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
want.
> 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 120Hz...it 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.
-Chris
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