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

From: Anders Carlsson <>
Date: Fri, 26 Aug 2011 11:25:37 +0200
Message-ID: <667A4B770E754B72990EC36CB7C6ABEE@ryds>
Marko Mäkelä wrote:

> If I remember correctly, my demo program would switch the display to an 
> interlaced 8x16 font on the 6560.

Hm, I always thought the interlace bit on the 6560 just toggles the chip 
from producing 30 full (progressive) frames @ 60 Hz to producing 60 half 
(interlaced) frames. Does the perceived vertical resolution really improve? 
On newer computers like the Amiga, interlaced mode really means double 
vertical resolution at the cost of flicker unless you have a monitor or 
flicker fixer than can handle it. I have read about NES and probably also 
C64 units that produce a progressive video signal which some newer TFT, LCD, 
Plasma etc TV's spew upon since they expect an interlaced signal.

Also I seem to remember the interlace bit on the 6560 was introduced only 
for Zenith TV's, which suggests to me they would have a problem with a 
progressive video signal. I doubt we ever had Zenith in the PAL world, so 
this bit would then not make any sense.

More interesting are the value ranges used on the 6560 and 6561 chips. 
Screen positioning in horizontal and vertical directions cover completely 
different value ranges, which is why all VIC-20 cartridges made for NTSC 
systems display the picture in the upper left corner on a PAL machine. Many 
of them let you center the display though. I understand the 6560 was 
completed before the 6561, so possibly due to the PAL display is larger they 
came up with different ranges for positioning. Also the granularity of 
screen positioning is different; the 6560 has far fewer steps than the 6561 

I own both types of VIC's, but as I only have PAL TV's of which some will 
accept NTSC composite video in colour, I can't say for sure how a real NTSC 
TV or monitor would act. At least I got no action at all by toggling the 
interlace bit on my NTSC VIC-20 using a PAL monitor.

Best regards

Anders Carlsson 

       Message was sent through the cbm-hackers mailing list
Received on 2011-08-26 10:00:19

Archive generated by hypermail 2.2.0.