RGB to VGA

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Ruptor
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RGB to VGA

Postby Ruptor » Wed Jan 08, 2020 12:59 pm

I came across this device that can solve the QL video output problem for modern monitors I think.
https://www.ebay.co.uk/itm/Arcade-Game- ... 0005.m1851
Do you think it is useful?


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vanpeebles
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Re: RGB to VGA

Postby vanpeebles » Wed Jan 08, 2020 1:06 pm

Covered in depth here :)

viewtopic.php?f=2&t=12&hilit=gbs8220

10 years ago :shock: :o


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Ruptor
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Re: RGB to VGA

Postby Ruptor » Wed Jan 08, 2020 1:40 pm

Ok I will shut and go away. :lol: I did search but didn't see that thread. There were some recent discussions about old monitors going bust and not being able to use VGA hence my post when I came across the board. I suppose it is a lot cheaper now for a while until VGA starts going obsolete. :roll:


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Pr0f
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Re: RGB to VGA

Postby Pr0f » Wed Jan 08, 2020 2:02 pm

Ruptor wrote:Ok I will shut and go away. :lol: I did search but didn't see that thread. There were some recent discussions about old monitors going bust and not being able to use VGA hence my post when I came across the board. I suppose it is a lot cheaper now for a while until VGA starts going obsolete. :roll:


It's easy to miss some of the older threads ;-) I use one of those with an old square LCD monitor that I rescued from the bin - seems to work well and I get my full 85 characters across.


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Peter
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Re: RGB to VGA

Postby Peter » Wed Jan 08, 2020 2:18 pm

For me it also works, but the picture quality is poor. It seems to vary somewhat, depending on the particular QL mainboard used.


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mk79
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Re: RGB to VGA

Postby mk79 » Wed Jan 08, 2020 9:31 pm

I have one of those, didn't work too well for me either, which is why I started QL-VGA (see other post just now). But damn that is unbelievably cheap :shock: I paid 10 times that back then and even that wasn't really expensive.


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bwinkel67
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Re: RGB to VGA

Postby bwinkel67 » Thu Feb 20, 2020 11:09 pm

mk79 wrote:I have one of those, didn't work too well for me either, which is why I started QL-VGA (see other post just now). But damn that is unbelievably cheap :shock: I paid 10 times that back then and even that wasn't really expensive.


This question likely oversimplifies some things. I've been searching online about discussions between RGB to VGA conversions just to understand what all is involved (not that I'm interested in solving it, just curious what the obstacles are). You are working on a card to give the QL a crisp VGA signal with no character loss on either side (which would be nice to have vs flipping a coin and investing in a converter that then ends up screwing things up during upscaling).

So my (likely oversimplified) question is, given that VGA basically wants Red, Green, Blue, V-Sync, and H-Sync signals and on top of that there is a refresh rate, what are the issues for conversion from QL RGB. I'm guessing there would seemingly be a need for C-Sync splitter circuitry and is the other part of that potential refresh rate incompatibilities or are there a slew of other things going on here like screen resolution (how does that manifest itself). There is also analog vs digital RGB but I think both the QL's RGB and VGA are analog. I guess my question is, what is that £3 board doing (and yes, that usually retails for like $20'ish so that's amazingly cheap ... unless it had high shipping cost).


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Pr0f
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Re: RGB to VGA

Postby Pr0f » Fri Feb 21, 2020 7:55 am

QL Video is a bit off because the timings are non standard (the display area is a little larger than it should be for broadcast standards, and the front / back porch are shorter - I believe the Sync pulses are ok for standard PAL / NTSC timings - but the problem this gives is that on a standard TV, the left and right edges of the display are off the screen. Monitors have much more forgiving electronics and are able to still lock the video signal and allow the display to be squeezed into the visible area by adjusting horizontal width and position.

With VGA the timing definitions are quite tight, and the approach of the converters is to take a broadcast standard and basically 'speed it up' to hit the VGA timings - because it assumes a broadcast specificaiton for the input - you are back to losing your left and right margins.

A better approach is to get closer to the signal generation itself, and take the scan line and retransmit it with the correct VGA timing - but this requires speed (expense) and if the input signal is noisy could result in noise in the picture.

QL outputs RGB in digital form, the VGA signal is specified for an analog voltage which allows a lot more colours - at the very least the QL signals need to be attenuated to hit the levels for RGB

Then you have Sync polarities to worry about.

That particular board uses a standard upscaling chip from brooktree I think it was last time I looked - which takes the video a scan line at a time and outputs it, correcting sync polarity if needed, and allowing adjustment of the front / back porch timing using push buttons and OSD menus. With the correct attentuators, clean power supply and good quality cable the output is not too bad from the QL video output, but you want to avoid ground loops and other noise sources. It also doesn't help that the QL video output connector is the other side of a long and electronically noisy pcb from it's video generator chip sat next to the main processor.


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mk79
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Re: RGB to VGA

Postby mk79 » Fri Feb 21, 2020 9:56 am

Yeah, basically 512x256 at 50Hz is no resolution ever supported by TVs or monitors, but you could adjust analog monitors to display it anyway.

Many computer monitors these days don't like 50Hz at all, they start working at 60Hz, so scalers that only work on a line-by-line basis might work or might not.

My hardware filters and samples the QL video (fixed timing for the QL ULA, so no adjustment necessary or even possible), reconstructs a memory image and outputs that image (double buffered) in a standard 1024x768 at 60Hz format by
a) doubling every pixel in X direction
b) tripling every pixel in Y direction
c) occasionally output the same frame twice

This way introduces minimal lag but maximizes compatibility. So it's supposed to be a "it just works" device.

(for fans of the ZX Spectrum 128, it supports that one, too)


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Pr0f
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Re: RGB to VGA

Postby Pr0f » Fri Feb 21, 2020 10:25 am

Marcel - with your doubling and tripling, what do the pixels / text look like on the monitor?

Are you using a 65 MHz 'dot' clock for the 1024x768 resolution?



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