The strange case of the Commodore 64 colour bands.

[stephen_usher]

That clock chip is the later revision board. This one has a 74LS629(?) generating the chroma clock and a divider circuit to create the dot clock.

The modulator does all the combination of the signals. I have touched a wire connected to the composite input on the telly to the luma and chroma outputs from the VIC-II. The luma is clean but the chroma shows the oscillations. However, the two are coupled via capacitors anyway, via the composite output in the PAL modulator, so there will be cross talk.

The reverse engineered schematic can be found here: http://www.zimmers.net/anonftp/pub/cbm ... UE3603.jpg

[stephen_usher]

I've just watched some captured video from someone else's PAL C64... They have the same chroma bands, but just a little bit less pronounced. Maybe it's a design issue?

P.S. I've just brought the C64 down stairs and plugged it into my SVHS recorder via S-Video and it's hugely better, but I can still see the colour fringing, but it's not as marked.

P.P.S. Here's how it looks on the plasma TV... (And the SID sounds incredible on the HiFi.)

IMG_2669.jpg (67.61 KiB) Viewed 2039 times

[rubber_jonnie]

I've just watched some captured video from someone else's PAL C64... They have the same chroma bands, but just a little bit less pronounced. Maybe it's a design issue?

P.S. I've just brought the C64 down stairs and plugged it into my SVHS recorder via S-Video and it's hugely better, but I can still see the colour fringing, but it's not as marked.

P.P.S. Here's how it looks on the plasma TV... (And the SID sounds incredible on the HiFi.)


IMG_2669.jpg

Wow that is really noticeable!!

Pretty sure I've never seen that on my 64C, so sounds like it could definitely be a design flaw. Mind you I do recall a friend with a breadbin back in the day and I really don't recall anything like that, and we played games together a lot.

Could it be that it's the TVs? Do you have an old analog one you could try, perhaps it needs slightly less fussy hardware to display on? Or have you tired it on a CRT already, I've lost track slightly.

[stephen_usher]

I've not tried on a CRT, no.

I think that the next time I order stuff from Mouser I'll add a 17.7334MHz crystal oscillator to the basket. You never know, it might help.

[rubber_jonnie]

I've not tried on a CRT, no.

I think that the next time I order stuff from Mouser I'll add a 17.7334MHz crystal oscillator to the basket. You never know, it might help.

Worth a try as if it is drifting it could be the cause of your problem.

Think I just watched the same Adrians Digital Basement video, quite surprising how the jailbars just went after a bit of power cycling.

[stephen_usher]

Someone on another site suggested that if I used a CRT display the issue wouldn't be there.

Well, I brought down my old Ferguson TX-RGB 14" telly and used the composite input...

The issue is harder to see, but so is the display!

IMG_2670.jpg (60.32 KiB) Viewed 2006 times

As can be seen from the BASIC display...

IMG_2678.jpg (37.3 KiB) Viewed 2006 times

But the issue is still there.

IMG_2679.jpg (104.56 KiB) Viewed 2006 times

Note: It looks better in real life. The iPhone camera was having an extremely hard time taking photos of the CRT display.
Note 2: Scan lines on scan doublers etc. get it all wrong. The visible lines on a CRT do not go left to right, they go up and down due to the shadow mask grid.

[rubber_jonnie]

Definitely looks better in the game, I presume the basic prompt is a bit fuzzy in the pics is due to the difficulty in taking photos?

Did you order a crystal, or are you waiting until you have enough for a decent order?

[stephen_usher]

Well, on the Ferguson the text on the BASIC is almost the same colour as the background! i.e. light blue on a slightly darker blue. I don't know who chose those colours. Still, I think I need to go into the back of the telly and readjust the colour balance on the guns, the red and green need boosting a bit.

I put in an order for the stuff last night, some DIP HCT logic chips, to act as bus level translators between the Pi Pico (3.3V) and everything else (TTL), and some 8Kx8 SRAM chips, so I added the crystal to the order. (I'm also trying to find some low capacity DIP dual-port 8 bit SRAM chips which are still in production and hopefully not as expensive as the Reneas ones I've found, £20 each for 1Kx8!)

[rubber_jonnie]

Well, on the Ferguson the text on the BASIC is almost the same colour as the background! i.e. light blue on a slightly darker blue. I don't know who chose those colours. Still, I think I need to go into the back of the telly and readjust the colour balance on the guns, the red and green need boosting a bit.

I put in an order for the stuff last night, some DIP HCT logic chips, to act as bus level translators between the Pi Pico (3.3V) and everything else (TTL), and some 8Kx8 SRAM chips, so I added the crystal to the order. (I'm also trying to find some low capacity DIP dual-port 8 bit SRAM chips which are still in production and hopefully not as expensive as the Reneas ones I've found, £20 each for 1Kx8!)

Wow that's some expensive RAM

IIRC there is a command to change the text to white, but you're right, whoever chose the colours wasn't thinking straight, the VIC-20 has a much nicer BASIC environment colour wise at least.

Be interesting to see what it looks like once the guns are adjusted, not to mention how it all looks with a new crystal.

[stephen_usher]

I may get time to take a look at the TV again soon. It's back up in the loft at the moment so it's a bit of pain to get down.