Not sure how frowned upon cross-posting might be, but I realized alt.comp.hardware.homebuilt seems to mostly be full of trivial things like video cards and 'XP or Vista' these days, so I'll post here as well, where a more appropriate group of folk will see and hopefully know something about the subject!
- - - - -
Hiya folks. I got all interested in project building last summer, only for winter to roll around and put a damper on my soldering (couldn't open the windows), so I pretty much fell out of the electronics mood for quite a while. But that urge to build has returned, and I'm back with idears on things I want to try and get back to making my own little computer!
I'm still really interested in being able to generate my own NTSC signal, and having poured over more documents on the subject again this summer, I think I'm pretty close to another attempt. Particularly since I've ordered a new microcontroller to try with, since my PIC I was attempting it on died last summer. But this time it's a 8052 variant, which I think I've fallen in love with due to the fact it can have both an externally accessible address/data bus as well as still have the all-in-one charm of a PIC. This one is also capable of one machine cycle per clock, as long as you stick to using the internal flash/sram.
Anyhoo, I'd just like to try this signal generation manually first, in black and white, pumping out the sync and levels myself. Only downside is the amount of misinformation on the internet. It would seem that some say the sync pulse is about -0.4v, blanking around 0v, black around 0.3v, and white at 1v. Other people say sync can be 0v, blanking/black 0.3v, and white 1v. I've also seen sync at 0, blanking
0.3, black 0.4, white 1, etc etc. I mean, does it really matter that much? Could I get by with just using 0, 0.3, and 1? I really don't want to have to try and start getting a negative voltage if avoidable, since to be honest, I'm not entirely sure how I'd do it at the moment. Televisions seem like they're fairly lenient, and with all the methods out there I've seen, I would assume most/all of them work, albeit possibly with less quality in some respect. I also found some clashing information on sending vertical sync, with some saying it's okay to just send repeating inverted sync pulses the whole time (and for the whole line, instead of doubles) to get back to the top instead of surrounding it with equalizing pulses.Aside from doing it the raw way, I also want to try using a 6845 I ordered last year to generate video. I think I've wrapped my head around how it works fairly well, but after looking at CGA register values, I found myself confused. It seems that if you're going to run in either 320x200 or 640x200 graphics modes, you don't set the number of vertically displayed rows to 200 as one might think, but 100 instead. I don't understand this, or how the screen would then be displayed properly, since it seemingly is only doing half the screen. As far as I know, CGA doesn't ever interlace the actual screen, despite the graphics memory being laid out interlaced (which I'm sure is related to this somehow). If anyone can shed any light on this for me, I'd sure appreciate it!