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 Post subject: SCART RGB filter boxes
PostPosted: Wed Jun 27, 2012 7:48 pm 
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Joined: Sat Jan 31, 2009 9:01 pm
Posts: 82
Location: Malta
Some time ago I got a request from AW72 for a SCART adaptor that makes RGB images softer, somewhat imitating a CVBS signal. Normally people want to use RGB for its sharpness, however for low-res games, the extra sharpness does not look nice.

After discussion and various design ideas we came up with a 3-way SCART switching box that can be used to connect a Minimig and two game consoles to a TV/monitor. The socket for the Minimig is unaltered, while the other two sockets give a low-pass filtered RGB signal.

I modified a SCART switcher from Conrad Electronics as in the attached image and carried out testing to verify the results. The prototype was sold to AW72 and he reports that it gives relatively good results and he's quite happy with it.

The reason I'm posting this here is that during my research I didn't find any devices that filter RGB signals, and maybe there are other people who are looking for such a filter.

If anyone's interested please feel free to contact me. I am currently looking for other SCART switching boxes to modify as the one from Conrad is somewhat expensive.

Cheers!
Edwin


Attachments:
Conrad SCART Switch 351102.jpg
Conrad SCART Switch 351102.jpg [ 5.48 KiB | Viewed 5454 times ]

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 Post subject: Re: SCART RGB filter boxes
PostPosted: Wed Jun 27, 2012 8:24 pm 
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Joined: Mon Dec 01, 2008 10:11 am
Posts: 197
wolfchild wrote:
Some time ago I got a request from AW72 for a SCART adaptor that makes RGB images softer, somewhat imitating a CVBS signal. Normally people want to use RGB for its sharpness, however for low-res games, the extra sharpness does not look nice.

The FPGA core can be modified easily so digital low-pass filter is also active when 15-kHz mode is selected. Should give similar softening effect as discrete filter.


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 Post subject: Re: SCART RGB filter boxes
PostPosted: Thu Jun 28, 2012 9:51 am 
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Joined: Mon Dec 01, 2008 9:58 pm
Posts: 1568
Location: .de
Until now I can't complain about 15KHz picture via Scart.
But Im only using an original C=1081 Monitor screen and the picture looks 100% like an A500 or any other Amiga picture.

At large LCD/Plasma screen the pixel may look bloggy due to INTERNAL scandoubler/flickerfixer in this screens! So a real Amiga RGB output will look the same.
Jakub already named an FPGA internal solution for this and I would suggest to do so :)

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 Post subject: Re: SCART RGB filter boxes
PostPosted: Thu Jun 28, 2012 10:04 pm 
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Joined: Wed May 27, 2009 10:51 pm
Posts: 480
No no no :P , WOLFCHILD don`t speak about a Scartbox-Modify for using it on Minimig (for the Minimig, i use the unmodified socket of the Scartbox). I tell the complete story.

I have here a Sega Mega-Drive which i equipped with a Switchless-Mod. This means, that it can be switched between PAL (50 FPS) and NTSC (60 FPS) and also between different language-types, over the RESET-Button.

But the problem then was this - all the time, i connected my Mega-Drive to the TV with a normal Cinch-cable (composite-signal) and this gave me a nice picture with a little unsharpness (looks like in Emulators when using an antialias or bilinear-filtering), so that the pixels of the graphics can not be seen to sharp. I like this composite-picture for 8-bit or 16-bit graphics.

But when i set the console in NTSC-Mode now, then the Mega-Drive don`t give out a color-signal anymore, when its connected with cinch over composite. Then i only had color-picture in PAL, but black/white-picture in NTSC-Mode. So i must use a RGB-Scart cable for it, to have colors in NTSC also. But even when i set the sharpness on my TV to extreme-low, the RGB-pic of the Mega-Drive looks ugly. way to sharp, graphics looks pixelated.

Then i had the idea, to ask WOLFCHILD (because i knew, that he makes cables for the Minimig), if he maybe can do something to my Mega-Drive-RGB-Scart-cable, that i have a little unsharpness, which would look close to composite-picture. The result was this modified Scart-Switch-Box, which had now the following:

Socket 1 - no filtering (unmodified) - (here my Minimig is connected, because my TV only has 1 Scart-socket, which can be switched to RGB-mode)

Socket 2 - strong filtering - (this is unused at the moment, but i will try with Atari VCS-2600)

Socket 3 - low filtering - (this is, where my Mega-Drive is connected now)

When i connect my Mega-Drive over socket 3 now, then i have the best result. The low-filter comes very close to a cinch-composite-picture. The strong filter had a very strong smoothness, could look good on very old consoles like the Atari-2600, when connecting it over RGB-Scart. i must try this, when i find my Atari-console in my parents-house :D (the Atari-VCS2600 also needs a mod for this, otherwise it can only be connected via antenna-cable, which is not comfortable).

So the result for the Mega-Drive with this Scartbox-Mod is very good. Now i have a nice smoothing effect and can use NTSC and PAL in full colors with it. Such a modified Scart-Box is senseful for people, who wants (or must, like in my case to have color in NTSC) to connect their Retro-Consoles on a TV via RGB-Scart and wants a little smoothing-filter on the picture.

The Minimig picture has its own little unsharpness, when connected over RGB-Scart. there no modifying is needed. because i connect the Minimig like i did it with the Mega-Drive, but here, the picture is not to sharp and good like it is. WOLFCHILD explained it somewhere, what the reason for this is.

But nevertheless, when YAQUBE wrote, its possible to change firmware, that the digital low-pass filter can also be used in 15-kHz mode, then this should be integrated. would be interesting to see, how Minimig-picture looks better then, turning filter on or off on the TV (on newer Monitors, it looks better turning Filter on). But normally i dont need to change something here, but it was necessary for the Mega-Drive picture.


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 Post subject: Re: SCART RGB filter boxes
PostPosted: Fri Jun 29, 2012 9:26 am 
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Joined: Sat Jan 31, 2009 9:01 pm
Posts: 82
Location: Malta
Thanks to all for the feedback and to AW72 for stepping in :)

Maybe I did not explain the use of the RGB filter box too well in my initial post. Probably the area of interest for this device is small and specialized, but the results are desirable for people who'd like to replicate the look and feel of composite video for consoles when using RGB. It's most useful for people using MAME with 15kHz output where composite is not an option.

Attached are a block diagram of the unit. Later on I'll post another message with attached comparative images of the filter effect against composite.

The filter is nothing fancy, just a passive low pass filter tweaked for the desired effect. It's been tested on CRT TV only since I don't have an LCD display that takes SCART input.

Cheers!
Edwin


Attachments:
RGB filter.png
RGB filter.png [ 51.04 KiB | Viewed 5407 times ]

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self-built minimig 1.1 :: 10ns SRAM :: 3.5MB RAM :: ARM board

VGA -> SCART cables currently out of stock - PM me for info
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 Post subject: Re: SCART RGB filter boxes
PostPosted: Fri Jun 29, 2012 1:04 pm 
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Joined: Sat Jan 31, 2009 9:01 pm
Posts: 82
Location: Malta
Attached are full size crops of the high score screen from Turrican, showing the RGB signal compared to a composite signal.

In my opinion the strong filter is a bit too strong and would do with some tweaking.

Edwin


Attachments:
Composite vs RGB Strong Filter.png
Composite vs RGB Strong Filter.png [ 1.01 MiB | Viewed 5407 times ]
Composite vs RGB Weak Filter.png
Composite vs RGB Weak Filter.png [ 1.04 MiB | Viewed 5407 times ]
Composite vs RGB Filter Off.png
Composite vs RGB Filter Off.png [ 1023.19 KiB | Viewed 5407 times ]

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self-built minimig 1.1 :: 10ns SRAM :: 3.5MB RAM :: ARM board

VGA -> SCART cables currently out of stock - PM me for info
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 Post subject: Re: SCART RGB filter boxes
PostPosted: Fri Jun 29, 2012 2:56 pm 
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Joined: Tue Nov 09, 2010 3:10 pm
Posts: 316
I'm no analog filter expert, but to me it looks like your "strong" filter shows quite large ringing artifacts. Perhaps you should use a different kind of filter (like Butterworth)? What kind of filter are you using now? Anyway, I don't think this is very visible from normal viewing distances.

edit: on a second look, this probably isn't ringing, it just seems that way, since the "pixels" are spaced apart. Nevermind ;)

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 Post subject: Re: SCART RGB filter boxes
PostPosted: Fri Jun 29, 2012 4:23 pm 
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Joined: Wed May 27, 2009 10:51 pm
Posts: 480
in real moving picture the filtered-effect can be seen much better, than on a foto-picture.

the edges of the pixels are smooth, so that the eye think it`s round. that`s how retro-graphics should look like, in my opinion.

before the unfiltered RGB-picture from my Mega-Drive, looked a little bit like a mosaik. now i like it.


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 Post subject: Re: SCART RGB filter boxes
PostPosted: Fri Jun 29, 2012 11:51 pm 
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Joined: Mon Dec 01, 2008 9:58 pm
Posts: 1568
Location: .de
Real FBAS/Composite signal has "echo" on Chroma signal only.
The RGB filter also has Luma echoing and this will be a bit too much.

But the result is OK from this close-up still image view.
I would use the weak filter if necessary.

In general on old CRT 5KHz screen the whole view is as on real Amiga or any other RGB output of the old days. Maybe a bit sharper due to new FPGA/sharp-edge tech. but still very native.

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 Post subject: Re: SCART RGB filter boxes
PostPosted: Sat Jun 30, 2012 11:36 am 
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Joined: Sat Jan 31, 2009 9:01 pm
Posts: 82
Location: Malta
@chaos
The filter is a simple R-C filter with a time constant equivalent to ~2 pixels for the weak filter and ~3 pixels for the strong filter. The effect is that the filtering smooths out transitions in the video signal, in effect blending pixels together horizontally.

@boing4000
The chroma artefacts of CVBS are not replicated by this filter and you still get stable colours (no shimmering effects) except that the image is smoother as it loses signal bandwidth.

Out of curiosity, I tried the filter on the Minimig and it does have a much greater effect on the video signal. The reason is Minimig's video output has a higher source impedance than normal (289.6 Ohms). This was done to simplify the design. The actual standard is 75 Ohm source impedance. Note that the Minimig 2 design by Illuwatar has proper amplification on the RGB output, as does the Replay board.

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self-built minimig 1.1 :: 10ns SRAM :: 3.5MB RAM :: ARM board

VGA -> SCART cables currently out of stock - PM me for info


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