Composite mod voltages

[PokeMon]

25thanni is a multipart demo written by Bodo Wenzel a good few years ago to celebrate the 25th Anniversary of our beloved Zeddies.

As the program has about 9kBytes I can not run this because I do not have the possibility to load programs at this time.
And the program uses HRG graphics as well (okay not for the first two screens).


But the picture is very clear, very good contrast and very silent background.

Picture from RF modulator / antenna input

Zwischenablage01.gif (202.63 KiB) Viewed 5447 times

Picture from video input.

Zwischenablage02.gif (209.25 KiB) Viewed 5446 times

[PokeMon]

Here full screen picture

over RF modulator / antenna input

Zwischenablage03.gif (359.5 KiB) Viewed 5449 times

and over video input

Zwischenablage04.gif (366.36 KiB) Viewed 5451 times

[PokeMon]

the scroller of which outputs black video during the backporch region of the video signal... on my original circuit using an LM1881 the video would distort a little on the scroller, but the newer version does not suffer from that.

I am not sure what you mean. I think the program can only use the standard D-FILE for output which effects only the screen area of 256x192 pixels.
So I think its not possible for the standard program / ULA to put something in the backporch region. Maybe you mean some problems when using many black pixels on the screen which will move the video signal voltage more up when coupled in via a condensator. Maybe you mean an effect when switching between black and white, maybe by inverting signal which could give some pump effect. I did not check this now. First I have to do the basics before using my ZX81 for playing and more.

The video output is not the goal of my interface because I want to digitize the video signal and transport this to my computer via USB and display the picture in a window. When the USB part is finished I could use the tape interface for putting programs on my computer.

[PokeMon]

By the way I had some problems with my plasma TV when putting out many invers characters on the screen.
It had problems with synchronizing.
But this effect was only via RF modulator / antenna input.
My video input was steady and clear with this picture.

Any more ideas what to test with video ?

Thanks, Karl


RF modulator /antenna with sync problems

Zwischenablage07.gif (200.32 KiB) Viewed 5440 times

same pic over video input with no problems at all

Zwischenablage08.gif (203.79 KiB) Viewed 5437 times

[Andy Rea]

Erm can't think of any other programs that put black pixels in the back porch area, that was the only one i found that did it.

Rezzurection does it too, but that requires hi-res also...

Time to build Ram expansion

The reason i say it, is like you my first circuit just used resistor matrix to mix the video and syncs and porch, parts back together and relied on the video part to be white that was then pulled back down to black level by the back-porch signal, BUT when the video part was black the back porch signal just pulled it down back to sync level, thus extending the sync pulse causing the disturbance.

Regards Andy

[PokeMon]

Erm can't think of any other programs that put black pixels in the back porch area, that was the only one i found that did it.

Rezzurection does it too, but that requires hi-res also...

Okay - hi-res could be maybe different.

But how can the program do this with normal video mode ?
If you have pixels in the back porch you are leaving the 256 pixel /32 chars per line window.
Maybe you can explain this part.

Yes you are right with pulling down black level to sync level.
But this can happen only with the old ULA with missing back porch.
The new ULA has build in back porch and this is done in the ULA hardware (i think so).

But as you know the ULA hardware better you can explain how software can set pixel outside the display area of 256x192.
I think video output is handled only via the DFILE in standard video mode.

[Andy Rea]

When operating normally, IE. 32 x 24 text mode all is fine and there is a small pause between Hsync (generated by hardware) and the first video cycles, this pause is by careful timing and syncronising of the video software in the ZX81 rom (it syncronizes to the last NMI, which happens to be the hsycn also)...

But since as we can take control of the software side of things, we can make video cycles happen whenever we like, even right over the top of the hsync ( although you won't see it as hsync & vsync clamp the output low).

ld hl,#scrolltext+0x8000
push hl
push hl
push hl
push hl
push hl
push hl
push hl
scf
jp (hl)
scrolltext:
.db 0,0,0,0,0,0,0,0,0,0
.db 0,0,0,0,0,0,0,0,0,0
.db 0,0,0,0,0,0,0,0,0,0
.db 0,0,0,0,0,0,0,0,0,0
.db 0,0,0,0,0,0,0,0,0 ; 47 characters visible!
ret c ; 49*4+11 = 207

This is part of Bodo's Code, the scroll text is executed 'as video' 8 times the RET at the end actually returning to the start, until the last time round, So there is only 11 clock cycles that video is NOT been generated, and we know that the Hsync is 16 clock cycles, so therefore the video MUST overlap some of the Hsync & backporch, as there are only 207 cycles per scan line.

Regards Andy

[PokeMon]

Thanks Andy.
I will think tomorrow of it, now it's a little bit late for that stuff.

By the way, I changed my schematic of video with logic gates for generating a back porch without LM1881 as a member request from the german forum.
Just for information. Two whom it may concern.

Video-HQPixBasic.gif (16.14 KiB) Viewed 5368 times

This should work with both ULA versions (with and without back porch, but regard desired Jumpersettings).
Back porch is done by a r/c combination, triggered from the sync signal with approx. 5 us duration.
So this use only one ic, one diode, 3 capacitors and one hand full of resistors.
It's maybe cheaper than buying a LM1881.

[PokeMon]

so therefore the video MUST overlap some of the Hsync & backporch, as there are only 207 cycles per scan line.

Regards Andy

There was a mistake in the last posted schematic from me yesterday (without LM1881).

By the way I changed now following in my video schematics:

1. it's no more important wether ULA supports back porch or not (or just from time to time or program to program) - back porch is always killed and generated new, so this is a workaround of bodo's program which might be a mistake in programming in general - but who cares. Since we know Microsoft we know "it's not a bug, it's a feature.

2. jumper setting is more easy for only inverting or noninverting picture with a 3-pin jumper to avoid unwanted shortcuts

3. the ULA type has to be known only in schematic with non capacitive coupling of ULA video output (because different voltage levels for sync)

Hope I did not make a mistake. By the way in the first (easiest) schematic I could spare on logic gate while using pixel video driver for back porch as well.
This trick can not be done with the inverting drivers.

By the way, I named it CCPIC for Crystal Clear Picture.

standard video with LM1881

Video-HQPix.gif (13.47 KiB) Viewed 5339 times

inverting or noninverting switchable with LM1881

Video-HQPixUniv.gif (15.51 KiB) Viewed 5332 times

inverting or noninverting switchable without LM1881 (own R/C combination triggered from sync)

Video-HQPixBasic.gif (17.14 KiB) Viewed 5327 times

By the way I did only test the version with 74LS07.
The version with 74LS06 is just a theoretical derivation but should work in the same was.
I hope I did not made a mistake.

[PokeMon]

One more correction, I forget a connection between sync and backporch because they are overlapping and backporch has to be avoided during sync.
I added one more schottky diode for this.

Video-HQPixBasic.gif (17.08 KiB) Viewed 5304 times

If I order electronics next time or stay in the near of an electronic shop, I will test it. Not tested yet. Or maybe someone is faster.