Slow Scan Television Experiments

[Harry Dalek]

Continuing... I was wondering yesterday if it's easier to approach this in a different manner. VCOs with their supply and temperature headaches, then the square wave needs turning into a sinewave which is usually done by heavy-handed filtering. The chip count was climbing too.

Thats interesting as todays tests it seems it works without turning the VCO square wave to a sine wave ,it makes me wonder if i should bother using another op amp.

Thanks for the info on the 565 pin out i gave it another go but still not a peep out of the thing i could have a dud ,that kept me fault finding for a while but i have put that away for the 4046 and tackled some more experiments on that today .

As this is such a low frequency application, using direct digital synthesis (DDS) might be possible with a simple micro. It is..

This is just the start, I have yet to work out the modulation method, but it looks promising. Below is the simple R2R D-A I used on the port output of the micro followed by an equally simple filter. The waveforms show a 1kHz sinewave output, pre and post-filter. The spectrum display shows the 2nd harmonic over 50db down, not bad for such a simple arrangement...and the filter at 2kHz isn't doing anything, it only cuts in above 5kHz.

This may just come down to two chips, a 14-20-pin micro and a 8-pin dual op-amp.

The frequency can be adjusted in 3.3mHz steps (0.0033Hz).

More to come.

Steve A.

It sounds like an elegant approach Steve ,bit beyond me and my low tech ways but i will be interesting to see what you come up with .

I had success today with the 4046 ,i took your advice on the LM324 ic and converted this 741 circuit to single supply ,there are a few changes to make it work with the 4046 VCO which i will draw up .
This part screen of the gray bars i screen grabbed first time i got it to work .
I will finish that off tomorrow and start testing a head amp and with light sensors with it .

[Harry Dalek]

I drew up the lm324 and 4046 circuit thanks again to Steve for the fault finding on the 4046 ! it can be an aggravating IC at times .

I am not sure how well i have done converting Steves advice of a Lm324 opamp unity gain used with the VCO i did my best here to do it and it works ,i have left out so far the square to sine wave filter after the 4046 as it seems to work without it , testing with my stair case generator it has not played up .

Any case advice welcome i have no problems changing things you learn from mistakes .

Also any one wanting to post SSTV stuff here welcome love seeing old SSTV circuits and information , so far as far as i know here perhaps Steve and i the lone hanger ons of the old 7 and 8 sec system i don't think the current Ham radio operators would lower them selfs to go back to the classic years of B/W.

Well if true they don't know what they are missing glorious 120 lines of shades of Gray .

[Steve Anderson]

A lot of ground to cover here Harry...

A square wave directly into something like MMSSTV will (should) work OK. Some software may have problems with the fast rising/falling edges, it is after all expecting a signal which has probably come from the output of a receiver/transceiver which will (should again) be filtered.

The reason for turning it into a sine wave is to eliminate the significant harmonics present in a square wave, if transmitted 'as is' will will cause splatter right across the band - you won't be popular. Now, one could say that any transmitter/transceiver should have a filter before the modulation system (note: should again), but it's best not to push one's luck.

If like myself you're going to mainly be doing this 'closed circuit', i.e. just at home for yourself then the above doesn't apply. But if you wish to record it via any digital method (PC sound card) there will be aliasing products within the signal. How much depends on sampling frequency and the quality of the anti-aliasing filter (if any) on the sound card amongst other factors. In short a simple filter will suffice.

Your circuit of the LM324 looks OK, but you don't need the three 100k resistors, remove and connect pin 7 of the LM324 direct to pin 9 of the 4046. The second section of the filter looks a little odd, but if it works who am I? Remember that the input signal must be positive only, not AC-coupled.

*****************************************

Continuing on with the DDS modulator...

By using the PWM module within the micro I have been able to eliminate the R2R ladder, this required a change to one resistor value. Considering the simplicity of the filter the result is quite good.

The PWM waveform at the bottom of the scope display seems to have 'teeth missing' at peaks and troughs, it's just the pulses are quite narrow and the sampling rate within the scope misses them.

Now on to modulation...

Steve A.

[Harry Dalek]

A lot of ground to cover here Harry...

OK

A square wave directly into something like MMSSTV will (should) work OK. Some software may have problems with the fast rising/falling edges, it is after all expecting a signal which has probably come from the output of a receiver/transceiver which will (should again) be filtered.

I was wondering the reason it was not causing and problems even some thing as simple as the ujt on its own seemed to work ok ,i suppose as you say when transmitting this is another story ,didn't think of that .

The reason for turning it into a sine wave is to eliminate the significant harmonics present in a square wave, if transmitted 'as is' will will cause splatter right across the band - you won't be popular. Now, one could say that any transmitter/transceiver should have a filter before the modulation system (note: should again), but it's best not to push one's luck.

I really had no intention to transmit it perhaps via light a laser might be interesting ,but i see your point ! if its whats needed to be correct SSTV signal i will construct it .

If like myself you're going to mainly be doing this 'closed circuit', i.e. just at home for yourself then the above doesn't apply. But if you wish to record it via any digital method (PC sound card) there will be aliasing products within the signal. How much depends on sampling frequency and the quality of the anti-aliasing filter (if any) on the sound card amongst other factors. In short a simple filter will suffice.

Well yes i would not go as far as transmitting it on the HF band i suppose really the internet is what ham radio is today just not a boys club any more .
I would like to record the slow scan video once its all done so a filter will be added back to the LM324 .

Your circuit of the LM324 looks OK, but you don't need the three 100k resistors, remove and connect pin 7 of the LM324 direct to pin 9 of the 4046. The second section of the filter looks a little odd, but if it works who am I? Remember that the input signal must be positive only, not AC-coupled.

I didn't change it much at the start apart from not using the 10k resistor on the filter out put ,the first op amp of the circuit worked fine no change ,but there was a few tiny spikes where the stair case waveform should be out of the second op amp ,i shorted out the .007 cap and the the signal emerged fine out of the second ic so i just left it i was thinking there gos the second half of the filter ! any case i first had the .015 on the out put of the second op amp ac coupled to pin 9 of the vco but it did nothing for the vco ,then i dc coupled with resistors slow and sure as the resistance increased band width and frequency of the VCO also came into range 100k was just right i tried a direct couple but it dropped the vco to the 300hz range i could see it was Frequency modulating thats why i started to try different resistances to see what effect it had on The VCO's frequency .

*****************************************

Continuing on with the DDS modulator...

By using the PWM module within the micro I have been able to eliminate the R2R ladder, this required a change to one resistor value. Considering the simplicity of the filter the result is quite good.

The PWM waveform at the bottom of the scope display seems to have 'teeth missing' at peaks and troughs, it's just the pulses are quite narrow and the sampling rate within the scope misses them.

Now on to modulation...

Steve A.

[/quote]



I am really glad your experimenting with SSTV OH your using the TLO71 i was nearly going to use this one...that circuit looks good i might pinch it for future reference !
On my bipolar board i was doing a PWM experiment i never finished i was going to do the same idea as laser modulation circuits talked about in the past feed the video to an opamp have a saw tooth oscillator both feeding the old LM311 then i knew i got it wrong the sawtooth oscillator should of been the VCO i suppose mixing them in the comparator might of worked .

I think you will finish yours quicker than mine

[Steve Anderson]

It is basically finished, at least in design. I have been using a development board for most of this to test the software with the filter(s) hanging outboard on a breadboard. The development board has a micro which is way too big/powerful for this application, I'm only using perhaps 5% of its internal hardware and less than 2% of the program memory. The final version will be much smaller but quite capable of this task.

The modulation results are shown below, it's not a particuarly good pair of waveforms, it's not so easy when the two are not in sync.

Now I need to port the software to the smaller device (quite easy - he says) then build the final version....and then knock up something to generate a SSTV baseband waveform for it to work on...I know, cart before horse...

Steve A.

[Harry Dalek]

Very good Steve it does not take you long to finish whats on your mind .

I came across this some time back but forgot about it around the time i was playing around with some transducers ,this fellow been experimenting with sstv via water transmitting .
http://www.acasper.org/2011/10/14/sstv- ... tion-link/

[Steve Anderson]

Well, not finished yet. I still have to build the final version, tweak the code for the simpler micro and draw up final circuits etc.. Given a few days...

The opposite of this is the demodulator...that is not so easy!

...this fellow been experimenting with sstv via water transmitting...

Some people have got too much time on their hands...

Steve A.

[Harry Dalek]

Well, not finished yet. I still have to build the final version, tweak the code for the simpler micro and draw up final circuits etc.. Given a few days...

The opposite of this is the demodulator...that is not so easy!

...this fellow been experimenting with sstv via water transmitting...

Some people have got too much time on their hands...

Steve A.

Oh what are you using to display the SSTV ?

I think you plan to use a digital picture and converting it for your test pattern ? i have never tried any thing like what you are doing .

I think transmitting SSTV via air is a bit harder than water but you might have a point there

I finished the VCOs out put circuit it looks like a sine wave now i recorded the test pattern uploaded here .

I have not been able to find a sstv program that will decode the wav file i suppose i could play it live via another device to the laptop and see.

I haven't added sync yet i suppose i need to get it via my test pattern clock and divide by 5 down to 15 hz .

[Steve Anderson]

Initially I'll be using MMSSTV for display, but that will be short-lived. I do plan to build a 'proper' P7 CRT monitor and/or a digital standards converter for display on 625 lines. It may be VGA rather than 625 as Graham Lewis has done for NBTV some years ago...haven't really made up my mind yet.

I've had to put work on this on hold as some 'real work' has landed in my lap.

In the meantime I'm wondering if it's worth building-in a test generator within the micro as well as being able to modulate it from an external source. Even in the smaller micro there's plenty of code space. The issue I have is what standard? 120/128 lines? US or European line rates? Given a couple more pins on the micro all four combinations would be possible, but I've run out of I/O pins...I assume MMSSTV expects US timing but can adapt to European rates too.

I downloaded MMSSTV and installed it on a Windoze PC a few days ago, as far as I can tell it appears to work OK, I just don't have a valid SSTV source for it yet.

Steve A.

[Harry Dalek]

Initially I'll be using MMSSTV for display, but that will be short-lived. I do plan to build a 'proper' P7 CRT monitor and/or a digital standards converter for display on 625 lines. It may be VGA rather than 625 as Graham Lewis has done for NBTV some years ago...haven't really made up my mind yet.

OH that would be a good use for a spare P7 CRT if you have one ,i wouldn't mind doing a updated version of my glow tin mechanical monitor with a few different ideas.
A standard P7 would be nice or you may even have one with a longer phosphor screen ? MMSSTV does 12 sec also and the robot modes which i am now sure about what the difference is ,any case a P7 type monitor must be rather rare these days .

I've had to put work on this on hold as some 'real work' has landed in my lap.

OK the other kind that gets in the way of the good stuff

In the meantime I'm wondering if it's worth building-in a test generator within the micro as well as being able to modulate it from an external source. Even in the smaller micro there's plenty of code space. The issue I have is what standard? 120/128 lines? US or European line rates? Given a couple more pins on the micro all four combinations would be possible, but I've run out of I/O pins...I assume MMSSTV expects US timing but can adapt to European rates too.

So your device could do both that would be good ,BTW silicon pixels chroma pix SSTV program thats good too.
The fellow who wrote the MMSSTV program is Japanese so i would think it would be USA standards, i am not sure about chroma pix.

I downloaded MMSSTV and installed it on a Windoze PC a few days ago, as far as I can tell it appears to work OK, I just don't have a valid SSTV source for it yet.

Steve A.

I am sure you will put good use once you get you gadget going ..i have been trying to find if those 2 programs do either standard that would be good and useful,i did use jxfax in the dos era and that did SSTV ,theres a windows version JVComm32 2.01 but it only does the robot modes ,the older JVfax70 does standard 8 sec found here http://www.pervisell.com/ham/dnjvfax.html its DOS but works fine on my lap top in windows 7.

[Steve Anderson]

...any case a P7 type monitor must be rather rare these days....

A complete P7 monitor either commercially made (e.g. Robot) or an operational home-brew version must be nigh on impossible to find. Thankfully when I was in the UK sometime in the mid-2000's I picked up a number of P7 CRTs, several 3" and two 5" as well as quite a few standard P1 versions. I was hoping to find a 5" (or better still a 7") electrostatically deflected and focussed P4 CRT. No luck. Anyway the price would probably have been ridiculous...and my baggage although OK on weight was bulky and large enough...how can airlines justify charging you for carrying a vacuum?

Steve A.

[Harry Dalek]

A complete P7 monitor either commercially made (e.g. Robot) or an operational home-brew version must be nigh on impossible to find. Thankfully when I was in the UK sometime in the mid-2000's I picked up a number of P7 CRTs, several 3" and two 5" as well as quite a few standard P1 versions. I was hoping to find a 5" (or better still a 7") electrostatically deflected and focussed P4 CRT. No luck. Anyway the price would probably have been ridiculous...and my baggage although OK on weight was bulky and large enough...how can airlines justify charging you for carrying a vacuum?

Steve A.

I have seen them for sale in Canada but the postage is a killer much more than the CRT costs.

You should put good use to one of them as you say not many of the vintage monitors still exist . Sounds like you have the magnetically deflected type ,i never liked the focus coil idea so i understand your aversion to copper wire on that front !

I have been playing around today with a head amp connected to the VCO and looks promising,i need to work out some experiments using the rotating mirrors or a mirror to start off with ,like sweeping across a test card image, the one line of shade variations should be repeated bit like the test card generator i have been using .
I hope to work out the focus and such this way .
The flying spot as i was testing the mirrors for scanning is the other option oh well see how it gos .

[Steve Anderson]

All my CRTs are electrostatically deflected and focussed, getting magnetic versions to work on several different line/frame rates cannot be an easy exercise, though CRT PC monitors (VGA, XGA etc.) managed it. What I was really hoping to find was at least one P4 (black & white) electrostatic CRT, but no luck as I said.

Steve Ostler went through the same process for his 'Argus' 405 TV, eventually he had to settle on a tube with a white-ish-violet mixture I think.

Ignoring the time and effort put into designing and building a SSTV-625 convertor, it would probably work out far cheaper than a P4 CRT, then add in the cost of the power supply which for CRTs is not cheap nor simple.

Steve A.

[Steve Anderson]

A few days to myself...

I thought I would check out the DDS SSTV modulator DC linearity, the result...as far as I'm concerned - perfect. It doesn't quite reach 2300Hz but that's a small software tweak. I'll work on that when the builders across the road down-tools. The chart below is from measured values, not calculated.

Steve A.

...a bit later... software tweak done, now it does reach slightly more than 2300Hz (magenta values), thankfully just as linear as the previous version (blue values).

[Harry Dalek]

Thats pretty good Steve nothing to complain about there ,in my case the ujt VCO is more than likely the best still as far as a voltage to frequency with that band width the 4046 still needs a little adjusting its a little more sensitive to what supply voltage its using to the end result .

I am going to start some reflecting tests off the mirrors with the horizontal mirror first so i hopefully get some results on line shade changes ,i am testing the Head amp i used for the nbtv mirror tests see how that gos, but with a new light sensor as always what stays and what gos depends on the results

As i said once before my heart is more in a direct light to sensor approach than flying spot so can not resist trying this .

UP date

The head amp testing was done this afternoon i found the 4046 circuit i need more amplification after the head amp it only changes the VOC bandwidth by about 2khz where as in the frequency graph from the mmsstv program posted below the The UJT VCO with the head amp will do the full bandwidth again depending on the amount of light .
The old fax system the light level is the same so not much of a problem ,in my case i suppose i would need a gain control to get it to work at different light levels or i would have to match the old fax system and adjust the light for the camera ....
So looking into this