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

Troubles with your machine? Just want to speak about the latest improvements? This is the place!

Moderators: Mug UK, Zorro 2, spiny, Greenious, Moderator Team

czietz
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Sat Sep 29, 2018 3:45 pm

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
You do not have the required permissions to view the files attached to this post.

mpattonm
Hardware Guru
Hardware Guru
Posts: 331
Joined: Mon Oct 21, 2002 8:52 am
Location: Czech republic
Contact:

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

Postby mpattonm » Sun Sep 30, 2018 7:32 am

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

czietz
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Sun Sep 30, 2018 7:49 am

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
Captain Atari
Captain Atari
Posts: 460
Joined: Wed Oct 24, 2007 7:52 pm
Location: France
Contact:

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

Postby BlankVector » Sun Sep 30, 2018 9:28 am

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.
Subscribe to my Vretrocomputing channel on YouTube and Facebook.

czietz
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Sun Sep 30, 2018 9:51 am

BlankVector wrote: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.

BlankVector wrote: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
Hardware Guru
Hardware Guru
Posts: 3589
Joined: Sat May 29, 2004 7:52 pm
Contact:

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

Postby ijor » Mon Oct 01, 2018 6:55 pm

czietz wrote: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
Captain Atari
Captain Atari
Posts: 237
Joined: Sat Jul 02, 2011 7:18 pm

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

Postby Silly_Pony » Sat Oct 06, 2018 3:59 am

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.
1040STe, 1040STf

czietz
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Sat Oct 13, 2018 11:32 am

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

czietz
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Mon Oct 15, 2018 4:41 pm

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
Hardware Guru
Hardware Guru
Posts: 773
Joined: Tue May 24, 2016 6:47 pm

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

Postby czietz » Mon Oct 15, 2018 5:30 pm

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
You do not have the required permissions to view the files attached to this post.


Social Media

     

Return to “Hardware”

Who is online

Users browsing this forum: No registered users and 16 guests