Dirty video on 6128 Plus RGB Scart

[endangermice]

Hi Guys,


I've just hooked up my Plus with RGB cable to my TV and I'm a bit disappointed with the display. The colours are not as clean or solid as they are on my CPC 6128. If you look at the blue background on the attached screenshot (a little bit hard to see because the camera seems to try filtering it out - but look to the bottom!). With the naked eye I would describe it as broken up by different shades of blue in vertical lines across the screen and is very noticeable - looks like a poor quality composite signal. This does not happen on the CPC and everything is solid.


I made the leads for both machines using exactly the same components apart from the differing DIN and wires for the plus. Everything is grounded correctly. I've made many leads for all sorts of machines in the past and this is the first time I have seen this.


Is this a known issue on the Plus? Is there anything I can do to fix it? It might be the failure of a cap or something on the motherboard but I'd like to know first whether or not this is normal before I start poking around inside!


Cheers,


Damien.

[Bryce]

Call me blind, but I can't see anything wrong there.

Bryce.

[endangermice]

I know it doesn't show up properly in the picture, I think the IPhone does some sort of funky filtering. I'll try another one with my D700.

[arnoldemu]

I know exactly what you mean and I see it on my plus monitor.
I think the gx4000 doesn't suffer from the same problem because it's clock has a different speed and the resolution is then more cleaner.

[Bryce]

The GX4000 is a completely different matter, because it has a proper SCART driver and circuitry.

Bryce.
(Still disappointed because I thought the "Dirty Video" was going to feature Georgia Salpa)

[arnoldemu]

The GX4000 is a completely different matter, because it has a proper SCART driver and circuitry.

Bryce.
(Still disappointed because I thought the "Dirty Video" was going to feature Georgia Salpa)

I was thinking the gx4000 connected to a colour monitor would not show it.

[endangermice]

(Still disappointed because I thought the "Dirty Video" was going to feature Georgia Salpa)

Hehe, sorry to dissapoint you! I thought the GX4000 clock speed only affected the composite video not the RGB. The CPC though is fine which I find odd seeing as it's older hardware.

I've taken a better picture using my Nikon, it shows the problem better. Oddly I've had to link to the picture from my webspace since the forum is now rejecting uploads due to a security check failure!?

I'm afraid I'm a bit of a stickler when it comes to video cleanliness, mainly I suspect becuase I work in the industry  .

Anyway this is what I'm talking about....

 
Don't get me wrong, I do like some streaky things, bacon being one of them, but alas video is not!
 

[arnoldemu]

Yes I see exactly that on mine. I don't see bacon though.

[endangermice]

No the bacon is only seen on very special occasions and usually only at the breakfast table!


Cool sounds like it is just how the Plus outputs video down the SCART, a bit of a shame considering the CPC does it much better!


Bryce are you able to confirm that you see the same issue on yours? Being the electronics genius that you are, any suggestions on how it might be improved or is this what comes directly from the ASIC?

[Bryce]

Strange. The blue on the border is fine, but not in the main window?

I have always had my Plus connected through my S-Video adapter, which definitely doesn't have this effect. I will measure the outputs on mine at the weekend and see where the problem lies. It looks like the Green signal isn't being pulled low enough. So the Blue isn't pure blue, but Blue/Green. But why doesn't it happen on the border too?

Bryce.


Edit: Silly experiment: If you change the text colour to the same blue as the background and do a CLS, is the background still this green or is it now normal blue?

[TotO]

Strange. The blue on the border is fine, but not in the main window?

You got the same effect on both, but it's more visible on lighten colours.

[endangermice]

Toto, you beat me to it . Yep exactly it is also present on the blue border, but the camera didn't pick it up very well. trying to shoot screen displays is really difficult, but it is there only less pronounced.


Sounds like this is a common problem, I find it very strange and also odd that your SVHS adapter seems to eliminate it. I presume it's taking the RBG video then multiplexing it into a chroma and luminance signal in which case unless it filters too I can't see how the conversion would eliminate the problem which might suggest your plus isn't exhibiting it....

[TotO]

What RGB cable are you using ?
Is, the video grounds are separated from others ?
What is the length of this cable ?
Do you get a better result by "touching/moving" the CPC video connector ?

[Bryce]

The S-Video circuit uses an AD724 to do all the RGB mixing trickery, so I'm not sure exactly what it does as far as filtering is concerned.

You could try a few things to find out where it's coming from:

1) Change text and background to pure blue and CLS. Then check pins 2,3,4,5,6,7,8 and 9 of IC115 (AMS48464). They should all be 0. If not, it's the ASIC that's generating the strange colour.
2) If all are 0 (as I suspect), then the problem is leakage between the analogue signals. The RGB outputs from IC115 are pins 15,16 and 17 - B, G and R. With no monitor connected and the CPC turned off, the resistance between 15 and 16 should be about 7.5K, the resistance between 16 and 17 should be about 7.2K and the resistance between 15 and 17 should be about 8.2K - Check if that is what you are getting.

If the values are lower, then you are getting signal leakage from one colour to the other.

Bryce.

Edit: good point TotO, if the cable is too long, you might be getting cross-talk in the cable.

[endangermice]

I'm using a cable I made myself. It's not individually screened but there is a screen around all 9 wires which is connected to the connector screens on both ends. I'm using exactly the same components that I have used to make cables for my CPC, Archimedes, Amiga and C64 (although C64 is SVHS). The cable is 2m long.


It works just fine on all the other computers and doesn't look like interference since the pattern is completely constant and is not in anyway affected by the sound.


Touching the ground doesn't help which suggests that the grounding is ok. It literally looks like some sort of filtering issue or lack of filtering on the signal. Apart from that it looks really good, sharp and clear just streaky in the solid colours!


I take your point about cable length, but these cables are professionally sourced, we use them in the office all the time and I've used them to make several RGB cables for other computers without issue so I don't think it is the cable but rather something funky at the Plus end....


I'll check out those voltages and let you know how I get on - thanks for the advice guys it's really appreciated .

[Bryce]

Hold on. Is it just the streaks, or is the colour also changed? The picture you've posted, looks like the window is almost touquise and not blue as it should be.

Bryce.

[endangermice]

No only the steaks. The colour discrepancies will be due to ambient light in the room and me not colour balancing the camera image. It was just a quick and dirty shot to show the streaking. Colour is absolutely fine and consistent with my CPC.


I presume you mean ic numbered 40464 not 48464 and that it's number 15 not 115..?

[arnoldemu]

Hold on. Is it just the streaks, or is the colour also changed? The picture you've posted, looks like the window is almost touquise and not blue as it should be.

Bryce.

I get the same display on a 6128 plus connected to an official Plus monitor, with no other cables between.

[Bryce]

Ah, ok, then that's an easy one - Yes, every Plus does that (except mine, because I've modded that bug out ages ago ). The streaks are due to the fact, that the cost cutting idiots at Amstrad in their infinite wisdom decided to delete the RGB output capacitors on the Plus. Add a 220pf capacitor to each colour signal (between colour signal and ground) and the streaks will disappear. Any higher than 220pf and the colours will run into each other and make the picture blurred.

Bryce.

Edit: Here you can see the difference:

[endangermice]

Typical - why miss these out? It's crazy they cost a few cents each. On well you live and learn. Bryce - thank you for coming to my rescue again! Hopefully this thread is going to help a lot of people who have had similar issues. I suspect arnoldemu, toto and I will be making a trip to our local component suppliers very shortly!

[Bryce]

Before you go out and buy any parts, let me check exactly what values I used. I remember experimenting a bit before I decided which value to use, but I can't remember what the final choice was.

Bryce.

[TotO]

I though that all your custom RGB video cables already got capacitors inside. :p

[Gryzor]

Typical - why miss these out? It's crazy they cost a few cents each.

A few cents here, a few cents there, plus the fact that probably nobody would notice it then and you get the idea.

[endangermice]

Cool no probs, no rush

[MacDeath]

the cost cutting idiots at Amstrad

why plurial...only one... Sir Alan himself.