Re: NTSC VIC-II timing

Date: Thu, 22 Jun 2017 01:28:35 +0000
Message-ID: <>
June 21, 2017 11:01 AM, wrote:
> On Wednesday 21 June 2017, 16:40:03 wrote:
>> According to the famous document by Chris Bauer, there are three timing
>> variants of the VIC-II. While PAL is always 63 cycles per line, with NTSC
>> there can be either 64 or 65 cycles per line and I am in fact able to
>> reproduce the difference with -model ntsc and -model oldntsc in the current
>> VICE.
>> The question (before I spend weekend on trial'n error counting cycles and
>> possibly reinventing the hammer ;-) is: do we have ane established,
>> reliable software method for detecting which NTSC VIC-II is installed in
>> the machine? I guess it must have been done multiple times by now and used
>> in some NTSC games/demos..
> most demos or games probably dont care about detecting the difference - as the
> 64 cycle VIC is extremely rare.
> however, iirc on codebase there are some detection routines. basically just
> count the cycles for a line (or a frame) using a CIA timer.

I wouldn't call them "extremely rare". They were in all the early C64's extending way beyond the silver badge era. Going by my serial database, I would say they were in the first ~300,000 C64's since they were found in all 326298 boards (all the S-series machines which stop before 00100000, and at least the first 2.5 hundred thousand P-series, which start at 00100000). I don't have enough samples to know how many of the Canadian machines there really were, but they were in those also.

Now, are they being used as "daily drivers" still- I doubt it. The video quality is greatly enhanced, even on the old board, but upgrading to a newer revision. The old 6567R56A chips have very grainy, low-contrast video.

Pete Rittwage
C64 Preservation Project

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Received on 2017-06-22 03:00:02

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