Posted By
TLC
on 2007-10-29
16:26:45
|  Re: Drean Commodore 16 details -- finally...
Degauss: thanks, it's been a long, but not at all that hard "project"... :-D ...I just kept my eyes open (for the last couple of years), as I knew the chance should come sooner or later as eBay gets more and more popular.
As I guess the machine won't even be hard to emulate (better said: darn simple, with all those things known by emulator writers today), the only thing needed is/was to finally obtain a computer (that nobody seemed to have inspected closely yet, as nobody had one), make notes of some info, make some simple tests -- all in all, to find evidence about a couple of anyway simple things. And after that, why not emulating it... it definitely isn't a well known platform, but it's part of the history, and it was a production machine anyway... IMHO a very interesting production machine, but that's probably just my point... 
(BTW: as I read, in Argentina, the situation has been slightly similar to that of Hungary... I mean, there were no real local software resellers that would have sold the known software titles. Instead, mostly everything, including a considerable amount of the computers (excluding those manufactured locally) were grey-imported to the country... The folks there spreaded pirated software from hand to hand, similarly to how it went in Hungary in the middle and late '80s... I wonder why didn't evolve a scene there, as it happened here... (unfortunately, I don't know the answer), it would have been interesting to meet software titles and demos made for effectively the same machine, from a country which/whose folks are that far from here so probably have their own great names...  ).
As to why Commodore had to change the clock: I think they just wanted to support PAL-N with as few modifications as possible. In the simplest case, for PAL-N to work, they just chose a crystal whose frequency is (similarly to how it's done in the PAL and NTSC models) 4 times the color subcarrier frequency of the tv-standard.
PAL-N's color subcarrier frequency is almost like that of NTSC's (3.582055 vs. 3.579545, the difference is almost negligible), but is higher nevertheless. That's why the PAL-N computer is a little bit faster than the NTSC one (assuming the same clock division chain). In other words: the only reason why the computer's speed doesn't match the speed of either the PAL or NTSC model, is the different television standard to be supported (whose color subcarrier frequency differs from both PAL and NTSC).
As to how much is the difference:
PAL --> NTSC: ~=0.92%
NTSC --> PAL-N: ~=0.07% (very small)
PAL --> PAL-N: ~=0.99% (not that small, but still almost unnoticeable, except for critical cases)
A quick test has shown that the 8365 generates 57 cycles per line, and 312 rows per frame in PAL mode (similarly to the usual PAL machines)... the overall speed and framerate is slightly higher than that of the European machines, but as long as the computer won't have to keep up syncing to the "outside world" (ie. the only known "time" it knows about is its own time), and unless there's some heavy modification in the 8365 that I'm not yet aware of, it should run all time-critical code well... I haven't made any further tests, as the computer is still in pieces, I just connected a keyboard to it to make those first-tests.
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