Thread: Recording TV with camera

This question might be a bit out of place here, but anyways...I recorded a Pokemon episode from tv today with my ancient PSP (640x480 with 15 fps, the best video camera I have...I know this sucks), just for the fun of it ya know. Both TVs I have are of the "box" variety..ie picture tube type. The recorded video had a lot of flicker and sometimes large parts of the screen went in phase with the "dark" time of TV ( = a big swathe of the video went black for a few seconds occasionaly). All this crap vanished when I reduced the brightness and contrast on my TV but the video still looks crappy.Also I'd built a stand so the camera stayed fixed in place without any "shaking".Now I'm asking these mostly out of curiosity...Would this effect, by any chance, be gone if the tv's "framerate" is made much higher?I've got an new LCD monitor in my home. If I somehow play tv on this monitor would it create better video recordings?Oh and what's the framerate for new box type TVs?If I'm able to record decent quality videos from tv with the stuff I have it'll be a cool thing lol.

Would this effect, by any chance, be gone if the tv's "framerate" is made much higher?

No, it would not. Most TVs and monitors run a refresh rate of 30 or 60 Hz. This is already faster than your camera's capture rate. This effect is compounded by the fact that your box TV is almost certainly interlaced (redraws only half the image per cycle). If you were to somehow increase the TV's refresh rate to something like 200 Hz, it would increase the number of redraws per second, and thus increase the chance that the camera would capture an image during a redraw. If you want to minimize this effect, you should use a TV that does progressive scan (instead of interlaced), and also runs at a lower refresh rate.

Of course your best course of action would be to use a capture device that could take in the TV's analog outputs. Something like this.

There are three things going on.

One, is the synchronicity between the TV frame rate and the camera capture frame rate, as you already thought of. Think of this in terms of Nyquist's sampling theorem. You need to sample the signal (TV image) at least twice as fast as the minimum signal change period. In other words you have to capture at the TV frame rate, or higher.

If you capture precisely at the TV frame rate, you are "critically sampling" and some phenomena can happen like the complete vanishing of signal components which are right at the Nyquist frequency. As you sample at higher frequencies this effect vanishes. If you sample at a LOWER frequency than the TV frame rate, you will get "temporal aliasing" which would cause higher frequency varations in the image to alias into lower frequencies. This is the source of the "backward turning wheel spokes" in some footage. It doesn't sound like this is what is happening though.

The second factor is the alignment of the pixel grid of the TV with the pixel grid of the camera. Here is another kind of aliasing: not temporal, but spatial. This can produce wave-like interference patterns on the captured image or Moire-style patterns when the orientation of the grids do not align with each other.

Third is the closed-loop gain control within the camera. Here is a circuit which tries to determine the overall scene brightness and adjust things like aperture and exposure time to maintain a visible scene. This closed-loop control process has a fundamental rate of operation, and so it can interact with the frame rate of the TV in strange ways. For instance if the camera capture aligns closely but not exactly with vblank, there will periods of time where the camera "sees nothing" and the closed-loop control drives the brightness to maximum. At other moments the camera sees the TV image and the controller drives the brightness down. So you may see an overall brightness modulation in the scene which can be quite drastic. This is most likely what you are seeing. Turning down the brightness on the TV caused the closed-loop controller to behave itself a little better.

Ok thank you both for your ideas.

So all this theory means that I need to make camera frame rate higher than tv frame rate. This certainly isnt possible given the equipment I have.

But I've recorded a gameplay video from a computer monitor (CRT) and it has appeared quite normal. Will I get progressive scanning if I'm somehow able to watch TV through my computer monitor? I know TV through computer monitor is possible with a certain electronic box that I might be able to borrow from a friend.

edit:

Here's a still from that video. A Moire pattern is certainly visible.

Experimented a bit and found out that only reducing the contrast removes those phase issues. I didnt change the color and brightness this time and the results are kinda better:

I noticed another problem with my PSP. It makes the videos corrupt that are larger a certain length of time so I'm forced to use DivFix++.
Also copying to computer is sometimes taking waaay longer than necessary and when I copy the same file twice, both copies are having different MD5 checksums. wtf! And also these videos are stopping on a frame after seeking to a certain point, most probably due to crappy copying.

This is making me really ........ing angry.

Edit:
Unrelated, I just found out about Meowth's song by searching for Meowth on this site. Cool!

As far as I know, no computer monitors are interlaced. I mean, they're typically put around a meter or so from the computer, so the signal only had to travel over the length of a wire instead of a large distribution network over the entire country, so bandwidth (primary reason behind interlacing AFAIK) have never been an issue. So I'm not an expert on the subject, but I do suppose it would help since it gets rid of the interlace issue.

primary reason behind interlacing AFAIK

O_o

The bandwidth question wasn't really the issue.

Sending 60 half-frames per second isn't meaningfully different than sending 30 full-frames per second.

Soma

I am going to disagree with that statement ("Sending 60 half-frames per second isn't meaningfully different than sending 30 full-frames per second."), but if we assume it was true back then, then why send only half the frames and having to invent the whole interlaced system instead of just sending all 60 frames at once?

I am going to disagree with that statement ("Sending 60 half-frames per second isn't meaningfully different than sending 30 full-frames per second."), but if we assume it was true back then, then why send only half the frames and having to invent the whole interlaced system instead of just sending all 60 frames at once?

O_o

What nonsense are you talking about?

1): The production is 30 frames* per second one way or the other.

2): The production does not send half the number of frames per second.

Soma

*) I don't want to discuss 29.97, PAL, or other standards specifically so just keeping general.

I don't know how it was back then, when interlacing was introduced, but I mean, today, 30 is much different from 60, depending on source.
I take it interlaced has something to do with the frequency of the voltage from the outlet. Therefore, if the source is 30, they would have to somehow "extend" it to 50/60. It still doesn't make sense, though, as they could just duplicate frames. So that leaves my question unanswered: if the source is 30 fps, then why bother with interlacing if bandwidth was not an issue?

I don't know how it was back then, when interlacing was introduced, but I mean, today, 30 is much different from 60, depending on source.

O_o

Truly, the mathematicians of the day had been constantly confusing their 30s and their 60s. I think that's why the GILF fetish is so popular.

Therefore, if the source is 30, they would have to somehow "extend" it to 50/60.

Reminds me of the time the cat fancier confused marketing bingo with science...

So that leaves my question unanswered: if the source is 30 fps, then why bother with interlacing if bandwidth was not an issue?

I find it strange that you can apparently conceive of the costs of overcoming bandwidth limitations in the existing infrastructure as being a factor, yet you can imagine no other limitations of the time that would have informed the decisions made during development.

Soma

Whatever then >_<

hmm I think my PSP is a chinese one (as in low cost china made devices, semi-legal of course).
If that's the case I doubt the members here or anywhere else might be able to help me with those video recording and copying bugs.