Re: Aw: Re: [stella] Krokodile Cart prototype review

Subject: Re: Aw: Re: [stella] Krokodile Cart prototype review
From: "Oliver Achten" <achten@xxxxxx>
Date: Fri, 27 Feb 2004 21:10:21 +0100 (MET)
> > Btw, those distortions are caused by the composite sync line of the TIA.
> Huh?  That can't be right...could you give details, Oliver?

Well, i have now modded many PAL Ataris in the past. My first mods were
rather lousy, they had distortion patterns which changed from game to game, and
sometimes when something on screen happens. I discovered that the distortion´s
origin was the CPU, and that the patterns are the state of the data lines of
the CPU. 

Then, playing to improve my composite mods, i discovered that when i added a
1k resistor between the TIA and my composite mod, those patterns magically
disappeared. It was not a poorly decoupled power circuit, as i thought, no, it
was the TIA passing the CPU signals through via the SYNC line. Talk about
cheap chip design here.

Btw, the TIA has many other flaws which are really a nightmare for me
getting a perfect signal. For instance, the hotter the chip gets, the more the
LUM0-LUM2 signal´s timing starts to vary. Result: soem colour combinations
produce small little white bars between colour changes. An effect which can only be
seen on a composite mod, since the TV modulator´s poor frequency resonance
filtes these sharp bars out.

The only way to overcome this mess is latching the LUM0-LUM2 signals using a
74LS374 or similar, clocked by the 3,58Mhz master clock.

Hope that clarifies some issues a little bit

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