SSTV-625 Up Converter MkV (Five)...

[Harry Dalek]

Well i started the devil monitor in 2017 i just finished days a go ,and i recall Klaas and he's P7 SSTV he built in the 70s recased and finished 2 or 3 years back so don't worry to much about the time every one does the same when you feel like it you work on it to your time table .
Some times you need a brake and try some thing else and come back to it with interest .
It looks to be going very well if i could help with advice i would but its beyond my minor skills so all i can do is watch and see how it goes .

[Steve Anderson]

I've found what appears to be the definitive reference for VESA monitor standards...if you intend to drive a VESA monitor you'll be needing this...though it might be worth checking for updates for some more recently introduced modes, but this'll do for me...Feb 2013...

For this project I'll be using 800x600/60Hz...page 26...definitions on page 14, section 3.1...

Steve A.

VESA-DMT-1.13.pdf

(5.12 MiB) Downloaded 261 times

[Klaas Robers]

,and i recall Klaas and he's P7 SSTV he built in the 70s recased and finished 2 or 3 years back so don't worry to much about the time every one does the same when you feel like it you work on it to your time table .

Indeed and then still it should have its texts to indicate the control knobs and I should finish the line input / outputs with a hidden volume control. Sometimes you need some time to get the better ideas for your projects.

And then suddenly there pops up a new project, that looks much more attractive than finishing the current projects .....

[Steve Anderson]

...And then suddenly there pops up a new project, that looks much more attractive than finishing the current projects .....

Yep, exactly my 'problem', except with this one I'm going to keep going until well and truly complete. There isn't much that could be improved upon as the basic display is constrained by the image parameters of 128x128. Some line and pixel interpolation could be tried but I doubt the effort would be worth it.

The SSTV demodulator is about the only sub-unit that could benefit from an upgrade and spending some time on. But that's after completing this first. Though I have already been thinking on it and made quite a few notes and sketches. To do A/B tests on various demodulation ideas the demodulator will be a plug-in unit for rapid comparisons [1]. Everything else, the unit, the VGA display and the camera used for screen-shots will all stay the same and all used under the same conditions and set-up.

Steve A.

[1] Perhaps with 'instant switching' between A and B.

[Steve Anderson]

Regarding using PC monitors for a display unit, be aware that about a decade ago the CVT (Coordinated Video Standard) was introduced. This a reduction in blanking times, clock speed (pixel speed effectively) and a few other parameters. It appears to only apply to resolutions greater than 1280x720 so my antique 800x600/60Hz usage here seems to have escaped this 'modification'. But no promises.

You should be able to get the spec. from the VESA website, though the most recent version I can find is dated Feb. 2013.

The latest spec. I can find for this mode is attached...all much the same concept as 525/625, similar terminology, just the numbers are quite different!

For better resolution, click the image...and 'Back' to return here...

Steve A.

I didn't worry about the copyright notice as it's an open and downloadable document from the VESA website. Many other sites with the same info., but not in quite so much detail...and it is 30 years old...

[Steve Anderson]

Right, I've made a start on the Read Proc. software - even though I haven't even started the build! I need to get back to Bangkok for that. Most of the exercise is porting it from the 18F2620 used in the MkI to the 18F26K22 used here, the devices in many ways are very similar - even down to the pin-out. Then re-writing/modifying the code, changing it from 625/50 to 800x600/60Hz VGA. I should be back home at the coming weekend...

The Write Proc. software should be simpler, it 'just' requires the change from a parallel RAM to a serial version. Hence the reduction of pins from 40 to 28. Likewise the RAM itself, two 8-pin devices instead of one 32-pin.

The output D-A isn't quite such a happy story mainly due to the old DA-08 D-A can't really keep up with the data-rate, though I'm still looking into a replacement for it. An ADV7128 would be ideal, but I'm not going to deal with the usual crap suppliers on the 'net - you know who...it's not in production anymore...though you would think something similar in current production existed...

Steve A.

[Steve Anderson]

A few 'sillies' in the CONFIG directives for the new 18F26K22 and the old 18F2620 code assembled with no problems at all in the newer device...it shows the commonality between these two devices...I'm hoping, perhaps unwisely, that things continue in such a manner...

2059 bytes for the old SSTV-625 code, potentially less for the VGA code as it's not interlaced (no half-lines). This VGA=628 lines as opposed to 625...a nice easy even number! Program bytes free? 63,477...slightly embarrassing! Perhaps a slight change to one of the 18F26K22's siblings is in order? Basically identical, just less program space and RAM...which is external anyway here, just a few needed for line-counting and housekeeping tasks, maybe 12 instead of nearly 4,000?

However, that commonality could set traps for the unwary...I'm not out of the woods yet!

Steve A.

The smallest memory version should be OK, PIC23K22. the DIL/DIP version (still 28-pins) GBP1.69++, say two quid...(1-off, Farnell/Element 14)...

24/03/21 Though looking at the UK prices for these devices, the difference isn't worth the bother. I'll stick with PIC18F26K22-I/SP chips (28-pin 0.3" DIL/DIP)...better the devil you know...and I have some in stock here...

[Steve Anderson]

Initial 'hacking' of 625 Read code to '628' VGA reduces size down to 1929 Bytes from 2059. I didn't expect a huge difference, and there's still a quite a way to go...most of which is the elimination of equalisation pulses, broad pulses and half-lines. Only three types of lines now, Vsync (Lines 1-4 inclusive), BlkLine (Blanking) and ActiveLine (those with video)....really simple...

Compare the VGA vertical interval waveform below with the usual 525/625 arrangement...this VGA standard uses positive syncs not the usual analogue negative...at 5V TTL, it's separate from the analogue RGB 0.7V video.

The first line with active video (as you'd expect) is line 28, it was developed in the days when most, if not all, displays were still CRT-based and needed some vertical flyback time.

Steve A.

SSTV-VGA MkIV Vert 1l.gif (4.79 KiB) Viewed 8178 times

[Steve Anderson]

In addition to the the above, we'll only be using 512 of those 600 VGA active lines (about 85%) and likewise a proportion (64%) of the horizontal pixels as the format is 1:1, square.

Steve A.

[Steve Anderson]

...still rationalising the circuit, the output D-A arrangement changed from 2x74AC157 (16-pin) to a single 74AC373 (20-pin), so no IC504 anymore...

I would have preferred to use a 74AC573 but just can't get them here, only difference is in the pin-out.

Update 28/03/21, Can get 74ACT573 here, so I'll be using that for IC503...no functional difference to an AC version in this application.

The difference between, say, the '373 and the '573 is the latter is laid out 'broadside', one side (pins 2-9) are all the 8 inputs, the other side (pins 12-19) are the 8 outputs. The '393 has all the I/O mixed in with each other, otherwise identical. The '573 was laid out for 8-bit buss orientated applications, like this one.

Steve A.

I'm doing what I can by 'remote control', the only tool I have here is this PC. For hardware construction I need to return to Bangkok...I'm hoping this weekend...maybe...later in the year we're hoping to move out of Bangkok on a permanent basis...though still in Thailand, about 160km away...

[Steve Anderson]

Copied and pasted from another SSTV thread...

Klaas Robers » Tue Jun 21, 2016 4:18 pm

Steve, it is already for quite some time that I wonder if an FM demodulator or discriminator shouldn't have an S-curve that is linear over the whole bandwidth of the FM spectrum. That is for SSTV it should be straight from 300 Hz (1200 - 900) up to 3200 Hz (2300 + 900). For the lower limit it could be 600 Hz (1500-900) because I allow the demodulated sync pulses to be distorted, as the sync is extracted in a different way.

Although, one might think that the frequency never goes lower than 1200 Hz (1500 for the video content) and higher than 2300 Hz. But the side bands are not in the signal just for fun. If you demodulate the FM with two tuned circuits, one at 1200 Hz and one on 2300 Hz, as ROBOT did, then you mis the side bands between 2300 and 3200 Hz, isn't it? At least they are attenuated in stead of boosted.

If I watch in this thread the page 10 then I see that for your demodulated "Blokgolf" signal, that in the upgoing black to white transients there is no ringing of the low pass filter, but a somewhat rounded shoulder, while at the white to black transients there is ringing and it looks as if the transients are steeper. In the same page a photo of an oscilloscope screen shows ringing up as well as down from a grey scale signal.

However, video signals are strange things. A picture has other requirements than audio.

I've looked into this with a view to an improved demodulator in general terms for SSTV...

This is not too onerous, if a 1MHz clock is gated by the SSTV FM waveform edges to generate a count, then use this as the index to a look-up table. As an exercise I've created the required look-up table from 300 to 3300Hz using a 1MHz gated signal. It came out as a total of 3,033 12-bit values, i.e 6,066 Bytes in program memory. maybe 12-bit is excessive, but it's easy to do, so why not? Of course it could be cut-down or simplified, but I'll start here first. 6k Bytes is nothing with todays micros.

I've already selected the micro, a PIC18F14K50, DIP/DIL, 20-pins. [1] Though it has a USB 'engine' within, I'll not be using it. It has 16kB of program memory, 6kB would be the look-up table, the remaining 10kB or so should be more than enough for the actual code.

Some thought should be given to what if the input is below 300Hz or above 3300Hz or simply not there at all? Hold the previous value and generate an 'invalid signal' indication? Those frequency bounds can be changed if deemed necessary.

The alternative is a reciprocal routine (1/n) which as ever in these simpler devices may be not as good - certainly not as fast - but this is SSTV - fast? - well, maybe not. I'll look into that sometime later...

I should point out this is just for the demodulator alone and potentially could be used with any SSTV display device.

Steve A.

[1] i.e. a separate micro, it's possible to incorporate this function within the Write micro, but until I get into the details of that code, I'm not sure...updates as and when...

[Steve Anderson]

Thinking on this further - it can be done within the Write processor. Though I may develop the 'stand-alone' version first, 1) Simply to prove it, 2) There'll be applications that might require a stand-alone version.

Steve A.

[Klaas Robers]

Steve, in case of an "overflow" or "underflow", you may give the output a fixed value. So for all frequencies of 300 Hz or lower, the output value of 300 Hz, and for all frequencies of 3300 Hz or higher the output value of 3300 Hz. The first one you need anyway in case there is no input signal, which is a occuring frequently.

And be aware that you use both edges in the input signal, the upgoing edges and the downgoing edges. In both there is information about the video signal. So in reality you are demodulating the frequencies of 600 to 6600 Hz........ In fact this is the sampling frequency of an FM modulated signal. And if you want a video signal with a bandwidth of 1000 or 900 Hz, you need a sampling frequency of at least 2000 Hz. If you don't do this "frequency doubling", you are having too few information samples at the input signals for dark areas, e.g. 1500 to 2000 Hz. So frequency doubling is needed.

[Steve Anderson]

Yes, I've been thinking about that! Just the positive going edges, just the negative going edges, or both?

It's quite a simple modification, so seeing you've suggested it I'll add that in...for both edges simply double the clock input to 2MHz..simply done by switching the counter pre-scaler...

As for below 300Hz or above 3,000Hz (or so), yes, limit the output to those values, no signal - somewhere mid-value?

Steve A.

More thinking...I must stop this habit!...keep it to both edges only...

[Klaas Robers]

No Steve, if there is no input signal, the frequency is below 300 Hz. Then limit the output signal to equal to 300 Hz.Then your demodulation characteristic is a straight line from 300 to 3300 Hz and horizonals going down and up. You may limit video signals in this way, it looks very natural. And with no input signal you have a black screen, why not?

If you like that you may limit the video signal, after the low pass filter, even more, to e.g 10% lower than 1500 Hz black, and 10% above 2300 Hz white. This is done too in digital TV, there black is 0F and white is F0. The remaining values are reserved for negative and positive ringing excursions. Remember that they give us a sensation of sharpness.