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.

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

Postby Steve Anderson » Thu Mar 11, 2021 8:33 pm

Having gotten a little confuddled with version numbers I've started afresh with V5, version 5.

From here on it starts...and requires a VGA 800x600 60Hz device as a display. A large percentage of previous ideas regarding this have been modified/scrapped.

Most areas have now been 'bolted down' and are unlikely to change, but there are a few still requiring further thought and design. Do not copy/build anything unless you don't mind wasting your time. It is a work-in-progress. I'm sorry to sound quite firm but the previous drawings did have, "NOT FOR CONTRUCTION" within them for good reason.

Right, having cleared the air, time to move on...

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

Postby Steve Anderson » Thu Mar 11, 2021 8:52 pm

In essence this uses 512x512 pixels of a 600x800 VGA 60 Hz display in monochrome. It's designed just to view the original 120x120 (or later 128x128) pixel/line displays with either a 50Hz/3 (16.7Hz line rate) or a 60Hz/4 (15Hz line rate). Colour and later formats are not supported at this stage, and probably won't be in the future. It's a modern dinosaur unit.

So where am I at the moment? Basic design, and to a large extent, detail design done. Just got to prove it to myself before I publish it. So as eager as some of you might be, would you want to publish something you've not even built yourself yet? I did before out of curiosity as to what others may have suggested as an improvement, however I didn't expect them to actually build it at such an embryonic stage. Hence the, "NOT FOR CONSTRUCTION" notices. Sorry to go on about it...

The design has mutated considerably in the last few days....updates once proven...basically, those that have started - you might as well start over...sorry, but you were warned!

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

Postby Steve Anderson » Thu Mar 11, 2021 10:52 pm

SSTV-625 MkIIV RAM 1 v5.gif
Here's an example of what I mean...THIS HAS NOT BEEN BUILT, wait until it has...

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

Postby Steve Anderson » Fri Mar 12, 2021 1:53 pm

...Less than 24 hours later, already there's a small change to the above schematic...(not yet shown)...

Quoting Hans Summers (QRP Labs), "Basically, it is hard enough to design a circuit that works and is reproducible by others, often involving complex hardware and firmware bound together in harmony." Succinctly put!

The reason for the 74AC157s (IC503/4) instead of the more usual HC series is they are faster, not a great concern here, but more importantly have a greater output drive ability reducing artifacts like the change of input value from 0x7F to 0x80 at the half-way point. An attempt to reduce contouring.

An alternative would be to raise the values of the R2R 'ladder' to 1k8/3k6 and adjust the feedback, I'm a bit concerned about speed though, so the use of the 'AC devices as above is the way I plan to go. You may have to shop around for the 'AC157s, but a quick check of major suppliers shows they're available.

I also DO NOT plan on using IC sockets on IC 503/4 in an attempt to reduce stray capacitance.

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

Postby Harry Dalek » Fri Mar 12, 2021 6:22 pm

Never heard of the AC versions i always thought HC were the best ,i will keep that in the memory on any future part orders
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Fri Mar 12, 2021 7:29 pm

There are others faster than AC - which is almost considered obsolete....it's the four-cylinder "moderately fast" logic family, though not your V8. The HC series is the Volkswagen or Nissan of TTL. The AC series - a 20 year old Jaguar...and I love old Jaguars...

Well "Best" depends on your application, speed, power consumption, and a few more considerations too...

But for 'run of the mill' stuff HC is good enough, but here it would be pushing it...mainly in terms of current handling ability...

I've resisted the urge to use FPLAs...but for how much longer?...

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

Postby Steve Anderson » Sat Mar 13, 2021 1:49 pm

I'm in the process of laying out the prototype board(s), it's quite tricky! Trying to keep stray capacitance and inductance to a minimum yet keeping it reasonably compact. Not so easy on stripboard! I must really get to grips with 'Eagle', the PCB design software. But this is a prototype, it'll be butt-ugly, though I hope to prove it's all possible...

Still working on the clock and CS gating between the two micros, not quite as straightforward as I hoped. Once that's resolved it's time to plug in the soldering iron.

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

Postby Steve Anderson » Sat Mar 13, 2021 4:16 pm

Here's a step in the right direction, how to do the CLK and CS switching (except the red bits, still under review)...almost there, but until I test - don't build....it's clear how things change during the design process, and it's sure not over yet! Wait until switch-on occurs! Also the 'cloud', which reminds me to come back and review that section sometime later...notes to myself during the design process...I do tend to document everything...though most is never published...

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SSTV-625 MkIV Main 1 v5-Ver 1-91.gif


Higher-res files (in time) can be sent by PM to members here...

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

Postby Steve Anderson » Sat Mar 13, 2021 4:27 pm

One thing I think I should point out, the initial version of this will use a simple zero-crossing (internal to IC101) demodulator. They work quite well but I feel they could be improved upon. So once this is an operational SSTV display device there could well be better performance external demodulators that could be added. i.e. Allow space and consider adding +/-12V supplies...it's totally 'up in the air' at the moment...

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

Postby Steve Anderson » Mon Mar 15, 2021 6:26 pm

I've decided to take a couple of days off away to just let my mind wander...nothing unusual many might say...a spin-off from this may be a NBTV-625 or NBTV-VGA version. Somewhat simpler as it wouldn't require the external RAM for NBTV only. Somewhat akin to what Graham Lewis did some years ago, basically NBTV sped up by a factor of four, a frame rate of 50Hz. No flicker. And the old DAC-08 D-A re-used (simple).

In Grahams case it required (I think) a mechanical display device rotating at four times the usual speed, 3000 RPM. I wouldn't want to be anywhere near that! For those in 50Hz countries (or those with more recent multi-standard TVs) this could be a cut-down version of this current up-converter. Converting to 60Hz...no, sorry, I'm not going there!

Update 16/03/21...I meant in the above converting to 525/60Hz, 800x600 VGA IS 60Hz!...or 56, 72, 75 or 85Hz...take your pick!! With pro-rata horizontal rates, bandwidths and pixel rates...what a mess!

I'll put it on the back-burner...along with a pile of other things...

Steve A.

For those in 60Hz countries, short of digging through (and finding in the first place) the manual, is there an easy way to determine if your TV works with a 50Hz signal? If it mentions PAL anywhere it probably is fine...unless you're in one of those countries in South America that used odd combinations...but I'm not going back to 625/50 on this, maybe other things, but if this turns out to be a success I somehow doubt it.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Wed Mar 17, 2021 5:21 pm

Looking to eliminate the 2x74AC157+resistor array D-A I found the ADV7128 (Analog Devices). Plenty fast enough, 10-bit (ignore two) and in SOIC package... 'acceptable'. But can I find a source? I don't trust the usual suspects that get thrown up in a search, only a reputable supplier thank you. But they are from the 90s so considered obsolete these days...oh well...

Or you could go with a triple (RGB) version, plenty out there, with a pin-spacing of under 1mm and usually around 48 of 'em - no thanks!

However, our old friend, the DA-08 could just be coaxed into coping, running it at maximum reference current, minimum resistive load (grounded base stage) and one or two other tricks...

...work in progress...though I'll probably go with the first option to start with...

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

Postby Steve Anderson » Thu Mar 18, 2021 6:46 pm

My next chore is to sort out all the conflicting VESA specifications for a 800x600 60Hz display. Most are very similar, but one or two sources raise questions in my mind. I guess the only way to find out is "To go for it!"

This is what I have to produce, conceptually no real difference to 525 or 625 except frequencies and bandwidths. But you know the 'net, so I'm taking my data from the VESA site and hoping my monitors, and yours, comply.

display-timings.png


With the timing data I have I'm hoping this is gonna be OK...the exact start of the vertical sync (and to an extent the end) concerns me...what waveforms are on the 'net are vague at best...but others have managed, I guess I will too...this is new territory for me!

I have two monitors to try this out on, a 23" Samsung and a 21.5" HP, we'll see...the in-built laptop screen as you'd expect isn't accessible...except via OS stuff...I'm not going there, though there are a lot of clever folks who have...

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

Postby Steve Anderson » Thu Mar 18, 2021 7:35 pm

Mentioning Grahams Lewis's 12.5Hz to 50Hz converter a couple of posts back, might have given me an idea for a CRT (simple) based NBTV project. What if we swapped out the mechanical display for a CRT? It's entirely possible, but first I have to get this thing done...

...yea, I know, easily distracted by a wandering mind...

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

Postby Harry Dalek » Sat Mar 20, 2021 4:39 pm

Steve listening to This bbc doc on JLB
https://www.youtube.com/watch?v=Sn-ljTKhGgQ&t=3s
He's mind did wander about from those that knew him so your in good company !
Nothing has to be finished to a time table if you feel like it do the crt project and come back to this ,no rules in doing what you feel like !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sun Mar 21, 2021 1:30 pm

Harry Dalek wrote:...Nothing has to be finished to a time table...

Well, this has already had a longer than expected gestation period, so I do want to get on with it.

I think the hardware design is basically there, so it's down to the build then the code.

However, I'm still dwelling on the SSTV demodulator which could almost be a separate project on its own. Initially I'll use the modified Robot 70 version, then swap it out for something else I've got rattling around in my skull...

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