Stedy's RGB to SCART cable

[Allen1]

Good luck with the cable, they aren't hard to make just a bit fiddly but certainly worth doing yourself

[Stedy]

Hi,

@Rocs9005

Are you using an LCD or CRT TV?

CRT tvs are more tolerant of the non standard 256 line progressive output of the PAL Amigas. With an LCD, it all depends on how the video decoder of analogue front end decodes the video. If the TV can handle VCR playback of a videotape with macrovision, it will work well with the Amiga.

You definitely need the resistor on the CSYNC (Amiga pin 10 to SCART pin 20) signal for an LCD TV. If you feed a TTL signal into a digitised analogue input, the analogue front end will have trouble decoding the sync information as the input amplifier gets overloaded and possible diode clamped.

HTH

Ian

[Rocs9005]

Hi Ian.

I'm using a new but budget LCD TV.
Having read the thread and your website, I got the impression the Scart Pin 20 resistor was important! Might the "trouble decoding the sync" cause the wobbly top part of the pic I was seeing? I realise different TVs behave differently, so might not be as simple as "wobbly pic = Sync problems".

When I get hold of a basic Scart to chop the end off, I'll be wiring a cable up exactly as your diagram, so will let you know how that works on my TV
Thanks for the reply, and all the effort you've put into the info in this thread.

Rocs9005

[Stedy]

Hi

For an example of the Video 'curl' effect caused by field sync issues, take a look at page 5 of this PDF, http://www.analog.com/static/importe...7357AN_850.pdf

What happens during the field blanking period is a pulse pattern, output by the source, in this case the Amiga that contains timing pulses to identify of the filed of video being output is an odd or even field. If this information is missing, as can happen with a VCR or with older computer systems, the receiver does it's best to interpret the video signal, sometime with less than perfect results.

When using non-interlaced screenmodes on the Amiga, no information to indicate odd/even fields of video is output, hence the issue with some TVs/monitors.

Note: two fields of video, one odd field and one even field, make a frame of video. So for PAL, you have 50 fields/25 frames a second and for NTSC you have 59.94 fields/29.97 frames a second.

Video decoder devices, like the ADV7184 from Analog devices, can be programmed to handle the non-compliant video from the Amiga and output a digitised stream ready for a scandoubler or a video encoder. I should know, I design video systems using these parts for a living!

Ian

[Rocs9005]

Hi

Finished making up my Scart cable today.
It seems to have trouble getting the TV to switch to Scart RGB,
as the TV switches to that mode, you get a flash of the WB (in black and white) with the TV OSD saying SCART RGB, and then the TV goes blank , and flashes it again. Does this over and over again. Not sure that it is autoselecting 4:3 either, although the TV has an aspect button where you can change that yourself. Putting it on Auto seems to leave it as 16:9 rather than 4:3

However, if you Ctrl-A-A reset the amiga for a few seconds, the TV usually stabilises, and at that point works fine. The duff cable I got from ebay didn't seem to have this problem (it had different, far worse ones instead!). Tried it with both my A1200s, same results with both. It doesnt have the HUGE image instability problems that the ebay cable had, once it settles on RGB scart, it seems perfect.

I have to admit to being slightly disappointed with the picture quality.
Running workbench in 640x256, text has a noticable "shimmer", almost reminds me of dot crawl. It looks very reasonable playing games though.

So I have 2 questions:

Any ideas on the auto select messing up? I notice the value in Alexh's post for the Scart 16 resistor are higher (220, rather than 75) and so was the duff cable I had before (177 measured).
Could it be that my TV wants a higher value in there?

Second question - Am I just too picky about the picture? Does everyone get slight shimmer through SCART connections?
I used to use a scandoubler/flickerfixer and a 17inch CRT that did 50Hz, but got fed up with the flicker of 50Hz, and started having to worry about the space it took up. That wasnt perfect (which I always blamed the external scandoubler for...) but I dont remember it having the same crawl effect.
Could my crappy soldering skills be to blame? The scart lead I chopped up was cheap, but fully wired. I did all the different grounds individually, and checked all the pins with a multimeter before daring to plug it into my Amiga, and it all seems correct...

Thanks,

Rocs9005.

[Allen1]

Come on Rocs9005,

I mentioned what worked for me after trying other descriptions.

[Rocs9005]

@Allen1 - I did read your posts, and don't worry, I was able to read the values you used the first 3 times you posted them, but thanks for posting them a 4th time anyway...

I used 75, because that's what is stated on the original post by Ian, and also on the diagram on his website. He doesn't appear to think 75 is "far too low", and seems to know more about this sort of thing than I am ever likely to.

I was looking for a 2nd opinion on the symptoms I saw before I go out and buy a different resistor, in case anyone thought it might be caused by something else.

I was also sharing my results just in case it helped anyone else, who like me might want to know what values to use, and find more than one answer given like I did. Also thought Ian might like to know how using his instructions worked out.

Rocs9005

[Photon]

Well, just compare to other consoles, borrow some later pre-HD console such as Playstation 1 or N64/Gamecube, and compare the pixel quality. "They should be equal" (c) The Office.

[Allen1]

@Rocs9005

the phrase horse to water springs to mind, Giddy up neddy

have removed the offending information to save confussion

[WDeranged]

Originally Posted by 8bitbubsy

Any difference on this and the one sold on Amigakit? I mean, in the hardware strategy/picture quality...

I noticed this old question and thought it's worth finding an answer as I recently got one from Amigakit and found my picture is all wobbly on my Bravia but rock solid on CRTs

[Stedy]

Hello,

@WDeranged

Do you have a multimter?
Would be great if you could measure any resistances in the cable from AmigaKit.

My suspicion is that the 5V sync level from the Amiga is upsetting the Sony LVD TV.

My cable was tested on a Sony Bravia TV and works fine, if we can find the issue with your cable, maybe we can work out how to fix it.

@Rocs9005

Thanks for the feedback. Have added a note to the page on my website about the different resistor value.

@ thread

I have been making enquiries to purchase 23 way d-type plugs, in quantity if necessary. Not had any luck so far, still got two vendors checking factory stock. Could use some of these for my upcoming video adaptor circuit.

Ian

[Rocs9005]

Hi Ian

I tried swapping the 75 resistor on Scart pin 16 for a 220, but it didn't make any difference at all. I'll try swapping the 330 on Scart 10 for a 270 as that's the other value that is different between your diagram and Alexh's suggestion in this post:
http://eab.abime.net/showpost.php?p=572128&postcount=18
I'll let you know how it goes.

Rocs9005.

[alexh]

Can we make this thread sticky?

[orange]

i am also having problems with scart pin 16..
it works with Amiga, but not with Atari. the problem is Atari has only +12V.
what resistor would you use if connecting it instead of +5V?

some people say its not wise to use Resistive divider, quote "as you simply can't predict how much current the TV will draw. Resistors ARE NOT a means to achieve a set voltage."
but i did manage to connect ZX+3 using correct ohms.
they say we should connect ataris 'vertical sync' to blanking but it doesn't work for me.

a bit offtopic, sorry..

EDIT: SOLVED, it actually works with vertical_sync + 75ohm on pin 16 (blanking). would it be good to use vertical_sync on Amiga too, instead of +5V? (it seems safer at least)

[Stedy]

Hi,

For the connection to the SCART blanking signal (pin 16) a 470 ohm resistor from +12V would suffice, this should give a voltage around 2V at the SCART connector, assuming a 75 ohm terminator is present on the input.

With regard to the query about resistive dividers, I suggest reading Ohm's law http://en.wikipedia.org/wiki/Ohms_law.

I am unsure which Atari system you are talking about, this is an Amiga forum.

@Rocs9005

Any luck getting your cable to work?

@Alexh

Thanks for making this a sticky thread.

[orange]

(we don't know whats inside atari 1024, some resistor in line with +12V perhaps, but never-mind cause it works..)

what about trying to connect Amiga pin 12 /VSYNC --> Vertical Sync (47 Ohm)
to pin 16 BLANKING ?

[orange]

oki, back to amiga, i have this EXACT same problem with my AMIGA:
http://ntsc-uk.domino.org/showthread.php?t=100786
(black bar on right of the image when Amiga is connected, but PS2 and wii display is OK )

there are some interesting technical details on that web page, they imply the problem is (composite)sync.
what do you think?

[Foul]

just made the rgb to scart cable, used 5m blinded cable with golden scart ... work perfect on my Samsung 80cm 4/3 CRT flat TV !

[Stedy]

@Foul

Good to hear the cable worked perfectly for you.

@Orange

I know that video decoders/encoders add 10-20us delay to the video signal due to the processing they undertake.

I would need to recheck this but the CSYNC output, should align with the RGB video signals. The only potential problem could be if you use the composite video output from the CXA1145 as the sync source to the SCART socket.

Next time I am in the lab I will measure the timing of the composite video signal to Hsync/Vsync.

Bye,

Ian

[orange]

well, according to that web page , using composite video output instead of csync should solve this problem.

(the same problem occurs with other computers that provide 'clean' CSYNC, such as amstrad and spectrum)