NBTV/SSTV-625 Up Converter MkIII

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: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Sat Dec 07, 2019 11:47 pm

Steve, a problem for 525 / 60 is that you cannot display 512 lines / frame, or 256 lines per field. You loose 20 lines per field due to the vertical blanking. So you loose some lines at the top or at the bottom, or both.

For 625/312 lines 25/50 Hz you loose 25 lines per field, so you still have 287 lines per field (non interlaced) free, so you can show 256 lines. That is what I made possible in my Robot scan converter with te aid of the small set-up PCB.

The CCIR and the RETMA did not have SSTV scan conversion in mind when they designed their TV systems.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sun Dec 08, 2019 2:08 pm

Klaas Robers wrote:Steve, a problem for 525 / 60 is that you cannot display 512 lines / frame, or 256 lines per field.

For 625/312 lines 25/50 Hz you loose 25 lines per field, so you still have 287 lines per field (non interlaced) free, so you can show 256 lines.

Yep, we've discussed that some time ago, it's one reason I'm sticking with the 625/50 system.

The result is that the up-converted SSTV fits nicely onto the screen with no cropping or overscanning.

The photo is the first 'real picture' I got out of the MkI, you can just make out the TV in the dim light... The TV is an old but functional CRT model, if I had used a flat-panel TV there'll be a bit more margin top and bottom.

The highlights seem a bit clipped (forehead, nose and chin), that's the still camera overexposing the screen due the dim surroundings, in person it was fine.

For the other screen-shots I moved the camera closer in and zoomed so that the display filled as much of the camera viewfinder as possible, that fixed the problem.

Steve A.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sun Dec 08, 2019 2:27 pm

Here is the same photo/TV set up with the still camera adjusted as above.

Steve A.

Note: The aspect ratio is slightly wrong (too tall), but that's down to the TV. There are no 'height' or 'width' controls like in the old days, there might be internally, but I wasn't going to open up the TV just for that.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Sun Dec 08, 2019 10:12 pm

Nice Steve, very little barrel distortion. I can just see the contouring on her left side of the forehead. But I am asking myself: where is the vertical line structure over the whole picture originating from? Is that the clock frequency of the A-D converter? However I see that it is not synchronous with the right edge of the displayed picture, see right top. Or do I see the TV-lines, was this photographed with the TV rotated over 90 degrees, was it lying on its side? Or is it just interference on the (RF) TV input signal?

But over all it is an almost perfect picture.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Mon Dec 09, 2019 5:30 pm

Very good Steve watching with interest you manipulating your circuits for SSTV wonder :wink:
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: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Dec 10, 2019 2:22 pm

Klaas Robers wrote:I can just see the contouring on her left side of the forehead.

Hmm, yes, there is a bit there, not sure why, I'll have a look at the original jpg, shouldn't be there, but you never know..

Klaas Robers wrote: But I am asking myself: where is the vertical line structure over the whole picture originating from? Is that the clock frequency of the A-D converter? However I see that it is not synchronous with the right edge of the displayed picture, see right top. Or do I see the TV-lines, was this photographed with the TV rotated over 90 degrees, was it lying on its side? Or is it just interference on the (RF) TV input signal?

But over all it is an almost perfect picture.

Thanks Klaas. The TV and the displayed SSTV conversion were as normal, as seen in the previous (wider view) photo. The input to the TV was baseband, not modulated RF, but it was a composite input not the Y channel of a component input...being an old and probably cheap TV it doesn't have a component input. The fast edges on the signal may have confused the colour decoder circuits...a long-shot, but possible. There was no colour sub-carrier burst so the colour circuits in the TV should have shut down. A true monochrome TV would have been better, but not many of them around these days.

Also being a colour TV maybe some artifacts are introduced by the shadow-mask CRT? It's a thought...it may have been banged around or dropped. Or it may simply need degaussing.. I should have tried it on the flat-panel TV downstairs which has component inputs and of course no CRT...

Steve A.

Added later...The TV also doesn't have a SCART connector (horrible things) which have pins allocated for component input(s)/output(s) - whether they are connected to anything internally is anyone's guess. No TVs in most of Asia have SCART connectors, thankfully! I think they're only found on European TVs.

Japan uses the same connector but with a different pin-out called JP-21, why? No clue...

The SCART/JP-21 is the video version of the DIN audio connector - also horrible...
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Tue Dec 10, 2019 8:55 pm

Steve,

The contouring looks to be the transition between 7F and 80. And it looks to be a step back in output signal. If you can make a sawtooth signal, you might see it in the middle of the ramp.

The vertical lines that I have seen, must be the shadow mask of the picture tube. I see fine vertical lines, no colour at all, so I did not expect that I was looking at a colour TV picture tube. And yes, I have a few B/W monitors, one of about 1960 (germanium transistors), the other somewhat younger. I will keep them carefully.

Good luck with your experiments
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Dec 10, 2019 9:16 pm

Klaas Robers wrote:The contouring looks to be the transition between 7F and 80.

That is a possibility, there should be a specification for that in the DA-08 datasheet...and also for the PIC A-D, though I'm only using 8 of the ADC's 10 bits.

Simplest thing is to just replace the chips, thankfully they're all in sockets...

Also the whole thing is built on stripboard (Veroboard), it may be an inadequate ground-plane and/or coupling between all those intertwined wires...remember there's quite a lot of fast switching in this. In the MkIII the RAM is serial I/O rather than parallel, again this may be a factor...less wiring, less coupling...

I'm generally obsessive with bypassing/decoupling on power rails, maybe that still needs improving...it can't harm...

I'll leave it be for now, we'll see if the MkIII displays the same problem....less wiring, less power and somewhat more compact. Also the MkI power supply regulators are not on the same board, but the MkIII has space for them...so I'll locate them where they're needed.

I'm tempted to start over and use a larger board so the demodulator is co-located rather than separate...in other words it's all on one board, not three.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Dec 10, 2019 9:48 pm

Klaas Robers wrote:...no colour at all, so I did not expect that I was looking at a colour TV picture tube.

Ah! I converted the files out of the camera to monochrome to remove the blue hue you see in the first photo...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Dec 18, 2019 4:47 pm

All connected with the MkIII SSTV-625 up-convertor and the revised demodulator will be continued here...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Dec 18, 2019 5:14 pm

With the revision I have done today it also makes the thing single-supply on the logic side, just +3.3V, no +5V. The +/-12V is still needed for the analogue sections though...

Suitable regulator is the LM1117T-3.3 (TO-220) or the LM317 (also TO-220) configured for 3.3V output. I'll add both versions to any circuits I do. They'll probably need a small heatsink too.

The usual 7812/7912 devices for the +/-12V, no heatsinks required. You could probably get away with a 79L12 for the -ve supply...

As always, be careful with the 78xx and 79xx regulators, they have different pin-outs. Build the PSU first and test it, better than frying other parts...and always safer than using bench-type power sources - it's too easy to knock or bump up the output voltage by accident - yes, I have done that myself!

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Dec 19, 2019 2:06 pm

Although I've now put a note on each drawing, I think it's a good idea to repeat it here:-

The logic supply is +3.3V, the micros have a maximum supply voltage rating of +3.6V.
Applying 5V WILL destroy them.

There is no requirement for +5V at all. When we think of logic we assume 5V, but not here...

Some micros are specified to use a +1.8V supply...including these ones, but the maximum oscillator frequency is limited to 16MHz, a 4MHz instruction rate.

Steve A.

Added later...all versions of the LM317 have a minimum load figure of around 5mA, when testing the +3.3V supply place a temporary load resistor on the output of 330 Ohm or less (within limits) on the output, a 330 Ohm 1/4W one will be fine. Remove when real load applied...

If that isn't done the output voltage can climb as the regulator 'loses control'. It might be wise to leave the 330 Ohm load in place permanently, just in case the real load gets disconnected accidentally.

Added even later...I've changed the circuit such that it doesn't need that 330 Ohm resistor...no load on the output is fine now...

The 78/79 series don't need this and are fine with no load at all.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Dec 19, 2019 6:51 pm

Here's (I hope) the final power supply. Items in red will be measured when unit is complete...don't take the pinning info as correct yet...

As boring as building power supplies can be, get it out of the way first...

Steve A.

Pinning info appears correct...

Make sure that R803 & R804 are really the values they say they are, use new components preferably...it is important!
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Dec 20, 2019 4:23 pm

Just a reminder...although it's not shown on the above diagram the live/active mains lead should have a power switch followed by a 0.5A fuse.

The 0V supply in all diagrams should be connected via the internal wiring to the mains Earth/Ground lead.

Steve A.

Some quick calculations...it seems the +12V current will be about half of the MkI at around 32mA, the +3.3V current substantially less than 100mA, possibly around 40mA, the -12V as shown. The main reason is most of the op-amps in the MkI have gone - and the drop in the logic voltage to 3.3V.

That means the whole device consumes less than 1W after the regulators...heatsinks? Maybe not.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sat Dec 21, 2019 1:36 pm

I've had a few exchanges via e-mail regarding this device, in my reply to one this morning here's my response...

There is no need for a separate decoder/demodulator board as I have incorporated that function into the write processor now. The whole device is still in the design phase and it is still 'work in progress'. There is no point in laying out PCBs or constructing the unit as it will change, maybe drastically as in the last few days. There are reasons I suggest it is kept to one PCB as a result of the MkI build and results. It will have to be double-sided with as much ground-plane as possible. The stripboard prototype version will be 12x16cm which includes the main unit and power supply. Not on the board are the transformer, panel controls, indicators or external connectors.

Often hardware changes are the result of the evolution of the software...and I haven't even started that! Anything posted on the forum is interim and shouldn't be built until a "For Production" 'sticker' appears on the drawing. I only post the interim/evolving drawings so others can follow the development of the unit.

When the "For Production" stage is reached I will post a note on the forum and also notify those included here. Maybe some are eager...just be a little patient please.

Steve A.


You'll note the mention of PCB(s), we'll see how that goes. If it does go ahead it should end up smaller than the 12x16cm prototype, and a single double-sided board only. I'll only specify components which are available in DIL/DIP packages, even if I cannot get them here, but I'll make sure they're available in the UK, Europe, US and Oz.

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