Oric on VGA monitors

If you want to ask questions about how the machine works, peculiar details, the differences between models, here it is !
How to program the oric hardware (VIA, FDC, ...) is also welcome.
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DrBob
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Oric on VGA monitors

Post by DrBob » Fri Apr 19, 2019 11:08 pm

I have noticed a lot of interest in a topic on SCART to TV. I tried this without success; I believe because the
Oric RGB synch signal is inverted (WRT UK TVs) and, anyway, taking over the home TV was not on. I therefore made
a simple circuit to convert the Oric output to VGA which is synch separated. This plugs into my desktop computer
monitor, an ASUS-VE248H series, giving a nice stable picture with little if any colour fringing. The problem
you may see if you look closely(!) at the picture is that circles are now ellipses. I expect that this is caused
by the monitor horizontal 88 dots/inch approx. I use it on the computer at 1920x1080. but it adjusts
automatically if the resolution is changed in software.

Q Why does the monitor adjust to the Oric synch signals to give a correct picture ratio?
DSC_0004.JPG
The attachment DSC_0004.JPG is no longer available
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Chema
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Re: Oric on VGA monitors

Post by Chema » Fri Apr 19, 2019 11:29 pm

No sure if your image is too stretched, but the original is. The Oric resolution is 240x200(+24 for the tree lines of text) instead of the usual 320x200, which gives pixels that are somewhat rectangular and ellipses instead of circles.

You can emulate the Oric real aspect ratio in Oricutron with a flag (can't remember which one now), just for comparison

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DrBob
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Re: Oric on VGA monitors

Post by DrBob » Sat Apr 20, 2019 1:49 am

Hi Chema
A circle on the Oricutron is 3.4"x3.4"...1:1, the same program generates a circle 8.3"x6.5"...~4:3 on the VEGA screen which is 7:4. I must say I am unable to think of a reason. Is the synch rates from the output socket different to those used for the RF output do you know?
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Symoon
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Re: Oric on VGA monitors

Post by Symoon » Sat Apr 20, 2019 6:17 am

I'm curious to know more about your circuit - as a potential user, not as an electronics expert ;)

Not sure this may help, but just for the record: before emulators, I had never, ever seen a real circle on Oric. On 4/3 TVs, they already were ellipses. That was using Scart on CRT TVs, in France.

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Dbug
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Re: Oric on VGA monitors

Post by Dbug » Sat Apr 20, 2019 8:36 am

DrBob wrote:
Sat Apr 20, 2019 1:49 am
Hi Chema
A circle on the Oricutron is 3.4"x3.4"...1:1, the same program generates a circle 8.3"x6.5"...~4:3 on the VEGA screen which is 7:4. I must say I am unable to think of a reason. Is the synch rates from the output socket different to those used for the RF output do you know?
Well, most emulators in software rendering mode are just showing non stretched pixels, so the 240x224 Oric screen gets rendered as 240x224 pixels on the PC, and if you are using a video mode where pixels are square, you get a perfect circle on the emulator.

As Chema said, the normal Oric display on an old CRT is squished vertically, because the resolution ratio is very different than on most machines, and you can somewhat see how it should look in Oricutron if you enable the OpenGL mode with Horizontal stretch:
OricutronAspectRatio.png
So basically your VGA monitor is showing the truth.

Now, thing to take into consideration, is that pretty much all the games and software made before 1996 was made on real Orics, so all the graphics were done on screens with squished appearance, so the artists compensated for that so it looks right... while after Euphoric was released, a large number of games were made on emulators, and we mostly did not really care for the fact it may look slightly different on the final machines... you mostly don't notice... except if there are perfect circles on the picture :D

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DrBob
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Re: Oric on VGA monitors

Post by DrBob » Sat Apr 20, 2019 5:01 pm

Hi Symoon, The circuit takes the RGB and composite synch from the Oric output socket and by digital means
produces buffered signals into 75 Ohms of RGB Vertical & Horizontal synch for VGA. In the old days this was
accomplished by taking a copy of the composite synch and low-pass filtering for Vertical and high-pass filtering
for Horizontal synch. Construction is on Vero board 70x50mm housed in a plastic hobby box.
In passing I have completed the first cut of my recovery of spoilt Oric tapes program which I am writing up at
the moment (Old tapes-analysis thread).
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Thanks for your reply Debug I think my candle has just guttered. I redid your experiment with a circle 80,1. on

the Oricutron software render 88x88. stretched it was 107x88 a stretch of~23%
On my monitor the same circle 80,1 measured 238x164 a stretch of ~45%. Each line is about 3.6pixels V or H.
The point is, can anything be done about it???

(info for those who find info fascinating)
See my diagram of the real Oric on my monitor, it seems to be nicely positioned. Some parameters would seem to
be:-, The monitor pixels are square and separated by 0.27675mm giving ~1080pixel vertical. The portion of the
screen being used by the Oric in the vertical direction is 224mm. This means the Oric is being represented by
224/0.27675 = ~809 pixels vertically and 359*0.27675 = ~1297pixels horizontally, an increase of 3.6 V & 5.4 H.
DSC_0006.JPG
Give me a fulcrum and a leaver long enough and I will move the world

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