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New "Active" Scart Cable Solves "Ghost Shadow" Image Problem on TFT Monitor

Started by LambdaMikel, 07:07, 16 March 18

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LambdaMikel

To connect my CPC's to the TFT monitor, I am using a SCART to HDMI Converter that is RGB-capable (the Scart input is - some aren't!):

https://www.amazon.com/CiBest-Converter-Adapter-Support-Set-top/dp/B06Y43RVLH/ref=sr_1_1_sspa?ie=UTF8&qid=1521180054&sr=8-1-spons&keywords=hdmi+scart&psc=1

Unfortunately, with the standard CPC-SCART-Cable that has the capacitor soldered in

http://www.cpcwiki.eu/index.php/File:Alt_Scart_Connect.PNG

I always got a "ghost shadow" picture. For it to go away and have a clear picture without ghost shadow, I always needed to power my CPC with 6 V. I did that for my videos etc., but only then, because I didn't want to damage the CPC (even though 6 V should be relatively safe).

Now I have ordered this "active" cable

https://coolnovelties.co.uk/coolnovelties/amstrad-video-cables/258-amstrad-cpc-464-6128-active-rgb-scart-cable.html?search_query=cpc+cable&results=15

and indeed, this fixes the "ghost shadow" problem @ 5 Volts! I am getting a perfect picture now. JFYI in case you are having a similar problem.

Bryce

But the ghosting has nothing to do with the capacitor/active part of the cable. The ghosting is due to parasitic capacitance and inductance on the RGB lines. By lengthening or shortening the entire cable you can "tune" the ghosting in and out. The new cable you've bought just happens to have got the length right for your setup. Try converting your own cable to "active" and the ghosting will still be there.

Powering a CPC from 6V is a really bad idea. Most of the ICs are only designed to work up to 5.25V, after that they start letting the magic smoke out.

Bryce.

The Equalizor

What Bryce said.


I have one of these exact units that I hacked into my 4128Plus and the picture is perfect. They're the best solution I've found for Amstrad RGB---> HDMI, those GB8200 things aren't a patch on these SCART adaptors...


Rob

LambdaMikel

Thanks for clarification.
Then this happens to be a well-designed cable that at least on my setup, resolves the "ghost image" problem.
The other cable was from a large cable seller on Ebay.de.

Audronic


@LambdaMikel

Have a look here (Just add 1 resistor)
Scroll down a bit This may work as i have modified the one you have purchased. and it WORKS



http://www.cpcwiki.eu/forum/hardware-related/has-anybody-used-hdmi-and-an-lcd-monitor-on-a-cpc-6128/msg119260/#msg119260


Good Luck


Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

LambdaMikel

Great, thanks for the link, wasn't aware of the thread!
I could do the modification and check if my other cable works then.
But the active cable above doesn't need the modification, so I'll
probably just keep using this from now on. Thanks anyway.

chinnyhill10

--
ChinnyVision - Reviews Of Classic Games Using Original Hardware
chinnyhill10 - YouTube

LambdaMikel

Quote from: chinnyhill10 on 02:46, 17 March 18

Really terrible idea to use 6v.

Guys, can you all read? I think it's a really terrible idea to waste your time telling me that over and over again when I admitted it at the top of the post myself.

I am telling you what was a REALLY terrible idea - to fry my CPC with 12 V. After all, only the RAM was damaged - this one completely though  ;D

Check out this one, you will feel better
http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/killed-my-cpc-464-help-with-diagnosis-please/

gotcha

I have the same 'ghosting' issue when using a 'passive' cable with a small LCD TV that I use as monitor.

I tried 2 different passive cables with a capacitor (the state of the art passive cable as far as I know).
One of the cable uses a 47uF capa, while the other uses a 100uF capa. The strenght of the ghosting depends on the capacitor: it is stronger with 100uF than with 47uf.

I finally decided to spend some time analyzing the issue.

For the passive cable with the 100uF capacitor, I've got a Scart 'RGB Blank' signal fluctuating between 800mv and 850mv when plugged into the TV. The Scart standard tells that RGB is active when when 'RGB Blank' >1v, so I assume that the ghosting is the result of a RGB mode not properly triggered.

Pushing further the analysis:

       
  • Without any cable, the 6128 outputs a sync signal at 4.8v when high.
  • With the Scart cable plugged, but not connected to the TV, 'RGB Blank' signal is 4.3v (I guess we loose 0.5v due to the capacitor that averages sync)
If the R137 resistor of the CPC was 220ohms as it should be and if the TV 'RGB Blank' input had an impedance of 75ohms as it should also be, I would have expected a 'RGB Blank' signal a bit greated than 1v when plugged in the TV,. But as I said earlier, I measure a signal between 800mv and 850mv. Then, either the CPC has a too high R137 resistor for the sync signal, or the TV has a too low impendancefor the 'RGB Blank' input, or both ... In any case, 'RGB blank' is lower than 1v what is not good.

At the end, the only way I've got a nice image is with an 'active' cable that uses the 5v from the power supply to set the 'RGB Blank' signal of the Scart. I think the passive cables approach (that try to use sync and a capacitor to set the RGB mode) brings the 'RGB blank' signal of the scart too close to the 1v in best case, and lower than 1v in many cases. I then don't think that it's a reliable solution.

gotcha

Note that in my case, if I was powering my CPC at 6v, I would get a 'RGB blank' signal around 1v and I guess the ghosting would disappear too. It's consistent with what LambdaMikel experimented.
But I will not make a try   ;D

LambdaMikel

Quote from: gotcha on 16:06, 30 September 18
Note that in my case, if I was powering my CPC at 6v
Dude, that is SUCH A BAD IDEA how can you do that, your CPC will explode ;D ;)

gerald

Quote from: gotcha on 15:20, 30 September 18
I have the same 'ghosting' issue when using a 'passive' cable with a small LCD TV that I use as monitor.

I tried 2 different passive cables with a capacitor (the state of the art passive cable as far as I know).
One of the cable uses a 47uF capa, while the other uses a 100uF capa. The strenght of the ghosting depends on the capacitor: it is stronger with 100uF than with 47uf.

I finally decided to spend some time analyzing the issue.

For the passive cable with the 100uF capacitor, I've got a Scart 'RGB Blank' signal fluctuating between 800mv and 850mv when plugged into the TV. The Scart standard tells that RGB is active when when 'RGB Blank' >1v, so I assume that the ghosting is the result of a RGB mode not properly triggered.

Pushing further the analysis:

       
  • Without any cable, the 6128 outputs a sync signal at 4.8v when high.
  • With the Scart cable plugged, but not connected to the TV, 'RGB Blank' signal is 4.3v (I guess we loose 0.5v due to the capacitor that averages sync)
If the R137 resistor of the CPC was 220ohms as it should be and if the TV 'RGB Blank' input had an impedance of 75ohms as it should also be, I would have expected a 'RGB Blank' signal a bit greated than 1v when plugged in the TV,. But as I said earlier, I measure a signal between 800mv and 850mv. Then, either the CPC has a too high R137 resistor for the sync signal, or the TV has a too low impendancefor the 'RGB Blank' input, or both ... In any case, 'RGB blank' is lower than 1v what is not good.

At the end, the only way I've got a nice image is with an 'active' cable that uses the 5v from the power supply to set the 'RGB Blank' signal of the Scart. I think the passive cables approach (that try to use sync and a capacitor to set the RGB mode) brings the 'RGB blank' signal of the scart too close to the 1v in best case, and lower than 1v in many cases. I then don't think that it's a reliable solution.
What about using the NSYNC signal (5V) and a diode + capa

NSYNC o-----|>|-+-----o RGB Blank
                |
               ---
               ---
                |
  GND o---------+-----o

gotcha

Quote from: gerald on 17:57, 30 September 18
What about using the NSYNC signal (5V) and a diode + capa
This is funny you propose this because I already did a try in the morning :)
But I got a lower signal on RGB Blank (between 680mv and 730mv), because the impedance of the TV RGB blank input is low : 75ohm (maybe even less with my TV).

The actual schematics is the following (220ohm inside the CPC and 75ohms of impedance in the TV)


NSYNC o-- 220ohms ----|>|-+-----o RGB Blank
                          |     |
                         ---    |
                         ---   75 ohms
                          |     |
  GND o-------------------+-----o


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