RGB to VGA

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mk79
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Re: RGB to VGA

Postby mk79 » Fri Feb 21, 2020 12:21 pm

Pr0f wrote:Marcel - with your doubling and tripling, what do the pixels / text look like on the monitor?
Exactly as they look on an analog monitor, just a lot bigger as monitors have increased a bit in size ;)
Are you using a 65 MHz 'dot' clock for the 1024x768 resolution?
Yes. If you want to be pedantic you could say that it's a 32.5Mhz pixel clock due to the pixel doubling, but nonetheless it's 100% VESA standard.

Marcel


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Re: RGB to VGA

Postby mk79 » Fri Feb 21, 2020 12:26 pm

Besides, there is a builtin screen server that outputs the QL-VGA logo if the input signal is lost or invalid. That does employ the full 65Mhz pixel clock. It helps the monitor not lose sync and also helps debugging as when you see the logo you know the output side is working correctly.


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Re: RGB to VGA

Postby Pr0f » Fri Feb 21, 2020 12:44 pm

That standard VESA frequnecy does seem to be acceptable to most of the HDMI monitors / my telly that I've tested with. I was also looking at 1024 x 576 at 50Hz as a possible, but there are black bands at the top and bottom as 576 > 512


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Re: RGB to VGA

Postby bwinkel67 » Fri Feb 21, 2020 8:33 pm

Pr0f wrote: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


So I was watching Adrian Black (Digital Basement youtuber) play with the SCART-to-HDMI converter (the one that actually does RGB to HDMI):

https://www.ebay.com/itm/Scart-HDMI-to-HDMI-720P-1080P-HD-Video-Converter-Monitor-Box-For-HDTV-DVD-STB/183803322670

He was hooking an old Tandy to it and said the Tandy was outputting digital RGB (I think CGA) and so he needed to add an additional component to convert it to analog:

https://www.ebay.com/itm/GGLABS-CGA2RGB-Commodore-PC-digital-to-analog-RGB-converter-C128-DAC-CGA-EGA-MDA/401220864442

I haven't heard anyone on this forum who uses that above SCART-to-HDMI also need to create an analog RFB signal. Adrian is pretty technically savvy so I doubt he screwed something up. Are we sure the QL RGB is digital?

Edit: this was the video: https://www.youtube.com/watch?v=HpfrelhokKQ


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Re: RGB to VGA

Postby tofro » Fri Feb 21, 2020 8:38 pm

bwinkel67 wrote:I haven't heard anyone on this forum who uses that above SCART-to-HDMI also need to create an analog RFB signal. Adrian is pretty technically savvy so I doubt he screwed something up. Are we sure the QL RGB is digital?

Edit: this was the video: https://www.youtube.com/watch?v=HpfrelhokKQ


You do. What people use is put serial resistors (~350Ohms) in the SCART connector that reduce the voltage from 5V TTL to ~1V analog.

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Re: RGB to VGA

Postby bwinkel67 » Fri Feb 21, 2020 8:54 pm

tofro wrote:
bwinkel67 wrote:I haven't heard anyone on this forum who uses that above SCART-to-HDMI also need to create an analog RFB signal. Adrian is pretty technically savvy so I doubt he screwed something up. Are we sure the QL RGB is digital?

Edit: this was the video: https://www.youtube.com/watch?v=HpfrelhokKQ


You do. What people use is put serial resistors (~350Ohms) in the SCART connector that reduce the voltage from 5V TTL to ~1V analog.

Tobias


Oh wow, mind is blown. I knew about putting those resistors on to cut the voltage (and have done exactly that with the cable I built) but I didn't realize that its purpose to to convert from digital to analog. I suppose maybe I'm understanding the term digital incorrectly when it comes to video.


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Re: RGB to VGA

Postby tofro » Fri Feb 21, 2020 8:59 pm

It's just that on the QL, it's absolutely easy, as it only has max 8 colours (That is, R,G, and B are either on or off).

On computers like the Spectrum that have an additional brightness signal that you need to mix into the R.G, and B signal, it is a tad more complicated.

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Re: RGB to VGA

Postby bwinkel67 » Fri Feb 21, 2020 9:51 pm

tofro wrote:It's just that on the QL, it's absolutely easy, as it only has max 8 colours (That is, R,G, and B are either on or off).

On computers like the Spectrum that have an additional brightness signal that you need to mix into the R.G, and B signal, it is a tad more complicated.

Tobias


Thank you Tobias, that really clears it up. So on the QL it just sends a high or low signal on each of the 3 lines giving you the 8 possible colors (black and white being on the extreme end of all-on or all-off). So sending this signal to an analog line you step down the voltage to have its intensity be something reasonable that appears on screen.


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Re: RGB to VGA

Postby tofro » Fri Feb 21, 2020 9:57 pm

bwinkel67 wrote:
tofro wrote:It's just that on the QL, it's absolutely easy, as it only has max 8 colours (That is, R,G, and B are either on or off).

On computers like the Spectrum that have an additional brightness signal that you need to mix into the R.G, and B signal, it is a tad more complicated.

Tobias


Thank you Tobias, that really clears it up. So on the QL it just sends a high or low signal on each of the 3 lines giving you the 8 possible colors (black and white being on the extreme end of all-on or all-off). So sending this signal to an analog line you step down the voltage to have its intensity be something reasonable that appears on screen.


Exactly. The 5Volts from the QL RGB are simply "eaten up" using a voltage divider consisting of the added resistor and the impedance of the monitor input, ending up at 0.7V or 0V on that input.

On the Spectrum 128, you need to add a bit of circuitry that pulls the R, G, and B even further down (by another 0.35V) when the BRIGHT signal is low, to give half brightness, thus 16 colors.


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