Hello!
Long time lurker, first time poster.
I have a question about my 6128+ and Scart/RGB that I hope one of the more knowledgeable folks can guide me on.
I recently added a 6128+ to my collection after probably years of thinking about it. It's my first Amstrad and the first time I've used an Amstrad since I used to play Ikari Warriors on my school friends 646 back in the day.
When I power up the machine with Burnin' Rubber, I notice that there are vertical lines visible. I've tried the usual adjustments to improve the picture quality, lower contrast etc. but nothing seems to help. I've attached some pictures to illustrate what I mean. Is this normal with a 6128+
The cable used is a passive variety from Cool Novelties and the TV is a Samsung flat panel. I've used the TV with other RGB capable computers, Atari ST, Amiga, BBC Master and so on with no vertical bands present that I can see.
Would the display benefit from an active rather than a passive cable ? or is it likely the machine is faulty or needs some work? I guess it could also be the cable as I don't have a spare to prove that one way or the other. I thought it best to check incase this is a common thing. I did a search but didn't see anything obvious.
The only other TV in the house does not have a SCART connector so I am struggling to eliminate that part of the chain.
I would appreciate any help or guidance :)
Thanks,
Welcome aboard.
Unfortunately, there's nothing you can do about this as it's the plus machine's RGB working as normal. My Plus machines do this on several TV's and with several cables.
It's just not noticable on a CRT screen but on an LCD via SCART you do see it, and the bigger the TV, the more noticable it is.
I think it's something to do with the way the ASIC chip handles the RGB, but I'm open to correction on that.
One of those things you have to just live with. :)
Quote from: Shaun M. Neary on 15:41, 03 August 20
Unfortunately, there's nothing you can do about this as it's the plus machine's RGB working as normal. My Plus machines do this on several TV's and with several cables.
It's just not noticable on a CRT screen but on an LCD via SCART you do see it, and the bigger the TV, the more noticable it is.
I think it's something to do with the way the ASIC chip handles the RGB, but I'm open to correction on that.
One of those things you have to just live with.
People call them jailbars and I'm sure this is common to many plus models. I'm pretty sure @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) provided a suggested fix using capacitors on the DIN socket.
Quote from: tjohnson on 15:49, 03 August 20
People call them jailbars and I'm sure this is common to many plus models. I'm pretty sure @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) provided a suggested fix using capacitors on the DIN socket.
If anyone can fix it... he certainly can! :laugh: :laugh:
Thanks for the welcome and the quick reply!
Must admit, it's a nice stable picture other than the jailbars and i'm really looking forward to getting an M4 and playing some games :)
Look forward to hearing if Bryce has any fixes to suggest on the RGB.
Thanks again!
Quote from: Muddyfunster on 16:07, 03 August 20
Thanks for the welcome and the quick reply!
Must admit, it's a nice stable picture other than the jailbars and i'm really looking forward to getting an M4 and playing some games :)
Look forward to hearing if Bryce has any fixes to suggest on the RGB.
Thanks again!
The M4 is a nice piece of kit, just bear in mind that it can't handle protected games very well, so find the cracked versions instead. They work like a treat though and load in a second. :D
I did think of adding a gotek but I didn't want to swap the drive out on this particular example as it's near mint.
I've looked again and I can't find any reference to the capacitor fix.
That's cos' it's lost inside an unrelated thread:
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/limited-batch-of-new-s-video-adapters/msg61860/#msg61860
Bryce.
Thanks @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225)
Reading through the thread and the related thread [size=78%]https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/video-not-clean-on-6128-plus-rgb-scart/25/ (https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/video-not-clean-on-6128-plus-rgb-scart/25/)[/size] it's just a case of adding those 3 caps to improve the picture.
Is that something that can be done via a cable modification?
Sorry if that's a daft question :) , i'm not that experienced with the electrical side of things and i'm pretty lethal (to myself) with a soldering iron!
Yes, it could be done on the cable, or in the SCART connector. You just need to connect each capacitor between each of the RGB wires/pins and GND. This smoothes the colour signals and merges the "jailbars" into each other. You can increase the capacitor value slightly to merge further, but this also starts to make the picture unsharp.
Bryce.
Thanks @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) .
That sound simple enough even for me :)
I've tried the capacitors, and they didn't help for me.
The GX4000 doesn't have the same issue, there are some differences in the circuit (see attached picture), maybe R199 drops the voltage too much?
Quote from: Overkill on 14:04, 10 August 20
I've tried the capacitors, and they didn't help for me.
The GX4000 doesn't have the same issue, there are some differences in the circuit (see attached picture), maybe R199 drops the voltage too much?
DIN RGB signals are straight out of the DAC, so R199 will not make any difference (it's only used roughly isolating the B/W composite signals generation).
But I think the main difference and explanation for the difference is L3 choke present on the GX4000 that prevent high frequency noise on the 5V to reach the DAC supply.
There is no equivalent on the Plus DAC supply.
I have found the LUM output is 340mV on the Plus and 460mV on the GX4000, I understand the LUM is just used as a video sync, but wouldn't R199 also reduce the voltage to each of the RGB lines via R188?
R199 is supplying the bias voltage via R188 for Q103. This has almost no effect on the RGB values especially as the base pin of Q103 will ALWAYS be around 0.6V + the voltage drop across R180 (which is of course current dependent). You need to calculate the currents throughout the circuit to know exactly whether voltages are being reduced (or boosted) by each resistor.
Bryce.
I'm not an electronics expert, but it just makes sense that the difference in picture quality between the GX4000 and a Plus, must be something to do with the differences in that circuit, because isn't every thing else on the digital side? I can't see just the choke making that much of a difference, but I might be wrong :)
See GX4000 vs Plus, with the same PSU, SCART cable and TV.
But there are many differences between those circuits, one of the main ones being the IREF, which is regulated on the GX4000, but just a simple (drifting) resistor on the Plus. Then there's the inductor that gerald mentioned which isolates the AVDD from the spikes on the general VDD rail. Then there's the fact that the 5V voltage regulation is in the GX4000, whereas the Plus has no internal regulation. All of these things make a difference.
If you want to try to get the Plus looking more like the GX, try removing R170 and replace it with a 4K7 resistor and a 4K7 variable resistor in series. This way you can safely adjust/experiment with IREF.
Bryce.
Nice idea, what does the IREF do?
Quote from: Overkill on 15:16, 10 August 20
Nice idea, what does the IREF do?
IREF is the reference current for the DAC. It value sets the voltage span of the DAC.
If you want to pay, you can also just short L3 in the GX4000 and see the effect on the screen.
Yes, I did think about shorting out the inductor. The GX I was trying is brand-new, so I don't want to mess with it, but I have a few other GX's lying about somewhere.
Quote from: gerald on 16:27, 10 August 20
IREF is the reference current for the DAC. It value sets the voltage span of the DAC.
If you want to pay, you can also just short L3 in the GX4000 and see the effect on the screen.
I'm going to be really pedantic here: Shorting the inductor will still have an inductance, all be it extremely small. To truely "short" an inductor, you have to remove it completely from the circuit and replace it with a bridge. Ok... I'll go back to what I was doing.
Bryce.
The Plus range does have it's own inductor L101.
oh, I see the GX has a few inductors, some just look like resistors
Quote from: Overkill on 19:20, 10 August 20
oh, I see the GX has a few inductors, some just look like resistors
That's what they look like to AC too ;)
Bryce.
Don't forget that the GX4000 has a slightly slower master clock, specifically to improve the video output:
https://www.cpcwiki.eu/index.php/Arnold_V_Specs_Revised#PAL_subcarrier_locking (https://www.cpcwiki.eu/index.php/Arnold_V_Specs_Revised#PAL_subcarrier_locking)
DOH! Anyone got a few spare days to spend on an oscilloscope to find out what's really happening? :D
[/size]I wonder if any of the upscalers would produce a better picture?
Quote from: Overkill on 09:09, 11 August 20
DOH! Anyone got a few spare days to spend on an oscilloscope to find out what's really happening? :D
I wonder if any of the upscalers would produce a better picture?
1 - If you need a few days to measure something on a scope, then you seriously need to learn how to properly use a scope.
2 - What exactly do you want measured and what's it going to tell you?
Bryce.
I'm sure that someone with the knowledge, tools and time could identify exactly why the GX produces a perfect picture on TVs but the Plus doesn't. whether it be that IREF circuit, smoother power, slower clock speed or something else.
Quote from: Overkill on 11:59, 11 August 20
I'm sure that someone with the knowledge, tools and time could identify exactly why the GX produces a perfect picture on TVs but the Plus doesn't. whether it be that IREF circuit, smoother power, slower clock speed or something else.
And then? What would we do with that information? It's unlikely that people will start replacing/adding parts to the video circuitry just to get a marginally better picture which can be attained with some careful tuning of their TV.
Bryce.
Well that would of course depend on the findings, people have tried adding capacitors already and didn't you say that you modified your own to overcome this issue? I wouldn't describe the picture difference as "marginally better", it's a lot better on the GX, and that's just on a 19" TV. There isn't any careful tuning that I can find apart from lowering the sharpness as low as it will go.
Personally I have a couple of CM14 monitors and a MM12, but I'd still be interested to discover why there is such a difference on a TV.
The problem is, it would be a different (and complicated) solution for every TV and even with the best result, there are people who aren't happy with the GX picture on their TV either.
Bryce.