NBTV/SSTV-625 Up Converter MkIV

[Harry Dalek]

Oh ok Steve ...just wait and see this time

[Steve Anderson]

Updated design coming on nicely... now I have to turn my attention to demodulation. There's three (maybe more) ways to do it, 1) Zero-crossing, 2) FM demodulation, 3) In software using mixing, FIR filters and a bit of calculus...

The MkI used #2, the results I think where quite good considering the relative simplicity of the design. I did experiment with #1, it worked OK, better in some areas, marginally less so in others, sadly I can't find the code I wrote for it. or even the concept as to how I did it...
.... I've yet to try #3.

Two of these methods may require an external demodulator, #2 and #3. Even #1 may benefit by using an external arrangement. I'm still dwelling on this but I don't want to spend too much time on it.

I may go with #1 first, and try the others later...

Steve A.

A fourth method that slipped my mind #4) PLL (Phase Locked Loop). Either analogue (e.g. our beloved 4046) or an ADPLL (All Digital Phase Locked Loop). The latter holds a lot of promise, but the maths required, "Does me 'ead in"...translation from 'saaf londin' speak, it's somewhat challenging...

Another musing, a blend of means is also a possibility...more to come...

[Steve Anderson]

Thought I'd hit another impasse this morning, but thankfully dodged around it. This VGA idea is testing this (and me) more than I expected! Though these micros can operate at a clock speed of 64MHz it's not compatible with a VGA 800x600 display, 40MHz it has to be.

I was hoping to have some simple screen-shots up this weekend, but I keep getting thwarted, plus I still have to do that outing to get some vital components...

Steve.

[Harry Dalek]

Never fun when it does not go to plan !
So now your clock will have to run at 40MHZ crystal change i suppose /

[Steve Anderson]

...crystal change i suppose /

Yes, oscillator changed to 40MHz along with a whole heap of other stuff, there's little similarity between the early versions and how it is now. The major components are the same, micro, RAM, etc., it's how they're strung together that is very different.

The reason for the speed change is the chosen flavour of VGA uses 25ns pixels (the inverse of 40MHz). However here they'll be in blocks of four at first (100ns) with a change, perhaps, to a pair (50ns) later. Hence the requirement for 74ACxxx series logic chips...as I see it at the moment just a few (3-4) 74AC00 and/or 74AC08.

Steve A.

[Steve Anderson]

Another point of note, the two micros are loaded with exactly the same software, both the read code and the write. Which code they run is determined by Pin 26 (RB5), if it's at +5V it runs the Write software, if at 0V it runs the Read software. There's no chance of mixing them up. There's vastly more program space available than both programs together...I wish the same could be said of the RAM!

Steve A.

[Steve Anderson]

I'm making a start on the Read micro, RAM and VGA output stage this weekend, it'll be a few days until I get some results to place here. I've still yet to do my shopping trip across town too. Also the power supply (quite easy) and case/chassis to do...and clear my workbench first! In addition make a start on the Read micro code.

Steve A.

[Steve Anderson]

I may have hit a major problem...the 800x600 VGA standard was designed for 4:3 CRT displays (mainly), but most flat-panel displays these days are 16:9. Although they might display an image it'll probably be the wrong aspect ratio, too wide. On my previous TV you could select the display aspect ratio, but not my current one, at least I don't think so. I'll have to investigate if PC VGA wide-screen monitors are similar. If not "Okay, Houston, we've had a problem".

There's only one way to find out...

If you're in a 525 country it may be worth checking if your TV can display 625/50, I would guess most under ten years old can, no need for PAL/NTSC/SECAM as this is monochrome. In other words I'm hinting that this may have to go back to standard-definition video, but I hope not!

Steve A.

This laptop PC has two monitors, the in-built 15" device and an external HDMI 24" monitor. I've been able to leave the internal display as is but I am able to change the external 24" to 800x600 VGA with no aspect ratio distortion....so we may be in business after all...it's not the PC we're testing, just how the display will behave...though that's via the DVI input, not the analogue VGA connector...here's hoping...

[smeezekitty]

I may have hit a major problem...the 800x600 VGA standard was designed for 4:3 CRT displays (mainly), but most flat-panel displays these days are 16:9. Although they might display an image it'll probably be the wrong aspect ratio, too wide. On my previous TV you could select the display aspect ratio, but not my current one, at least I don't think so. I'll have to investigate if PC VGA wide-screen monitors are similar. If not "Okay, Houston, we've had a problem".

There's only one way to find out...

If you're in a 525 country it may be worth checking if your TV can display 625/50, I would guess most under ten years old can, no need for PAL/NTSC/SECAM as this is monochrome. In other words I'm hinting that this may have to go back to standard-definition video, but I hope not!

Steve A.

This laptop PC has two monitors, the in-built 15" device and an external HDMI 24" monitor. I've been able to leave the internal display as is but I am able to change the external 24" to 800x600 VGA with no aspect ratio distortion....so we may be in business after all...it's not the PC we're testing, just how the display will behave...though that's via the DVI input, not the analogue VGA connector...here's hoping...

4:3 monitors are common as dirt in thirft stores here. Personally, a VGA output would be more useful than a PAL television output IMO.

[Steve Anderson]

4:3 monitors are common as dirt in thirft stores here. Personally, a VGA output would be more useful than a PAL television output IMO.

Quite agree. which is why I intended (and still do) to go in that direction, mainly to negate the 525/625 and 50Hz/60Hz worldwide divide. VGA is VGA whatever country you're in. I just wanted to see yesterday if a 600x400 VGA 60Hz display presented a 4:3 aspect ratio on a 16:9 screen correctly, on one monitor I tried it does, I've yet to check another one.

I'm not sure if we have 'thrift stores' here, but there is a shop near me piled high with second-hand CRT TVs, I guess they may have a few VGA PC monitors too. Worth asking the next time I go past.

The wife will say, "What? More junk?"

Steve A.

[Steve Anderson]

Think I've gotten confused between version numbers for this device...from here on it'll be Version V (five)...I'll start a new thread...

Steve A.