Shifter video interference

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exxos
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Shifter video interference

Postby exxos » Thu Feb 19, 2015 10:34 pm

I have seen this problem more noticeable on my small LCD monitor than anything. Though A few people have mentioned interference to me over the past few months also. So I started this thread to start work on solving these problems.

What I have found is there is a lot of noise on the RGB lines. I first tried adding various capacitors and resistors to smooth it out. which it did, but it also made the image look a bit blurred. The shifter itself seems to be the cause of the problems. I think ideally the RGB outputs need buffering with a Schmitt buffer to clean up the lines before they go into the transistor drivers. Its not a easy task exactly, but doable.

There is also noise on the 5V rail as it travels from each RGB driver transistor. A 1uF capacitor on the collector of Q3 to 0V cleans the 5V line up, but does not really help much with video quality.

So far, I have swapped out the 2N3904's with a generic BC548 transistor and this greatly reduced the problem. So it is something for people to try here as it's a easy fix to see if it helps people with such problems. I have tried a BC337 but there wasn't much different between that and the 2N3904's.
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Re: Shifter video interference

Postby mc6809e » Thu Feb 19, 2015 11:53 pm

Maybe the noise is being introduced by shifter itself. There could be noise on the power/ground to shifter. Could there be a failing decoupling cap somewhere?

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Re: Shifter video interference

Postby exxos » Fri Feb 20, 2015 9:29 am

mc6809e wrote:Maybe the noise is being introduced by shifter itself. There could be noise on the power/ground to shifter. Could there be a failing decoupling cap somewhere?


Thats what I mean, the noise is coming from the shifter itself, the supply is stable. But the signal output on the RGB lines isn't stable, it bounces on the HI's up to half a volt. It looks like 32mhz noise on the HI's. this would explain the odd patten generated on the screens , like the green desktop has a patten of noise across it. I will take and take some images later.
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Re: Shifter video interference

Postby troed » Fri Feb 20, 2015 10:41 am

exxos wrote:
mc6809e wrote:Maybe the noise is being introduced by shifter itself. There could be noise on the power/ground to shifter. Could there be a failing decoupling cap somewhere?


Thats what I mean, the noise is coming from the shifter itself, the supply is stable. But the signal output on the RGB lines isn't stable, it bounces on the HI's up to half a volt. It looks like 32mhz noise on the HI's. this would explain the odd patten generated on the screens , like the green desktop has a patten of noise across it. I will take and take some images later.


IIRC correctly the STE always had more ghosting on RGB compared to the STF. This was "known" back then.

But I'm not sure that ghosting is what you mean?

/Troed

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Re: Shifter video interference

Postby exxos » Fri Feb 20, 2015 11:06 am

Ghosting seems to happen when the voltage is a fraction to low on the RGB transistors. I have replicated that during my tests yesterday.

shifter_op.png

That is the a output on one of the RGB lines. The noise is actually 32mhz, and is not on the supply rail. So the noise is being generated internally most likely. The only way to smooth it out is to buffer all the RGB lines.

The image problems..

noise.JPG


After changing the switchers to BC548..

noise2.jpg


Still not perfect but does improve things a lot. So defiantly on the right track.
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Re: Shifter video interference

Postby exxos » Fri Feb 20, 2015 11:25 am

Looking though the video circuits..

The ST schematic it uses 15K,7.5K,3.6K resistors to feed the base of the transistors.
The STFM uses 3.9K, 2K, 1K.
The STE uses 12K, 9K, 1.5K.

The STE is not a totally fair comparison as its has its own version of the shifter. Though has anyone noticed ghosting on the original ST video ?

I would make a guess that if those STE resistors values were lowered. The ghosting problems might go away. Maybe someone can verify that ? Maybe 9K, 6.75K, 1.125K (25% less value over original).

In anycase, Its not the STE I am working with, its the STFM. Totally different problems to the ones I am referring to :)
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Re: Shifter video interference

Postby dlfrsilver » Sat Feb 21, 2015 1:22 pm

exxos wrote:Looking though the video circuits..

The ST schematic it uses 15K,7.5K,3.6K resistors to feed the base of the transistors.
The STFM uses 3.9K, 2K, 1K.
The STE uses 12K, 9K, 1.5K.

The STE is not a totally fair comparison as its has its own version of the shifter. Though has anyone noticed ghosting on the original ST video ?

I would make a guess that if those STE resistors values were lowered. The ghosting problems might go away. Maybe someone can verify that ? Maybe 9K, 6.75K, 1.125K (25% less value over original).

In anycase, Its not the STE I am working with, its the STFM. Totally different problems to the ones I am referring to :)


Hello Chris :)

Now that you mention it, all my STs, either STF or STE have some kind of "waves" rolling over the screen, from the left of the screen to the right, and this after reworking all the PSUs.

It would be nice indeed if you find a solution for this, in order to get ride of this effect on screen..... I'd be pleased to correct this effect on my computers :)
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Re: Shifter video interference

Postby exxos » Sat Feb 21, 2015 2:30 pm

dlfrsilver wrote:
Hello Chris :)

Now that you mention it, all my STs, either STF or STE have some kind of "waves" rolling over the screen, from the left of the screen to the right, and this after reworking all the PSUs.

It would be nice indeed if you find a solution for this, in order to get ride of this effect on screen..... I'd be pleased to correct this effect on my computers :)


The ST STF computers have a part of the video circuit missing which causes "dot crawl" on the screen. The STFM's do not suffer from that problem. But the images I posted I would imagine most ST's in all variants suffer from.

I haven't looked at the STE yet, but noticed some problems on the MEGA1 video. Though that one seemed to be supply rail noise. But after changing the PSU it solved it. So the video suffers from a few problems. I think its solvable on the STFM's as I have found what the problem is. Probably similar issues the STE. Its on my to do list of things to fix :)
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Re: Shifter video interference

Postby paul92706 » Sat Feb 21, 2015 5:01 pm

Hi Chris, I emailed you about a few months ago about having this same problem with my TT030, video interference. I see it looks like you made nice progress on your pictures. Could this be implemented on a TT030 also? after a while that interference gets on my nerves i would love to fix this issue, anyhow great work buddy!
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Re: Shifter video interference

Postby exxos » Sat Feb 21, 2015 5:21 pm

paul92706 wrote:Hi Chris, I emailed you about a few months ago about having this same problem with my TT030, video interference. I see it looks like you made nice progress on your pictures. Could this be implemented on a TT030 also? after a while that interference gets on my nerves i would love to fix this issue, anyhow great work buddy!


Yeah I think I remember you mentioning it :) various machines suffer from all sorts of problems. First is to check the supply is smooth as that will be a common problem. I've got 10uF accross my shifter (SMT ceramic) and it makes the supply solid. Though bad PSU's do not help the video circuit by far. Buffering the shifter's outputs will cure the noise and the related patterns on the screen as shown in my images. I need to build a buffer board to look into the IMP chip issues also. So will also try it on the shifter after.
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Re: Shifter video interference

Postby paul92706 » Sat Feb 21, 2015 8:39 pm

exxos wrote:
paul92706 wrote:Hi Chris, I emailed you about a few months ago about having this same problem with my TT030, video interference. I see it looks like you made nice progress on your pictures. Could this be implemented on a TT030 also? after a while that interference gets on my nerves i would love to fix this issue, anyhow great work buddy!


Yeah I think I remember you mentioning it :) various machines suffer from all sorts of problems. First is to check the supply is smooth as that will be a common problem. I've got 10uF accross my shifter (SMT ceramic) and it makes the supply solid. Though bad PSU's do not help the video circuit by far. Buffering the shifter's outputs will cure the noise and the related patterns on the screen as shown in my images. I need to build a buffer board to look into the IMP chip issues also. So will also try it on the shifter after.

I actually just installed a new PC power supply to my TT, with a little moding and cutting of extra wires, and tweaking, it all worked out great, so its basically a new PSU in my TT. I did notice the problem did fix a very little with new PSU but the interference is still present. Can't wait till you get that buffer board going as i can test it on my TT and report results here :-) keep up your good work buddy! :D
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Re: Shifter video interference

Postby exxos » Sat Feb 21, 2015 9:14 pm

Do you get a moving type distortion on the image, or is it a still distortion like in my images ? :cheers: You could probably scope the supply rail on the shifter see whats there, also scope pins 27,28,29 as they are the red outputs and see what you get there. If you can get a screenshot of the problem it would possibly help too :)
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Re: Shifter video interference

Postby paul92706 » Sat Feb 21, 2015 10:31 pm

exxos wrote:Do you get a moving type distortion on the image, or is it a still distortion like in my images ? :cheers: You could probably scope the supply rail on the shifter see whats there, also scope pins 27,28,29 as they are the red outputs and see what you get there. If you can get a screenshot of the problem it would possibly help too :)

Its pretty much distortion like on your images, its funny cause on the lighter colors it isn't as bad, but its more visable on darker colors, on White its Good!(not visable) and Black of course is also Good (not visable). As you can see on examples on pictures. I am waiting for my new scope to come in from ebay, as soon as i get it i will take some measurements.
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Re: Shifter video interference

Postby troed » Sun Feb 22, 2015 9:42 am

paul92706 wrote:Its pretty much distortion like on your images, its funny cause on the lighter colors it isn't as bad, but its more visable on darker colors, on White its Good!(not visable) and Black of course is also Good (not visable). As you can see on examples on pictures. I am waiting for my new scope to come in from ebay, as soon as i get it i will take some measurements.


That's down to the cable. I'm lucky to have a flat screen capable of 15kHz RGB input so I've created my own switchable color+mono cable. However, the well shielded cable I was planning on using turned out to be a nightmare to solder to the connectors and so I had to opt for a cheap cable with plain wires throughout - no shielding. I get exactly the same vertical bar banding as you have.

exxos' interference seems to be different.

/Troed

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Re: Shifter video interference

Postby exxos » Sun Feb 22, 2015 11:06 am

White and black are fine on mine. My problems seems to show on the green desktop more than anything. Though if I add a some pF onto the Red driver line, it goes away. Well not totally. But like I said in my first post, adding capacitors smooths out the digital waveforms shown in my images, the picture is better, but the capacitors add delays which causes other problems like blurring & ghosting. Capacitors prove cleaning up the digital line is the way to go. Cleaning them up with a buffer will solve the problem.

Only way is to try the shifter outputs on a scope, you can also try the outputs of your cable to see whats there compared to whats being outputted from the ST end. I think on my girlfriends large 32" LCD, I think the white also suffers , but that is on a different ST anyway.

Like everything there is normally more than one problem. Though if the shifter output is bad to start with, all other problems become secondary. As I still have not got my 4MB shifter adapters made yet, I will looking into adding the buffers on there. Then the board has a dual purpose :)
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