Re: 6561 (PAL VIC-I) interlace mode bit

From: Segher Boessenkool <*segher_at_kernel.crashing.org*> Date: Sat, 27 Aug 2011 00:47:18 +0200 Message-Id: <40710F72-DAD5-462E-87D3-4A4E63B5CE79@kernel.crashing.org> · segher_at_kernel.crashing.org

> Looks like they've intentionally disabled the function of the INT  
> (interlace) bit. Maybe it interfered with the PAL encoding system  
> in some way.
>
> On the C128 VIC-IIe, you can really mess up the PAL colour encoding  
> by stealing (IIRC) odd numbers of lines using the TEST bit. On the  
> TV I tested this on several years ago, the colour disappeared and  
> would occasionally flicker back on but mostly stayed off. In PAL  
> the alternation of the R-Y component of the colour burst and colour  
> information must be maintained each line. On the PAL C128 the  
> number of lines is even (312) and so a complete sequence of 156 PAL  
> switch alternations occurs every frame. Possibly the PAL switch is  
> tied directly to the LSB of the Y counter. If this were the case,  
> it wouldn't work well with interlace mode.

I cannot really check the 6561 for this now, but at least the 8565
(new PAL VIC-II) uses Y0 for the alternating colour phase generation,
indeed.

Segher

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