Flicker Free Filming of TV and PC Monitors

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JBP
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Joined: Nov 3 2000

Probably a question that has been asked and answered many times in the past, but can anyone tell me how to have a working TV screen or PC monitor appear in a video without flickering or lines running vertically down it please?

Ron Chapman
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Joined: Apr 17 1999

I think you can only do it with a camcorder that has a variable scan rate, most domestic cams don't have it.

Alan Roberts
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Joined: May 3 1999

The only trick you can use is to change the camera's exposure until you find a value that works. You might not find one at all, or at best one that works only part of the time. There's no easy way out of this one.

Get my test cards document, and cards for 625, 525, 720 and 1080. Thanks to Gavin Gration for hosting them.
Camera settings documents are held by Daniel Browning and at the EBU
My book, 'Circles of Confusion' is available here.
Also EBU Tech.3335 tells how to test cameras, and R.118 tells how to use the results.

Hawk
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Joined: Nov 5 2000

I have a Sony 100 htz TV and can video that as you suggest and a LCD monitor and that has no problems.

So it maybe to do with refresh rates and the camera (just thoughts)

TBL is I can video with a DV camcorder my TV and LCD monitor no problems.

------------------
Mick

Mick

Hawk
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Joined: Nov 5 2000

As a after thought to prove my previous post.

If you look at some of the News progs with the presenter in front of people working on computers behind and a LCD screen to one side, the LCD screen is rock solid but the others flickering.

Next on some of the chat programs now days they have big 100 htz wide screen TVs beside the presenter and they don’t flicker (although that could be the camera).

------------------
Mick

[This message has been edited by Hawk (edited 05 November 2000).]

Mick

Alan Roberts
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Joined: May 3 1999

Hawk, your observations on LCD and CRT are correct. The reason is to do with the impulse response of the display.

LCD panels are comparatively slow, typically many milliseconds. That means each pixel will remain at a set level for many milliseconds after being opened (or closed etc). A CRT, by comparison, is very fast. A typical phosphor has a time copnstant of only a few microseconds.

So, a LCD panel tends to blur information from one field to the next, while a CRT keeps them firmly apart.

Also, CRTs are scanned at constant speed, whereas LCDs are (usually) scanned a line at at time, all pixels together.

The net result is that the CRT has a response rather like a Dirac pulse (of zero remporal width but containing the information in a pulse of infinit amplitude) whereas the LCD has a smooth remporal function. So, the LCD is always on and "fades" from one field to the next, whereas the CRT makes temprally sharp fields, the information is only there for a few microseconds, relying on integration in the eye to produce a smooth impression of brightness. Consequently, CRTs flicker but LCDs don't.

Get my test cards document, and cards for 625, 525, 720 and 1080. Thanks to Gavin Gration for hosting them.
Camera settings documents are held by Daniel Browning and at the EBU
My book, 'Circles of Confusion' is available here.
Also EBU Tech.3335 tells how to test cameras, and R.118 tells how to use the results.

Hawk
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Joined: Nov 5 2000

Very good Alan on LCD,

Could you please expand on 100 htz TVs?

(to learn is to know)

------------------
Mick

Mick

Alan Roberts
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Joined: May 3 1999

Certainly. 100Hz tvs show the fields at double the data rate by a local up-conversion. The display is still a crt, so the impulse response is still in the order of microseconds. The algorythm by which the incoming fields are interpolated to produce the outgoing fields has no effect on flicker, only on the smoothness of motion-portrayal and on preservation of vertical detail (that's a trade-off, better motion protrayal usually thorws away vertical detail unless a motion-adaptive algorythm is used, and they're expensive in silicon).

One result of 100Hz displays is the double display of incoming fields, and that can result in film-like motion judder. Remember that film on tv (625/50-PAL) is shown at 25fps, two fields from each frame. Each fra,e is a single instant in the scene but the fields are shown as if they belong to two successive instants in the display. That leads to "judder", the phenomenon in which the eye cannot resolve the irregular motion and tracks motion as two separate motion time-lines, separated by one field, we call that judder.

Hope that helps.

Get my test cards document, and cards for 625, 525, 720 and 1080. Thanks to Gavin Gration for hosting them.
Camera settings documents are held by Daniel Browning and at the EBU
My book, 'Circles of Confusion' is available here.
Also EBU Tech.3335 tells how to test cameras, and R.118 tells how to use the results.

ChrisBitz
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Joined: Jun 8 1999

I'm hereby giving myself a pat on the back for searching and finding this post, instead of posting a new topic, but it seems that there never was a solution posted.

I have a project that will involve interviewing someone in front of a bank of CCTV screens. Does anyone know of the cheapest thing I can hire that will allow me to film this? I normally use a TRV900, and I've found that adjusting the shutter speed will allow me to make the horizontal line stationary, with a part of the screen broght and the other part dim...

Any new good ideas?

Chris.

Alan Roberts at work
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Joined: May 6 1999

Chris, there was a solution, use a shutter setting and experiment. That's really all you can do. When you're experimenting, make sure that you view the camera signal on a genuine crt display, not on lcd, because the lcd will give a misleading impression of the interlace (being a long exposure display).

To record a bank of tv monitors, the ideal way is to have them all locked to the same sync train, and to lock your camera to it as well. That's what would happen in a real tv studio. Then it all stays together and works.

With CCTV that may not be possible because the cameras usually free-run. One solution is to get hold of a quad-split arrangement (Panasonic do a decent one, that's about £30/day from tv kit hire shops, try Metro or Fine Point or any one of dozens more), then you'll automatically have them sunk together and have won half the battle. IIRC, these units also have external sync so you can locke them all together and win all of the battle. Then all you have to do is lock the camera to the same source and away you go (or find a shutter setting that works).

Hope that helps.

DAVE M
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Joined: May 17 1999

Assuming that you're shooting "drama" and are only using one "PC", you can cheat using an Amiga.
Zen Computers used to (still do?) provide Amigas to replace IBM's in Broadcast Drama, as they can be Genlocked and be synced to the camera.
If you can, find anyone who still does Linear editing using an Amiga for titles, and either a genlockable camera, or mixer that will take sync from an external video source (MX50).
using the shutter to sync can lead to really dark pix, and odd effects.
dm

nattt
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Joined: Aug 14 2000

HTTV in Ottawa, Canada use our 4 Apple Cinema displays to film their off screen stuff with. This picture is always superb!!

Even with the best digibeta with clearscan and all that on a CRT, it never looks as good as off our monitors.

[This message has been edited by nattt (edited 21 March 2001).]

nattress.com - Filters for FCP & Color
red.com - 5k Digital Cinema

StevenBagley at Uni
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Joined: Oct 31 2000

I was filming my Grandad's Birthday party on saturday with my Sony VX1000 and in the background there is a tv screen which is completely free from flicker (I guess I got lucky The best thing you can do with a computer screen is to make sure that the refresh rate is an even multiple of 50Hz

One interesting thing I noticed with my old vhs camera -- if I pointed it at a CRT display and rotated it a black band would develop on screen (its size being relative to the angle of rotation)

I guess the simplest method to getting a great recording of a tv screen -- is to not bother. Just drop in the images on top -- I've seen this done so many times on TV (sometimes well, sometimes appallingly). If you've got AfterEffects you got use a bluescreen with a white cross on it to motion track where to place the overlay -- nicked that idea for a documentary on 'Dawson's Creek' of all places... With a few choice filters over the 'onscreen' footage say the 'pinch and punch' one to distort it more like a tv screen (unless its Flatscreen one of course)....

See you earlier,

Steven

nattt
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Joined: Aug 14 2000

If you're comping in a screen afterwards, you can 4 point motion track the corners of the screen, and then, use a "screen" blending mode, and that will keep the bright reflections on the real tv screen and make it look like the picture was really there. I've done this in extreme closeup with a hand reaching for the centre of the screen, and with the reflections, you couldn't tell it from real.

nattress.com - Filters for FCP & Color
red.com - 5k Digital Cinema

Dusan
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Joined: Mar 25 2001

To ahieve what you are after you have to synchronize telly/monitor with camera/camcorder. Some equipment (probably professional but not only)beside the internal it accepts also external sync control. I used b/w Sony camera pointed at titles which was driven by pulse from main camera so that they were scaning in sync. This allowed superimposition of titles over a live picture. I was doing this about 20 years ago! As the technology progressed very fast, by now I would think it is nearly impossible to buy a camera/camcorder accepting external sync for anything less than gold.

Mad_mardy
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Joined: Oct 19 2000

it really easy
all you have to do is set the shutter speed for the camera to the refesh frequency of the screen.
Television in this country use 50hz.
shutter=1/50
100hz=1/100

pc screen is easier to set the refresh rate to say 75hz or whatever your cam can achieve
then set the cam to 1/75

System 1: AMD X6 2.8, M4A79 Deluxe, 4GB DDR2, ATI HD4870 1GB DDR 3, 2TB total drive space, Matrox RTX 2, Premiere Pro CS4

System 2: AMD X2 5600, M2NPV-VM, 2GB DDR2, Geforce 8600GT 256 DDR 3, 450GB Total drive space, RTX100 with Premiere Pro 2

Camera's: JVC HD200, JVC HD101, 2X Sony HC62

Jim Bird
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Joined: Sep 15 2000

Hi,

As well as setting the camera shutter speed, you can also, at the same time adjust the refresh rate of your PC monitor.

These settings are adjustable within the Control Panel of your windows PC.

Jim Bird.