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 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 - Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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