Filming 50/60Hz CRT television at 25/50fps - flicker?

[Reinis Traidas]

Hey. I'm rather new here. I'm shooting a short student film next week on the RED ONE (first time). There are a few scenes that involve a mock TV show running on a few different television sets (all old tube sets). This is in Australia, so the TV's would have 50Hz refresh rate. (?) Some of the scenes will be 25 fps, some 50 fps. The question is - will it flicker? We need a clear image of the TV picture. Should we get LCD's or just go the VFX path and comp the footage in there in post? Thanks guys!

 

[martinnoweck]

are all the tube tvs fed by the same signal or do they show different program?

are you shooting red?

if you want to play save i would sync your shooting camera to the tv programm via genlock.

best wishes,
martin

 

[martinnoweck]

yes, australia is PAL, so the tvs should run 50 Hz interlaced.

 

[Andrew Gentle]

I vote for VFX. Put some tracking markers on the screens if you want to move the camera.

 

[Stepan Kment]

VFX

VFX

I also vote for VFX - You'll have hard time syncing images together, You'd need some way to adjust phase of a clock to sync with the TV -

so imho -
- go for 25fps on RED and then slowdown these few scenes to your project's actual framerate adjusting audio pitch, if You really need live picture.
- try LCDs. Note that even these may not be the best.
- and should the content on the TV set really look fine, then consider VFX, which means probably using Mocha planar tracker - that means ensure You'll have something to track, feed some trackable pattern on the TV (avoid pattern touching the TV screen boundary, just put some squares or so in the middle of the screen, but on the other hand, not only at the centre - just ensure boundary will be completely black to avoid probs with keying VFX layer in). Also, should anyone cover the screen at any moment, adjust the background so that something around the TV remains static.. You have to have something to track at any moment, keep in mind!

Some in-advance testing may come in handy, too.

:thumbup1:

 

[Reinis Traidas]

Thanks guys. I'll test it out on a few LCD's and CRT's before I shoot.

If I can get flicker free images of the TV image on both 25 fps and 50 fps, I think we'll shoot them like that. If not - VFX.

Would it work if we simply put a nice, visible tracking marker (like an X) in the middle of the TV?

Also, because the TV is going to be a dark room, I sort of imagine a bit of TV light spilling on the edges of it - like it would in a darkened space. This is probably something that is going to require more difficult VFX though.

I sure hope we can simply get flicker free images straight from the TV feed. I honestly don't understand why it would flicker if it's 50Hz and we're shooting 25 fps... Maybe someone can explain...

 

[Casey Green]

You should be fine with running 25fps on the Camera with a 50Hz CRT source and 180 degree shutter. The main issue, as someone already mentioned, is phase.

You basically need to have the the Camera's shutter fully exposing the CRT screen at the point in time when each frame is fully "painted" on the CRT monitor. The RED has phase controls which should allow for adjustments.

Another method is to use a sync generator and genlock both the Camera(s) and Source(s) feeding the CRT monitors. This is usually done by having another video processing framerate converter, which is capable of manual phase control, in between the video source and the CRT(s).

If you do not successfully genlock / phase the Camera and Video Source, you may notice a horizontal "tear" in the CRT monitor's image, depending on how much of a difference there is between the two. You may not notice this at first if you happen to get lucky and your shutter and CRT are fairly close, but this usually isn't the case, and will normally shift throughout the day.

 

[Reinis Traidas]

I've just done a very brief test with an older CRT and a LCD (laptop screen).

I had very bad flicker at 1/50th running at 25 fps on the CRT. Upon switching to 1/60th most of the flicker was gone with just a bit of subtle horizontal tearing happening. Does this mean that the CRT television is actually 60Hz? With the shutter at 1/60th the tearing was the same at 50 fps too. It's definitely usable for wide shots and/or extreme close-ups.

LCD was fine at any shutter or frame rate. The only catch with that was keeping the LCD angled toward the lens properly as it's angle of view was pretty shabby.

 

[Stepan Kment]

I've just done a very brief test with an older CRT and a LCD (laptop screen).

I had very bad flicker at 1/50th running at 25 fps on the CRT. Upon switching to 1/60th most of the flicker was gone with just a bit of subtle horizontal tearing happening. Does this mean that the CRT television is actually 60Hz? With the shutter at 1/60th the tearing was the same at 50 fps too.

Depends on what you're feeding to the TV set. Note that though PAL is 50i, still there is so called PAL60, which is actually electrically encoded as PAL, but the image is NTSC, so 60i. They invented this to allow NTSC video tapes and DVDs to play on European TVs (on the other hand, NTSC TVs are not required to play 50i, that's why all all region DVDs have to be encoded in NTSC).. maybe this is your case? If not, then it's really weird :willy_nilly:

 

[lediana]

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[Reinis Traidas]

Depends on what you're feeding to the TV set. Note that though PAL is 50i, still there is so called PAL60, which is actually electrically encoded as PAL, but the image is NTSC, so 60i. They invented this to allow NTSC video tapes and DVDs to play on European TVs (on the other hand, NTSC TVs are not required to play 50i, that's why all all region DVDs have to be encoded in NTSC).. maybe this is your case? If not, then it's really weird :willy_nilly:

We were feeding image from a laptop via a composite cable using one of those Apple mini DVI to composite adapters. Could that explain it? Will try feeding from a regular DVD player to see what happens.