VGA output black and white speckles or bars?

SurfFerret · 02 March 2022, 00:12

Returning Amiga user here! Been using a CD32 with an OSSC (via an SX-1) for some time, but got my A1200 recapped. Output through CD32 RGB->SCART is smooth, but there are some artefacts on the A1200 even after filtering and I need help understanding what to look at next.

I tried the VGA output in case HSYNC helped. The image is technically crisper, except there are vertical black and white bars all over the place! At 15KHz it is a 'snowy' pattern (black and white alternate) and 31KHz they are vertically aligned. This is the same on an old monitor and TV, except they have awful phase issues too.

Examples here: https://imgur.com/a/fbXaHPf

The one that matters most of course is 15KHz

unfortunately it's frozen in time so you can't see how incredibly unusable it is.

The artefact exists over SCART too, except the black and white pattern presents horizontally after filtering and rolls down the screen like an old VCR recording. But it at least is a solid image in most of the screen most of the time.

I've tried this with different VGA cables (more shielding = crisper, but all have the same pattern), unbuffered and buffered adapters, plus an old VGA adapter that I used 20 years ago on a CRT. All have the same effect. I also checked with a 4-colour screen mode to try and rule out bad DMA memory access. Is this a problem with the adapter or the DAC? I have an Indivision on the way, so basically I'm just hoping it's not Lisa/Alice/chipram.

Torti-the-Smurf · 02 March 2022, 13:01

I never have seen dots in that context but its essential to adjust

CLOCK and PHASE !

Did you do that allready ?

Best place to do that is in a paint programm with an Dark Grey background.
(try also some different very muted colours. The CLOCK and PHASE has to be EXACT otherwise you get all kinds of noise and frong scaling/stripes)

So, its essential to adjust both and then , write them down, just in case

Hope that helped.

NoX1911 · 02 March 2022, 22:12

Not sure if helpful but here is someone who seems to have had the same problem:
http://www.iljitsch.com/2020/09-16-u...-over-vga.html

SurfFerret · 03 March 2022, 01:44

That’s pretty much it, same pattern but less intense than mine so it disappears under dblpal. I never tried VGA on the CD32 - OSSC couldn’t sync - and there’s no low pass filtering that way. Knowing I’m not the only one seeing this is helpful enough.

I would play around with double sampling (like for superhires modes) but the Indivision arrives tomorrow so it should be moot. Hopefully.

02 March 2022, 00:12	#1
SurfFerret Registered User Join Date: Feb 2022 Location: London Posts: 4	VGA output black and white speckles or bars? Returning Amiga user here! Been using a CD32 with an OSSC (via an SX-1) for some time, but got my A1200 recapped. Output through CD32 RGB->SCART is smooth, but there are some artefacts on the A1200 even after filtering and I need help understanding what to look at next. I tried the VGA output in case HSYNC helped. The image is technically crisper, except there are vertical black and white bars all over the place! At 15KHz it is a 'snowy' pattern (black and white alternate) and 31KHz they are vertically aligned. This is the same on an old monitor and TV, except they have awful phase issues too. Examples here: https://imgur.com/a/fbXaHPf The one that matters most of course is 15KHz unfortunately it's frozen in time so you can't see how incredibly unusable it is. The artefact exists over SCART too, except the black and white pattern presents horizontally after filtering and rolls down the screen like an old VCR recording. But it at least is a solid image in most of the screen most of the time. I've tried this with different VGA cables (more shielding = crisper, but all have the same pattern), unbuffered and buffered adapters, plus an old VGA adapter that I used 20 years ago on a CRT. All have the same effect. I also checked with a 4-colour screen mode to try and rule out bad DMA memory access. Is this a problem with the adapter or the DAC? I have an Indivision on the way, so basically I'm just hoping it's not Lisa/Alice/chipram.

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02 March 2022, 13:01	#2
Torti-the-Smurf Registered User Join Date: Dec 2018 Location: Earth Posts: 1,086	I never have seen dots in that context but its essential to adjust CLOCK and PHASE ! Did you do that allready ? Best place to do that is in a paint programm with an Dark Grey background. (try also some different very muted colours. The CLOCK and PHASE has to be EXACT otherwise you get all kinds of noise and frong scaling/stripes) So, its essential to adjust both and then , write them down, just in case Hope that helped.

02 March 2022, 22:12	#3
NoX1911 2064 Join Date: Jul 2005 Location: de Posts: 231	Not sure if helpful but here is someone who seems to have had the same problem: http://www.iljitsch.com/2020/09-16-u...-over-vga.html

03 March 2022, 01:44	#4
SurfFerret Registered User Join Date: Feb 2022 Location: London Posts: 4	That’s pretty much it, same pattern but less intense than mine so it disappears under dblpal. I never tried VGA on the CD32 - OSSC couldn’t sync - and there’s no low pass filtering that way. Knowing I’m not the only one seeing this is helpful enough. I would play around with double sampling (like for superhires modes) but the Indivision arrives tomorrow so it should be moot. Hopefully.

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