SSTV-VGA Display Up-Converter.

[Steve Anderson]

This thread picks up where "New SSTV Demodulator" left off as it now encompasses more than just the demodulation of SSTV.

Using a VGA display has the advantage of a single worldwide standard, VGA is VGA no matter where in the world you may be. No more global split between 50Hz or 60Hz power-line frequencies. However it does present a few challenges, mainly the increase of video bandwidth required. Today it's possible to construct a SSTV-to-VGA up-converter yourself, though it's not really suitable for the beginner. As far as I know there are no commercial nor home-made up-converters of this type in existence, though I'm bound to be proven wrong!

So as it is said on YouTube, "Let's get started!"

Steve A.

[Steve Anderson]

Here's my initial go at the SSTV-VGA read processor, RAM and output stage of the up-converter.

The two 8-pin RAM devices are initialised in SQI mode such that they present a byte-wide I/O interface to the micro and the D-A.

The rest is fairly conventional. I have my reservations in re-using the DA-08 again as it really isn't specified for operation at these speeds, though it did give a surprisingly good account of itself at 625-lines, better than I expected. I'm looking at faster D-A's as mentioned in the drawing - time will tell.

I mentioned the 'read processor' above, but it may be possible to negate the need for a 'write processor', making the device a single processor design. Maybe, maybe not.

Note:- The logic supply is 3.3V, not 5V. Applying 5V will fry most of the devices!

The output stage has been 'beefed-up' as it has to drive three 75R VGA inputs (RGB) rather than one (Y).

Fig. 8.0 mentioned will follow...

Steve A.

[Steve Anderson]

One thing I don't have is some way of testing the monitors I have is if they'll work on 800x600 60Hz VGA standard. So, to that end I'm going to knock up the following circuit. It's simple and the code should be quite easy. I'd rather build this than find out later that after building the whole up-converter the monitors don't work with this signal. It could be adapted to other VGA standards too - mainly a software exercise.

Steve A.

Yes, I know I could simply change my PCs settings as a test, but this could also provide more information, much like a 'proper' test pattern.

The three 5V to 0.7 output attenuators will follow...done...

Steve A.

Yes, I also know it's Christmas, but here it doesn't happen, it's just another normal Sunday....but for those who do celebrate it - have a good one!

VGA Test Attenuators V1.gif (5.16 KiB) Viewed 3362 times

[Steve Anderson]

Because of the speeds involved (even at such comparatively low VGA screen resolutions) I'm considering migrating this to a 16-bit processor. It's about time I got to grips with them and their 32-bit counterparts. There's a limit to what you can do with 8-bit chips...I think I've reached that limit...at least in terms of speed...

So this may be slow in gestation as I learn the habits of these new (to me) devices.

...though having downloaded a number of datasheets for the 16-bit devices I can see this is going to be a steep learning curve....but having got to grips with the 16-bit devices, the 32-bit versions seem a simple upgrade...we'll see...

The first devices I'll 'cut my teeth on' are the PIC24EP32GP202-I/SP, inexpensive and good for 70 MIPS, 140MHz internal clock, nice and simple 28-pin DIL packaging...forget 5V, all 3.3V nominal (Max 3.6V), but still quite useable at slower speeds down to 1.8V.

Contrast that with some 8-bit PIC18 devices which max out it 64MHz (16MIPS) internal at 3.3V. Devices that are rated to work at 5V are becoming rarer these days. Some of these PIC24 devices date back to the late 90s, time we caught up...

Steve A.

[Steve Anderson]

Like so often happens this has been 'on hold' for the past six months, but I do have some time now to restart this. I have confirmed my two PC monitors both operate at 800x600 @ 60Hz, so that's one unknown out of the way.

The 'paperwork' side of the design is mostly done and I have all the major chips, just a few 74AC versions to obtain. The only unknown is if the RAM I have is fast enough, if not I'll have to order a few 25ns versions. I'll build the little 'VGA test generator' first (as above, though with some minor changes), then move on to the up-converter proper. I'll also add H and V blanking to the above as a programming exercise, they may come in useful. Note, this so far is just the VGA test generator, the up-convertor is yet to come...

Another addition to the overall use for this is not just SSTV, but NBTV at the usual 12.5Hz frame-rate, and maybe 15Hz too....so really the thread title should be changed...

Steve A.

[Steve Anderson]

As mentioned above I'll start a new thread in the "Electronic NBTV" section of this forum entitled "NBTV & SSTV to VGA". A short title as a longer one won't fit.

It will appear in due course.

With that this thread is concluded...

Steve A.