Re: [stella] Re: 2600's TIA

Subject: Re: [stella] Re: 2600's TIA
From: Kevin Horton <khorton@xxxxxxxxxxx>
Date: Tue, 28 Aug 2001 00:28:01 -0500
At 01:08 8/28/01 -0400, you wrote:
On Mon, 27 Aug 2001, Kevin Horton wrote:

> The CLK input line would be A0, and the divide by 6 would generate A1,2,
> and 3. the ROM would be 12 bits wide, and 12 words deep. Each bit line
> could be used to generate the required phases by "storing" a square wave (6
> 0's then 6 1's in a row); each off by 30 degrees (or exactly 1 ROM location).

Yeah, ROM lookup table are Good Stuff(tm).  The need for 12 words is obvious.
But why 12 bits?  Do you need to generate 12 signals? I don't know anything
at all about NES architecture.  Oh...duh.  Nevermind I got it.  When I do this
type of thing to create a phased waveform, I usually use a loadable barrel
shifter driving a regular shift register.  Was in the wrong mindset for ROM
lookups.  :)

> This same idea could of course be condensed down into logic, too.  This is
> more likely than a lookup table which is "expensive" in regards to chip
> real estate.

Yep.  ROMs are good for experimenting with until you know exactly what you

> If you're seeing a large AC signal, that tells me you didn't ground your
> scope, so you're seeing the AC line on it due to the open ground loop.  Fix
> that and check again.

No, it really was just a ripple.  Like the chroma in a composite NTSC signal.
A couple hundred mV at most.  It had a crawling phase was crawling, but it
was very frequency stable.  And it wasn't at 60Hz or was somewhere
around 7.5MHz.  So it wasn't line noise from poor grounding.  I don't remember

Hmm, oh I bet I know what you were seeing... probably switching power supply ripple. It would be superimposed on the video if it's not filtered out properly on the video card. The ripple can approach 50-200mv, and would be at a frequency determined by the regulator. Current draw in regular patterns (i.e. memory accesses or something) could cause the ripple to change in synch with it, superimposing odd harmonics and other junk. On a perfect RGB signal, this wouldn't be there.

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