Techy television question.

[machasm]

Completly off topic this one,
Why is it that when viewing an image of a monitor on a television do we see lines rolling down the monitor?

My guess is that both sets are running at the same or similar frequencies (50Hz mains). This causes harmonics which manifest themselves as the rolling lines.
I'm not sure if this is right and would like to know what the real cause is?
I also am quite techie so hopefully will be able to understand a techie answer I hope!
Thanks.

[blackbrabus]

this is all down to the hurtz the TV is and the frame rate of the thing recording it.

its all about the flicker baby.

[Unsung Hero]

:iagree::iagree::iagree:
:cheers2: J

[blackbrabus]

smae reason the blades on this heli look like they are not moving.

[ame]http://uk.youtube.com/watch?v=Dyisg9-Mwjw[/ame]

[phockzie]

it takes 1/50th of a second (i think, or 1/25th) to fully draw a single frame on a CRT TV. A video cameras has a varying shutter speed as most of them use the shutter speed to control the amount of light entering the sensor. so the shutter will stop the camera seeing the whole picture drawn on the tv.

Bit rough explanation, but i havn't got much time ;)

[cjcj1949]

It's called sampling theory. In order to capture all info in a frequency band you need to sample at twice the frequency. I can remember doing the work but not understanding it. It has nothing to do with harmonics which are to do with fourier series,ie representing a signal as a series of sine waves at f,2f,3f etc. I can aslo remember Nyquist but not sure what that was about. The sampling error was called aliasing when the sampling frequency was not high enough.



Chris

[adamwantstodoaeros]

[quote=cjcj1949;230063]It's called sampling theory. In order to capture all info in a frequency band you need to sample at twice the frequency... I can aslo remember Nyquist but not sure what that was about. [/quote]

The sampling theory is Nyquist's theory (or at least one of them). :)

But yeah, its because it takes 1/50th of a second to draw a frame on a CRT TV, so the bit of the TV screen currently being 'drawn' as the camera takes the photo gets much more exposed than the rest of it. Depending on the exposure time and rate of the camera, you'll get different thicknesses and speeds of lines scrolling down the screen.

Im not sure I ever saw this effect on TFT's or LCD's (because they have a constant backlight).

Or at least, thats what I think...

[cjcj1949]

Have you got a link to his sampling theory and a simple explanation. I can see in terms of helicopter blades that if you sample at exactly the same frequency as the blades rotate that they will seem to be standing still.

Chris

[pchristy]

Its because the monitor and camera are not running at exactly the same frame rate. Although they are both nominally 50 Hz (60 Hz in the US and Japan), there is always a slight difference between them unless thay are locked to the same sync source.

In a professional TV studio, you can point the studio camera at the monitors and there will be no flicker, because they are all locked to the same source.

Professional broadcasters use a Rubidium Vapour Phase-locked oscillator to generate the sync pulses. This is one of those things that is accurate to 1 second every billion years (or something equally ridiculous!).

You can't expect a £200 hand held from Dixons to achieve the same level of accuracy......!

Even a broadcast hand-held will show flicker if its left free running, rather than locked to the station syncs.

--
Pete

[adamwantstodoaeros]

[quote=cjcj1949;230083]Have you got a link to his sampling theory and a simple explanation.[/quote]

I think this site shows it quite well, with plenty of description.
[url]https://redwood.berkeley.edu/bruno/npb261/aliasing.pdf[/url]

[cjcj1949]

[quote=pchristy;230148]Its because the monitor and camera are not running at exactly the same frame rate. Although they are both nominally 50 Hz (60 Hz in the US and Japan), there is always a slight difference between them unless thay are locked to the same sync source.

In a professional TV studio, you can point the studio camera at the monitors and there will be no flicker, because they are all locked to the same source.

Professional broadcasters use a Rubidium Vapour Phase-locked oscillator to generate the sync pulses. This is one of those things that is accurate to 1 second every billion years (or something equally ridiculous!).

You can't expect a £200 hand held from Dixons to achieve the same level of accuracy......!

Even a broadcast hand-held will show flicker if its left free running, rather than locked to the station syncs.

--
Pete[/quote]

I think that's cheating. It's not frequency related if your picking up the frame sync pulse. If you can't pick up a sync pulse then your back to sampling. Now cameras can recognize faces perhaps they'll be able to get framesynced to a TV image.

Chris