameters?


Volker