Stedy's RGB to SCART cable

[Allen1]

Hi there ikonsgr,

you are right about the Amiga having a common/shared ground (GND), so if one wire is connected to a ground pin on the Amiga, it can then be connected to all the other ground pins on the scart socket.

I think from reading your posts that you haven't wired the other grounds in the scart plug, this is probably where your problems are coming from. Different manufacturers of TV's may not use or connect a common ground for signals, so it would be easiest to wire up the existing ground in your scart connector to all the other grounds apart from the shield. That's the way mine are wired up including the audio ground, if you do have spare wires in your cable then they could also be used as extra grounds but are not essential in this instance as there is a shared ground on the Amiga.


Worth a try anyway

[FOL]

Originally Posted by Allen1

Hi there ikonsgr,

you are right about the Amiga having a common/shared ground (GND), so if one wire is connected to a ground pin on the Amiga, it can then be connected to all the other ground pins on the scart socket.

I think from reading your posts that you haven't wired the other grounds in the scart plug, this is probably where your problems are coming from. Different manufacturers of TV's may not use or connect a common ground for signals, so it would be easiest to wire up the existing ground in your scart connector to all the other grounds apart from the shield. That's the way mine are wired up including the audio ground, if you do have spare wires in your cable then they could also be used as extra grounds but are not essential in this instance as there is a shared ground on the Amiga.


Worth a try anyway

100% Spot on. Grounds on 23 pin seem to be commoned, but on the scart you need to connect them all (except the outer shield).

[orange]

well pin 13 is separated: http://pinouts.ru/all/AmigaVideo_pinout.shtml
also check on schematics, A500-Denise-Video-Mouse-Joy.jpg

even though some grounds are 'commoned' that doesn't mean you should use a single wire (if you want to do it by-the-book).
I am sure there is a reason why Stedy left them in his diagram.

[FOL]

Originally Posted by orange

well pin 13 is separated: http://pinouts.ru/all/AmigaVideo_pinout.shtml
also check on schematics, A500-Denise-Video-Mouse-Joy.jpg

even though some grounds are 'commoned' that doesn't mean you should use a single wire (if you want to do it by-the-book).
I am sure there is a reason why Stedy left them in his diagram.

I think they were talking about the group 16 to 20.
I have never used pin 13, never intend to. Stedys diagram is fine as it is.

[ikonsgr]

Allen1, the next thing i'm going to do is what you are saying. As i already have a scart cable with only one ground connected (amiga pin 16 to scart pin 17 only), i intend to short circuit all the grounds on the scart connectors ,so ALL scart grounds will be connected with the common amiga ground, and on amiga's side all grounds except pin 13 are most probable connected together as in the A500 manual all ground pin's are referred as plain "ground" and not "analogue red ground" or "CSYNC ground" etc.).It will be very interested to see if there will be any improvement.

Orange, the pin 13 is indeed a "special" separate ground. But according to THE book (A500 user's manual ) it's the special return/ground for the "external clock enable" (pin 2) which of course has nothing to do with the scart cable!
stedy's design also uses this "special" ground as return/ground for the pin 16 of the scart (rgb selected) which is fed by amiga's pin 23 e.g the 5volt power supply!It will be very interesting if stedy can explain us why he used this "special" amiga ground and what are the benefits of doing so.
Now, i've finished a first version of stedy's design, i made it exactly as the diagram in his site shows ,except from connecting the "special" ground (amiga pin 13 to scart pin 18) and also i used the 220Ω (as the NOTE says) instead of 75Ω in pin 23, to reduce the "rgb select" voltage to about 1.2volt (as 2.5volts maybe will be too much on some modern lcd tv's - 1-3volt is the range but 1volt is the "nominal", anyway).
Now, the first test on the crt tv shows that THERE IS an overall improvement in the quality of the picture:
1) The "noise band" that i told you,when audio rca not connected, is completely disappeared with stedy's design.
2)Some small interferences i had, when floppy drive was moving its heads, also disappeared.
3)Small flickering especially around yellow letters in black background, also disappeared.
On the other hand, i couldn't notice any difference in picture quality on the lcd tv (all the noises and interferences that are present on the crt tv with the "one ground" cable, with lcd tv are vanished) . Of course i use an old amiga 500, maybe with an AGA AMIGA there will be more improvements. Anyway, even from the above observations, it is clear that using separate ground cables for r,g,b, as in stedy's design, gives you overall better picture quality (although in the old crt i tested it, i didn't notice any difference in "clearness" of the picture as for example in the sharpness-distinctness of the letters, maybe in a better tv will show...)

[ikonsgr]

Allen1, i've just did what you are saying. i short circuit all the grounds on the scart connectors ,so ALL scart grounds will be connected with the common amiga ground, and on amiga's side all grounds (except pin 13). Unfortunately there was no improvement. So, separated r,g,b grounds are a "must" if you want a better picture quality.BUT from my experience, if you are only playing games, the difference is practically unnoticeable. I was using the "one ground" cable for months and played many hours and didn't notice anything ;-)


Orange and stedy, instead of using both 5volt and 12 volt supplies that require 2 cables, wouldn't be better if i connect both scart pin 8 and pin 16 (with a 1ΚΩ resistor on pin 8 and 680Ω on pin 16 to take ~1.2volt for "rgb select") from the amiga's 12volt pin 22 and so save a pin/cable?

UPDATE: I made the cable with the upper rewiring (essentially exactly as stedy's design,except i take the "rgb select" from the 12volt supply instead of 5volt) and cant see any difference with the previous design (without amiga pin 13).

[orange]

perhaps he used pin 13 because RGB select is actually DC signal (as we use it).
btw, ikonsgr, what kind of cables do you use? if you really must have best possible picture, you should probably find (full) SCART<->SCART cable and replace one end.

yes, I think it doesn't matter if you use only +12V. if that pin can provide enough power.
I never connected SCART pin8, my plasmaTV has an option to change AV. (don't they all?)

Originally Posted by FOL

I think they were talking about the group 16 to 20.
I have never used pin 13, never intend to. Stedys diagram is fine as it is.

so, everybody is right but me?

[Bamiga2002]

I have the RGB-scart cable from Amigakit and a quick test showed nice picture quality on an LCD tv . Further testing ensues when I (again!) have my tower put together and in working order.

[ikonsgr]

I've just finished testing on all 3 tv's with my amiga 500 .The results for the stedy's design cable and the simpler one with "one ground",are:

-With the lcd tv, picture quality is exactly the same with both cables.
-With the crt 28" LG (2003 model) also, picture was exactly the same.
-With the old (whatever brand) 14" crt there was indeed an improvement with stedy's cable, but only regarding interferences from floppy drive activity and the "noise band" that faintly rolling on picture if you don't connect the audio rca's.Of course this improvement is practically unnoticeable with games and very slightly annoying with workbench applications (as the picture quality of the old 14" crt is a little crapy anyway...)

The funny thing with the above results is that, the picture improvement showed only on the oldest crappy 14" crt ,and not on the much better 28" crt and lcd tv's!

So my final conclusion is:

if you have an amiga 500, the simpler "one ground" cable is a cheap and good solution,especially if you only play games.Practically there is no benefit of using stedy's design.

BUT if you have a newer aga amiga (1200/4000) (or a flicker fixer etc), maybe stedy's cable will indeed show better picture quality. Unfortunately i don't have an AMIGA 1200 to prove that, so if anyone has, can try and tell us the results!

[ikonsgr]

orange, i use a shielded round 10branch low loss special cable which i think is ok since the length of cables are less than 2meters.

Originally Posted by orange

yes, I think it doesn't matter if you use only +12V. if that pin can provide enough power.

Both 12volt and 5volt supplies on the amiga's 23pin can handle 100mA max. The suggested 75Ω resistor on the 5volt by stedy's design requires ~33mA on 5volt feed. Using the 1KΩ and the 680Ω from the 12volt as i suggest requires in total less than 30mA, away below the 100mA limit and even lower than stedy's 5volt!

Originally Posted by orange

I never connected SCART pin8, my plasmaTV has an option to change AV. (don't they all?)

I believe so. The only good thing about putting 12volt on scart pin 8 is that you don't need to turn channel to "av", all channels are giving amiga's picture! And of course all 16:9 modern tv's have a button on the remote control to change the aspect ratio... Anyway i had a spare branch on the cable, so i used it there

[Stedy]

Hi,

A lot to cover here so apologies if I miss something.

I used multiple grounds for the various signal types because that's the way I have been taught to do it. I design equipment for electrically noisy environments and part of the job when working on the interconnects is sorting out the grounding and earthing. RGB Analogue Video has 3 pairs, red/red ground, green/green ground and blue/blue ground. The grounds are shields around the signal. This considerably reduces crosstalk in the cable.

I separate digital ground (SYNC return) from Analogue grounds (Video) as digital grounds have higher noise than anlogue grounds. They of course need to be connected together, at one star point, this is near the Amiga PSU connector IIRC.

How you generate the 1-3V for the RGB select (SCART pin 16) is up to you. I did it my way in case of a failure of the terminating resistor or if the resistor inside the TV was not fitted for some reason. 5V should be handled easier than +12V. The current on the +5V supply is within limits and the power dissipation on the resistor is 80mW, no need to worry when using a 250mW or better resistor.

The 1K resistor on the +12V signal, used to select the aspect ratio, is in case you make a mistake and short circuit the wire, it limits the current to a reasonable level.

I don't have a specification for the SCART socket but the Video decoder devices, used on modern TVs, I have been designing with for a number of years now. I understand what signal levels they expect. Feeding the 4.4V TTL CSYNC signal, into a decoder that expects a 1V signal and operates of a 3.3V supply is not good for long term reliability! This is the reason for the 330 ohm resistor. On older machines, like the A500, the TTL CSYNC level may be lower, so reducing the value could help. When I get around to resurrecting my A500, it needs a re-cap, I will measure it.

I made my test cable using a SCART lead purchased from a pound store and cut one end off. As the cable did not have a metalised backshell or 360 degree screening, I do not bother with connecting the shield wire, the RGB and Audio signals are protected.
Once a cable screen is broken, even once, no matter how small, it adds 15-20dB to the noise emissions and in turn increases the susceptibility. As it works adequately as-is I do not bother with the overall shielding. Finally finished some MIL-STD-461E EMC testing today @ work, cable shielding and grounding was very important in getting acceptable results.

If you want a quality ready made cable, buy it from Amigakit. When my next video project comes along, you can re-use it.

Ian

[FOL]

Originally Posted by orange

perhaps he used pin 13 because RGB select is actually DC signal (as we use it).
btw, ikonsgr, what kind of cables do you use? if you really must have best possible picture, you should probably find (full) SCART<->SCART cable and replace one end.

yes, I think it doesn't matter if you use only +12V. if that pin can provide enough power.
I never connected SCART pin8, my plasmaTV has an option to change AV. (don't they all?)

so, everybody is right but me?

Think im missing something here.
I just said I have never used and never intend to use pin 13.
I have made 100+ cables without issue.

Alot of next gen tvs have issues. Due to the manufactures not being bothered about standards.

[ikonsgr]

Originally Posted by Stedy

Hi,
I used multiple grounds for the various signal types because that's the way I have been taught to do it.I design equipment for electrically noisy environments and part of the job when working on the interconnects is sorting out the grounding and earthing.RGB Analogue Video has 3 pairs, red/red ground, green/green ground and blue/blue ground. The grounds are shields around the signal. This considerably reduces crosstalk in the cable.

In the specific case we are talking, of a short length scart-amiga cable (1.5m-2m) and on a common household which i think it's not considered a "noisy" environment, you think that crosstalk is really an issue which can deteriorate picture quality so that it can be visible? So you suggest that the use of this type of cable (with internal pairs of signal-ground around the signal) is really a MUST in order to achieve the best possible picture quality? You think tha i will see a visible improvement with my amiga 500 on crt or lcd tv with a cable of that kind?

Originally Posted by Stedy

I don't have a specification for the SCART socket but the Video decoder devices, used on modern TVs, I have been designing with for a number of years now. I understand what signal levels they expect. Feeding the 4.4V TTL CSYNC signal, into a decoder that expects a 1V signal and operates of a 3.3V supply is not good for long term reliability! This is the reason for the 330 ohm resistor. On older machines, like the A500, the TTL CSYNC level may be lower, so reducing the value could help. When I get around to resurrecting my A500, it needs a re-cap, I will measure it.

As my amiga 500 is in perfect health i can assure you that TTL signal is not lower, i measured it about 4.35volt, so the 330Ω fits just fine (already done it anyway and works perfect! )

Originally Posted by Stedy

If you want a quality ready made cable, buy it from Amigakit. When my next video project comes along, you can re-use it.
Ian

The only problem is that, to buy it from amigakit i must sell my kidney!

[Mr B]

Originally Posted by Stedy

The Amiga's video sync is not compliant with either the PAL or NTSC specifications in non-interlaced mode and is better in interlaced mode, not ideal for games. Some TVs can not handle the non-standard Video sync. If the TV can handle a VCR playing back a macrovision protected video tape, it should work better with the Amiga.

Not much of a list, but my LG, a 42LX650n, is one of these that wont accept the Amiga video sync. The Sony Bravia lineup seams to do better.

Originally Posted by Stedy

Am I the only one here who still has a VCR?

No, i still have one, and have been contemplating to unpack it to try and see if the Amiga would work "through" it, so to speak. In the end i sort of gave up, and pretty much decided to wait for Jens new breed of the Indivision.
B!

[ikonsgr]

@stedy. I found a multi branch cable where the grounds inside,are shields (e.g. twisted) around the signal cables, but every signal's ground is not isolated from one another, they all make contact together as one "big" ground(something like this http://www.technokit.co.uk/proddetail.php?prod=E649 )! Did you meant that kind of cable, or a cable where each inner signal cable+shield ground, is completely isolated,from the others? (although i couldn't find a cable like that so far...). You think this kind of cable will give me better picture quality than of a 10branch cable where each ground is one inner small cable, isolated from the others across the cable, but of course they are not "wrapped around" every signal cable.

P.s i cant find anywhere a multi core individual screening cable with more than 4 cores!Do you know where i can find a 5 or 6 core? To build AMIGA-scart cable according to your diagram we will need at least a 5 core individual screnning cable, right?

[Allen1]

Hi there ikonsgr,

the link below shows the connections that I followed when I made my cables. I used a shielded cable which if I remember correctly, I used the shield as an extra GND and it worked fine. I wonder if you have a mobile phone or wireless house phone near to the TV that could be getting picked up by the cable, if you have then maybe try moving it away and see if that sorts out the interference.

I had tried a few cable wiring designs that simply didn't work before I found alexh's version in the link below, that one worked great on all my TV's (I don't have an LCD one) and Amigas so I never looked any further and I didn't know about Stedy's thread till much later. Compare your cable to the one in the link and see if by chance one of the wires is soldered to a wrong pin, its oh so easy to do as I have found out myself on more than one occasion

http://eab.abime.net/showpost.php?p=572128&postcount=18

[ikonsgr]

Allen1, i don't have any problems with the scart cable! Even the simpler one i made so far (with only one ground) works good enough, games and workbench looks fairly good, at least with my amiga 500!
The thing is that, if you try this cable with a better aga amiga model, on a high definition modern lcd tv and you want to use applications and not just play games, the picture quality of this cable may not satisfy you.
I don't now if, in the just 1.5-2meters the amiga scart cable is, crosstalk and interference is really an issue, which can deteriorate picture quality to the point that is fairly visible (visible ghosting,moire etc),and if the use of some other special cables , (like the one stedy says, with the separate ground around each signal cable) can give you the best possible picture in any case.That's why i'm asking stedy who has the knowledge and the experience to tell us about!

[Allen1]

Thanks for clearing that up a little, the problems you have going on with the AGA computer could be down to bad capacitors or similar problems in the computer or could be something as simple as your HD TV and 28" CRT TV doesn't like the input it is getting from the Amiga. The common problems are often that the scart socket on the TV needs to be an RGB one and if there is more than one socket on the TV you may have to try the others as they are not always wired up the same, failing that try your on screen menu and check that the input is set to RGB.

[ikonsgr]

As, i have post it before, i gave one of my cables to a guy that has an amiga 4000 (lucky one! ), and said he was getting picture but with something like "shadow". He also told then that he ordered a cable form amigakit and works perfect.
So in these "hard" cases, where you want to use your "heavy" amiga in a more professional way, you expect to get the best picture possible. For what stedy told us so far (and a little research i made on the issue... ) it seems that if i had used the special "individually screened" cable, most likely the "shadows" the guy told me, would be disappeared! I will make another one scart cable using this "special" cable in the weekend and tell you my impressions.Of course if i cant see a difference with my little amiga 500 on the tv's i have, this doesn't mean that "heavier" amiga's with better tv's will not see a difference too!

p.s. i don't know if you ever try the "individual screening" cable but i think it will be more easy to construct it . As you only have 4-5 colored signal cables and the correspondent grounds, it's much more easy to make the connections on both amiga and scart connectors as pairs (cable-ground) , than using a 10core cable with 10 different colours!

[Mr B]

I had ghost shadows on a mates Sony Bravia, with a cable from Amigakit. Doesn't work at all on my own Telly, but i blame that on the TV. It's a long cable, so i guess that can have something to do with it. Just saying it may have something to do with the TV as well as the cable, so your never going to know until you try it.
B!