Video to NBTV converter

Forum for discussion of narrow-bandwidth mechanical television

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Postby Steve Anderson » Thu Feb 27, 2014 7:07 pm

Back onto the converter whilst I await some parts to arrive for the monitor.

Pulse outputs are now active and the timing needs some 'tweaking' to get it spot-on.

Gif below shows composite NBTV out at the frame interval (yellow), the HCTTL composite syncs (magenta), the frame sync (green) and the continuous 400Hz line drive for direct-driven CRT timebases for example (cyan).

...so progress is being made...

Steve A.
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Postby Steve Anderson » Fri Feb 28, 2014 1:23 am

I wanted to check two things with the rather odd looking waveform below. First was the PWM/filter combination was linear, second was that the memory addressing which has to work backwards on reading was functioning correctly - which it would appear to be. Traces and their colours are as above.

The ramp is the full 256 levels from black to white, the syncs are generated in the software and not stored in memory.

The reverse (ascending ramp) of this was fine also.

There are some small glitches I need to sort out and of course still the timing to do. Best I attack that with a fresh mind tomorrow, it is 2120 here.

Steve A.
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Postby Steve Anderson » Fri Feb 28, 2014 7:46 pm

Timings are now sorted, it reads RAM in both reverse directions translating L-R, T-B into B-T, R-L.

The pattern generation, RAM reading and output path can be said to be done...now comes the fun part!!

Steve A.
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Postby Monroe Lee King Jr. » Sun Mar 02, 2014 11:06 pm

Hummmm this looks interesting! It may help out if we get to the hardware phase with NBTV after some testing done with software. Great work being done here!

Progress!
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Postby Steve Anderson » Mon Mar 03, 2014 5:52 pm

This converter (and the others) could be adapted to produce any sane standard as an output. Unless you have an already defined standard I would suggest keeping the scanning directions the same as standard 525/625 TV, left to right, top to bottom for line and frame/field respectively.

It would be quite feasible to get 32 or 36 lines at 10fps in a reasonable audio bandwidth. Converting this to a serial digital stream would require a much greater bandwidth unless compressed - which is possible.

A lot of this hinges on your ground equipment, I guess this 32/36 line system would require up-converting to 525/625 for display and recording, though you could easily record the down-linked signal on a PC.

Anyway, food for thought.

Steve A.
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Postby Steve Anderson » Thu Mar 13, 2014 2:58 pm

Work has sporadically continued on this - confounded interruptions all the time.

I'll tell you one thing - there's one heck of a lot of things happening during the 625 VBI and all the while it has to keep pumping out the NBTV signal smoothly.

If, and it's a big if, I don't get any more distractions I'm hoping to have at least a beta version ready before the end of this month (March).

The hardware is pretty much fixed in stone now.

The metal-bashers have finished (at long last) the mechanical aspect of the 5" CRT monitor so I should have that up and running in a few days.

Steve A.
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Postby Steve Anderson » Fri Mar 14, 2014 4:47 pm

If it's not one thing it's another...

The CRT monitor metalwork was delivered yesterday evening - and they've made a complete cods of it. There's only three items for them to fabricate and all were wrong in some manner or another.

We went over the drawings and they conceded they got it wrong.

So I'll have to probably wait weeks to (hopefully) get a corrected version.

Damn!

A phone call was all that was needed for clarification - even so, it shouldn't have been necessary if they actually read the drawings instead of just looking at them!

Steve A.

Relevant drawings and a photo attached...
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5ADP1 Mech 202-Model B.gif
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5ADP1 Mech 203-Model B.gif
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Postby Harry Dalek » Sat Mar 15, 2014 1:32 pm

Hi Steve did you get to keep this one that has mistakes ? that will make them pay for it.

Even so i like the look at in its form the L shape idea is the way to go if you are doing a CRT scope or monitor ...it is hard to make holes and such i hope one day when i finish my cnc i might get it to do stuff like that .

Any case looking forward to seeing the monitor come together do post schematics if you do them .
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Postby Steve Anderson » Sat Mar 15, 2014 2:28 pm

Hi Harry, Yes, under normal circumstances one would want to get some form of recompense from a supplier like this. But I have to be careful - the owner of the company is one of my brother-in-laws.

As such I do get it done at cost paying only for the materials used, but have to wait until they get a quiet period. So I just have to bite my tounge. Ho hum...

Steve A.

Added later...As for schematics etc., I'll put together a .pdf as I have in the past. I will submit it to Jeremy, but having had a few CRT monitors published previously I'm not sure he'll be that interested. Whatever, I'll eventually post it in the 'Electronic TV' section.

This is not a dedicated NBTV monitor per se, but really an WYZ monitor which with the use of different plug-in cards/modules will be able to display a wide variety of standards, NBTV being just one of them.

A large proportion of the circuits are designed, but as yet untested.
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Postby AncientBrit » Mon Mar 17, 2014 6:42 pm

Steve,

I like the concept of plug in cards to test various scan formats.

Cheers,

Graham
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Postby Steve Anderson » Tue Mar 18, 2014 11:47 am

Thanks Graham. I have an interest in other TV formats aside from NBTV, it seems daft to have to build one monitor for each standard when one could be made flexible. Of course if only one standard is required you simply hard-wire the one card in place. It doesn't really reduce the amount of work though, the extra is one multi-pin plug and socket.

My major headache is getting the higher-bandwidth signals from near ground to the grid-cathode circuit at some -1400V, yet still being DC-coupled. Opto-isolators are ideal but many don't have the bandwidth in either linear versions or logic...at least those I can get here. Even Farnell UK have a very limited selection of suitable candidates.

I've thought of AM or FM modulation of a low-ish RF signal, say 10-20MHz, fibre-optics - but those with suitable bandwidth are expensive - and a few other ideas too. Suggestions welcome.

I'm looking for 6MHz of analogue bandwidth or 60Mb/s for serial digital to handle standard 625.

Steve A.
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Postby Steve Anderson » Tue Mar 18, 2014 4:48 pm

The demons are out to thwart me. I originally intended to use D-types as the plug-in interface, cheap and I have plenty of them. I chose the 37-pin flavour as this would provide more than one pin per supply rail and several ground pins leaving still plenty for the actual signal and control connections.

I have done this before but not with so many pins - guess what I found. See below.

Who was the pervert that put D-type pins on a 2.77mm pitch instead of the usual 2.54mm (0,1")?

I have been using/specifying D-types for decades but was totally unaware of this.

So I'm going to have to buy (heavens forbid) some DIN41612 connectors which I KNOW are on a 2.54mm pitch. I have double-checked!!

What is 2.77mm? Something like 7/64"?

The spacing between rows is also oddball at 2.84mm. OK, let's just say the designer was having a bad day.

Yours incredulous of Bangkok.
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Postby AncientBrit » Wed Mar 19, 2014 1:34 am

Hi Steve,

Yes, D pin spacing is a pain.

Depends on how robust you need the connection to be, and how many insertions.

You might consider a strip of 0.1 inch headers on the unit 'PCB' and matching sockets similar to that used for DIY DIL sockets.
The socket pins would be mounted in a narrow strip of Veroboard aka Stripboard.

You would need probably to provide guides for the module, say flanges to either side plus some retention bar at the top to hold the whole in position.

Or dare I say it a separate D plug/socket on flying lead.

Good luck.

Cheers,

Graham
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Postby AncientBrit » Wed Mar 19, 2014 1:42 am

Hi Steve,

Re coupling signals to an elevated (or depressed) electrode I have seen an opto coupler used with a small value cap in parallel to improve the HF response.

My Hitachi scope uses an interesting method in the Y amp.
The signal is split in two using a very LPF (near DC) and a matching HPF.
The signals are then amplified in amps tailored to the these filters and finally combined near the Y plates.
This gives low DC drift but with a high gain/bandwidth product.

You might try something similar with the video mod signal.


Cheers,

Graham
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Postby AncientBrit » Wed Mar 19, 2014 6:36 pm

Hi Steve,

Further overnight thoughts for Z mod.

If the Beast is to be used only for picture display of television signals, and these signals have/will have a defined line blanking period, one solution might be to employ a line by line clamp following ac coupling to the grid/cathode.

The main concern is that on first application of power the ac coupling may introduce a massive negative spike into the base/gate of the clamping device so some form of snubbing will need to be added.

The clamp pulse would be ac coupled to the clamp device.

Depending on whether you need to maintaing the integrety or otherwise of the viewed signal the clamp could be soft or hard.

The grid base of the CRT determines the signal amp. but you are probably talking of a modulation signal of 75v pk pk so a good start would be to lift a cct from a colour TV receiver for tube drive.

These tend to use an HT of 140v or so with a collector load around 5 or 6k.
Some employ inductive peaking but the -3db point is usually 5MHz. Not sure if that would suit you.

The inductor is sometimes in the collector load, sometimes in the feed to the tube acting like a Percival coil, splitting the stray capacity into 2 and forming a pi LPF.

Somewhile ago a did an investigation into making a small scope using a tube with very poor Y deflection sensitivity.
The problem I found even employing comp. emitter follower drives was getting sufficient current to charge/discharge the stray C of the Y plates.
Small signals were okay, large signals suffered poor rise/fall times.
In other words the gain/bandwidth product was not up to the job.

Not sure if any of the above is of use,

Cheers,

Graham
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