Video to NBTV converter

Forum for discussion of narrow-bandwidth mechanical television

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Postby Steve Anderson » Sat Jan 18, 2014 10:37 pm

AncientBrit wrote:If you migrate to a 40pin device you might consider using one of the extra pins to switch to a test pattern on the NBTV ouput. Graham


The 28-pin version does include a test pattern generator, currently just two patterns on the absence of 625 input. Deleting that selection function and having only a single pattern would free up one pin.

Doing away with a 'No Signal' LED drive would free up another.

Turning the Vpp/#MCLR into a standard input and moving a few others around would free up a third pin.

Deleting one of the Gamma settings so it's either linear (1.0) or Gamma decode (2.0...or any other value) would free up a forth.

So, in all it could be done.

As reluctant as I am to move in time to a 40-pin device I feel it's the way to go. For those a little wary of wiring up anything with more than 16-pins, it's really just the same exercise, just on a larger scale. concentrate and eliminate distractions...turn the damn phone off!!

Steve A.

It's a shame, perhaps a blessing, that no-one produced a package mid-way between 28 and 40 pins. Say 34...though 32-pin packages are common in parallel RAM devices.
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Postby Steve Anderson » Tue Jan 21, 2014 7:50 pm

I found a couple of hours today to lay out, drill and assemble the front panel. I still have the internal wiring between the two boards and the front panel to do, but there's not that much of it.

Tho photo was taken without the LEDs, they'll be here probably tomorrow.

Where time has allowed I've been resurrecting the mono 625 SPG in another thread as a known stable and static source of 625.

Steve A.
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Postby AncientBrit » Wed Jan 22, 2014 7:43 pm

That's nicely presented Steve, well done,

Graham
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Postby Harry Dalek » Wed Jan 22, 2014 10:58 pm

Yes i agree very well done ! a great build :P
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Postby Steve Anderson » Thu Jan 23, 2014 5:45 am

Thanks gents,

No rocket science used, I just lay out the hardware in AutoCad on a 1=1 scale and print the same way. (see attached) I use adhesive tape to fix the drilling template first, drill, rip that off and add final panel escutcheon again held in place by 5cm (2") adhesive tape.

Bang the holes into the escutcheon, fix hardware - job done.

You could use any drawing package where you can get a 1=1 printed output, even kindergarten software like Visio to do this....not tried any other than AutoCad though.

But it's not that easy, if you look carefully you can see the wrinkles...still, it's better than nothing I guess...

I forgot to add a name to the real device, but I've added it into the drawing below, I'm not going to pull it apart just for that!!

Steve A.
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Postby Panrock » Fri Jan 24, 2014 5:59 am

I'm impressed Steve A. Well done! Useful bit of kit you now have built too.

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Postby Steve Anderson » Fri Jan 24, 2014 7:29 pm

Thanks Steve,

The biggest challenge has yet to be addressed, i.e. the software. I have some thoughts in mind but it's a case of getting on with it now...if only I didn't keep getting interrupted by 'real work'! This should have been completed at least a year ago!

Steve A.
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Postby Steve Anderson » Sun Jan 26, 2014 8:00 pm

I found an hour or so today and thought, "Oh sod it." and changed the front panel...no wrinkles this time, a bit more care taken...

The LEDs arrived on time.

I guess the name is now set in stone...

Steve A.
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Postby Steve Anderson » Wed Jan 29, 2014 6:05 pm

Grabbed a couple of microseconds today to wire up and check out the front panel controls.

Now it's a case of chip-stuffing and getting on with the software...

Steve A.

Yea, I know, it's a bit of a rat's nest, I'll tidy it up some other time...
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Postby Steve Anderson » Wed Jan 29, 2014 7:17 pm

Well, the input side of the system works well, the screen-scrape below shows the clamped video input to the A-D after removal of colour information by the input filter and after the gain stage.

The cyan trace is the clamp pulse output of the LM1881. The yellow trace stays right where you put it, it doesn't wander up and down with changes in APL (Average Picture Level).

Steve A.

I just noticed I originally put D-A, now corrected!
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Postby Steve Anderson » Sun Feb 02, 2014 5:12 pm

The micro is now fitted and software work has commenced.

First though I wanted to check out the PWM-to-analogue filter. The screen-scrapes below shows an internally generated 250kHz PWM modulated at 5kHz from 10-90% amplitude.

The filter is a 4-pole Bessel function with minimal overshoot, -3db at 20kHz. Works better than I expected!

Steve A.

Later...a few lines of code later and the first waveform that resembles a NBTV signal emerges...(third screen-scrape). This is only the pattern generator part, not a conversion.
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PWM & Filter 5kHz Square Wave 1.gif
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PWM & Filter 5kHz Square Wave 2.gif
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Basic NBTV Staircase waveform 1.gif
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Postby Steve Anderson » Wed Feb 05, 2014 11:49 pm

A third iteration! I forgot to add a PWM output, my 5" CRT monitor* can process this and apply it to the grid/cathode circuit directly, thereby bypassing any analogue stages.

The lazy part of me thought, "I'll just stick it on the rear panel." But the not-so-lazy part of me won, so it's now a front-panel output. As a result a couple of other things have been moved around.

Steve A.

*The 5" 5ADP! monitor will appear in due course. At the moment it resembles a pile of rubbish. I'm awaiting the metal-bashers to do their thing. We're not scared of some 3kV are we?
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Postby Steve Anderson » Sun Feb 23, 2014 4:00 pm

Work on this converter has had to take a bit of a halt simply that I'm at the stage where I need a monitor to display the results on - which I don't have. So I need to build one.

It's a cut down version of the abandoned 'Brute' project of a few years ago. It uses the same 5ADP1 5" CRT but is vastly more compact. The previous incarnation almost took up all the available workbench space - it was simply too big.

The power transformers have been relegated to an external enclosure which can sit on the floor. Also does away with potential magnetic field problems.

I have also chosen to make it all solid-state (except obviously the CRT), this has also reduced the size of the power supply without the heaters.

I'm not including an audio path - multimedia speakers can perform that task.

Steve A.
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Postby Harry Dalek » Wed Feb 26, 2014 3:42 pm

Steve Anderson wrote:Work on this converter has had to take a bit of a halt simply that I'm at the stage where I need a monitor to display the results on - which I don't have. So I need to build one.

It's a cut down version of the abandoned 'Brute' project of a few years ago. It uses the same 5ADP1 5" CRT but is vastly more compact. The previous incarnation almost took up all the available workbench space - it was simply too big.

The power transformers have been relegated to an external enclosure which can sit on the floor. Also does away with potential magnetic field problems.

I have also chosen to make it all solid-state (except obviously the CRT), this has also reduced the size of the power supply without the heaters.

I'm not including an audio path - multimedia speakers can perform that task.

Steve A.


That will be an interesting project to see this CRT monitor being made .
Oh yes i always find its best to make things small if you can .
Just an idea as you know far more than me but could you do away with the transformer power supply with a switch mode and use something like a laptop fluro inverter for the high voltage part .
You more than likely can knock these up but these days for scrappers like me little dvd players are a good area for a free switch mode power supply once these gadgets play up and very small.
Just thinking a nice way to do away with the magnetic field problem .
Any case it will be nice to see that monitor constructed.
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Postby Steve Anderson » Thu Feb 27, 2014 12:06 pm

Harry,

Thanks for the input. Even though I did say this new version of 5" CRT monitor will be 'vastly' smaller, it's all relative. The original was..

500mm W, 500mm D, and 210mm H.

This cut-down version is...

220mm W, 500mm D, and 180mm H.

The depth is primarily set by the length of the CRT which is 430mm, you then have to add space for the socket and wiring and a bit of clearance to the rear panel.

This CRT requires 3kV (+1500V and -1500V) at about 3mA, most of which goes in side-chain components like the focus and astigmatism controls etc. It's operating at the bottom end of its specification, ideally it should be around 4kV.

I feel more confident/comfortable using conventional transformers for this but have often looked into switched-mode in the past - maybe one day.

Steve A.
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