Re: [stella] Re: 2600's TIA

Subject: Re: [stella] Re: 2600's TIA
From: Chris Wilkson <ecwilkso@xxxxxxx>
Date: Tue, 28 Aug 2001 01:08:51 -0400 (EDT)
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
the exact frequency, but it didn't seem to be a multiple of anything else that
I was seeing.

> Anyways, There's a couple types of RGB.  There's RGB+S which is RGB and
> separate synch, then there's RGB with synch on green.  It is just that- the
> synch info is on the G line.  The synch is of course the standard old synch
> as used on NTSC.  long ~60Hz for vertical and short ~15Khz for the
> horizontal.  There is of course no colour burst or phase information on the
> waveform- it is a purely analog signal; the brightness of the colour is
> directly related to the signal level.  More voltage = brighter.  I dunno if
> black level is sent like on NTSC; if it is, then it would work the same.

Yeah, this is the way I understood it.  And that's why what I saw was so
confusing.  I'll have to look again at some point.


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