MiSTer FPGA Forum

Hi all,

Am I right in saying ao486 cannot output via RGB? Presumably because RGB is not digital?

mist-rich wrote: ↑Mon Oct 31, 2022 6:17 pm Hi all,

Am I right in saying ao486 cannot output via RGB? Presumably because RGB is not digital?

It can, but at 31khz since it's VGA.

You can do 15kHz using modelines. It looks quite amazing.

Its better to say that AO486 doesn't have native, analog output. It's more like you got vga_scaler=1 on all the time, so you need to set up some video modes to make the scaler target a compatible resolution for your display (probably 15khz in this case)

I didn't realise that , i'll have another look at my settings. The Mister really is an amazing device

- I need to read up more on the workings of frequency as I don't fully understand it at the moment. Would a 15khz mean the screen is refreshing 15 times per second (also know as hertz)?

No, it's kilohertz, thousands of hertz. It's a measurement of video bandwidth to the display. 15KHz was the signaling rate used by the earliest 80-column screens, like CGA, Amiga graphics, or Atari ST color mode. It could send a wide variety of signals, because the bandwidth could be split many ways. The Amiga, for instance, could do 320x200 or 640x200 in varying color depths. But 400 lines was too much, so 320x400 and 640x400 had to be sent interlaced, sending the even and odd lines alternately. (which caused nasty flicker on most monitors.)

VGA doubled that video bandwidth, so it could do 400 lines without flicker. If you use a modeline to output a 15KHz signal from the Mister with AO486 as a source, the image will be downsampled, meaning that the edges of things will be a little blurry. For videogames, this often looks pretty good, but if you're trying to do 80-column text, it might potentially be annoying.

Cheers Malor You're a wealth of knowledge and that has made things clearer.

I'm starting to see how modern (digital) eradicated those issues and the reason why shaders/filters are used to replicate those 'quirks '
..Also probs goes someway to explaining why when I take a photo of a CRT I can see lines in the image?

- when things moved to digital (DVI era?) presumably the data was all 1&0's and there was nothing frequency-wise being sent in the cable any more?

Video bandwidth is definitely still a thing. It's just enormously larger than it used to be. That's true even on digital monitors, which still receive data just like way back when on 15KHz monitors; each frame is sent left to right, top to bottom, one pixel at a time. There's still a virtual raster beam in every display. They just draw a lot more lines and columns, and on more modern screens, do so at much higher framerates.

Most LCD-type screens still update their pixels top to bottom, too, although sometimes the panel is set sideways, and thus draws in some other direction, perhaps left to right. I'm pretty sure OLEDs also do this. But they update in stripes, with bands of multiple pixels all changing simultaneously. I'm not sure how big the bands are.

The earliest 80 column modes used a 15KHz signal; modern displays use far more. The most recent standards can drive 80 gigabits. I could probably figure out the actual conversion from kilohertz to gigabits, but it's early and I have to leave soon. Maybe I'll post back later, but for now, "much faster" should suffice.