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 Steve Anderson » Fri Sep 01, 2017 10:25 am

For the moment I wouldn't spend any more time on this illusive problem, it is doing the job even if not quite as well as hoped.

Now I'm back home I'll have a look myself before and after I have modified my build to the '3.3V version'. I expect some sub-carrier bleed-through on the sync-tips where the carrier is at its lowest frequency, but from black to white it should be somewhat cleaner.

Given the low resolution of the system generally this degree of noise is unlikely to be visible on-screen unless really trying to see it.

As mentioned before I intend to replace this demodulator at some point in the future, so I wouldn't worry about this too much.

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

Postby Harry Dalek » Fri Sep 01, 2017 2:49 pm

Steve Anderson wrote:
As mentioned before I intend to replace this demodulator at some point in the future, so I wouldn't worry about this too much.

Steve A.


Steve is it worth waiting as i have not started on the demodulator yet mainly because a seller on Aliexpress had not sent the TL071 2 8 ics i ordered they say they had a problem didn't send them if i had not asked i would still be going to the letter box each day ....buggers !

Did you say your back from Europe now ?
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 acl » Fri Sep 01, 2017 4:45 pm

Thanks for all the ideas lads. I will wait Steve.

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

Postby Steve Anderson » Fri Sep 01, 2017 9:11 pm

Harry, well there's not going to be a mad panic rush, so wait a while for the chips...make sure you've ordered the correct parts though. I wouldn't wait for the replacement demodulator design as that will happen after this MkIII version is complete along with the NBTV side of it. So that could be quite a way down the line.

There is also the chance that a better demodulator could be too complex and/or expensive, unlikely, but possible.

Steve A.

Yes, back in Bangkok now.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Sep 01, 2017 9:38 pm

Steve Anderson wrote:Harry, well there's not going to be a mad panic rush, so wait a while for the chips...make sure you've ordered the correct parts though. I wouldn't wait for the replacement demodulator design as that will happen after this MkIII version is complete along with the NBTV side of it. So that could be quite a way down the line.

There is also the chance that a better demodulator could be too complex and/or expensive, unlikely, but possible.

Steve A.

Yes, back in Bangkok now.

OK i will get some Tesla DAC's off to you next week .
Sorry yes they were a mix the TL084 2 TL072 and 71s opamps some for this and stock up a bit .
Ok Steve i will make the original Demodulator understand no worries .
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 Klaas Robers » Tue Sep 05, 2017 8:22 pm

Steve, I am trying to devellop a PIC software FM-decoder, with a counting FM-detector followed by a software 7-pole Butterworth FIR filter. This is something I never did before, in the past I only got something of the theory behind it. To begin with I plan to DA-convert the calculated video output, because then I can measure the frequency response and other characteristics and see principal (rounding?) errors.

Until now I plan a sampling frequency of 20 kHz for the FIR-filter and a cut off of 1 kHz. With a X-tal clock of 20 MHz I hope to have enough clock pulses to run the FIR-filter. The input of the PIC will be a limited SSTV audio signal.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Sep 06, 2017 11:51 am

I too have looked at FIR filters for the demodulator, but so far only as a paperwork exercise. More to do here when I replace the Robot 70 demod.

The counting/zero crossing demod was what led to my problems with sync-jitter, so for the time being it's also on hold. But strangely, subjectively the actual video appeared better.

In the background I have another concept for this but first I need to review some theory which I have forgotten.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Sat Dec 02, 2017 4:48 am

Klaas Robers wrote: (At Sat 13 May 2017, in this thread, page 1)
Yes Steve, you are right......., if the counter of the Robot 400 wasn't counting further. Let me explain:
The counter proceeds once in two lines. This is the counter for addressing the memory vertically. Each memory line is displayed twice.

268 / 2 = 138d = 86h = 1000 0110b
The bold ones can be gated out. Then on line 268 (and 269) there is the sync pulse, and
the not bold bits are don't cares, so
270 / 2 = 139d = 87h = 1000 0111b
is also gated out, so lines 270 and 271 are also giving the sync pulse. 4 lines V-sync, fine.

However...... the V-counter continues to line 312, so:
284 / 2 = 142d = 8Eh = 1000 1110b and
286 / 2 = 143d = 8Fh = 1000 1111b together,
will generate a second V-sync pulse of 4 lines, and

300 / 2 = 150d = 96h = 1001 0110b and
302 / 2 = 151d = 97h = 1001 0111b together,
will generate a third V-sync pulse of 4 lines.

The current situation is that also bit 3 is incuded in the AND , so the V-sync is only on lines 284 - 287 (second example).
So for a V-sync on lines 268 - 271 I need to include the zeros of b3 and b4 inverted in the AND. May be that, when everything is working, that I try to glue a 7402 on top of the 7430, input b3 and b4 in one of the 4 gates and connect the output to one of the non used 7430 inputs. It looks silly, but after the box is closed, nobody sees it.

I did it ! In stead of using an extra circuit, I made a small plug-in board with the 7430 and two inverters, which included the two bits in RED. This eliminates the second and the third sync pulse. The inverters are made each of a BC548 and a base series resistor of 22k.

I made a small plug-into-an IC-socket with the original 7430 8-input NAND and the two transistor-inverters. That circuit could be made small enough to fit into the IC-socket of the original PCB. This places the active field nicely vertically in the middle. I will make a photo soon.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Thu Dec 07, 2017 12:13 am

I made a screenphoto of the "Testbeeld" after the circuit changes. Unhappily it is not really sharp.

Robot_Testbeeld.JPG
Robot_Testbeeld.JPG (63.66 KiB) Viewed 80 times

Only line 1 and 2 are not visible, but that is a consistent sync-error of the Robot. You see a part of line 2 at the bottom of the frame, just above the (hard ware) Robot grey-scale.

Then I made too a photo of the insert for the Robot-PCB, including the two inverters and the 7430, which was originally directly in the board's socket, with some of its pins hanging outside.

Robot_inzet.JPG
Robot_inzet.JPG (53.66 KiB) Viewed 80 times

At the bottom right you see how the Robot is originally programmed for the different line numbers, by placing IC-pins outside the socket.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Thu Dec 07, 2017 1:52 pm

I like your plug in circuit board Klass ...Is there an example of a picture or test pattern from the original unchanged device could do ,it would be good to see side by side original and changed .
I looks pretty good for a 70's scan converter ...as with NBTV everything looks better smaller screen ..what was the display monitor ?
I always thought these things would have to have a little tv does it have signals out luminance and V H sync ? for its own monitor like an early computer screen idea .
BTW have you seen this issue below
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Screen 00006.jpg
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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 Klaas Robers » Sun Dec 10, 2017 6:51 am

In the old situation the picture was shifted up to the upper edge of the TV (monitor) screen. Then the upper black and white bar was patially hidden under the bezel of the TV or monitor. You can see that in http://www.taswegian.com/NBTV/forum/viewtopic.php?f=24&t=2254&start=30. That picture is sharper. I don't know why the camera didn't autofocus better this time. But the active area is now better centered, in the vertical direction. That was the only improvement.

No, I have not seen the CQ TV magazine 136 of the BATC. Is the article on the web somewhere? I am more or less curious what they improved.....
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Sun Dec 10, 2017 7:47 am

Klaas Robers wrote:In the old situation the picture was shifted up to the upper edge of the TV (monitor) screen. Then the upper black and white bar was patially hidden under the bezel of the TV or monitor. You can see that in http://www.taswegian.com/NBTV/forum/viewtopic.php?f=24&t=2254&start=30. That picture is sharper. I don't know why the camera didn't autofocus better this time. But the active area is now better centered, in the vertical direction. That was the only improvement.

No, I have not seen the CQ TV magazine 136 of the BATC. Is the article on the web somewhere? I am more or less curious what they improved.....


Yes the magazine is a free download

http://www.batc.org.uk/cq-tv/archive/index.html

http://www.batc.org.uk/cq-tv/archive/1986.html
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 Klaas Robers » Sun Dec 10, 2017 11:07 pm

Thanks Harry. It was nice to read, but no interesting modifications. Just gimmicks like four pictures in one.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Sun Dec 10, 2017 11:24 pm

Steve, I started to program an SSTV audio FM detector with a FIR low pass filter into a PIC12F683. This is a 8 pin small PIC.
- It has to run on a 20 MHz quartz
- and will accept a limited audio signal (from an 8=pin opamp?).
- May be the built in comparator (pin 6 and 7) is also suitable.
- The FM detector will be an ultra linear counting detector.
- To start with the output is analogue video via the PWM output (pin 5).
- The PWM runs at 165 kHz, with 128 levels (7 bits),
- when you connect an RC low pass to pin 5,
- 12k and 10nF to ground, the residual 165 kHz is very low in amplitude.
- The FIR filter runs at a sample frequency of 19.5 kHz.

And yes, if we use a more-pin PIC, the video can also be outputted as a parallel 8 bit data. But then the free running sample frequency might cause synchronisation problems.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Tue Dec 12, 2017 3:46 am

Steve, I made a photo of the impulse response of the FIR low pass filter in the PIC software. The cut-off frequency will be slightly under 1 kHz.

impulse response.jpg
impulse response.jpg (20.28 KiB) Viewed 21 times

You see the ringing before and after the band width limited impulse. They should be equal, but due to the RC at the output, to convert the PWM signal into an analogue wave form for the oscilloscope, the overshoot after the impulse is slightly decreased. The feeding impulses were not an input signal, they were produced by a few instructions in the software. Similar impulses will also be the output of the software counting detector, but then much closer together, so that they give a continuous DC signal, no more pulses.

I don't know the frequency response yet, that is the next item for me to measure. But it should be a 6th or 7th order Butterworth low pass filter (42 dB/octave) with no group delay distortion.
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