NBTV without any moving parts. THE NIPTRIX

Forum for discussion of narrow-bandwidth mechanical television

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Postby gary » Fri Oct 12, 2012 3:06 pm

Steve Anderson wrote:Not wishing to throw a spanner in the works, has any thought been given to sync pulse time? If using 48 pixels during the active video time of a nominal 2.375ms (allowing 5% of 2.5ms for syncs) results in a pixel rate slightly higher than 19.2kHz....around 20.21...kHz...

I'm gonna make a run for it...

Steve A.


Yeah but if you are using the NBTVA standard format the sync forms part of 48 pixels - put it this way, you can use as many pixels as you like along the line but if the signal is bandwidth limited to the NBTVA standard of 9.6kHz you don't achieve any greater resolution by increasing the number of display pixels beyond 48 - sync. I suppose you could in fact save some LEDs by dropping off the LEDs at the bottom of each line that would normally only ever be displaying black if that was all that was ever going to be shown on the display.

I suppose I could have summed all of that up by just saying that the sync is included in the aspect ratio.

Video2NBTV alpha produces video with the full bandwidth of a 48 kHz (24 kHz bandwidth) so that the line of NBTVA video actually has a resolution of 120 pixels - but that is simply because there is no need to limit the bandwidth in the file itself - it should only be limited by the transmission system itself. In the release version of Video2NBTV the video is the Brown/Robers colour format so automatically has half the display bandwidth of a B&W signal, on top of that it is 44.1kHz, and on top of that, the anti-aliasing filter DOES limit the bandwidth to 9.6kHz because it needs a slower roll off to prevent ringing that is clearly evident in the picture (but wouldn't be if it was listened to as audio).
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Postby Steve Anderson » Fri Oct 12, 2012 4:50 pm

Totally agree Gary. But I wonder why the sync period was included in the aspect ratio figure? In all other systems that I'm aware of the aspect ratio is that of the displayed image without consideration to sync/blanking time in either line or field/frame.

In which case your points lead to the acceptance of sticking with a 19.2kHz pixel rate...colour aside.

I tend to use whatever bandwidth is available to me, I'm not trying to be historically correct, I'm not going to transmit my signals nor try and squeeze them down a phone line.

With the optical constraints of mechanical NBTV I wonder how many cameras/monitors can resolve more than 9.6kHz? Given precision engineering facilities I'm sure it can be done but how many here have access to such or are willing to pay one-off charges to have it done by someone else.

So Albert, stick with 19.2kHz...

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Postby gary » Fri Oct 12, 2012 5:13 pm

Steve Anderson wrote:Totally agree Gary. But I wonder why the sync period was included in the aspect ratio figure?


I am not sure. I am guessing it was to keep it in line with Baird's 30 line format whereby sync was really just a masking of the bottom portion of the picture. I would LIKE to think it was because sync is optional (at least as being part of the picture signal) but I don't think so. Mind you, it wouldn't make any difference as far as this matter goes - the aspect ration would merely be defined as square.

Steve Anderson wrote:In which case your points lead to the acceptance of sticking with a 19.2kHz pixel rate...colour aside.


Colour too as it still has a bw of 9.6kHz, and good point too I should have made it clear that the timing would still need to be that even if the part of the picture reserved for sync was removed.

Steve Anderson wrote:With the optical constraints of mechanical NBTV I wonder how many cameras/monitors can resolve more than 9.6kHz?


Blasphemy! rank Blasphemy! Stewards take this man and pack him off to RCA where he belongs!

I'll have you know a mechanical monitor EASILY displays 9.6kHz and is capable of much higher! In fact, with the right shaped aperture it actually out performs a CRT, at least in the direction of scan.

So mister glass bulb man - put that in your pipe and smoke it!

;-)

PS: there is a great deal of literature on this web site, mostly posted by Chris Long, that gives the mathematics to back up my assertion (note that aperture correction is assumed as it is with a CRT).
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Postby Steve Anderson » Fri Oct 12, 2012 5:49 pm

Seems like I've backed myself into a corner here. Indeed I have downloaded most of what Chris posted or linked to, I just haven't really studied it. The day I get around to a mechanical camera or monitor then I will. I was just wondering...now I have my answer...

I did start on a dual-spiral drum monitor a few years ago, there's a picture of it here somewhere but something else came along and distracted me and it's been shelved ever since.

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Postby gary » Fri Oct 12, 2012 7:48 pm

LOL - Don't worry Steve, whilst every thing I have stated there is *theoretically* true - "There's many a slip 'twixt the cup and the lip" and it is no doubt difficult to obtain great pictures on a mechanical TV but, surprisingly, not due to inability to reconstruct the frequencies (well if your circuit design is poor that's another matter).

Have you actually seen a mechanical televisor in action Steve? One frustrating thing for me is the incredible difficulty in photographing or videoing a televisor picture in a way that does it justice. When I look at my (CNC made) bead disk pictures I am astounded by the clarity and brightness of the picture - yet it doesn't seem to come over anywhere near as well when I try to video it. Of course I don't have access to good equipment, and never will, but the annoying thing is it doesn't have to be all that good, but they just don't design cameras of any kind to take pictures of NBTV televisors.

I must say your drum televisor project was ambitious - it is much more difficult to get "perfect" than a Nipkow disk (IMHO), and yes access to good equipment would be a major plus.

Never-the-less, a man with your skills, I'm sure could knock over quite a passable televisor in a weekend so you really should consider giving it a go, at least then you would know what the rest of us are going through... ;-)
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Postby Steve Anderson » Fri Oct 12, 2012 9:04 pm

No, never seen a real mechanical TV 'in the flesh' as it were. I almost did last year, I arrived in the UK one week before the 2011 convention. But with getting over jet-lag, getting my feet under the table on the contract and generally sorting my life out the convention had to be 'another time'.

If I had arrived 2-3 weeks earlier I would have been there.

There is a slim chance I'll make it for the 2013 one, but at this stage it is slim...

...anyway, let's let Albert return us to the matter at hand...

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Postby Viewmaster » Fri Oct 12, 2012 10:32 pm

Grant Dixon uses 19.2 kHz so I just copied as it seemed the correct freq to use. (400line x 48 pixels)

BUT I am puzzled about these big power supplies mentioned.
I am not going to try to put more than about 40m/a through each LED as a starter, after grossly over running just a few to see how they stand up.

So why do I need a big supply? One LED on at a time + circuitry supplies
wouild be less than 100m/a.

What am I missing here?

BTW, it now has a name........ "The Niptrix." (NIPkow + maTRIX)
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Postby Steve Anderson » Fri Oct 12, 2012 11:06 pm

Viewmaster wrote: What am I missing here?)


Nothing Albert...my fault, with the twists and turns in this thread I got confuzzled...ignore the reference to PC power supplies...

Steve A.

I's been one of those days....with the post below I feel it's best I 'down tools' for the day...I'm obviously not with it today...
Last edited by Steve Anderson on Fri Oct 12, 2012 11:24 pm, edited 1 time in total.
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Postby gary » Fri Oct 12, 2012 11:17 pm

hang on. It comes from the concept of over driving the leds to ensure adequate brightness. The required PS comes from Klaas's post:
I know at least that the LEDs in a TV remote control run at about 1A when they are on. However they run at a mean duty cycle of 1/80, so the mean current is still no more than 12 mA. So if you leave the LEDs time to cool down, they can be overdriven considerably if you do that for a short time.

So if you plan to have one NBTV-line on for 1/32 of a frame, that is 1/400 sec, then you may run it at a peak white current of 500 mA per LED. For 48 LEDs in one line that is 24A from your power supply (as there is always a line in operation) for a white frame.

But then you will see the flicker very well. It would be better if you had a memory (capacitor) per pixel that will see that the LED keeps the required brightness the full 1/12.5 sec. Then you would have a flickerfree picture. However that implies 1540 capacitors and 1540 transistors and 1540 resistors besides the 1540 LEDs.

Then the current for each LED can be say 10 mA peak white again, but for a white frame still 16A is needed from the PSU.
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Postby Viewmaster » Sat Oct 13, 2012 12:59 am

gary wrote:hang on. It comes from the concept of over driving the leds to ensure adequate brightness. The required PS comes from Klaas's post:
I know at least that the LEDs in a TV remote control run at about 1A when they are on. However they run at a mean duty cycle of 1/80, so the mean current is still no more than 12 mA. So if you leave the LEDs time to cool down, they can be overdriven considerably if you do that for a short time.

So if you plan to have one NBTV-line on for 1/32 of a frame, that is 1/400 sec, then you may run it at a peak white current of 500 mA per LED. For 48 LEDs in one line that is 24A from your power supply (as there is always a line in operation) for a white frame.

But then you will see the flicker very well. It would be better if you had a memory (capacitor) per pixel that will see that the LED keeps the required brightness the full 1/12.5 sec. Then you would have a flickerfree picture. However that implies 1540 capacitors and 1540 transistors and 1540 resistors besides the 1540 LEDs.

Then the current for each LED can be say 10 mA peak white again, but for a white frame still 16A is needed from the PSU.


Grant Dixon said his display was a bright display, having a single LED on at a time running at 80mA. This is what I shall do but try lower currents.....As Xmas is approaching, for me, it's a case of,
"In his master's steps he trod, where the LED was........" :)

Only done 10 joints today.....gotta cook the din dins.

edited...Could one just put a small electrolytic C in series with a small R across each LED me wonders to keep the lighs burning a bit longer or would a lot more circuit be required?
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Postby Klaas Robers » Sat Oct 13, 2012 6:50 am

The point is: when you iluminate one LED at the time, the LEDs will be on always for 52 us and then off for 80 ms. So the duty cycle is 1/1500. If you then run the LED to only 20 mA you will have a very dim display.

However you can overdrive the LEDs considerably for this short time. Then your display will get much brighter. If you drive them up to 500 mA, the LEDs will be 25 x as bright. But then still one LED will give the same light as if you run it continuously at 0.33 mA. You can easily try that by connecting one LED to 12 volt via a resistor of 30k. if you are satisfied with that brighness drive the LEDs up to 500 mA.

On the other hand the brighter you make the display, the more annoying the flicker becomes. So a dim display is not always a bad idea.
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Postby Viewmaster » Sat Oct 13, 2012 6:23 pm

Klaas Robers wrote:The point is: when you iluminate one LED at the time, the LEDs will be on always for 52 us and then off for 80 ms. So the duty cycle is 1/1500. If you then run the LED to only 20 mA you will have a very dim display.

However you can overdrive the LEDs considerably for this short time. Then your display will get much brighter. If you drive them up to 500 mA, the LEDs will be 25 x as bright. .


Thanks for that Klaas. Driving at 500mA would result in blown LED's if the scanning ever stopped as has been mentioned before.

Maybe in this case one could monitor the line and row frequencies and if they ever stopped, the video modulator, which controls all the LED's,
be switched off.
But it might not be in time to save an LED.
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Postby gary » Sat Oct 13, 2012 8:07 pm

The thing is though Albert, NOT overdriving the LEDs will probably lead you to the same ultimate situation - i.e. a black picture...

There must be a way to protect the LEDs - a retriggerable monostable or something similar perhaps.

Still I suppose if you start at 20 mA (or whatever) and work upwards you will find out if it's needed or not - a controllable clock rate would be useful for initial debugging.

This should be interesting :-)
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Postby gary » Sat Oct 13, 2012 8:19 pm

hmmm - just thinking out loud here - but if the led was being driven by a signal modulated by a pulse train synchronised to the clock rate such that the duty cycle was always low whether or not the shift register clock was present or not - in other words you would need TWO clock sources to fail for a fail situation to occur.

OTOH if the *signal* was modulated by the shift register clock then if the clock stopped (assuming logic zero) then a fail situation could never occur. Theoretically this would have no effect on the picture as the frequencies would be maintained... hmmmm
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Postby Viewmaster » Sat Oct 13, 2012 10:52 pm

Trust you Gary, to come up with such elegant, clever solutions.
My clumsy solution was to use two LM331's (F to V converters), one for monitoring the column clock and the other for the row clock.
The two voltage o/p's from these to be ANDed together and if the AND gate o/p went LOW then shut down the video modulator.

As my 4017 cascade board is being soldered up my options for changes grow less by the day, but your solution is still 'buildable in' if I ever grossly over run my NIPTRIX. :)
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