RGB to VGA

Nagging hardware related question? Post here!
User avatar
mk79
QL Wafer Drive
Posts: 1349
Joined: Sun Feb 02, 2014 10:54 am
Location: Esslingen/Germany
Contact:

Re: RGB to VGA

Post by mk79 »

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


User avatar
mk79
QL Wafer Drive
Posts: 1349
Joined: Sun Feb 02, 2014 10:54 am
Location: Esslingen/Germany
Contact:

Re: RGB to VGA

Post by mk79 »

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.


User avatar
Pr0f
QL Wafer Drive
Posts: 1298
Joined: Thu Oct 12, 2017 9:54 am

Re: RGB to VGA

Post by Pr0f »

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


User avatar
bwinkel67
QL Wafer Drive
Posts: 1187
Joined: Thu Oct 03, 2019 2:09 am

Re: RGB to VGA

Post by bwinkel67 »

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- ... 3803322670

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 ... 1220864442

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


User avatar
tofro
Font of All Knowledge
Posts: 2685
Joined: Sun Feb 13, 2011 10:53 pm
Location: SW Germany

Re: RGB to VGA

Post by tofro »

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


ʎɐqǝ ɯoɹɟ ǝq oʇ ƃuᴉoƃ ʇou sᴉ pɹɐoqʎǝʞ ʇxǝu ʎɯ 'ɹɐǝp ɥO
User avatar
bwinkel67
QL Wafer Drive
Posts: 1187
Joined: Thu Oct 03, 2019 2:09 am

Re: RGB to VGA

Post by bwinkel67 »

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.


User avatar
tofro
Font of All Knowledge
Posts: 2685
Joined: Sun Feb 13, 2011 10:53 pm
Location: SW Germany

Re: RGB to VGA

Post by tofro »

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


ʎɐqǝ ɯoɹɟ ǝq oʇ ƃuᴉoƃ ʇou sᴉ pɹɐoqʎǝʞ ʇxǝu ʎɯ 'ɹɐǝp ɥO
User avatar
bwinkel67
QL Wafer Drive
Posts: 1187
Joined: Thu Oct 03, 2019 2:09 am

Re: RGB to VGA

Post by bwinkel67 »

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.


User avatar
tofro
Font of All Knowledge
Posts: 2685
Joined: Sun Feb 13, 2011 10:53 pm
Location: SW Germany

Re: RGB to VGA

Post by tofro »

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.


ʎɐqǝ ɯoɹɟ ǝq oʇ ƃuᴉoƃ ʇou sᴉ pɹɐoqʎǝʞ ʇxǝu ʎɯ 'ɹɐǝp ɥO
User avatar
Cristian
Aurora
Posts: 960
Joined: Mon Feb 16, 2015 1:40 pm
Location: Veneto

Re: RGB to VGA

Post by Cristian »

I've just found this device I didn't know.
It accepts some kind of RGB input. Probably it's worth investigating...

https://www.serdashop.com/MCE2VGA

Image


Post Reply