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 » Mon Jan 20, 2020 10:49 pm

Steve,

generates your DDS half sinewaves? That is, is it readjusting the frequency of the output sinewave each half sinewave? Or is it adjusting it also during the generation of the sine wave form? It could have been that only at the beginning of a new complete sine, that it samples the voltage of the input signal. In that case you lose information, because for an FM modulated carrier, there is modulation information in each zero crossing. This is the more important because the carrier frequency is low in respect to the highest modulation frequencies.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Jan 21, 2020 1:28 pm

The DDS system of generating waveforms is very flexible and not just limited to sine-wave output. You could have several different look-up tables within the processor to generate square, triangle or any arbitrary repetitive waveform. An overview of the system is here:-

https://www.analog.com/en/analog-dialog ... esis.html#

In this device the 256 8-bit video samples are sequentially fed into the 'Tuning Word' register(s) in 55ms (EU standard), followed by those required for the sync pulse. An equivalent 3.25Hz subcarrier resolution between 1500-2300Hz, only dependent on the crystal frequency accuracy, usually between 30-50ppm (Parts Per Million) for a cheap crystal. (0.003-0.005%). All so much easier and more accurate than an analogue VCO or PLL. The frequency change is instant with no trace of overshoot/undershoot as a result of analogue loop-filters.

The result is loaded into a PWM module after conversion by the look-up table. The output is a 78.12kHz PWM-modulated square-wave, the following filter removes the harmonics and the 78.12kHz, ending up with a close approximation to a sine-wave output. Any remaining harmonics should be removed by the filtering in a transmitter, they are quite low anyway.

For a more accurate approximation to a sine-wave (or whatever) the result after the look-up table could be sent to a 'proper' external D-A instead of using PWM, then filtered. But that's not really necessary for this application. For a good audio test oscillator it certainly would be required if you needed to keep the harmonics (distortion elements) very low.

For this modulator I'll measure the harmonic levels (distortion) next time I use it, I'm surprised I didn't previously...the worst results will be at 1200Hz undoubtedly...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Jan 21, 2020 4:49 pm

Coding size for the DDS modulator, uses just this amount of code...

Code: Select all
MEMORY USAGE MAP ('X' = Used,  '-' = Unused)

0000 : X---X---XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX
0040 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX X--------------- ----------------
MPASM  5.42                SSTV 683 DDS 3.ASM   11-19-2014  13:20:01         PAGE 13


MEMORY USAGE MAP ('X' = Used,  '-' = Unused)


01C0 : ---------------- ---------------- ---------------- ---------------X
0200 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX
0240 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX
0280 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX
02C0 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX
2000 : -------X-------- ---------------- ---------------- ----------------

All other memory blocks unused.

Program Memory Words Used:   348
Program Memory Words Free:  1700


Errors   :     0
Warnings :     0 reported,     0 suppressed
Messages :     5 reported,     0 suppressed


The Playing Card 'spade' symbol is how the ASCII interpreter reads it, should be a page break code if you were printing it. The first chunk of code is the actual processing, the second is the 256 byte look-up code for a sine wave. The single byte at 0x01FF is an instruction that points to and loads the appropriate data at the location within the following 256 byte data field, then returns to the main program. It's the data within these 256 bytes that determine the waveform of the output, here it's for a sine-wave, but it could be anything...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Wed Jan 22, 2020 7:56 am

I can imagine how it works. It is indeed possible to change the frequency constantly, while the input signal changes in voltage. That is what happens too in my analogue FM-modulator. However there the oscillator (UJT-type) gives only short discharge pulses after a capacitor reached a certain value.

The charging current is immediately proportionally to the input video voltage. The oscillator runs on the double frequency, so 2400 to 4600 Hz, and the output is divided by two. Then this square wave signal is sent through a 5th order Butterworth low pass filter, with a cut off frequency of, I guess, 3 kHz.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 22, 2020 1:07 pm

Klaas Robers wrote:The oscillator runs on the double frequency, so 2400 to 4600 Hz, and the output is divided by two.

I did consider running the modulator in the same manner which means the filter only has to remove the odd harmonics, the strongest probably being the third of 3600Hz during a sync pulse. However I had committed myself already and as it turned out it wasn't a problem. However I may implement this anyway...it would require only some minor changes in the coding.

I haven't seen Unijunctions used in many years...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Wed Jan 22, 2020 8:52 pm

Steve Anderson wrote:I haven't seen Unijunctions used in many years...
Steve A.

Very good! Because that implies that you have seen them used, and may be you even have been using them yourself. I fear that many younger hobbyists in electronics never have seen them, nor that they know what they could do with them. But in my circuits they work already for about 50 years and without any failure. Also in my monitor the oscillators for horizontal and vertical deflection are UJT-oscillators. Simple and reliable.

It is remarkable to see that certain components are popular and well known for many decades, while others live only for several years. Transistors BC107, BC108 from 60 years ago.... yes, they got a new housing, but as BC 547 and BC548, they are still made. My DTL circuits did not live for long, while TTL 7400 series were there for many decades. In reality C-MOS took over as the technology for the 7400 series .... It took over for everything. Sometimes I am amazed that very old vacuum tubes, say A415, from 1927, are still alive and the vacuum is remarkably high. Good technology. Well designed.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 22, 2020 9:58 pm

As I recall, unijunctions were used in the PRF (Pulse Repetition Frequency) oscillators in the Decca Radar (Marine) sets I worked on in the late 70s, For short-ranges (0.25 nautical miles when berthing) the PRF was 3400Hz, then divided to 1700Hz for medium ranges, and 850Hz for up to 60 nautical miles. More than that the curvature of the Earth tends to get in the way, even on a tall mast/vessel. This triggered the Magnatron pulse and also the screen timebase - you waited for a reflection, there's the distance to the object (land or another vessel). Typically shown in movies - though rather poorly. In the test labs we could watch aircraft arriving and leaving Heathrow airport.

Before the days of digital storage/processing a P7 type phosphor was used in the display CRTs. It wasn't P7, but a more modern development of it. They were either 9", 12" or 16", depending on the system you purchased, all round screen as you'd expect. Magnetic deflection, even so I wouldn't mind having an example of the 9" version though today! EHT typically those found in CRT TVs, 16-25kV.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Thu Jan 23, 2020 6:33 am

Wasn't that a phosphor with an orange afterglow? I have seen those screens with orange on the screen and a longer afterglow than the P7 phosphor that I know from SSTV.

By the way, I experienced that the afterglow of the 7BP7 is longet than of the 5FP7. So P7 isn't always P7.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Jan 23, 2020 1:41 pm

I have several CRTs from the 50s so they are getting on to being 70 years old, agreed, well made. Some are P1/DG, some P7/DP.

Although the P7 phosphor is well known to us there were others too with a long persistence, don't ask me what the 'P' numbers were...

The CRTs used at Decca didn't use the standard(?) system for numbering, they were 'house numbers' made specially to Decca's specification, likewise the phosphors. The light was a bright blue 'flash' followed by the long orange afterglow, similar to a P7 tube. The CRT screen had an orange perspex cover that masked the blue 'flash'.

These long-persistence CRTs usually use dual-layer phosphors. The electrons first hit a short persistence layer that mostly emits UV and a bit of blue, the blue 'flash' we can see. The UV then excites the long-persistence orange layer. All a bit of a 'black art' to me, but it works and that's all I need to know.

The duration of the persistence is down to the chemistry of the phosphors, another 'black art', possibly also the final anode voltage, the impact speed of the electrons, i.e. their kinetic energy. These accelerated electrons move at around 10% of the speed of light!

Anyway, for this we don't need long persistence...

Steve A.

CRT 'P' numbers here:-

https://www.valvecollector.uk/documents ... cteristics

Long persistence tubes for radar mostly were P7, P12, P14 and P19, some others too within the list...scroll up & down the list, there's some other data that might be useful to some...
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Jan 24, 2020 8:24 pm

Steve Anderson wrote:
Long persistence tubes for radar mostly were P7, P12, P14 and P19, some others too within the list...scroll up & down the list, there's some other data that might be useful to some...



Looks like the Persistence number go's up to 56 sec !
Attachments
Cathode Ray Tube Phosphors-1.jpg
Cathode Ray Tube Phosphors-2.jpg
Cathode Ray Tube Phosphors-3.jpg
Cathode Ray Tube Phosphors-4.jpg
Cathode Ray Tube Phosphors-5.jpg
Cathode Ray Tube Phosphors-6.jpg
Cathode Ray Tube Phosphors-7.jpg
Cathode Ray Tube Phosphors-8.jpg
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 » Sun Jan 26, 2020 1:41 pm

Very good find Harry, not seen a listing as comprehensive as this before...

I've been unable to connect to this forum for the last few days, anyone else have the same issue? All other sites were no problem.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Sun Jan 26, 2020 7:10 pm

Steve Anderson wrote:Very good find Harry, not seen a listing as comprehensive as this before...

I've been unable to connect to this forum for the last few days, anyone else have the same issue? All other sites were no problem.

Steve A.


No i have had no problems connecting to this one ,The Secret Society of Lathe Trolls forum that was off for a few days .

I was thinking the 56 sec Tubes must be for vintage Radar displays you would need that for the antenna rotating sweep
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 Dave Moll » Sun Jan 26, 2020 8:08 pm

Steve Anderson wrote:I've been unable to connect to this forum for the last few days, anyone else have the same issue? All other sites were no problem.


You are not alone. I too was getting something along the lines of "server not found".

I had assumed that the site was down, but if Harry was able to access it, that's clearly not the case. Maybe it was only accessible from Australia for some reason.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Tue Jan 28, 2020 8:38 am

No, me as well had no problems in accessing this site from the Netherlands. And I look almost every day at least once.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Jan 28, 2020 3:45 pm

Dave Moll wrote:...but if Harry was able to access it, that's clearly not the case. Maybe it was only accessible from Australia for some reason.

Well, for the time being it seems to be fixed...until the next time...

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