Faint vertical gray lines in STE/MSTE in high-res - Am I the only one?

[czietz]

Hello,

both my 1040STE and my MegaSTE show flickering faint vertical gray lines in high resolution (monochrome) video mode every 16 pixels. This happens on a VGA monitor as well as on the SM124. It particularly visible on a mostly white screen but can also be seen in the desktop background pattern.
See picture below. (Contrast has been enhanced in the photo to make the lines more visible; ignore the circular bands caused by moiré.)

I think I have now understood what causes this. However, I'm wondering if I'm the only one seeing this? If I was, I'd only build a quick-and-dirty fix that suits me. If this is a more common problem with (Mega)STEs, I'll spend some time designing a fix that can be used by others as well.

ste_lines.jpg

[mpattonm]

I am too lazy to do a calculation, but could this be 50Hz PSU noise?

[czietz]

50 Hz noise manifests itself as more overall flickering of the screen with ca. 21 Hz (71 Hz - 50 Hz). I had that as well, but fortunately I was able to fix it by selecting a more suitable opto-coupler for the PSU.

The gray lines however correlate with the RAM access of the GSTSHIFTER.

[BlankVector]

Could it be a problem on some data bus line between Shifter and RAM?

BTW, what I see is that you have managed to display 3 different colors (white, black, grey) on supposedly bi-color hardware, that's quite amazing.

[czietz]

Could it be a problem on some data bus line between Shifter and RAM?

Ah, no the data lines are all fine, it's just some 100 mVolts of noise that get overlaid on top of the mono output of the Shifter. I think this already happens within the GSTSHIFTER.

BTW, what I see is that you have managed to display 3 different colors (white, black, grey) on supposedly bi-color hardware, that's quite amazing.

Not intentionally, though and not really under my control. (The intensity of the lines depends on the intensity of RAM access. E.g. on the TOS desktop they are more visible when TOS itself runs from RAM, not from ROM.)

[ijor]

Ah, no the data lines are all fine, it's just some 100 mVolts of noise that get overlaid on top of the mono output of the Shifter. I think this already happens within the GSTSHIFTER.

Interesting. I never realized the mono signal is treated as analog and not digital. I never had a SM124, but that means it could be used as a true monochrome monitor (multiple shades of grey), and not just black & white.

[Silly_Pony]

Making it digital would've been more expensive. SM series monitors have separate brightness/contrast control for this reason, useless in ST Mono but they were trying to future proof.

[czietz]

The fix I designed seems to work in my MegaSTE. Still need to confirm this in the 1040STE before going into more details.

[czietz]

On the 1040STE the results were (only) partly promising. The lines were visibly reduced, but contrary to my MegaSTE they didn't fully vanish. As requested on FB, I want to document my findings, though, while I think of solutions for the 1040STE.

After a lot of measuring, I've come to the conclusion that the lines you see are caused by impedance coupling from the power supply on the mainboard. As you might know, the STE/MegaSTE Shifter contains the multiplexers/latches to moderate between memory access for the CPU and for video output. When these multiplexers switch, momentarily an increased current flows through Vcc and GND, i.e. the IC's power supply pins. This causes a phenomenon known in literature as ground (or Vcc) bounce where the supply rails within an IC change in voltage.

Obviously, such a change of the IC-internal supply rails will be overlaid on any output signal (e.g. see Figure 2 in the PDF linked above). This is far less critical on purely digital outputs (as long as no high/low threshold is crossed). But the video output signals from the Shifter (including the high-res one) are essentially treated as analog signals and so any ripple on them will be visible. Because the switching of the multiplexer occurs every 500 ns (1/2 MHz) you can see the effect every 16 pixels in high-res mode.

Since this is an effect internal to the Shifter, no amount of recapping will fix it -- different from other power supply related effects. Hence, my fix consists of a digital buffer that will take the mono video signal as input and output a ripple free version of it -- assuming the buffer itself is supplied from a clean power supply, such as directly from the big buffer capacitor on the mainboard.

As this fixes the issue on the MegaSTE, I'm confident I'm on the right track. Currently, I think that in the 1040STE also power supply variations outside of the Shifter (e.g. at the transistor which the high-res signal passes) overlay further ripple and thus also cause (less visible) vertical lines.

[czietz]

PS: You can see how the buffer cleans up the signal in the attached scope screen shot -- measured on the 1040STE. Yellow trace is the mono output from the Shifter. Notice the high amount of noise plus one very visible ripple due to GND/Vcc bounce. Blue trace is the output from my buffer board: much cleaner. That's one gray line prevented or reduced.

NewFile9.png

[moonferret]

Sorry to revive an old thread but did you ever publish the fix for this? I've got a MegaSTE with the same issue.

Cheers,
Dave

[czietz]

I did publish the design as Eagle files in the German forum: https://forum.atari-home.de/index.php/t ... #msg251298. Google Translate can give you a translation of the brief description I posted there.

[mikro]

First time I see this thread. :) Will check on my MegaSTEs and STE.

[TheNameOfTheGame]

I missed this thread also. So the fix is a small board to condition a signal?

[czietz]

I missed this thread also. So the fix is a small board to condition a signal?

Correct: a board that basically fits in the spot of a resistor and conditions the signal. But see above: I had mixed results. It works fine on my MSTE; but there are still lines on my 1040STE. Someone else reported success on his 1040STE. YMMV.

[TheNameOfTheGame]

Hmm, seems there are additional variables involved? Or is it down to just slightly different internal characteristics of each shifter chip?

Do you have any extra boards made up? Might be worth trying out on my STE.

[czietz]

I haven't touched this project since 2018, so I don't remember if I tried to figure out the differences.

No, sorry, the one board I hadn't used myself is long gone to someone else. But the Eagle design files are in the German forum thread (link above).

[TheNameOfTheGame]

Ah, how did you get the boards made? I didn't think eagle files were straight uploads to jlbpcb or am I mistaken?

[czietz]

Ah, how did you get the boards made? I didn't think eagle files were straight uploads to jlbpcb or am I mistaken?

Looking at my order history from 5 years ago, it appears I had them made at Aisler, where one can in fact upload the Eagle .brd file.

[moonferret]

I did publish the design as Eagle files in the German forum: https://forum.atari-home.de/index.php/t ... #msg251298. Google Translate can give you a translation of the brief description I posted there.

Brilliant! Thanks for the link. Hopefully, it will fix the issue on my machine

Cheers,
Dave

[TheNameOfTheGame]

ah, ok, I'll look into it. Ouch is it really 12euros for a board with Aisler? jlbpcb is $2 for 5 I think.

Imported the .brd into Aisler and it reported an error:

aisler.png

Next, I tried to bring the brd and sch into eagle, but I only have 6.20 and it says your design is from a newer version.

eagle.png

So no luck for me. Any way to get jlpcb compatible project files?

[czietz]

Any way to get jlpcb compatible project files?

Ask someone to make whatever file format JLPCB prefers. The project is CC-BY-SA-licensed, so people are free to do this.

EDIT: Or go to another PCB manufacturer, like OSH Park, where 3 of these boards cost 0.70 USD.

[tzok]

Here you have the CAM (Gerber) files, which are required by most PCB fabs:

atari_ste_mono_signal_conditioner_GERBER.zip

[TheNameOfTheGame]

Here you have the CAM (Gerber) files, which are required by most PCB fabs: atari_ste_mono_signal_conditioner_GERBER.zip

Great, thanks so much. Got an order in for 5 boards at jlpcb.

[TheNameOfTheGame]

The boards and parts arrived and finally found a moment to build and install on a spare STE.

monovideomod_markup.png

I can say this mod makes a very noticeable difference. Can't see any lines in mono after install.

monoscreen.png

So @czietz I'm wondering if another board be put on the COLOR line? I see on the schematic that the COLOR line goes through a 5.8k resistor. Would it be enough to change the 1K resistor on the board to a 5.8k one?

shiftercolorline_markup.png

*EDIT* Switched to color output via the ST2VGA adpapter. Might be my imagination but I swear it has a better picture now. Is there some way buffering the mono signal could have helped the color output signal also?