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 » Tue Dec 12, 2017 8:46 am

I had to lower the sampling frequency, 20 kHz is too high for the PIC on a 20 MHz clock to perform a proper FIR low pass filter. I came on 13 kHz now, still 13 times higher than the highest frequency in the SSTV video. Yes, I had to change all factors in the FIR calculation, but once this is done (this evening) everything runs again.
I could make a photograph of a hundred Hz square wave that goes through the FIR filter (base band).

Squarewave_response.jpg
Squarewave_response.jpg (19.32 KiB) Viewed 2633 times

You see that the ringing is almost symmetrical. The cut-off frequency of the filter should be again about 1 kHz. However, I have not checked that. It is the next thing that I am going to do.

To give you an impression: one SSTV pixel has the width of half a centimeter (box) on the screen of this oscilloscope.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Dec 12, 2017 11:34 am

Very well done Klaas. The FIR filter was the path I was going to try next - but other matters have frustratingly kept me away from doing anything on this.

I intend to use a 20-pin 18F14K22 which has an internal clock which with a x4 PLL means a 16MHz instruction rate. If you need a more accurate clock you can use an external 16MHz crystal and the internal x4 PLL. The 14K22 has more pins than required for the demodulator function but there are none with the speed or internal peripherals required in a smaller package. It also has more program space and RAM should you need it, also a more extensive instruction set.

Anyway, good progress and remarkable for such a simple device, the waveforms are as good as textbook.

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

Postby Klaas Robers » Wed Dec 13, 2017 8:28 pm

I checked the frequency response of the FIR-low-pass-filter and it behaves as expected: flat to 1 kHz and then attenuating steeply.

But I observed strange things. This is the first time that I connected an external tone generator, and the oscilloscope shows remarkable irregularities seen. This can be caused by the sampling of the square wave input signal. Unil now all input signals were generated internally in software, and every thing happened synchronously with the sampling frequency, of 13 kHz. But now, with an external signal source, this sampling introduces a jitter on the edges of the input waveform. I saw this on the sine wave output of frequencies below 1 kHz as jitter on the upgoing and downgoing edges. But in the stop band this became more severe. This can be, because time jitter can be seen as frequency jitter and the sharp FIR-filter translates this into amplitude variations. And that is what I got.

While the input signal will be an FM modulated SSTV-signal, this also introduces phase errors or phase noise. I have no sharp idea how bad this will be, so I am going to program the counting detector, not so difficult, and look what it brings.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Dec 14, 2017 11:27 am

Well, I guess it depends on how much jitter, if in this application it's less than one pixel or simply not resolvable is it actually a problem? I don't know.

Presumably raising the sample rate reduces the jitter but you end up with a long filter with more taps than would be desirable. Always a compromise.

We await developments...

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

Postby Klaas Robers » Thu Dec 14, 2017 9:04 pm

It looks rather dramatic. As far as I see the problem:

- A FIR filter is a shift register that shifts the signal in a fixed frequency
- then there are many taps, with a multiplication with the successive coefficients.
- This runs at a fixed speed, sampling frequency.
- So far so good.

- The input of the FIR-filter is the output signal of the counting detector.
- Such a detector gives a fixed pulse of one sample time length for each zero crossing of the input signal.
- As the input signal is a FM modulated tone, only the zero crossings are important.
- So I designed the shiftregister of the FIR filter as a binary shift register,
- 80 bits long, 10 bytes,
- which shift simultaneously each sample clock.
- A zero crossing of the input produces a "one",
- no zero crossing produces a zero in the shift register.
- This is nice because the multiplications of the coefficients are only by 1 or by 0.

Suppose a sampling frequency of 20 kHz:
- then with an input frequency of 2 kHz,
- there are 4000 zero crossings per second,
- so in the shift register we see ..... 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 .....
- this is perfect.

- but at a slightly higher input frequency, e.g. 2010 Hz,
- we see now and then (20 times per second),
- a ..... 1 0 0 0 1 .... passing by, so 3 zeros in stead of 4 zeros.
- That can be seen as a short duration of 2500 Hz,
- and that gives a positive pulse in the output of the FIR filter,
- in practice 20 of these pulses per second.
- Yes, the pulse is short, so the FIR filter suppresses it heavily,
- but it is clearly visible.
- It is even more visible with the sample frequency of 13 kHz that I have at this moment.
- I will make an oscillograph photo of this later on this day......
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sun Jan 07, 2018 6:41 pm

For those awaiting developments on this up-converter I apologise. Another 'real work' project landed in my lap in early December, I'm hoping to be clear of it by the end of this month. I'm keen to get back on the case too...

I have been reviewing the folder contents to remind myself where I had got to...six or so months and you (or at least I) forget...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Sun Jan 07, 2018 9:48 pm

Steve Anderson wrote:For those awaiting developments on this up-converter I apologise. Another 'real work' project landed in my lap in early December, I'm hoping to be clear of it by the end of this month. I'm keen to get back on the case too...

I have been reviewing the folder contents to remind myself where I had got to...six or so months and you (or at least I) forget...

Steve A.


OK Steve i am glad it is not just me ! i was reviewing stuff i didn't remember doing or very vaguely..dam this aging ..
I was just checking funny enough today where i had put my parts for this ..when your ready fine with me .
off topic
I was also looking at your CRT monitor project 1 today here http://www.hanssummers.com/nbtv.html funny i don't think it is on the forum does the monitor still exist ?
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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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Mon Jan 08, 2018 1:59 am

No problem Steve, I assumed you had some 'paying' work on your plate.

All the best Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Jan 09, 2018 11:45 am

Harry, yes, rather surprisingly it does exist exactly as built in October 2009, it hasn't been tampered with at all. It should still work, one day I should power it up. First I'd need to clean off eight years of dust!

I still have all the folders for it on the PC, all the circuits and the final pdf.

I would upload the pdf and some photos but my browser (Chrome) has gone 'funny' on me in the last couple of months - I guess I need to do a re-install.

Chris, thanks for the understanding...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 10, 2018 11:54 am

I've found the thread, it has the pdf attached in the opening post, otherwise there's very little in it.

viewtopic.php?f=1&t=935&hilit=cute

I thought I built it in 2009, but as you can see the first post is March 2007! Almost 11 years ago!

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Wed Jan 10, 2018 3:31 pm

Steve Anderson wrote:I've found the thread, it has the pdf attached in the opening post, otherwise there's very little in it.

viewtopic.php?f=1&t=935&hilit=cute

I thought I built it in 2009, but as you can see the first post is March 2007! Almost 11 years ago!

Steve A.



Hi Steve you must of constructed in secret ,as it popped up constructed .
Unlike me and others i suppose its a slow slog to get things to work and you post more for help ...well it might be 11 years but i like the circuit and i will use parts already constructed and tested to work for me ...and seems fine up to 128 line for my needs..but i am just using it for sync detection and crt modulation
I Think you mentioned it was a bit dusty now .needs a dusting and a power up after all these years be nice to see it run .
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 » Sat Mar 03, 2018 9:05 pm

At last! I have the time to resume work on this project. I've been reviewing where I had got to and what is next to do.

First I need to sort out my office/workshop - it looks like a landfill. That's probably a weeks work alone.

Then I need to go across town to get the revised parts needed and stock up on those that have got low. That's a day on its own.

Sometime in early September last year I boxed all this up and I have to find where the heck I put it!

I'll be interleaving this in with a paid project so I won't be able to spend 100% of my time on it, but progress is on the horizon!

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

Postby acl » Sat Mar 03, 2018 9:26 pm

Well done Steve, I look forward to carrying on with converter

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

Postby Harry Dalek » Sat Mar 03, 2018 9:42 pm

No worries there Steve i can imagine the mess getting a work shop back as well as missing stuff ,i still have stuff in boxes from the 2000 move .
I will keep an eye out for more on this project when you have time .
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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