Amiga FPGA and video signal, is there any good FPGA?

[balrogsoft]

Hello.


Recently, my interest in buying an FPGA with an Amiga core has increased due to the drop in the price of the FPGA. I bought a cheap fpga based on UnAmiga fpga (with MIST AGA core), but I found many problems with the video, at the point that the seller returned my money after testing it on 4 monitors and 2 tvs with same problems.

Using the FPGA with VGA connector, shows strange vertical lines with more or less brightness forming bars.






Then I tried the scart connector with RGB signal, but the television shows a ripple distortion.





This is normal? The seller says it works on some TVs and monitors, but I tried 4 monitors with the same problem and 2 TVs. Does anyone have a Fpga with an Amiga core and have these problems? I can understand some problems with vga monitors, but the rgb signal should be more compatible.

[hooverphonique]

The top case is very normal and happens a lot when using lcd's with real Amigas as well.

[balrogsoft]

I haven't connected my Amiga to the vga port of a monitor, I do not know how it should look. But I have an RGB cable to SCART and I never saw this distortion problem using a flat lcd tv.

[robinsonb5]

Originally Posted by balrogsoft

I haven't connected my Amiga to the vga port of a monitor, I do not know how it should look. But I have an RGB cable to SCART and I never saw this distortion problem using a flat lcd tv.

The brightness bars are normal, both for real Amigas and FPGA replicas. The ripple distortion isn't normal, and real Amigas won't do that. The cause is probably just that the core's clocks are slightly off. It's not possible to create any arbitrary clock frequency with an FPGA - you have to get as close as you can (usually pretty close) by multiplying and dividing a master clock, but you often end up slightly over or under the target frequency. In this case it looks like the end result is just out of spec enough to cause problems for your TV. It might be possible to tweak the clock settings and get them close enough for the TV to handle, but unless the complete source for this port is available for this particular board's Minimig core, the publisher would need to do it.

[balrogsoft]

Thanks, it makes sense, this fpga uses a modified MIST AGA core to operate at different clock speed, and it could affect the generation of rgb video, and probably the MIST fpga do not have this issue.

[Stedy]

The Mist and Unamiga Video outputs are not compatible with SCART video inputs. It's the same problem that afflicts real Amigas with SCART, the 'video' sync signal is 2-3x larger than the TV expects. Look for the Amiga SCART cable design for further details.

[balrogsoft]

Well I must be very lucky, all the TVs I've had in recent years, worked perfectly with the rgb to scart cable. I never had this problem.

[Hewitson]

Originally Posted by hooverphonique

The top case is very normal and happens a lot when using lcd's with real Amigas as well.

It does? I've never encountered it on any of my Panasonics.

[MartinW]

Originally Posted by Hewitson

It does? I've never encountered it on any of my Panasonics.

Yeah, it's not that unusual and is well documented particularly with the otherwise great BenQ LCDs (702 / 912). If there is a pixel clock adjustment then it should be able to sort the problem in the first picture out - just increase (usually) the pixel clock value on the monitor and by then you will need to shift the picture horizontally one way or the other as well. As others have already said, the 2nd picture is not normal.

If the Panasonics are not showing any artefacts like that then they may just have much better auto-adjustment.