CCTV, Camera and Circular Reference

[quark]

Guys,

Hold on your redflags for a moment(if possible) and think over.

I came across this excellent(IMHO) question somewhere and here it goes.

Suppose, if we focus our video camera to a CCTV screen, just focusing the screen only and not its rim, switch on the TV and camera. What do we see in the TV? Somebody suggested infinite no. of screens in the screen like we see in two parallel mirrors. I doubt. I think it will show up blurr image. What do you guys think?

 

[IRstuff]

Been there, done that

> infinite screens

It's actually pretty cool, particularly if they're not perfectly aligned, you get kind of a spiral effect.

TTFN

 

[VEBill]

Geesh, didn't everyone do that when cameras first became available to the average schmoe ? I certainly did - within seconds (!) of connecting the camera to the monitor...

 

[MacGyverS2000]

The fun part is seeing how close you can get to the TV while still including a bit of the bezel in the picture... and then trying to count the number of copies you see


Dan
Owner

http://www.Hi-TecDesigns.com

 

[SolderingGunslinger]

As with so much in this world, the answer is "Depends"

First off, I assume (Yeah, I know make an ass out of you and me) that we are talking about strictly one of those El Cheapo CCTV surveillance type cameras and there isn’t any ancillary equipment between camera and monitor.

How long is the cable interconnection cable? In video, timing is everything. If the camera is out of phase with the monitor due to cable length, you may indeed see the blanking. TV stations keep everything in sync by using specific length cables, a central timing source (Sync or “Black”) and when something cannot be timed, a TBC (Time Base Corrector) to synchronize the source to house black.

Now, real world stuff like velocity factor of the cable and imperfections in the optic system thrown to the wind and we live in a perfect world, The camera would be scanning NTSC line 23, Field one at the same time the monitor is scanning line 23 field one, sync pulses would be within a millionth of a femtosecond of each other, video levels would be a perfect 100 IRE for white and 10 IRE for black and all you would see is a white picture on the monitor.

Zoom backward and you would see monitors going into the distance to the resolution of 525 line NTSC video.

I remain,

The Old Soldering Gunslinger

 

[johnwm]

It's called video howlround (by analogy with audio howlround) and it's been a standard 'trick shot' effect in TV for at least 40 years. UK viewers will be very familiar with the effect as it was use extensively in the 'Dr Who' opening title sequence, as well as other SF type programs, and stuff like TOTP.

The length of cable and other timing issues are of course irrelevant, as the TV camera sensitive element, whether old-style vidicon or modern silicon, act as a time integrator over the frame period (approx 16 - 20ms, depending on which part of the world you come from)

Good Luck
johnwm
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[VEBill]

"...camera is out of phase with the monitor due to cable length..."

This is a non-issue for consumer equipment since the camera provides the sync pulses embedded in the video signal. The cable length might matter with respect to attenuation, but not synchronization.

Even for studio equipment, I can only assume that they've moved away from the old central timing architecture and replaced it with RAM-based buffers to align the timing.

 

[quark]

Thanks all for the replies. Nothing beats a practical observation but I have two questions in mind.

1. If we are printing the captured image of the screen on the same screen, will it take less than a screen area? If not, how can we see another screen in it?

2. If the image is exact replica of the blank screen(blue one) how can the replica be actually seen?

Thanks again.

 

[VEBill]

1) If you include the bezel, then the 'endless' screens get smaller all the way down.

2) If you zoom in closer, then you get to see some really weird effects (like huge pixels).

You should borrow a camcorder and TV and try it. It's good for about 20 minutes of amusement.

 

[Warpspeed]

You are right of course, the displayed image can be small or large depending on the focal angle, and the distance of the camera from the screen. It may even be angled well off to one side !

The effects you can get are endless and quite entertaining.

There is no one correct answer. If the camera is held really close, the feedback just overloads the system, and the whole screen can just go completely to peak white.