KD2BD SSTV convertor

Forum for discussion of SSTV topics. Slow Scan television (SSTV) is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or colour.

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Re: Compatability

Postby Steve Anderson » Sun Sep 12, 2021 12:39 pm

acl wrote:...I did invest 7 GBP including postage and packing from China for a NTSC to PAL converter as shown below but it didn't seem to work with a monochrome signal.

Ah! You may have been caught out as I was some 25 years ago. I bought something similar in HK. It (probably) is a PAL-NTSC convertor (and vice-versa), but that's all. It decodes PAL into YUV (Y, Pb, Pr) and then re-encodes it to NTSC. It does nothing to the number of lines, frame rate and so on. A monochrome signal (or the Y) passes through (largely) unchanged.

It's a colour format convertor, or a colour system convertor, it is not a standards convertor. A standards convertor does it all, not just NTSC-PAL but at the same time 525-625 (or the opposite). Hence that's why they call it a 'system convertor', hoodwinking using semantics. They are technically correct, that's how they get away with it.

And at seven quid, no surprise. The line conversion (all three signals, YUV) requires a lot of RAM and fast signal processing, signal and line interpolation, adding lines or deleting them, all that costs money, lots of it.

PAL/NTSC/SECAM are acronyms for colour encoding standards, not line and frame rates. Recall the BBC experimented with NTSC on 405-lines, but with a lower sub-carrier frequency than 3.58MHz at 2.6578125MHz. It's still NTSC, but with 405-lines at a 25Hz frame-rate and a different sub-carrier frequency.

Some South American countries have/had odd mixtures too, NTSC on 625-lines, well, any permutation you can think of really...

Steve A.

Another example of NTSC is here - and right 'on topic' for this forum...

http://authorityfile.co.uk/NBSC/Home/About
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Re: KD2BD SSTV convertor

Postby Steve Anderson » Sun Sep 12, 2021 4:24 pm

A follow-on to the above...most TVs with an analogue (composite) input made in the last decade or more are multi-standard and should work on 525/60 or 625/50 and auto-switch between colour standards or B&W as required. From what I see Johns convertor is 525/60 output which a recent UK TV should lock to correctly. Maybe not the smaller portables though.

Likewise a recent US TV should work with a 625/50 signal and decode PAL correctly. Time to consult the manual...if you can find it!

All the screen-shots I did for my MkI SSTV-625 convertor were done on a 25-year old CRT version, including my current avitar. It works on any combination of 525/625, and PAL/NTSC/SECAM, any of the six combinations...though I'm not able to test the SECAM decoder...

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Bench testing analogue board in isolation

Postby acl » Mon Sep 13, 2021 12:27 am

Please note all the technical information was supplied by John, I am just writing up our email exchanges. With all ICs removed power up the board and verify zero, Vcc (positive 5 Volts) and Vcc/2 (Positive 2.5 Volts) are present on the correct IC pins. Power down and insert all devices. Verify that approx. 60mA is being drawn from the 5 Volt supply.

The only alignment involves adjusting trim pots R62 and R63 associated with the LM555 oscillator to produce an output of 4151 Hz for reception of 15 Hz SSTV, and 4654 Hz for 16.666 Hz SSTV. Output from the oscillator can be measured on Pin 3 of the LM555. Should the TriplePIC display SSTV images having any degree of line-to-line jitter when receiving video from a clean, stable source, a slight re-adjustment of these trim pots will eliminate the instability. John at some intends to refine this section by introducing a PIC to replicate the 555 timer thus eliminating any pot adjustment. In practice I felt the current arrangement was perfect. All the pots do is set the vertical height of the screen image.

IMG_3409.JPG
The cheap and cheerful one I used off Uncle eBay



You will now need to inject a suitable audio signal into the convertor. I used a cheap signal generator but you could also try an on-line version although I haven’t tried it yet. https://onlinetonegenerator.com/

So, at U3D Pin 14 (Demodulated Composite SSTV Video) you should measure the following DC voltages under the following conditions:

No signal: 2.6 volts
1200 Hz: 1.6 volts
1500 Hz: 1.9 volts
1900 Hz: 2.4 volts
2300 Hz: 2.9 volts

By this stage correct operation of the video output signal amplitude against input frequency has been verified.



Next, we need to prove correct operation of both the vertical and horizontal synch signals emanating from this board. Both of these signals have a standing dc signal on them and are fed into two analogue input channels on the 16F88 PIC on digital board. When the pulses appear the dc level increases and if it exceeds a pre-defined value in software a synch pulse is deemed to have occurred. This works really well in practice. Below are the output waveforms you should be observing when feeding the convertor with a suitable SSTV signal.

pic_294_2.bmp
Vertical synch output U5D pin 14
pic_294_2.bmp (1.1 MiB) Viewed 327 times

pic_294_3.bmp
Horizontal synch output U5A pin 1
pic_294_3.bmp (1.1 MiB) Viewed 327 times



My thanks goes out to John for both his time and patience.
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KD2BD SSTV converter static testing digital board

Postby acl » Mon Sep 13, 2021 2:22 am

This is probably the fiddliest board to build and it is both worth the time and effort performic static confidence tests before fitting the semiconductor devices. As there is a lot of interconnected wiring it is important to verify the integrity of all connections. The board has probably been constructed over several days and during breaks wires may have been missed.
We could use a digital multimeter to check ‘point to point’ wiring however with this we cannot differentiate between an open circuit and a grounded connection. So armed with an eBay ‘special offer’ logic probe, a nice brightly coloured highlighter and a clean copy of the schematic diagram we will proceed to perform static wiring checks on this board.

d4739d0a-104d-48db-be45-0edd74a48eda_1.9ef8976e0741599267ef3c336c54d9f3.jpeg
Logic probe



For readers unfamiliar with operation of a logic probe it is shaped like a pen with a test probe for connecting to pins on unsuspecting logic ICs to determine their logic status. Usually, a green LED indicates a zero-logic state whist red one indicates a logic ‘1’ or a high logic state. An open circuit is normally indicated with both LEDs extinguished. Often a high and low audio tone sounder is incorporated corresponding to logic state.
Anyway, with all devices removed and the digital board on the bench without any other connections apply the five-volt supply and verify minimum dc current being taken and no smoke or distressed wiring are present. Remember, smoke conducts electricity not electrons. If the smoke ‘escapes’ from cables and components the current stops flowing.

Connect the supply to the logic probe and verify all power supply connections to all of the active devices. Mark off on the schematic diagram. Using a resistor on a flying lead connected to the positive five-volt supply verify remaining connections are open circuit until the resistor is place on the connected pins. By placing the logic probe tip on adjacent pins to the one you are testing, you can verify the present of solder ‘splashes’ between tracks.


The next posting will include programming the three PIC processors and bench testing the digital board in isolation from the analogue board. Anybody contemplating building the converter without programming facilities we can arrange to produce them for you as I do all the ones for NBTVA. During initialisation John has included a clever confidence test that produces a half screen grey-scale wipe routine which proves the integrity of all devices on this board including all three processors so it is easy to test.
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KD2BD SSTV converter Analogue section PCB design

Postby acl » Mon Sep 13, 2021 11:04 pm

Capture1.JPG
First attempt
Capture1.JPG (90.59 KiB) Viewed 282 times


I have been toying with the idea of producing a PCB for the analogue section of Johns SSTV converter but have become 'stumped' trying to fit it in even to the area I built mine on stripboard. I know I can stand resistors on end to some degree but I am struggling as I wish to produce PCBs to encourage more interested people to join us. Any help or advice is welcome. I'm sure we should be able to cut and paste circuitry as many of the filters share the same layout.

Anyway here's hoping.
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KD2BD SSTV converter Software Installation

Postby acl » Tue Sep 14, 2021 12:31 am

Software for Johns converter is located on his WEB page

https://www.qsl.net/kd2bd/TriplePIC.html

Capture.JPG
Capture.JPG (50.17 KiB) Viewed 280 times


In order to program up the three PIC processors you will need some sort of programming module in order to transfer the Hex files into the appropriate devices. Luckily these are relatively cheap although those not wishing to outlay money for this, I am sure we can program them up ourselves.

Ralph Taggart and myself use the TL86611 Plus as a stand-alone USB device it not only programs PIC processors but EPROMS used for NBTV projects. The software driving it from the PC allows you to clear, download code to the device, program and verify device is programmed correctly. Please remember to select the Intel .Hex file option rather than the standard binary file when programming PIC for the converter.

s-l500.jpg
s-l500.jpg (21.49 KiB) Viewed 280 times



A cheaper option is the now superseded dedicated PIC programmer from Microchip. A cloned Pickit 3 is still available on eBay at around 20GBP but to will need a 3 GBP adapter to plug your devices into as it is designed to program PICs in situ. Steve Anderson uses this one extensively for his software development.

t.jpg
t.jpg (77.67 KiB) Viewed 280 times


The next posting is dedicated to proving correct operation of the standalone digital board.
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KD2BD SSTV converter digital board testing

Postby acl » Tue Sep 14, 2021 3:32 am

Confidence testing of the digital board.

IMG_3441.JPG
IMG_3441.JPG (165.29 KiB) Viewed 273 times


IMG_3501.JPG


To verify correct operation of the digital section board all we need to do is connect out video monitor into J7 (FSTV output) connector and apply our five-volt supply. During the power up sequence we should see a blank screen. Soon after the picture will scan down to form a split screen. The top half left to right black to white bars and the lower reversed. After a second the picture will then scroll down again displaying the reverse image. Assuming this is correct we can now power down proceed to interconnect the two boards together and wire push buttons and switches and LED.



Positive five volts
Zero Volts
Demodulated SSTV
Vertical synch pulse
Horizontal synch pulse
Horizontal synch out
Pin 3 555 timer
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Re: KD2BD SSTV converter Software Installation

Postby Steve Anderson » Tue Sep 14, 2021 3:00 pm

acl wrote:Steve Anderson uses this one extensively for his software development.

The PicKit 3 has been discontinued and superseded by...guess what?...The PicKit 4! A far better device (Yes, I bought one). The PicKit 3 is still available from some suppliers but it cannot program newer devices like the PICxxQxxx series. Also the '3' cannot program some 16 or 32-bit devices, actually there are very few it can.

https://www.microchip.com/en-us/develop ... l/PG164140

Available everywhere via the usual major distributors...though not cheap, in the UK just under 70 quid inclusive (US$95...) that's for the genuine article...but it's worth it. I wouldn't even consider a rip-off copy...

PG164140A.jpg
PG164140A.jpg (32.75 KiB) Viewed 253 times


As for programming EPROMs, no problem, I haven't used them since the '80s. If I were to have a go at my retro MC6809 system (exactly for what I have no idea) I would use EEPROMs or flash memory. Many simple designs on the 'net for programming those...though you would probably make that part of the final devices functionality anyway...i.e. it can re-program itself via a simple 'bootloader'...

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KD2BD SSTV converter Final testing

Postby acl » Tue Sep 14, 2021 8:06 pm

Having successfully verified operation of both cards we are ready to go ahead with final testing. I found it easy to mount both boards on a scrap piece of plywood in order to gain access whilst testing before mounting in an instrument case.
Upon applying the five-volt supply the initial screen should appear as described earlier. After applying a SSTV signal to the unit, the synch LED should start to ’blink’ in sympathy with the horizontal synch signal and the SSTV image should appear on the screen. The blinking of the LED acknowledges the horizontal synch pulses have been received by the U7 PIC. You could add an extra LED via a suitable 300 Ohm dropper resistor between pin two of U7 and ground to get indication of start of the frame although the existing LED does produce a longer ‘blink’ during this period.

finish.JPG
Final testing arrangement


IMG_3484.JPG
Start of frame sequence ( they are not her ears)
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KD2BD SSTV converter results

Postby acl » Wed Sep 15, 2021 10:32 pm

Results from John Magliacane's Triple PIC SSTV to 525 line convertor in conjunction with signals generated using MMSTV. Well done John.

IMG_3527.JPG
Monitor screen shot


SSTV demo.mp4
MMSSTV Demo
(20.26 MiB) Downloaded 19 times
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Re: KD2BD SSTV convertor

Postby kd2bd » Thu Sep 16, 2021 1:16 am

Well done, Chris! Enjoy your new scan converter in good health! :D

It looks like there is still some digital noise in the image. It appears I neglected to illustrate it in the schematics, but in addition to the PIC16F88, the PIC16F77 and PIC16F76 also need 0.1uF capacitors from +Vdd to ground. In fact, every chip on the digital board should get one, as close to the chip as possible.

73 de John, KD2BD
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KD2BD SSTV converter

Postby acl » Thu Sep 16, 2021 2:50 am

I see the vertical noise lines but it is a lot better than before. The only possible way forward is to produce a PCB to give us consistent results. Still impressed with the performance though. There are already 100nF disc ceramics across the power lines for each IC although these have been mounted on the component side underneath the turned pin sockets.

Regards Chris Lewis
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Re: KD2BD SSTV convertor

Postby Steve Anderson » Thu Sep 16, 2021 1:55 pm

Indeed, well done Chris and John.

A PCB with a ground-plane and nice fat grounds/0V will definitely help, but it would be nice to figure out where this junk is getting into the system in the first place.

I had a similar problem on my MkI version, but to a much lesser extent. See screen-scrape. Note the 'glitch' at the 50% grey level, and lesser at the 25 and 75% voltages. It turned out to be a defective D-A. Replacing it and it went away. Putting the original back - and there it was again. Though I doubt that's the reason here.

Initial Line 1.gif
Initial Line 1.gif (42.06 KiB) Viewed 195 times


The HF 'burst' at the end of the line was just to check the frequency response. Note the date, six years ago!

I wish I was aware of the TLC7524 D-A at the time, the DAC0800 I used needs two supplies, +12V and -12V. Plus the TLC has internal latches which would have meant I could have done away with the two 74HC08s used for blanking. Thanks John, I'll not be using the DAC0800 in the future!

The amount of RAM is overkill, but they were what I had...

Note: My copy of Autocad was playing up at the time, hence the 'strange' text..

SSTV-625 D-A.gif
SSTV-625 D-A.gif (18.47 KiB) Viewed 195 times


For the SSTV-VGA up-convertor I'll be using discrete resistors to make up the R2R D-A driven by a octal buss latch (74AC574), designed for higher output current drive. I can't find a higher speed version of the D-A. Though they must exist. The DAC0800/TLC7524 are OK for 525/625-lines, but too slow for 800x600 VGA. That requires each pixel be 25ns, or 20MHz of video bandwidth.

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KD2BD SSTV converter Possible CPU PCB

Postby acl » Fri Sep 17, 2021 7:13 am

Thanks Steve,
Just found out how to add ground plane onto my attempt at the CPU section The question now is is it worth adding another ground plane on the reverse side of the board and do I just connect the ground plane(s) up to zero volts in one place?

Regards Chris Lewis

Capture.JPG
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KD2BD SSTV converter

Postby acl » Fri Sep 17, 2021 7:31 am

Hi all,

I am using exactly the same PICs as John, I have tried changing the RAM as this caused problems with Robot 400s . If it's not my wiring then it must be the D to A. The picture is quite acceptable but for the purists I will investigate further.

Regards Chris Lewis
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