Hedghog L Standards Converter

[`FRANK.C]

Hi Trevor
Jeffrey has answered that far better than I ever could.
Colour encoding is something I know little about. At one stage I was going to do some experimenting. One year I even picked up a vectorscope at the NVCF.
But the FPGAs got scarce and I lost interest.

Frank

[Murphyv310]

Thanks Jeffrey for the information and thanks Frank as well.
If anyone does decide to do a RGB to NTSC encoder I'd be interested, even if there is PCBs I'd certainly be happy to buy them. I'm finding some work getting tedious due to eyesight and arthritis in fingers, still positive enough to give it a go though.

[Mike Watterson]

Three hedgehogs fed by 625 RGB (SCART?) and then an analogue 405 RGB to NTSC using 2 x SA612 8 pin DIL or SMD. Single supply, double balanced mixer and on-board oscillator with buffer and on-board regulator. One IC can have the crystal and feed the others Osc IN from Osc Out. The 1496 needs a more complex regulated supply and more parts. I've used both. The SA612 is widely available and cheap.

Obviously the 405 NTSC SC crystal isn't as cheap as 525 or 625 NTSC.

A bit expensive on the hedgehog usage, but the NTSC is quite simple compared to SECAM or PAL, which are also quite simple to generate.

[Mike Watterson]

Block diagram and schematics of an analogue PAL coder I designed for G2 Systems in 1986/7. My first proper project when I went freelance. Desinged to code the output of a BBC micro. It predates low cost video opamps. Easily modified for NTSC (I've done NTSC 525). I still have the handwired prototype and it still works. A production version is coding the RGB output of the COW rotating BBC globe at the Dulwich museum.

Obviously RGB to B-Y, R-Y and Y using video opamps feeding 2 x SL612 is simpler. And the NTSC is simpler than PAL. I can't remember if the NTSC needs the color burst that PAL has for reference phase. You can actually buy RGB to component video adaptors cheaply now, but I've no idea if they care about frame & line spec.

I've two DVD players that do component, but I think both are progressive only.

[ppppenguin]

NTSC has a colour burst that's the same phase on every line. Almost anything commercially available with a composite PAL or NTSC output will be specifically designed for 625 or 525 respectively. I think some of the encoder chips (601 type digits to composite analogue) were fairly flexible but it was still hard to make them do 405.

Not sure why anyone would think of using 3x standards converters. The computer can be made to deliver RGB at 405 so what you need is an ordinary analogue coder. Like the one I designed back in 1986 and showed above. Parts of that were loosely based on the Cox 153 PAL coder which which in turn was something of an industry standard.

[Mike Watterson]

I was thinking more of DVD players, setboxes etc rather than a PC.

I agree if using a PC then your design with more modern parts is all that's needed.

[ppppenguin]

The number of 405 NTSC sets was never very large. I doubt it was more than 100, possibly much less. A handful probably survive. Of course it's possible to convert an ordinary PAL or NTSC set to 405 NTSC. Easier if it's a dual standard colour set you're starting from. But I can't see why you would want to.

[Mike Watterson]

Dual standard colour (405-625) can't have been common? I only ever saw dual standard mono. I think it's now 50 years since I worked on a 405 or even saw a dual standard 405-625 set.
In late 1990s the Barco were the ultimate multi-standard set used for Secam, PAL, NTSC and variations.

[ppppenguin]

All UK colour sets from 1967 to 1969 were 625/405. They didn't do colour on 405 though it would be entirely possible to modify one. For example the Thorn 2000 chassis is modular, so if you have a spare decoder panel you could modify it to 405 NTSC and swap as needed. A true switchable set would be harder. Probably a modern PAL decoder and separate NTSC405 decoder using a spare decoder panel as a carrier.

The biggest outstanding question would be whether the IF response on 405 would be good enough for colour.

[Geordie McBoyne]

An NTSC 405 line sub-carrier and sync pulse generator is easy to implement with a hand full of TTL chips.
The master oscillator operates at 10,631,250Hz. This is supplied to two divider circuits, one divide-by-four to produce the sub-carrier frequency of 2,657,812.5Hz. The other is a divide-by-525 network to realise the H2 frequency of 20,250Hz. Two further divider networks are required to produce the standard 405 sync pulse train. Divide-by-405 = 50hz. Divide-by-2 for the line sync pulses.

Geordie McBoyne.