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 » Sun Jan 05, 2020 8:18 am

Steve, is the clock a fixed clock? I don't see an x-tal for that processor. But the internal clock will do, as long as it doesn't follow the zero crossing speed. That is a trick that I have seen now and then used by Karen Orton.

I would separate both processes completely. On a zero crossing I would reset and start a timer after I have read the value (half period time) of the previous half cycle. Then calculate the video voltage that would be the result of the measurement and use that as the current video voltage. In this process the computer clock is only used to run the timer.

Then completely separate from that I would make a free running process that generates 256 or 128 events pro line to clock the video value into the frame memory. This process should be slowly adjusted in timing to synchronise itself onto the received sync pulses. Of course you can start with two default values, one for the 50 Hz process, the other for the 60 Hz process. After two sync pulses you know which one to set as your default value......
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sun Jan 05, 2020 2:38 pm

Yes, the write processor (IC101) uses it's internal oscillator at 16MHz which with the x4 PLL yields an effective oscillator frequency of 64Mhz. At the temperatures and voltages expected this to be used at the frequency is +/-2%, which is near enough in this application and can be 'tuned' if required, but not here. Of those where I have used the internal oscillator it's mostly 10 times better than that without 'tuning'. There's no linkage between the internal oscillator or any I/O or internal process unless programmed...also applies to where an external crystal is used.

The second paragraph - agreed, the simplest way to do it.

The third paragraph I'm still thinking about!

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Wed Jan 08, 2020 3:09 am

Again about the contouring.... It might be also possible that what I am seeing is a timing problem. Because the DAC08 is an asynchronous DAC it might be that when it switches over from 7F to 80 or reverse, that when the MSb goes from 0 to 1 and all the other bits from 1 to 0 that, if this not happens exactly on the same moment, that you see for a very short moment the video level of completely different numbers. That will show a short glitch. In wildly changing video, we will not see this, but in slowly changing grey value, we might see it. Please check this carefully.

Quite some DAC circuits have an input register, that can be clocked at some high frequency and that prevents for time delay differences. It is a form of data resynchronisation. But when you can program an upgoing and a downgoing ramp in the memory, you can check this. A problem is that once you have seen it, you will see it forever.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 08, 2020 1:19 pm

That is all possible, called propagation skew, where an output when rising takes 50ns, but falling say 30ns. It's neighbour which is going the other way is going to be either faster or slower depending on the data value. This leads to a glitch of 20ns which I wouldn't have thought to be visible, but perhaps here it is. The difference is usually the active devices used, PMOS for '1' output, and NMOS for '0', and the difference in electron and hole mobility. The devices also will have a difference in internal capacitance which doesn't help either. Likewise the NPN and PNP junctions in the DAC. Add to all this what is called 'settling time' which is the time the DAC takes to assume the correct analogue output voltage.

I don't think the DAC-08/DAC0800 were designed with video applications in mind, but as we've seen they do a reasonable job of it. Perhaps the chip I used was out of specification or faulty? It'll be a bit embarrassing if that's the case! (No, I didn't try changing it).

I don't think we'll see this with the serial RAM (I hope), or it may be reduced if the same family of DACs are used. These memories by their very nature do have a shift register/latch on the input and output. Time will tell. I may allocate some space on the board for a latch, but I hope it won't be needed. Also Klaas's suggestion of using a common-base transistor on the output of the DAC may help, isolating the DAC current output from the rest of the world, instead of the resistive load used previously.

Most DACs also have a specification for 'digital feedthrough', how much of the digital signals appear on the analogue output, this is usually very low though caused by (again) chip internal capacitance.

If it should turn out to be the DAC...what to replace it with? I did have a look around for serial input 8-bit (or more) DACs but didn't really find any suitable, most were too slow.

And as I mentioned before it may simply be the method of construction I used which isn't really suitable for high speed digital or analogue signals. It's honestly quite surprising it worked as well as it did!

It may also be a culmination of all of the above!...plus a bit more?

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 08, 2020 6:17 pm

However, we must also keep in mind the linearity of the ADC within the MkI write processor used. With the zero-crossing method proposed here that potential problem should go away.

Klaas's suggestion of doing a ramp (software, both rising and falling) test output should assist in determining the culprit, both at slow speeds and full speed...

So, my plan is to build up the output stage including the read processor and DAC (without the RAM) and with a bit of simple test code and see what there is to be found...

There (where possible) will be a lot more ground plane than previous, with really thick lo-z ground leads..I'm sure I have some offcuts of 2.5mm squared solid mains cable somewhere! Stripped of course...excessive? Perhaps, but let's get to the bottom of this...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Wed Jan 08, 2020 8:26 pm

Very good! You don't need to make a video signal with sync and so on. A simple ramp, fast enough to see data skew and a good oscilloscope will do the task. This is also the way I did this in the past.

We will see.....
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Jan 10, 2020 3:48 pm

Here's a very interim look at the potential layout of the MkIII up-converter,(right), compared to the MkI (left). Simpler and all on one board...click image for higher-res version. The final version (a PCB I hope) will be much better...

Now all have to do is finish this off, build it, write the code and get it working! Not much really!

Even simpler if you don't need the baseband SSTV output.

If you can get hold of the MCP4821 (IC401) in a DIL package (I can't here without the hassle, cost and delay of importing) then that makes things a little easier.

Note: IC203 is rotated 180 degrees, I hate doing this, but it does make sense on this occasion.

The blank stripboard/Veroboard I use here is 92 x 110mm.

Steve A.

By the way, I'm not expecting anyone to copy this layout, it's here just for information. In fact I suggest you don't, I can already see a few things that could do with being moved around...
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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Sun Jan 12, 2020 9:57 pm

Keep up the good work Steve . Look forward to progress reports on the converter .

Kindest regards Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Mon Jan 13, 2020 6:34 pm

Update to the stripboard layout to reduce coupling between sub-circuits...more work still to be done before construction starts...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Mon Jan 13, 2020 7:14 pm

I've already made a few changes again, best I not post anymore for a while until I'm happy....

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SSTV

Postby acl » Tue Jan 14, 2020 2:03 am

Hi there Steve,

I am a little confused by the optional 'slow scan base signal' output. You probably described it previously but perhaps you could put my mind at ease. As I see it all the decoding is done now within the PIC processors allowing the audio output from the source rather than the original decoder based on the ROBOT decoder. The output of the device is a standard CCIR monochromic signal to feed the display device. My question is the optional analogue output signal to clean up the received signal and echo it out or, are you planning to be able to create SSTV signals from this output in the future.

Another thing that confuses me is the original Cop McDonald's system is somewhat slightly different from the SSTV being used by amateurs in the 1980s and I found that I could not decode the SSTV files on this forum using a PC based decoder but I could decode the following -:

https://www.youtube.com/watch?v=u3k6Xt30Z7g

Although there was a parameter I had to change in the configuration this still failed to display the pictures correctly. I would like to hear your thoughts Steve
Kindest regards Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Jan 14, 2020 1:24 pm

The base-band output is optional, it's not required for the unit to do its assigned job. It's the SSTV FM signal demodulated/decoded to it's original form, i.e. without the 1200-2300Hz FM subcarrier, from DC to around 1kHz.

Below is an example from the MkI device which used the ROBOT 70 demodulator. The yellow trace is the base-band signal input to the modulator whose FM output is the 1200-2300Hz rather piercing sound of SSTV, similar in sound to a Fax. The magenta trace is the output of the demodulator/decoder back at base-band. Note the US standard line rate of 15Hz, well, 14.9971Hz - near enough!

The manner in which this MkIII up-convertor works doesn't need this signal, though the MkI did, that's why it's optional. However it's useful to be able to see the video waveform on a 'scope and diagnose any problems with the source signal. It could also be used to drive a conventional P7 phosphor CRT monitor with a base-band input.

The time delay between the waveforms is down to the modulation/demodulation process and the delay introduced by filters.

If not required no changes are required to the rest of the MkIII circuits, simply leave pins 22, 23 and 24 of IC101 open-circuit...it could be added later with no changes (hardware or software) required.

Conceptually SSTV it's no different to NBTV or FSTV, just a lot slower...and because of the very low frequencies within the base-band signal it's modulated onto a sub-carrier.

I hope this helps...

Steve A.
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Re: SSTV

Postby Steve Anderson » Tue Jan 14, 2020 1:49 pm

acl wrote:Another thing that confuses me is the original Cop McDonald's system is somewhat slightly different from the SSTV being used by amateurs in the 1980s and I found that I could not decode the SSTV files on this forum using a PC based decoder but I could decode the following...

The majority of PC based SSTV viewers require a VIS code at the start of each frame, the original Cop McDonald system didn't have or need them, so most of the PC-based software fails on a signal without a VIS code 'header'.

The VIS code conveys data about the following picture frame, resolution, colour/B&W, standard of coding (Martin/Scottie/etc), line and frame rates and so on.

I believe that JVFax software can be used with this original format, it's mainly for Faxes, but does 8-second B&W SSTV too...but I'm no expert on it, Harry may want to add something here...also being DOS-based you may need to run it in a DOS-box...again I'm no expert here...

I should remind all, this device handles only the original Cop McDonald B&W format, both US & EU line/frame rates at 120 or 128 lines. None others. It doesn't understand VIS codes or handle any modern format. The base-band analogue output mentioned in the previous posting should provide a signal for an external unit to handle those other formats. At this stage I have no intention of developing such a device - though that may change one day...but there are plenty of PC apps (and some smartphone apps) that do handle the more modern formats, so I doubt I'll ever get around to it...

I believe there is a VIS code for the 8s 120/128 B&W format - more research required...that could be added quite simply into a modulator...even though this device doesn't need it and would simply ignore it.

Steve A.

JVFax is these days called JVComm32, whether it still supports this original 8-second B&W mode I don't know...it's now Windoze based...
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Re: SSTV

Postby Harry Dalek » Tue Jan 14, 2020 7:01 pm

Steve Anderson wrote:
The majority of PC based SSTV viewers require a VIS code at the start of each frame, the original Cop McDonald system didn't have or need them, so most of the PC-based software fails on a signal without a VIS code 'header'.


Most software even if your lucky to find it has 8 sec sstv use's the vis code ...he started off with AM SSTV from memory as you know Steve ..who has ever made the original system !

I believe that JVFax software can be used with this original format, it's mainly for Faxes, but does 8-second B&W SSTV too...but I'm no expert on it, Harry may want to add something here...also being DOS-based you may need to run it in a DOS-box...again I'm no expert here...


I think the problem is with jvfax have not played with it for a while it needs a sound blaster card and DOS as well yes a need also a dos emulater to run it ,i had more luck with Mscan230 its dos as well but only one i have ever found that does 7.2 sec and 8 sec SSTV


I should remind all, this device handles only the original Cop McDonald B&W format, both US & EU line/frame rates at 120 or 128 lines.


Hell that's all you want all the rest is frankenstein sstv !

None others. It doesn't understand VIS codes or handle any modern format. The base-band analogue output mentioned in the previous posting should provide a signal for an exter nal unit to handle those other formats. At this stage I have no intention of developing such a device - though that may change one day...but there are plenty of PC apps (and some smartphone apps) that do handle the more modern formats, so I doubt I'll ever get around to it...


Yep no need there's enough out there that does the other 70s sstv there are so many modes its a wonder any one could keep up back then ! i always wondered in the earlier b/w development why the next step was 12 sec and 36 and 72 why not 16 sec 32 and 64 ..any case early sstv is the best .

I believe there is a VIS code for the 8s 120/128 B&W format - more research required...that could be added quite simply into a modulator...even though this device doesn't need it and would simply ignore it.


Not seen any robot 7.2 sec ?
Steve A.

JVFax is these days called JVComm32, whether it still supports this original 8-second B&W mode I don't know...it's now Windoze based...


I was just looking it starts as 12 sec B/w but i think this is Robot system again....i just don't know why they all just ignore the classic SSTV ...

I never thought of it till now because i have been using the vis code sstv to record on my records have to see if that really does record when i stat up testing again ... i will know on play back ...i will pay more attention this time!
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Re: SSTV

Postby Steve Anderson » Tue Jan 14, 2020 8:46 pm

Harry Dalek wrote:....i just don't know why they all just ignore the classic SSTV ...

Good question, to which I don't have the answer....the 8/7.2 second monochrome mode might sound slow and limited to many, but compared to those that require minutes to transmit it's actually quite brief and fast and conveys the basic photo acceptably. Remember when newspapers were monochrome only? I do, OK, I'm an old fart!

I can imagine many youngsters asking. "What's a newspaper?" I never see on the trains here anyone reading a newspaper...most have got their heads buried in a cellphone...sad...no wonder why many languages are suffering in the wave of recent ignorance, poor education and linguistic apathy...

...OK. I'll get down off my soap-box now...

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