Bouncing screen on STF with RGB Scart

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Bouncing screen on STF with RGB Scart

Postby EstTeeEfEm » Fri Oct 16, 2015 7:31 pm

Ok, after purchasing a proper RGB cable for my STF and hooking up to a CRT TV, I have a very good picture (aside from the problem itself) and the sound works fine.
I connected to the actual RGB scart, the other two do not show anything and the desktop loads fine, i can play games and the sysinfo stuff shows exactly what the machine is 1Mb / TOS 1.0 Atari STF

Basically, the screen bounces... all the time...
it only bounces like a centimetre up and down but its really annoying! Has anyone else experienced this before? If so, what can you recommend to fix this?

Thanks in advance (One day ill get everything sorted!)

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Re: Bouncing screen on STF with RGB Scart

Postby siriushardware » Fri Oct 16, 2015 8:19 pm

Couple of things: Are you able to manually choose between RGB and Composite Video mode on your SCART input? Assuming you are using RGB now, what happens when you choose Composite? (The picture will be slightly less good than RGB, but does it clear up the picture stability problem)?

The composite signal should be present in your cable because it is usually used even in RGB input mode to provide the composite sync signal required by SCART RGB inputs. It might be that the maker of your cable has used some sort of passive network (a couple of diodes or whatever) to create the composite sync signal from the separate vsync and hsync outputs on the ST. I wouldn't do it that way because the sync outputs on the ST don't have a lot of drive power and the composite sync / composite video input on a scart socket usually has a resistance of 75R from the input down to ground.

Composite video outputs are designed to drive into a 75R load, but the two sync outputs on the ST are not.

Second, a lot of old TVs didn't get on well with the video signals generated by VCRs when they were replaying from tape and produced an unstable picture even though they worked perfectly with off-air signals.

Some makers, notably Philips, often had a special 'VCR' mode switch which could be engaged to improve the picture stability during replay from VCRs. My Philips CM8852 monitor (which is the main colour monitor I use with my STs) has exactly this type of switch on the rear panel beside the SCART socket.

If your TV has such a switch, then switching it from the state it is in now to the opposite state may clear up your problem.

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Re: Bouncing screen on STF with RGB Scart

Postby EstTeeEfEm » Fri Oct 16, 2015 8:29 pm

I know the composite signal is present because my STFM works without a problem, but this STF shows no picture that way. The guy I got it from sounds like he's had a lot of experience building these different types of cables so I couldnt presume he'd forgotten anything or added anything extra.
I'd assume he used the extra resistors too from what I can gather. He does produce separate scart cables for STFM's and STE's knowing that they have an RF present.

If your TV has such a switch, then switching it from the state it is in now to the opposite state may clear up your problem.


It has no such switch and is a SHARP if that makes any difference?

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Re: Bouncing screen on STF with RGB Scart

Postby simonsunnyboy » Sat Oct 17, 2015 1:03 pm

STF machines do not have a composite video signal, only RGB. IIRC composite video is only available on machines equipped with TV modulator like the STE or STFM.
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Re: Bouncing screen on STF with RGB Scart

Postby siriushardware » Tue Nov 03, 2015 8:03 pm

simonsunnyboy wrote:STF machines do not have a composite video signal, only RGB. IIRC composite video is only available on machines equipped with TV modulator like the STE or STFM.


Ah, sorry, I missed that it was an STF, not an STFM. In part, I think I was misled by the OP's 'handle' :-)

STF, that means it has composite sync coming out of the pin which, on an STFM, would be composite video? EstTeeEfEm, have you got a scope? Can you have a look at the sync signal to see what size it is, whether it looks OK?

This diagram is a section which includes both the modulator section (as a self contained box, not fitted in your STF) and the sync combiner Q12 which is fitted instead when there is no modulator present. This combines the motherboard VSYNC and HSYNC signals which are then output as composite sync to pin 2 of the monitor socket instead of composite video.

STF_STFM_Sync.jpg


Vertical instability on an otherwise OK picture suggests there's something not quite right with the sync - when it's jumping, what happens if you tweak the vertical hold control on the TV - (does it even have one?) Too far the wrong way and the picture will start rolling upwards or downwards, but a small adjustment in the right direction might stabilise it. Is your TV equally happy to work on 50Hz or 60Hz? Does the framerate make any difference to your problem?
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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Mon Dec 07, 2015 11:04 pm

Hi,

If you are handy with a soldering iron, add a 330 to 510 ohm resistor, in series, with CSYNC signal from pin 2 of the DIN connector.
With the STFM design and the TV termination, you are injecting approximately 3.5V into a device expecting at most a 1.4V signal. Not good for the long term health of the TV.

However composite sync is created, you must never feed Digital TTL signals into a SCART socket, they are too big for the Analogue Csync/composite input on pin 20 of the SCART socket. Google Atari ST SCART cable and you'll find the modified SCART cable design of mine that fixed all the Amiga issues, the same applies to Atari machines.

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Re: Bouncing screen on STF with RGB Scart

Postby exxos » Mon Dec 07, 2015 11:23 pm

stedy wrote:Hi,

If you are handy with a soldering iron, add a 330 to 510 ohm resistor, in series, with CSYNC signal from pin 2 of the DIN connector.
With the STFM design and the TV termination, you are injecting approximately 3.5V into a device expecting at most a 1.4V signal. Not good for the long term health of the TV.

However composite sync is created, you must never feed Digital TTL signals into a SCART socket, they are too big for the Analogue Csync/composite input on pin 20 of the SCART socket. Google Atari ST SCART cable and you'll find the modified SCART cable design of mine that fixed all the Amiga issues, the same applies to Atari machines.


That is interesting. I wonder if the voltage being to high are related to other video problems I have in this thread http://www.atari-forum.com/viewtopic.php?f=15&t=28859#p283632
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Re: Bouncing screen on STF with RGB Scart

Postby bartek030 » Tue Dec 08, 2015 10:15 am

Schematic for proper atari rgb (scart) cable:

http://info-coach.fr/atari/hardware/int ... itel_cable

Add 150 ohm resistors on R,G,B signals.
DO NOT add resistors on V-sync (blanking signal).

Boucing screen solution - add 100uf / 10v (or 16v, 25v) capacitor between Vsync (blanking) and groud (pin16 + / pin18 - in scart socket).
It works every time with any STF, STFM, STE even on new LCDs/Plasmas (in most cases LG are problematic).

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Re: Bouncing screen on STF with RGB Scart

Postby exxos » Tue Dec 08, 2015 1:24 pm

I did add resistors on my scart cable, not sure what value, but I guess its possible that the voltage is to high then it could be oscillating causing the noise in the display.. I will try and investigate this more...
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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Tue Dec 08, 2015 11:02 pm

Hi,

Try this SCART cable:
Image

This fixes all the levels, provides a ~2V signal on SCART PIN 16 to select RGB mode and porivides a simple means to work with machines with/without RF modulator.

I don't have an ST but does anyone have measurements of the Mono video on pin 11?

Ian

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Re: Bouncing screen on STF with RGB Scart

Postby siriushardware » Thu Dec 10, 2015 10:17 pm

I'm interested in your method of deriving the force-RGB signal from the resistor-limited (1.2K internal) +12V out on the Atari monitor socket - my Sharp LCD TV has no menu options to force RGB on on its SCART sockets and assumes Composite by default. Drives me nuts as I hate having to use composite when the source can supply RGB.

The ST isn't the only retro kit I use with it though, so I'm considering taking the TV apart and fitting a switch next to one of the SCARTS to force-supply the RGB enable signal to the SCART input locally, regardless of whether the source does or does not supply a suitable force-RGB signal down the lead.

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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Wed Dec 16, 2015 1:04 pm

Hi,

@siriushardware

I only created a simple potential divider to get a ~2V signal on the Fast Blank pin. Most retro systems have +5V somewhere and the TV has 10K to ground, so I can make the potential divider with only 1 resistor in the cable. I'm surprised all Atari SCART cables have this missing.

Do the colours look correct on the TV with your cable?

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Re: Bouncing screen on STF with RGB Scart

Postby siriushardware » Wed Dec 16, 2015 6:34 pm

My cable (self made, years and years ago) is much simpler than those most people seem to use - R, G, B are direct to the relevant pins on the SCART plug (no resistors). Sync is just the composite output from the STFM / STe fed into the composite video input of the scart socket. - The Philips CM8524 CRT monitor I've always used with the STe / STFM has a hardware input mode switch (RGB / Composite) - when in RGB mode it uses the composite sync portion of the composite video input signal as composite sync and ignores the video portion, instead using the R / G / B signals. The picture on the Philips has always been pretty OK in Composite mode, but fantastic in RGB mode.

I can't say what the colours would look like in RGB mode on the Sharp TV because I haven't tried your RGB mode force dodge yet, so I've only seen it when using composite video on the TV - the TV has a much larger screen so the lack of resolution and general fuzziness inherent in composite makes it look pretty poor. That's why I was considering applying your force RGB mod to my old lead, but it would only solve the problem for the STs and still leave me with the same problem for other retro kit which doesn't output any handy voltage to use for this purpose.

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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Sun Jan 17, 2016 11:27 pm

Hi,

Has anyone tried the SCART cable design I posted earlier?
I saw this image appear on the Atari Wiki, would be nice to remove the watermark if it successfully works.

Ian

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Re: Bouncing screen on STF with RGB Scart

Postby Atari030 » Mon Jan 18, 2016 2:09 am

If I can get a SCART plug (not common in Aussie) I'll give it a go with my LCD.

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Re: Bouncing screen on STF with RGB Scart

Postby walterg74 » Sat Sep 10, 2016 5:15 pm

stedy wrote:Hi,

If you are handy with a soldering iron, add a 330 to 510 ohm resistor, in series, with CSYNC signal from pin 2 of the DIN connector.
With the STFM design and the TV termination, you are injecting approximately 3.5V into a device expecting at most a 1.4V signal. Not good for the long term health of the TV.

However composite sync is created, you must never feed Digital TTL signals into a SCART socket, they are too big for the Analogue Csync/composite input on pin 20 of the SCART socket. Google Atari ST SCART cable and you'll find the modified SCART cable design of mine that fixed all the Amiga issues, the same applies to Atari machines.



Hi Ian,

Could I trouble you to take a look at this thread I made? Having an issue with the video cable with an STF and not really sure what the problem is or how to fix it.

http://atari-forum.com/viewtopic.php?f=15&t=30295


Thanks!

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Re: Bouncing screen on STF with RGB Scart

Postby czietz » Mon Sep 12, 2016 6:44 am

stedy wrote:Has anyone tried the SCART cable design I posted earlier?


Ian, I have some issues with the schematic, particularly with the voltage divider meant to supply pin 16 (fast blanking) with the RGB mode switch signal.

Note that the TV's input impedance on this pin is nowhere near 10K. In fact, according to the standard (EN 50049) it is 75 ohm. I just measured it: my TV's SCART input conforms to the standard, so the divider would not work as intended. Also note that the 12 V are passed through an 1.2 K series resistor inside the ST so you can't fix the problem by decreasing the resistor values of the divider.

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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Fri Oct 14, 2016 8:51 pm

czietz wrote:
stedy wrote:Has anyone tried the SCART cable design I posted earlier?


Ian, I have some issues with the schematic, particularly with the voltage divider meant to supply pin 16 (fast blanking) with the RGB mode switch signal.

Note that the TV's input impedance on this pin is nowhere near 10K. In fact, according to the standard (EN 50049) it is 75 ohm. I just measured it: my TV's SCART input conforms to the standard, so the divider would not work as intended. Also note that the 12 V are passed through an 1.2 K series resistor inside the ST so you can't fix the problem by decreasing the resistor values of the divider.


Another good plan foiled!
I measured two of my Tvs, they had around 10K on the fast blank input. I re-checked some reference designs from Analog Devices, one had the 10K input impedance, the other had 75 ohms!

I missed the 1.2K on the +12V signal, when I find my ST schematics, I'll see what I can do to work around this. Will have to play with a simulator or maybe a breadboard soon 8)

Thanks for the feedback.

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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Sun Oct 23, 2016 2:35 pm

Hi,

Updated SCART cable diagram, Image

Feedback appreciated, this should fix the level select issue, also added a note about the need for potentially changing the RGB output resistors.

I'll have some updated GBS-8200 diagrams shortly, will make one for the Atari ST/TT/Falcon.

Ian

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Re: Bouncing screen on STF with RGB Scart

Postby walterg74 » Sun Oct 30, 2016 9:18 pm

stedy wrote:Hi,

Updated SCART cable diagram, Image

Feedback appreciated, this should fix the level select issue, also added a note about the need for potentially changing the RGB output resistors.

I'll have some updated GBS-8200 diagrams shortly, will make one for the Atari ST/TT/Falcon.

Ian



Hi Ian, should this one work on an STf ? I could not get it to work with any of my cables, on a 1084S-D2 monitor (not with scart + scart/db-9 adapter, not with direct philips type cable), not with a PVM using scart cable.
Today I luckily acquired an SM124 and image came up fine, so Atari should be working correctly (side note, the othe monitors and cables work fine with my other Ataris).

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Re: Bouncing screen on STF with RGB Scart

Postby Oge » Thu Nov 03, 2016 9:40 pm

Rolling screen is also caused by the TV having problems with Blanking. Atari ST has a very rough +12V signal and I believe that a lot of issues would be solved just by feeding +12V by an external 100mA AC Adapter. I was going to offer this solution on a dedicated Cable but the lack of request for ST SCART Cables made me desist (and the Cable requests a lot of time itself).

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Re: Bouncing screen on STF with RGB Scart

Postby Maeke » Fri Nov 04, 2016 8:42 am

simonsunnyboy wrote:STF machines do not have a composite video signal, only RGB. IIRC composite video is only available on machines equipped with TV modulator like the STE or STFM.


Sorry to say that but not all ste have a modulator, by example french stes doesn't have one.

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Re: Bouncing screen on STF with RGB Scart

Postby simonsunnyboy » Fri Nov 04, 2016 4:51 pm

Maeke wrote:
simonsunnyboy wrote:STF machines do not have a composite video signal, only RGB. IIRC composite video is only available on machines equipped with TV modulator like the STE or STFM.


Sorry to say that but not all ste have a modulator, by example french stes doesn't have one.


I didn't imply "all STEs" if you mean that.
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Re: Bouncing screen on STF with RGB Scart

Postby rj1 » Mon Nov 07, 2016 8:41 pm

stedy wrote:Hi,

Updated SCART cable diagram, Image

Feedback appreciated, this should fix the level select issue, also added a note about the need for potentially changing the RGB output resistors.

I'll have some updated GBS-8200 diagrams shortly, will make one for the Atari ST/TT/Falcon.

Ian


I had similar issue on a Mega 1 as OP, where the display would jump diagonally by about 1-2cm every few seconds on one TV (Philips LCD) but would be fine on another TV (Sharp LCD).
I added a 1N4148 diode and a 10uF capacitor for the BLANKING SCART pin 16 as on your schematic and it solved the issue.

Thanks for this idea.

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Re: Bouncing screen on STF with RGB Scart

Postby stedy » Mon Nov 07, 2016 11:39 pm

rj1 wrote:I had similar issue on a Mega 1 as OP, where the display would jump diagonally by about 1-2cm every few seconds on one TV (Philips LCD) but would be fine on another TV (Sharp LCD).
I added a 1N4148 diode and a 10uF capacitor for the BLANKING SCART pin 16 as on your schematic and it solved the issue.


:D it appears to work.

walterg74 wrote:
Hi Ian, should this one work on an STf ? I could not get it to work with any of my cables, on a 1084S-D2 monitor (not with scart + scart/db-9 adapter, not with direct philips type cable), not with a PVM using scart cable.
Today I luckily acquired an SM124 and image came up fine, so Atari should be working correctly (side note, the othe monitors and cables work fine with my other Ataris).

Give it a try, the switch + 2 resistors on the CSYNC signal should cater for all eventualities.
A Sony PVM should work fine with Hsync/Vsync, they do tolerate a reasonable amount of variation on TTL Csync too.


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