Starting at the beginning with Mirror Screws

Forum for discussion of narrow-bandwidth mechanical television

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Postby kareno » Fri Sep 23, 2011 1:59 pm

Cummulative error puts a skew on the picture - annoying but not terrible. But I do think it would be very difficult to get the mirrors set right when a line is only 3 degrees.

I've found the notes I made when deciding on a memory card for my 405 line player, and the reasons I ruled out MMC/SD: they need 3.3V. I would need level shifters to interface to my PIC. Also, the maximum transfer rate the PIC could handle is 5Mbit/sec which is not enough for 405 lines.

Now I think about it, my CF player could be made to work on any standard up to 405 lines with just a firmware change. But it would be monochrome and at least one line would need to be blank to accommodate CF card access time. Other than that, any standard is possible.
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Postby Viewmaster » Fri Sep 23, 2011 6:24 pm

If Steve did have trouble with cummulative error he could always scrap the pin location.
Then use a 120 tooth spur gear to index each of the 120 slats in turn, locking each one as he went. As spur gears do not suffer from this error, nor then would his mirror.
I did this with a 32 line mirror, using a 96 tooth gear and indexing on every third tooth as shown, with each new slat to be positioned held against a fixed stop.
I used 32 small clamps to lock each slat and these were all removed on the final 32nd position, after final lock nut tightening.
Steve would have to design a better locking system than this though for his 120, as making 120 clamps is rather daunting.
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Postby Panrock » Fri Sep 23, 2011 8:48 pm

Thanks to everyone for their various ideas in support of this project, and of course a big welcome to Darryl too.

A few points:

I would have thought it virtually certain there will be some accumulated error. Darryl's allowance of 9 minutes of arc seems generous to me. I had been working on the basis of 1½ minutes (1 pixel of error). This equates to an error of just 0.05mm at the locating pins and this is 'pushing it', because it's about half of what I understand to be the working tolerance on the laser cutting. Moreover, (unless I can correct for this by adjusting the CAD files after seeing the test pieces) there may also be errors in the diameters of the locating holes caused by other laser cutting effects.

So getting this mirror screw to perform well in practice might involve some 'rough techniques'. In particular, at some point I can see myself having to judiciously 'tap' each of the individual slats back or forth by tiny amounts against their stops to achieve a smooth rendering of a test waveform. A laborious process - but not impossible. Even worse, tapping one could then affect its neighbours. But maybe this effect would prove useful when correcting for cumulative error. There will doubtless be problems, and perfection will be hard or impossible to achieve. But I won't know until I try!

Darryl's idea of allowing the user to shift the position of each line individually to correct for mirror position errors would be a wonderful feature to have, if this were indeed possible. Some raggedness at the edge of the raster would be a price well worth paying in my opinion. And a video-to-motor synchronising feature would be luxury indeed.

As for synchronising this heavy beast, apart from a stout motor I had been assuming using the 'club' circuit, a sync wheel somewhere on the shaft with 120 black and white marks and a opto pickup of some sort. Experience at 30-lines has shown me that if a few syncs (about 10%) are removed every frame, this circuit will not only lock to line but will automatically handle frame phasing too. Since (as I understand it) there are no integral syncs in Darryl's mechanical waveforms, I envisage a separate sync line with a bunch of the pulses missing every frame, just as I used in the Grosvenor and its mechanical camera.

Whatever source(s) are eventually used, I think we should base the signal spec on the assumption that Darryl's World Converter will be the 'fallback' standard, and any LED driving arrangements we incorporate should be based on the feeds available from his converter. Keeps things simpler! :-)

One thing that is already coming across strongly is how much an ambitious project like this relies on the help of other members here, who are each contributing their own unique specialisms. Thank you all!

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Postby M3DVQ » Fri Sep 23, 2011 10:37 pm

I've been pondering the problem of synchronising mechanical tv to the video signal for a while. I reckon there should be a way to use a multi-phase motor (for something heavy like a big mirror screw or giant nipkow I guess that'd mean a three phase pump motor or something).

A stout power supply board would generate the necessary phase shifted sine waves to energise the motor coils, their frequency and phase derived from the sync on the incoming video signal. Once up to speed the speed and phase of the disk/screw should be inextricably bound to the incoming signal.

No doubt there are many reasons why it won't work, the first of which is that I've still not found the time to do any experiments.
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Postby tubesrule » Fri Sep 23, 2011 11:35 pm

Viewmaster,
That is an awesome jig you came up with to align the mirrors. There would be no cumulative error, and by using a large diameter gear, the absolute position accuracy would be extremely good.

Steve,
You probably will have enough adjustment range in each mirror to be able to reduce or eliminate any cumulative error. As you say, it could be tricky to tap one mirror without affecting it's neighbors, but maybe with a jig like Viewmaster's it won't be that difficult.

While there are no specific sync signals in most of the mechanical standards for historical accuracy, there is a separate Sync output from the WC-01 that can provide a line, frame or mains square wave or sine wave. This provides a clean sync for the motor control and eliminates the need to filter the video signal trying to recover a fundamental sync frequency.

For adjusting the individual lines, this would probably not be practical to add using just the character display and encoders on the unit itself. The WC-01 has an undisclosed USB interface that will allow full control and setting of parameters from a computer which would make features like this much easier to use. I guess I need to find some time and finish the USB interface and client software.

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Postby Viewmaster » Sat Sep 24, 2011 12:07 am

tubesrule wrote:Viewmaster,
That is an awesome jig you came up with to align the mirrors. There would be no cumulative error, and by using a large diameter gear, the absolute position accuracy would be extremely good.

Steve,
You probably will have enough adjustment range in each mirror to be able to reduce or eliminate any cumulative error. As you say, it could be tricky to tap one mirror without affecting it's neighbors, but maybe with a jig like Viewmaster's it won't be that difficult.
Darryl


It's just occured to me that if an acceptable error were over, say, 6 slats
(maybe 1/2 pixel error ?) then 'sets' of sixes could be pinned as per Steve's designed.
Then each of the 20 slat sets (20 x 6 =120) could be aligned along my gear indexing idea. So only 20 such clamps/fixes would be required, not 120
......and pro rata for any other 'set' numbers whose errors were acceptable.

PS. There are plenty of DC high power (12v to 36v) golf buggy motors for sale on Ebay, from £12 to £45. Running up to 8 amps so the club's
motor circuit using the IRF512 would have to be changed to a bigger beasty
(or 2 in parallel?)..........over to the other Steve!
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Postby tubesrule » Sat Sep 24, 2011 12:13 am

Viewmaster wrote:It's just occured to me that if an acceptable error were over, say, 6 slats
(maybe 1/2 pixel error ?) then 'sets' of sixes could be pinned as per Steve's designed.
Then each of the 20 slat sets (20 x 6 =120) could be aligned along my gear indexing idea. So only 20 such clamps/fixes would be required, not 120
......and pro rata for any other 'set' numbers whose errors were acceptable.

Now that is a clever solution! Even just pairing 2 or 3 mirrors together and then using the jig would tremendously ease alignment. Nicely done.
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Not a master, just old !

Postby Viewmaster » Sat Sep 24, 2011 12:23 am

As there are many newcomers to this topic and forum now I wish to make clear that although I call myself 'Viewmaster' this is not because I consider myself to be a master of NBTV in any way. On the contrary my knowledge of electronics doesn't go much passed ohm's law . Even then I forget where the I,E and R goes. :-)

The reason for my name is that in the early 1950's I built a kit of parts for
the TV receiver, the 'Viewmaster' as it was known as (TRF with EF50s).
So, simply in nostalgic memory for those good ol' days I call myself 'Viewmaster.

There, that's given my old age away :-)
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Postby Steve Anderson » Sat Sep 24, 2011 4:36 pm

The WC-01 is a very desirable instrument, it's versatility and specifications justify its price-tag. Much of that must have gone into the hardware R&D effort and software development. A broadcast quality standards converter (say from Snell & Willcox) costs vastly more and generally they don't produce such a variety of different formats.

Its huge advantage is that it handles moving video and doesn't just produce boring test-signals (though it can).

The reason that I'm interested in all this is that for many a year I have had the desire to dabble in 120-lines, partly as a moving-picture version of SSTV. Originally SSTV was 120-lines as proposed by Copthorne McDonald in the 50s, but nowadays it's usually some multiple of the binary-friendly number 128.

As desirable as the WC-01 is I'm afraid I couldn't justify the cost and would have to go down the road of a custom test-pattern generator.

The mini-NBTG could easily be configured to produce 120/25p signals but using the same chip would mean monochrome only...it simply doesn't have enough ports for RGB colour...though I may have a solution to that...the operative word being 'may'.

Shown below is what it outputs both in composite and non-composite mode, these are for standard NBTV video, the waveform is for 'Reversing Bars', haft the screen of ascending bars (on the right), and half the screen of descending bars.

In addition to the analogue video waveform it has Frame Drive (cyan trace), NBTVA composite/missing syncs (magenta trace) and continuous 400Hz Line Drive (green trace). The yellow video trace is after a 15kHz filter.

The timing I mentioned earlier in this thread for 120-lines was a suggested starting point. If I can find some data on the TeKaDe system I'll perhaps go with that rather than introduce yet another 'standard'.

Steve A.

P.S. Having a hard time of finding on-line TeKaDe data, anyone have any they can post here?

P.P.S. Sorry, at this stage no screen-shots...I've gotta build another monitor as the last one was again plundered for parts...
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Postby Panrock » Sat Sep 24, 2011 6:33 pm

Steve A, it looks like a 120-line variant of your generator could be plugged in for immediate results, and would include the ability to sync it up from the pulses provided.

Anyone with 120-line capability would be welcome to try plugging their source in at the convention. Of course, it's as yet unknown which convention this would be. I'd like to have it working by the next (2012) one. All this, when I still haven't had the first slats cut... :oops:

If I can get this rig successfully built - and it is still a fairly big if - I intend to watch programmes on it on a regular basis. For me, the romantic appeal of historic television (in this case a whizzing assembly of magic mirrors!) lies as much in the 'message' as the 'medium'. I already have in mind some vintage 'technicolor' recorded materials that would look wonderful on it.

So I think I can justify purchasing a World Converter. That is, unless an equally effective solution - but perhaps with fewer bells and whistles - comes up in the meantime. :D

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Postby Steve Anderson » Sat Sep 24, 2011 7:10 pm

Panrock wrote:...That is, unless an equally effective solution - but perhaps with fewer bells and whistles - comes up in the meantime. Steve O


Well it is an item on my To-Do list (along with a shed-load of others). Because the frame rate and scanning directions are directly related to 625 TV it should basically become a buffering exercise, slightly easier than having to rotate things as per the NBTVA standard.

My initial concern would be sourcing a suitable A-D that samples at 3-4MHz at a sane price. The micros are getting faster and cheaper but their internal A-Ds still lag well behind that sort of figure.

Something to ponder the next time I have to spend 12 or more hours on a ruddy plane...

Steve A.
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Postby Steve Anderson » Sat Sep 24, 2011 7:36 pm

One of things I think could well be a stumbling block when using a PWM system at low MHz rates may be the response time of the LEDs/Luxeons. I think the red and green ones may be OK, but I'm not sure about the blue variety.

I recall Klaas mentioning that there is a visible delay in the switching speed of supposedly white LEDs due to the UV-excited phosphor. Whether this applies to the blue-only LEDs I don't know...I guess I should bone-up on their operating mechanism and/or pour over some datasheets.

I could dust-off the PMT noise rig and measure it, the PMTs are more than fast enough, even if disappointingly noisy...which would be of no concern here.

Any thoughts on this?

Steve A.

Steve, I had a look at the datasheets for the LEDs you plan to use, but as usual there's no mention of switching speed. It is only usually quoted for those feeding fibres (usually IR) or within opto-couplers...
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Postby gary » Sat Sep 24, 2011 8:22 pm

For what it's worth Steve O, if you had an old Pentium 4 or better computer lying around (or could pick one up from a recycling place) you could generate a fairly reasonable test card useful for initial testing at least.

Here is an example or two of a 312.5 line 50 fps progressive scan signal (PAL) I created using just a 2 resistor ladder (more resistors more greyscale). 120 line should be much better. If not the best solution, it is certainly the cheapest.

(Note: in this case the output is displayed on a small monochrome PAL tv).

PS:
Does anyone know how to get attachments to appear in the post in the order you attach them (it appears that they are ordered according to size)

PPS: Solved! all you have to do is add a PS and it swaps them! lets see if a PPS reverses that - sheesh
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Postby Panrock » Sat Sep 24, 2011 8:42 pm

Thanks Gary, this is still more to think about when I decide how best to fire up this mirror screw, when/if it gets built.

For me, computers are just tools and I don't know much about 'em. Pentium 4 - what period would that be? I do have an old 1996 computer (with Windows 98 ) knocking around in the bedroom. In fact I used it for this morning's posts!

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Postby M3DVQ » Sat Sep 24, 2011 9:49 pm

Panrock wrote:For me, computers are just tools and I don't know much about 'em. Pentium 4 - what period would that be? I do have an old 1996 computer (with Windows 98 ) knocking around in the bedroom. In fact I used it for this morning's posts!


Pentium 4 would be an early 00s PC I would say
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