Subject: Re: [stella] Cosmic Ark scanline pixel doubling From: Andrew Towers <mariofrog@xxxxxxxxxxx> Date: Sat, 12 Mar 2005 12:47:48 -0500 |
At 09:05 AM 12/03/2005 -0500, you wrote: >Andrew Towers describes the HMOVE handling and the Cosmic Ark >starfield effect quite nicely in the document that you linked >to in one of your last messages. He only doesn't mention the >pixel doubling for the missiles. I have had some thoughts on this, but I haven't had a chance to write them up yet. Well, here we go. I haven't been able to find anything specific in the schematics either for this one; all I can tell you is the doubling/missing missile pixels must be due to interactions between the normal movable object clock (MOTCK) and the extra pulses sent by HMOVE during the visible part of the scanline. In particular, I have reasoned thus: the effect repeats itself every four lines which is interesting because it corresponds to the starting phase of the two-phase clocked logic for the missile position counter. There are 68 clocks of HBLANK time, and each HMOVE moves the missile 17 pixels left. If I were to do this four times, I would have used up 68 clocks (17*4=68) so I'm back where I started. The position counter counts once for every four clocks, and each line I add 17 -- so I actually add 4 and 1/4 counts to the position counter each line. This puts the position counter 1/4 out of phase with the previous line each time. This does not explain the doubling/missing effect, but it lays the groundwork. The 17 pixel left shift is due to extra HMOVE clocks during the non-visible part of the scanline; this is quite straight-forward. I belive the strange pixel behavour is due to "undefined" interactions between the normal MOTCK and the extra HMOVE pulses during the *visible* part of the scanline only. I call these "undefined" because the interactions depend entirely on the precise phase between the two clock signals (the HMOVE pulses are twice as wide as the MOTCK pulses, and only come once every 4th MOTCK). The phase depends on the gate delays introduced on the HMOVE signal (approx. 4 gates worth), which in turn depends on the exact transistor layout of the gates and the manufacturing process used! With this in mind, it can be seen that the HMOVE clock might simply extend a normal MOTCK pulse, causing a double-wide pulse. This might coincide nicely with two pixel clocks, causing a double-pixel to be displayed, but only if it falls on the correct 1/4 cycle of the position counter. Alternatively, with different transistor sizes in a later revision (the TIA-1A schematics mention sizes changing for that revision), the behaviour might be completely different, with the extra HMOVE pulses plugging up MOTCK pulses instead of widening them. I suspect this is also why some of these "borderline" effects appear/disappear as the machine warms up. I haven't looked into this very thoroughly, but these are my current thoughts on the matter. Regards, Andrew. Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://stella.biglist.com
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