..

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