SSTV-625 Up Converter MkV (Five)...

Where it all started as far as most are concerned and saw heavy use from the 60s through to the 80s. Colour and Hi-res modes have unfortunately pushed this system into the backwaters of SSTV. Time to resurrect interest in this simple analogue system.

Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sat Sep 18, 2021 7:16 pm

I don't think I recall seeing that. I remember his 12.5Hz-to-50Hz NBTV convertor, but not this one. Was it published in the newsletter, or elsewhere? I may have missed an issue if it were...then again, I may have seen it, I have the original NBTVA back-issue CD, (I don't know if it's been updated for more recent issues) if it were on there I have it. Vol. and Issue No. would help...

The last issue on the CD I have is N33_3i.pdf (March 2008, the total for all up to then is under 400MB)...I also have a few later issues I scanned myself...

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Re: SSTV-625 Up Converter MkV (Five)...

Postby acl » Sat Sep 18, 2021 10:08 pm

Steve,

The block diagram is in Vol 31.3. Due to lack of space in the newsletter and of interest to only a few members Graham posted circuit diagrams and PICs to interested parties.

Regards Chris Lewis
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sun Sep 19, 2021 12:15 pm

Thanks Chris. Yes, I have that overview, if anyone has copies of Grahams detailed work on this I'd be thankful in receiving them.

Graham also mentions an item in 30/3, I'll have look at that too...done, sadly another block diagram.

I'm interested in how he arranged the two sync outputs to the monitor. What data I can find (a lot) is often vague and/or conflicting. Perhaps I should make the effort and measure the waveforms from a PC. It's not the timing info I need, but the levels and impedances. From what I've seen it's not that critical or important.

The actual RGB video is thankfully well defined.

Steve A.

I found a Tektronix document that basically says 2.4V-5.5V at 8mA...i.e. a basic logic signal...I'll go with that and see what happens.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sun Sep 19, 2021 1:47 pm

With what I can find r.e. the VGA sync outputs to feed a monitor I'll give the following a try...somehow the +5VL for IC522 got chopped off...

Steve A.

SSTV-VGA MkIV Main 1 v6A.gif
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VGA driver circuitry

Postby acl » Sun Sep 19, 2021 11:57 pm

Every time I build something I always plan to keep a folder with all documentation in it. Unfortunately I never do.

Anyway I can't find any of Grahams circuits but I photographed the VGA output area. It looks like many of the pins on the VGA socket are linked and four wires(thin green)go to the driver board. The outputs seem to be driven by discreet transistors and resistors. Knowing Grahams excellent experience in design there must be a very good reason for this.

IMG_3536.JPG


IMG_3537.JPG
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Re: SSTV-625 Up Converter MkV (Five)...

Postby acl » Mon Sep 20, 2021 1:37 am

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Re: VGA driver circuitry

Postby Steve Anderson » Mon Sep 20, 2021 12:48 pm

acl wrote:It looks like many of the pins on the VGA socket are linked and four wires(thin green)go to the driver board. The outputs seem to be driven by discreet transistors and resistors. Knowing Grahams excellent experience in design there must be a very good reason for this.


Well, you'll note that the output circuit in my previous posting is the same idea, there's only one channel, not three as this is monochrome with the RGB monitor inputs simply fed with the same signal. That portion is of no issue, it's basically done. There's quite a few ground pins too, so they all need linking together.

Thanks for the photos Chris, you'll note there's quite a few 150R resistors in parallel to create the 75R video output impedance, again, well documented on the 'net. Nice to see those old metal-can transistors again, haven't used any in many, many years. I think I have a few kicking around...somewhere...

Where information is vague/conflicting is regarding the two separate H and V sync signals, the whole lot sometimes called RGBHV, five signals in total.

Also thanks for the two links, but as expected they focus on the RGB portion, and the syncs are an afterthought, if mentioned at all.

acl wrote:Every time I build something I always plan to keep a folder with all documentation in it. Unfortunately I never do. Anyway I can't find any of Grahams circuits but I photographed the VGA output area.


I learnt my lesson many years ago, I keep and archive everything and the NAS backs it all up in a RAID array. I'm still hesitant to use cloud storage...plus you need Internet access and that's not always possible.

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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Mon Sep 20, 2021 1:53 pm

OK, found what I need...

VGA Syncs 1.gif


With that data it is possible to drive the sync line direct from the micro though I think I'll keep some form of simpler(?) buffer/driver in there for insurance.

So it should be a doddle to generate this...the first line with 'active video' is Line-28.

SSTV-VGA MkIV Vert 1l.gif
SSTV-VGA MkIV Vert 1l.gif (4.79 KiB) Viewed 8984 times


Note 800x600 VGA syncs are positive-going, inverted to the usual 525/625 arrangement...

Steve A.

...from this document, great bedtime reading material...

vesapad.pdf
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Mon Sep 20, 2021 3:39 pm

Quoting myself...
Steve Anderson wrote:...I think I'll keep some form of simpler(?) buffer/driver in there for insurance. Steve A.

Really, just one 14-pin chip and four resistors? I don't think it's possible to simplify that!

Though realistically the 'AC125 could be replaced with an 'AC00 (NAND) or 'AC08 (AND), both capable and just used as a buffer/driver....and probably easier to obtain.

Something so simple, yet creates all this consternation - well, at least to me...anyway, one way or another there is a plan...and a local supplier (to me) has them all...

Steve A.

Next day...I've decided to replace the 'AC125 with an 'AC00, no major change, I'll update the schematic but not post it yet. Done.
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Re: VGA driver circuitry

Postby Steve Anderson » Tue Sep 21, 2021 6:32 pm

acl wrote:Knowing Grahams excellent experience in design...

Yes, it's a pity we have to refer to Graham in the past tense, it was he that led me onto the path of resuming software writing, in particular with PIC devices...via this forum...

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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Wed Sep 22, 2021 4:30 pm

A large percentage of the required parts are now ordered. Except all of the 74ACxxx chips, there's only one place in Bangkok that has them and it's right out the north of the city (I'm in the south-east) and they don't deliver, cash only (nothing unusual here). A pain, especially at it's the wet season now. I'll keep an eye on the weather forecast and make a 'run for it' soon. Farnell and RS do have a presence here but their delivery charges are a killer!

Started laying out the board, much simpler than the MkI, no large 0.6" 32 or 40-pin chips. Largest are the two micros, 0.3" 28-pin.

I may be able to delete IC103 (74AC157), but I need to get the hardware partially built first and do some testing...but I'm not that optimistic...but you never know...it depends if you can do do modifications to the CONFIG parameters 'on the fly', possible, maybe, but not certain...and how long the changes take, they are in most cases just used at start-up...not dynamically...

Later...No, not possible it seems, CONFIG directives aren't copied into RAM, oh well....

Maybe I should push this 'into the future'...

Steve A.

...maybe I should have stuck with 525/625? Perfectly adequate, but unfortunately now obsolete...that would been a comparatively easy task otherwise...
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Wed Sep 29, 2021 6:06 pm

I'm calling a pause on this for a while simply 'cos I can't get the chips I need. Delivery dates are into mid-next year to the distributors...it will resume when things ease up...

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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sat Oct 02, 2021 10:34 pm

This SSTV-VGA up-convertor is stalled as my old Tek 100MHz quad channel 'scope isn't up to the task....another investment, but it's needed for 'real work' too. I might have to invest in a decent logic analyser too...and I'm supposed to be retiring at my age...

The chip supply situation only serves to compound the headache...though with a bit of a re-design I might be able to negate a large part of that...I do tend to keep an eye on what is and what is not available in Europe.

(Similar posted elsewhere too).

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