Video recording rates for 32-line NBTV

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Video recording rates for 32-line NBTV

Postby Ralph » Mon Dec 07, 2015 2:19 pm

NBTV_Test_Web.wmv
Video recording at two different frame rates of an NBTV Nipkow display
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I have a video camera that I use for planetary imaging that gives me greater control ovr the exposure settings. The attached video includes two different exposure settings:

11 Frames/Second - this frame interval allows exposure of on complete NBTV frame and enough of a second frame to create a white bar (the over-exposed zone) including coverage with both frames

13 Frames/Second - this frame interval does not completely expose a single NBTV frame, leaving a black bar where the missing lines were located. This bar is much narrower than the one produced with the typical 15 fps imaging rate.

I am looking for additional control software for this camera so that I can get precisely 1/12.5 of a sec. frame exposure, which should eliminate any black or white aliasing of the video image. Until I can locate the software I need, the question of the day is "Which version - white bar or black bar, provides the least distraction when viewing the NBTV display?"

Ralph
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Re: Video recording rates for 32-line NBTV

Postby Klaas Robers » Tue Dec 08, 2015 8:15 am

Ralph,

I prefer the white bar. But what puzzels me is why both bars run from left to right? I would suppose that one runs from right to left, while the other runs from left to right. This is not the case, so there should be some "dead" time in the process.

But the images are remarquable perfect.
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Re: Video recording rates for 32-line NBTV

Postby gary » Tue Dec 08, 2015 9:11 am

I also prefer the white bar as not only is it easier on the eye but, of course, there is at least some of the information still present.

Of course in a moving picture this may not be as pleasant (if you could call any defect that) as the information within the overlap may be substantially different.

I agree with Klaas I would have expected the "apparent" direction of the bar to reverse as the recording frame rate from less than to greater than the picture frame rate. Curious.
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Re: Video recording rates for 32-line NBTV

Postby Ralph » Tue Dec 08, 2015 2:35 pm

Guys,

I think the answer to the direction of movement is tied into the attributes of the camera. The frame rate that is selected is independent of the exposure. So, I was using two different exposures, but, as I remember, the output frame rate was set to 15 fps. I suspect it is the frame repetition rate that is controlling the apparent movement of the bars. Next time I set up the camera, I will try different frame rates and we shall see what happens.

I also need to find a video editor that will let me crop, but it's a start.....

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Re: Video recording rates for 32-line NBTV

Postby gary » Tue Dec 08, 2015 4:31 pm

Interesting, I can't quite get my head around how that works but from what you are saying it implements exposure control by changing capture frame rate and then up sampling?

VirtualDub with allow you to crop your video.
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Re: Video recording rates for 32-line NBTV

Postby Ralph » Wed Dec 09, 2015 1:09 am

Gary,

I have used the camera for several years now and never thought deeply about how it works. I can say that frame rate and exposure appear not to interact and that exposure times can be set to a very wide range from milliseconds to a minute or more. I'm pretty sure I have a tech manual around the house or out at the observatory and I shall look into the system architecture.

Thanks for the software pointer.....

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Re: Video recording rates for 32-line NBTV

Postby Ralph » Wed Dec 09, 2015 3:13 am

NBTV_Test_Web.wmv
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OK, this should be a properly cropped version of the video file.

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Re: Video recording rates for 32-line NBTV

Postby gary » Wed Dec 09, 2015 8:51 am

Ralph wrote: I'm pretty sure I have a tech manual around the house or out at the observatory and I shall look into the system architecture.

Ralph


Well I for one would find that interesting, so when you have time an update would be great.

I still prefer the white bar BTW but I am warming to the black bar ha ha.

Will you be doing a moving picture experiment as well? - it would be interesting to me to see how the white bar looks in that context (the black bar being familiar to all of us ;-))
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Re: Video recording rates for 32-line NBTV

Postby Ralph » Wed Dec 09, 2015 2:45 pm

Gary,

Here is a very short set of videos with my talking to the camera. Unfortunately, with this particular software there is no way to add the sound. It probably would not work as you will note the very rapid movement in the videos. This almost certainly reflects time delays to reconcile exposure rates with the constant 15 fps frame rate. The software I have been using is all about astronomical imaging, so you can play games with the video timing because there is no audio to reconcile with the video stream.

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Re: Video recording rates for 32-line NBTV

Postby gary » Wed Dec 09, 2015 3:26 pm

Ralph this site is all about video - who needs sound? LOL!

It looks as though the 15 fps is actually the wrong rate - in VirtualDub you can change the sample rate to anything you want - and also merge audio to it - so with a little effort you could probably sync up your audio - if nothing else you could add music etc. It's just something to keep in mind that may come in useful down the track.

That's a very nice clear picture Ralph, especially when there's movement - is that your modified Daily Express Kit Televisor?
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Re: Video recording rates for 32-line NBTV

Postby gary » Wed Dec 09, 2015 3:40 pm

In fact, VD reports that is 30 fps - when changed to 15 fps it looks more "normal" - but it is hard to tell given the material - you could be talking very fast or slow ;-)

Anyway that's a diversion it matters little to the topic at hand.
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Re: Video recording rates for 32-line NBTV

Postby Klaas Robers » Thu Dec 10, 2015 1:57 am

Strange.... How could a live camera take more frames per second than 1 / exposure time? E.g. how could you get 25 frames per second with an exposure time (for each frame I suppose) of 1/10 seconds? I CAN think of the opposite: a camera that takes 10 frames per second where each frame is exposed for 1 / 25 second. And I think this is more or less the situation in CCD cameras.

For taking Nipkow NBTV pictures an exposure time of 1 / 12.5 second is needed. In that case the "split" is zero, that is the black or white bar.

Then the camera should grab and process the frame in zero time. Then the "split" will be stalled, always at the same place. In reality this is not possible, you always need time for processing in CCD cameras. The first possible setting might be 1/12.5 sec exposure time and 6.25 frames per second. Yes, then you are missing one out of two frames. But the digital memory system at least avoids the severe flicker.

Then I can imagine that in the digital world you can play back faster than you took the pictures. Is is what I see in the short film of Ralph talking. The movements are too fast for a human's head. That is something that you can do afterwards. But in some way it should be possible to set the frame rate before you start your film taking. That then defines the processing time.

I did some simple calculations:
- 1/11 sec is an exposure time of 91 msec.
- One NBTV frame takes 80 msec.
- One NBTV line takes 2.5 msec.,
- So in 1/11 sec exposure time 36.4 NBTV lines were scanned
- that is 32 + 4.4,
- So 4.4 lines are exposed twice.
When I pause the video file, I can see a white band of 4.5 lines.
Not bad.

But the white band moves over the frame in about 1 second.
- It moves against the scanning direction,
- so the number of stored frames per second = 12.5 / 2 + 1 = 7.5 f/sec
- could that be 15 fps / 2 ?
- or 30 fps / 4 ?
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Re: Video recording rates for 32-line NBTV

Postby Ralph » Thu Dec 10, 2015 4:44 am

evolution.jpg
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Actually Gary I am speaking at a normal pace and it was the apparent speed increase that I first noticed when using the astronomy software to prepare the full-motion video. The same thing was undoubtedly happening with the earlier video, but it was not obvious because the images were not moving. I do know that the acquisition of the moving images had a 15 fps frame rate, so if the output AVI from VirtualDub was at 30 fps, the first iteration this evening will be to convert that back to 15 fps and see what it looks like.

I also made a full-motion/sound video using another piece of software but the same camera. I suspect that the exposure and the 15 fps frame rate were linked, so the black-bar artifact was more obvious and annoying. However, motion and sound sync were perfectly acceptable. I will look for that one and post a sample. More diversions as we sink into winter up here! Klaas, your math is impeccable and the task at hand is to find a mix that is as pleasing and hopefully as accurate as possible.

As far as my "Daily Express Televisor", that is a complicated story. I had one of Denis's kits and it was beautifully done. The left image (above) shows the first assembly of the mechanics, the only change being the use if one of Peter Yanczer's 32-line (Club Standard) discs. Denis provided the modern electronics to make it workable, but I was profoundly disappointed in the engineering of the original Daily Express system (not Denis's faithful reproduction). I wanted a classic open-frame televisor, but I was only going to build one Nipkow machine. It had to be robust and I intended to use it as a test-bed to investigate a variety of video and drive options. I ended us taking the two original motor support piers and modified then very slightly to mount a superb 12V DC permanent-magnet motor. It had a 1/4-inch SS shaft, double sealed ball bearings, and a 12 oz./inch torque rating.The motor is almost totally silent (the only noise the televisor makes is from the aerodynamics of the spinning disc (above, center image). The basic mechanics of the final system are shown in the right-hand image. The only Daily Express components are the two motor piers and the magnifying lens and its support. It is mounted on an attractive Acacia wood base (above, right). It works well as you can see and has provided the fodder for several contributions to the Club's Newsletter which are starting down the pipeline.

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Re: Video recording rates for 32-line NBTV

Postby Ralph » Thu Dec 10, 2015 10:24 am

Slow_Ralph.wmv
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OK Guys,

I found the speed-up source in the last video. VirtualDub was not the culprit. The original AVI was shot at 15fps and that is how it emerged from VirtualDub
after cropping the video. Unfortunately, my AVIs coming out of VirtualDub and not readable by the Windows video player. Any ideas Gary? However, they can be read by Windows Live Movie Maker and there is the problem. WLMM will boost the 15 fps AVI to 30 fps, hence the .wmv files are running at double speed. You can reduce the playback speed by 50% in WLMM and then the .wmv file will play properly at 15 fps. That is what the Slow_Ralph video is all about.

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Last edited by Ralph on Thu Dec 10, 2015 10:33 am, edited 1 time in total.
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Re: Video recording rates for 32-line NBTV

Postby Ralph » Thu Dec 10, 2015 10:32 am

Talking_Ralph.wmv
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The Talking_Ralph video was shot with the same camera but other software, so that I could include an audio track. With this software there are far more issues with anomalies from the exposure setting (whatever it was) and this AVI was recorded at 30 fps. VirtualDub cropped in at 30 fps and Windows Live Movie Maker kept that rate, probably because of the audio track. While this is my first "talking" video, I would like to have the sound along with either the 11/sec or 13/sec exposure. Anyway, it is me and it talks......

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