AV modded Famicom - Screen blinking on light backgrounds in games

[averre]

I recently acquired my first Famicom and AV modded it according to the instructions by JPx72.
Video and sound output are great, and there are hardly any noticeable jailbars.

However, occasionally the screen starts to blink and I'm not 100% sure, but it seems that it is caused by "light" backgrounds.
I have been able to reproduce it in Super Mario Bros. 3, on World 1-3, when you get to "Coin Heaven".
Link to image of that part of the stage: http://cdn.wikimg.net/strategywiki/images/1/11/SMB3_W1_S3_c.png

I tested the same cartridge on my european NES, and it didn't have any issues.
The famicom's revision is HVC-CPU-GPM-02.

Does anyone recognize the issue?
Or does anyone have an idea of what the cause might be, and perhaps a solution?

Thanks in advance.

[famifan]

However, occasionally the screen starts to blink and I'm not 100% sure, but it seems that it is caused by "light" backgrounds.
I have been able to reproduce it in Super Mario Bros. 3, on World 1-3, when you get to "Coin Heaven".

what version of SMB3?  NTSC or PAL?

I tested the same cartridge on my european NES, and it didn't have any issues.
The famicom's revision is HVC-CPU-GPM-02.

obviously, PAL hardware has slightly more time to *render the scene*. Just compare the amount of scanlines http://wiki.nesdev.com/w/index.php/Clock_rate

More scanlines means more smoothly rendered scene.

So, the question is, what version of SMB3 you put into your famicom?

Besides SMB3, have you noticed that any other game was affected by the same issue?

[averre]

I've tested it with my original PAL copy, US release through an everdrive and an NTSC bootleg copy.
Same results with all three.

I've also experienced the blinking in some other games.
For instance in Gimmick! (through the everdrive), near the end of the first stage before the boss fight, there is mostly a bright waterfall in the background, with only a few squares of platform.
That's when the blinking starts. As soon as I'm passed it and there is more solid ground rendered on screen, it stops.

I'll see if I can grab some footage.

[P]

Since you have an Everdrive it's easy to test colours. You could try this color test program by Rainwarrior and see if certain colours makes your screen blink.

[chowder]

Double check all of the soldering on your mod.

[averre]

Thanks for your help and suggestions!

Did some tests with the colour test program.
The screen started to blink with these settings:
C: 0D
E: 111 (also 101)
S: 0

The weird thing is that it's a dark colour. There goes my theory.
After the test I booted up Gimmick!, got to the waterfall part that I mentioned in my previous post, but there was no screen blinking.

I guess the next step is to recheck the soldering.

[P]

Colour $0D is the "forbidden black" that sets a voltage that is "blacker than black" and glitches on certain TVs. It only glitches when you have the emphasis bits red and blue on (E: 101) though?

But Gimmick isn't using $0D as far as I know so maybe that is a different problem. Or maybe some error in the modding is causing some colour to become $0D.

[averre]

It glitched the most when I had E: 111 on. And a bit less with E: 101.
No glitching with any other E settings.

I checked the soldering on the mod, and it seemed pretty ok, but I still redid some of the soldering points.
But it had no effect when testing it on the tv. Still the same screen blinking.

However, yesterday I went over to my parents home, and tested it on a CRT tv.
I could not reproduce the blinking on that tv!

I've previously only used the famicom on a plasma tv, since that is all I have.

[P]

I was just going to suggest trying another TV. OK maybe your TV doesn't like colour $0D, but it's strange that it only glitches when using that colour together with emphasis bits.

If you set the background colour to white (either $20 or $30), what colour tint do the screen get if you set the emphasis bits? The bits are E: BGR (Blue, Green, Red) in that order. So for example if you only set the red bit (E: 001) the screen should get a red tint. If you set both green and red (E: 011) you should get a yellow tint. Does that work properly?

[averre]

When settings the emphasis bits on the colours $20 and $30, I get the tints that you described.
They are very light though. Should they be?

001 gave me a light red colour.
011, a light yellow colour.
100, a light blue colour.

I also tried using a SCART adapter, instead of the composite input, but the behaviour was the same.

[P]

Sounds like the emphasis bits behaves as they should. The tints are very light on my LCD TV too (though colour $0D never glitches on that TV).

Using a SCART adapter shouldn't change anything. It's still the same composite signal, you just change the shape of the plug.

I guess your plasma TV just doesn't like certain combination of colour and emphasis bits. I never heard that SMB3 or Gimmick is using emphasis bits or colour $0D though.

One ROM I know that is using colour $0D is the Gameie Genie BIOS ROM. It uses colour $0D as the backdrop colour.


Edit: I asked around at Nesdev and they seem to believe it's the TV too:

I don't believe that either of those two games uses $0D or emphasis.

I've seen modern TVs do various stupid things because of their post processing, motion interpolation, etc. Maybe just try switching your TV to "game mode" if it has one? Usually that disables a lot of extra operations in the display chain that can mess things up.

[chowder]

Probably a long shot, but are you using the same AV cable for the NES/Famicom/other TV tests?

If you're willing to try another mod, give this one a shot.  I must have modded a dozen or more consoles using it for other people and never had the issue you describe:

[averre]

Thanks P for the help, and for asking on other forums.
I also tried adjusting the settings on the TV, but it didn't help.

But it doesn't bother me that much (yet).
Like I mentioned before, I've only experienced it in very few games, so far.

Next time I buy a TV I might have to bring the famicom to the store and hook it up to test
Post Merge: November 01, 2015, 11:25:56 am

Probably a long shot, but are you using the same AV cable for the NES/Famicom/other TV tests?

If you're willing to try another mod, give this one a shot.

I'm using different cables on the systems. I modded the famicom with a 4 pole 3.5 mm jack.
I have another famicom, as backup, that I haven't modded yet. I might try your mod when I get around to modding it.

Thanks for the schematic.

[P]

Hmm now it sounds like it could be the AV-mod that is at fault after all. Check the Nesdev thread.