The 'Hellcat MK I' 625-32 Converter.

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Re: The 'Hellcat MK I' 625-32 Converter.

Postby acl » Wed Nov 04, 2020 6:05 am

Hi there Steve,

As far as I know there have been two 625 to NBTV converters designed orther than Peter Smiths. One by Graham lewis and the other by Karen Orton. I have built all three converters and they all give outstanding results.

Stay safe Chris Lewis
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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Wed Nov 04, 2020 11:30 am

Thanks for that Chris, my memory has been jogged! I recall Karen's version now as it was quite recent...2014...is that recent? As I mentioned a few days ago, where does time go?

I'll have to go on a rummage to find Graham's version.

I recall Graham doing a NBTV 'speed-up' converter, 12.5Hz NBTV in, still NBTV out, but at 50Hz to eliminate that flicker. Ideal for CRTs, not sure if I'd like a disc spinning at 3000 RPM anywhere near me!

Steve A.
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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Tue Nov 17, 2020 1:38 pm

Things are moving along on this, there have been a few changes to the circuit posted in this thread on the third of this month, mostly minor so there's no point in re-posting it again (yet). The most obvious change is the removal of the 525/625 input standard switch, the micro now auto-detects which standard is to be converted.

A left-right pan-pot has been added to select which part of the FSTV frame is to be converted due to the difference in aspect ratios.

The input circuit is virtually unchanged from the design of long ago, but as above there are bound to be a few alterations. This has a 2MHz 75-ohm low-pass filter to suppress any colour subcarrier on the input signal, and also performs FSTV pulse extraction with black-level clamping of the video.

Hellcat Input v1-02.gif


The memory within the micro is enough to store two 32x48 pixel NBTV frames [1], if need be, along with the required 'housekeeping' RAM.

As ever, anything in a colour aside from black means it's not 'set in stone' yet and may change...

Also, here's the plot of the input filter, despite its simplicity it does quite a good job...plus the following LF356 at a nominal gain of 3.6 will add a gentle roll-off starting at around 1.4MHz.

Steve A.

The PIC internal RAM usage is also below...you would be correct if you guess that I haven't started on the software yet...

[1] If (a big IF) only one frame store is required the line resolution could be increased to 64 pixels, perhaps more...
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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Thu Nov 19, 2020 3:00 pm

Going back a little while, Peter Smiths 625-NBTV converter was published in the newsletter, Vol. 25 No. 3 sometime in the year 2000. I even mentioned it a few postings back! Senility...it creeps up on you!

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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Sat Nov 21, 2020 1:55 pm

I have a bit of a dilemma between the A-Ds for this. Using my original choice of the 8-bit A-D (AD7822) has timing issues between it and the micro, this can be solved by adding an eight-bit latch between the two. Or the 6-bit CA3306? No timing problems but you need to shop around to find it (it's obsolete), and only 6-bit.

I've spent quite a bit of time trying to find alternatives, but none really leapt out at me. Also many (most) are in SMD packages.

More recent serial output devices require speeds to/from the micro which it cannot attain.

Though Klaas did provide this link, be careful of the shipping charges:- https://www.digipart.com/part/CA3306

Utsource (scroll down) have over 190,000 in stock!! But it seems they're used...buyer beware...

If anyone can suggest a sane alternative - I'm all ears!

Steve A.

Also, those that might consider building this, any preference as to 6 or 8 bits?

Later...Decision made! 6-bits at first, 8-bits when a suitable A-D device is identified...
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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Sat Nov 21, 2020 2:52 pm

Initial interface between CA3306 and the micro...

Steve A.
525-625 Camera CA3306 1.gif
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Re: The 'Hellcat MK I' 625-32 Converter.

Postby Steve Anderson » Sun Nov 22, 2020 1:45 pm

Quoting myself..."In the interim I may be able to use the fibre set-top box which in addition to the HDMI interface has YUV outputs, I'm hoping at 625/50." ...It wasn't to be. It's analogue HD complete with tri-level syncs. But the composite output is 625 PAL...Oh well...

The horizontal timebase below is at 5us/div, so the line rate is much faster than 625, which is to be expected...this is just the Y (luminance) output, I didn't bother looking at the U & V.

Steve A.

Yes! An A: drive! What the scope calls the front USB port when using a flash-drive, the rear one becomes the B: drive. Presumably you could use an external hard drive instead, though I've never tried it. If you need that much storage there's also a RJ45 Network interface on the back.
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