New SSTV Demodulator.

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: New SSTV Demodulator.

Postby Steve Anderson » Sun Dec 11, 2022 3:31 pm

Replacing the micro with a PIC18F14K22 which runs at 64MHz Fosc is easy, the pin-out is almost the same as below, pin 17 which becomes RA2 (an I/O) rather than the USB regulator bypass capacitor connection. The '14K22 loses the USB function which is no issue here. But they're not available until March next year or later it seems. I'm really fed up with this 'semiconductor drought'. So for the time being here's my interim plan...

Not sure what the UART function may do, but it's there if need be...

Next step is board layout and build...then the software...

Steve A.

(Drawing has been updated yet again with the logic supply changed to 3.3V).

SSTV V6 Demodulator 11.gif
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Re: New SSTV Demodulator.

Postby Steve Anderson » Mon Dec 12, 2022 8:27 pm

I've hit a bit of a brick wall, not so much this demodulator but the up-convertor that was due to follow it. It hit home that the serial RAM I was planning to use simply isn't fast enough, and it seems to be the same for all of them. I've been working with Flash memory recently which goes beyond 100MHz clock rates, I forgot that serial RAM seems to be limited to a 20MHz clock rate, which is too slow for 625, forget VGA!

I was hoping to avoid parallel RAM, but it seems I may be stuck with it! Ideas, suggestions and enlightenment welcome...

Steve A.
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Re: New SSTV Demodulator.

Postby kd2bd » Tue Dec 13, 2022 2:24 am

Maybe some ideas can be gathered by the type of video RAM used by PC video cards having analog VGA video out?
Or, (possibly) an inexpensive (but standard) PC video card can be used directly, if it's given the proper data and commands?

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Re: New SSTV Demodulator.

Postby Steve Anderson » Tue Dec 13, 2022 11:25 am

Yes, that's a possibility, something I'll look into. It should be able do a whole load of other modes/resolutions too, which might have other uses. Though I suspect not standard 625/525, composite PAL/NTSC colour even less so, though RGB or YUV would be OK.

These days the display part of a PC (desktop or laptop) is usually integrated within the motherboard/mainboard, but surely separate display cards are still available? I assume I have one in my relatively new-ish desktop PC as it has dual HDMI outputs for two screens. I often have (say) Excel on the left screen to generate the components parts list while Autocad running on the right screen showing the schematic so I can easily cross-corelate the two. Flipping between two applications on a single screen is prone to errors. Even my previous laptop, now deceased, was able to do the same, its in-built screen showing Excel and the external (much bigger) screen showing the schematic.

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Re: New SSTV Demodulator.

Postby Steve Anderson » Tue Dec 13, 2022 6:22 pm

Well, for the moment this is as far as this subject will go. I obviously need to build and test this version of the demodulator, and at the same time create an updated SSTV-625 convertor, maybe a VGA version, but even at 800x600 VGA it does push the limits of home construction. The devices used in commercial designs take a while to filter down to being available to mere mortals such as us. However, as time goes on it is becoming more possible.

Steve A.

For newcomers here, this is what's possible with a 8-second B/W 128x256 SSTV image, under admittedly ideal conditions....it's a screen shot with the camera pointed at a standard JVC TV, nothing special...that 'child' is now an ER specialist in a local hospital...

Nong Jeab 2.jpg
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Re: New SSTV Demodulator.

Postby Steve Anderson » Sat Dec 17, 2022 7:21 pm

Here's how I see the 1:1 128-line SSTV frame fitting into a 800X600 VGA monitor screen area...this has advantages in reducing the horizontal data rate...

The area outside the SSTV frame could be any shade of grey from black to white, though I'll probably set it to black as previous.

Don't forget this is an RGBHV standard, though there is some blanking during the vertical interval, it's much less as a proportion compared to 525/625. If PC monitors were specified in the same manner as TVs then 800x600 VGA would become 628 lines with 1056 pixels in the Hsync-to-Hsync time, i.e. 256 pixels are blanked. All the data is below...

Steve A.

SSTV V6 Demodulator 12.gif


800x600-60Hz.png
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Re: New SSTV Demodulator.

Postby Klaas Robers » Sun Dec 18, 2022 10:57 pm

Steve,

do you think that you can make the horizontal SSTV-area 480 pixels? I still have the impression that a picture of 120 lines is a square. The extra 32 lines that Robot added are used by them for a grey scale, which is not part of picture.

I think that they did this because the digital memory of the ROBOT 400 scan converter already had this extra capacity and it was more difficult to hide that. I also think, but don't know for sure, that the SSTV camera they had, also gave just 120 lines. Is there anybody that can confirm / deny this?

Another option could be to give a line 496 pixels. That is a compromise between 480 and 512.
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Re: New SSTV Demodulator.

Postby kd2bd » Mon Dec 19, 2022 10:37 am

Klaas,

As far as I am aware, the Robot 80 and other similar video sampling cameras from the early 1970s generated 128 line video and used all 128 lines for picture information. I believe that the digital scan converters that followed several years later used 120 lines for picture information while reserving the last 8 lines for a grey-scale pattern only.

Notice the "Divide by 128" circuit in the Robot 80 block diagram:

robot80.png


venus.png


The Robot 300 storage tube scan converter was capable of several different "binary-friendly" scan line formats, including 128 (but not 120). If I am not mistaken, all 128 lines were used for picture information:

Image

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Re: New SSTV Demodulator.

Postby Steve Anderson » Mon Dec 19, 2022 1:48 pm

Klaas Robers wrote:Steve, do you think that you can make the horizontal SSTV-area 480 pixels? I still have the impression that a picture of 120 lines is a square.

Yes, entirely possible, the display area whether 120 or 128 lines would have a visible aspect ratio as displayed on the screen of 1:1 in both cases.

For example using the 128-line versions with the active portion of a line being sampled 256 times in 55ms (EU standard), each pixel is effectively repeated to fill 512 pixels of the 800x600 VGA line. Vertically each one of 128 lines is repeated four times to make 512 VGA lines, much in the same way the original 625 up-converter worked, except there it took advantage of the interlacing to double the effective lines. Most, nearly all, PC display formats are non-interlaced, including this proposed one.

The result should be a displayed 1:1 aspect ratio as per the revised sketch below. for 120 lines most factors are simply pro-rata.

I should really start a new thread as this has now departed the theme of the thread and is encompassing more than just a demodulator...

DONE - A new thread created in the SSTV folder/subject heading, "SSTV-VGA Display Up-Converter." I suggest continuing this subject matter within that new thread...

Steve A.

Note: The sketch below is an approximation, it's not exactly to scale...though the aspect ratios are correct...

SSTV V6 Demodulator 13A.gif
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