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: KD2BD SSTV converter Possible CPU PCB

Postby Steve Anderson » Fri Sep 17, 2021 12:48 pm

acl wrote:The question now is is it worth adding another ground plane on the reverse side of the board...

In this comparatively low-speed application a single ground-plane is enough. If this were running in the GHz region and using stripline techniques, then yes, but that's not the case.

It appears that generally the top (component) side is given over to the ground-plane, and the bottom (solder) side to as much of the actual trackwork as possible. There will be need to run some tracks on the component side, but that's only if it can't be done on the solder side. If your auto-router has priority settings set them up accordingly.

With a ground-plane I strongly advise using the solder resist option, otherwise accidental shorts while testing are almost inevitable. Silk-screening, nice to have, and doesn't add much to the cost (if anything). Likewise plated-through holes and vias. For components/pins connected to the ground-plane add 'thermal breaks' around the leads/pins to aid in reliable soldering...I'll try and find an example...

https://en.wikipedia.org/wiki/Thermal_relief
https://en.wikipedia.org/wiki/Copper_pour

Here pin 8 has a variation of the thermal break...more conventional, the larger pad, upper left...

Thermal Break 1.jpg


Photo acknowledgement Hans Summers, QRP Labs.

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Re: KD2BD SSTV convertor

Postby acl » Sun Sep 19, 2021 7:50 am

Thanks
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Re: KD2BD SSTV convertor

Postby kd2bd » Fri Sep 24, 2021 5:09 am

I'm very happy this is a "PIC Friendly" group. (Not everything in life requires the complexity of an Arduino or a Raspberry Pi.) For what it's worth, this is the device I use to program my PICs:

jdm84ne3.png
jdm84ne3.png (4.4 KiB) Viewed 289 times

It is powered through a PC's serial port. Mine looks like this:

picprog.jpg
picprog.jpg (142.59 KiB) Viewed 289 times

It was originally built with an 18-pin ZIF socket for a PIC16F84. For PICs of other sizes, I have a 5-pin connector at the bottom that allows me to make the +5v, +Vpp, Clock, Data, and Ground connections to a PIC temporarily plugged into a protoboard. (Eventually I intend to build a separate board containing sockets for the other PICs I often work with.)

I'm a hard-core Slackware Linux man. The programming software I use can be found here: Picprog 1.9.1

The assembler I use is gpasm, which is part of the larger gputils package. There's a PIC simulator available as well, which can be very useful in stepping through a program's instructions to see what's actually going on inside the chip as the program executes.

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Re: KD2BD SSTV convertor

Postby Steve Anderson » Sun Sep 26, 2021 2:49 pm

I prefer using the PicKit as it allows for in-circuit programming etc, no need to remove the chip from the board and replace it. Lessens/removes the risk of bent-under pins or static damage while handling (the latter rare these days). The photo below shows the two six-pin headers for the PicKit, one each for the two micros in the MkI version.

The resistor hanging off the side is a 'dummy TV', just a 75R load for the video output. The two pots are for setting the sync and video levels.

MkI SSTV-625 B.jpg

Here is the PicKit being used in anger during early development of the MkI.

SSTV-625 Status 03-08-2015 1.jpg
SSTV-625 Status 03-08-2015 1.jpg (75.31 KiB) Viewed 259 times


The demodulator sat underneath the main board, the power supply here shown in its final position, the transformer turned out to be far larger than need be, a smaller one could have been used. I think that the heatsinks weren't necessary, not sure, it was six years ago...

Demod and PSU1.jpg
Demod and PSU1.jpg (173 KiB) Viewed 259 times


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KD2BD SSTV convertor

Postby acl » Mon Oct 11, 2021 6:16 am

John sent me some updated software to allow his converter to produce a 625 line PAL system output instead of the original NTSC system. This works fine as shown in the attached photographs. In addittion I fed the output through a Video to VGA converter and the results were quite impressive on a VGA monitor. The converter was found on eBay priced 15.99 GBP.


Video to VGA.jpg
Video to VGA adapter

IMG_3577.JPG
Display of normal Philips CRT television

IMG_3586.JPG
Another

IMG_3589.JPG
Klaas on Philips VGA monitor

IMG_3588.JPG
VGA monitor

IMG_3591.JPG
VGA monitor
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Re: KD2BD SSTV convertor

Postby Klaas Robers » Tue Oct 12, 2021 7:56 pm

Very good! I see my video back. However I see some fine spurious spikes / lines. Those were not in my SSTV wave-file.

And indeed, circles are no more circles. But any way, that is difficult. My ROBOT-400 shows the same circle to oval distortion.
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Re: KD2BD SSTV convertor

Postby kd2bd » Wed Oct 13, 2021 3:06 am

Some switching noise from the RAM address lines is getting into the FSTV output, causing those faint vertical lines. It is probably a lead dress and/or grounding issue as my unit using multi-layer point-to-point wiring on a perforated circuit board is absolutely clean:

clean_fstv.jpg
clean_fstv.jpg (74.81 KiB) Viewed 146 times

Getting a 1:1 aspect ratio SSTV image displayed accurately on a 4:3 FSTV monitor is a bit of a challenge due to a variety of compromises taking place. My unit will fill the screen with a 128 line image. Anything less than 128 lines will cause further aspect ratio distortion. The good news is that many (if not all) FSTV monitors have height and width controls. In fact, I just recently added some additional FSTV scanning lines to my NTSC firmware to prevent the displayed SSTV images from being cut off at the very top.

Klaas, here is your test pattern as a 128 line 60 Hz image using a PC as a display device. Even here there are some compromises made in my generation of the SSTV signal as the original image I used contained something other than an even multiple of 128 horizontal scanning lines:

555-int.png
555-int.png (49.51 KiB) Viewed 146 times

After I get my new horizontal sync detection circuitry finalized, I will have the freedom to perform further testing with 50 Hz SSTV images, and even add a "high-resolution" 256 line mode of operation.

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Re: KD2BD SSTV convertor

Postby Klaas Robers » Thu Oct 14, 2021 2:55 am

Yes John,

I know the compromizes. However yesterday I calculated the pixelspeed for a standard CCIR 625 (312) line scanning system.

If you want to have a square display window of 128 pixels (horizontally) by 120 lines (vertical), you should have a pixel frequency of 15,7 MHz / 4.

If you want to have a square display window of 128 pixels (horizontally) by 128 lines (vertical, the ROBOT deviation), then you should have a pixel frequency of 14,7 MHz / 4.

The / 4 is because
1. you have not 625 lines but 312 lines and
2. you display every line twice, occupying 256 or 240 lines of the visible 288 lines in the CCIR TV-system field.
(A frame is 625 lines, 25 frames per second interlaced, a field is 312 lines, 50 fields per second non interlaced, so progressive scan.)

There are more simplifications that you can use, a non interlaced scanning is also easier for the V-sync pulse.

About the 120 lines / 128 lines problem, I had yesterday the idea to choose for a pixel frequency of 15,2 MHz, that is in between 14,7 and 15,7 and then the deviation of the squareness is no more than 2%, both for 120 lines and for 128 lines. Does this affect your x-tal frequency for the read-out PIC? I think so.

Good luck,
73, Klaas PA0KLS
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KD2BD SSTV convertor

Postby acl » Thu Oct 14, 2021 4:06 am

Hi there all,

Klaas ,The fine vertical lines are a problem with my prototype that we know about. The PCB I have designed for Johns PIC processor section initially the waveforms look a lot cleaner so , hopefully when the replacement op- amps finally arrive from China I can prove this to the final output stage. I will send you a PCB once I am happy with this. John has provided me with the PAL code as well as NTSC and they both work well. I have the Gerber files for the board so these can be sourced cheaply from the Far East should anybody wish to build the converter. John did talk about a SECAM version as many of the SSTV fraternity are French.(La fraternité SSTV est française )Personally I am quite impressed with the PAL unit feeding a commercially available converter to VGA. I don't know if ther is a converter to offer the same but with a HDMI output.

Regards Chris Lewis G6ACL
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Re: KD2BD SSTV convertor

Postby kd2bd » Thu Oct 14, 2021 10:49 am

If there's any interest, I can illustrate the composition of each FSTV video line my scan converter generates.

In general, I digitize each SSTV video line into 256 pixels and display them on the FSTV monitor at a 6 MHz rate in a Pillarbox format. For PAL (and SECAM since both are based on the same CCIR video timings), it will appear that the up-converted SSTV image is stretched horizontally by about 9%. Overscan would increase this distortion even further, but it doesn't appear to be objectionable.

Adjacent SSTV lines are averaged to produce 256 FSTV lines from 128 SSTV lines. The top and the bottom of the FSTV image are padded with blank lines to center the SSTV image vertically on the visible portion of the FSTV display. With no PAL monitor of my own, I was pleased to see Chris's PAL CRT monitor displaying a well-centered SSTV image. At the risk of drifting further from accepted standards, I recently added some additional blank lines to my NTSC firmware to allow more "headroom" at the top of the displayed SSTV image. So far, all monitors tested tolerated the slower vertical sweep rate and the additional scanning lines without issue.

FSTV vertical sync is serrated and is preceded and followed by equalization pulses of the proper duration to produce a non-interlaced raster. For PAL, I measured a horizontal sync rate of 15626.5 Hz, and a vertical sync rate of 50.085 Hz, yielding 312 lines. I'm happy with that. :D

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Re: KD2BD SSTV convertor

Postby Steve Anderson » Thu Oct 14, 2021 4:33 pm

Well done guys, shame my SSTV-VGA up-converter has come to a halt due to component shortages. I've looked at alternate suppliers and alternate parts and frankly got almost nowhere. So frustrating!

However predictions are that by the end of the year things "should be almost be back to normal..." a vague prediction if ever there was one! We'll see.

That's from the production point of view. Outstanding orders need to be fulfilled first (they have priority), then getting them to distributors, and eventually us lot. Say April (2022)...maybe? I don't see things starting to improve before then. But I hope I'm wrong!

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