I am curious is it possible to show a 256 line picture on a CGA monitor? It is quite easy to set the 6845 to show 256 lines (the BBC Micro uses such modes by default, the Amstrad CPC standard monitors can show 256 lines) but can a typical CGA monitor show all of them? Thank you.
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A CGA monitor resolution
- Thread starter vol.litwr
- Start date
I don't think so - those machines use PAL timings, but CGA was designed for NTSC. Total frame length is 262 scanlines, and 16 are already dedicated to vertical retrace, so even with no blanking at all you're 10 lines short.
You can of course increase the frame length, which reduces the vertical refresh rate - I've done that (entirely by accident) on my CRT TV which happens to support both PAL and NTSC, and got a black and white PAL picture, but a true CGA monitor won't sync to that.
You can of course increase the frame length, which reduces the vertical refresh rate - I've done that (entirely by accident) on my CRT TV which happens to support both PAL and NTSC, and got a black and white PAL picture, but a true CGA monitor won't sync to that.
kdr
Experienced Member
You can of course increase the frame length, which reduces the vertical refresh rate - I've done that (entirely by accident) on my CRT TV which happens to support both PAL and NTSC, and got a black and white PAL picture, but a true CGA monitor won't sync to that.
I was under the impression that most CGA monitors are (essentially) just television sets and thus their vertical deflection IC should sync up to anything in the 47-63hz range; it's the colour decoding that's wildly different between PAL and NTSC displays. So now I'm wondering if NTSC colour decoding is based solely upon detecting the colourburst or whether vertical sync frequency also factors into the decision. Unfortunately I don't have the One True CGA Monitor from IBM with which to confirm this. :D
Eudimorphodon
Veteran Member
An actual “CGA”, IE, RGBI monitor, certainly isn’t going to care about NTSC color decoding. (I’m sure how dual-standard TVs figure out which mode to be in varies, and maybe refresh rate is a convenient shorthand. An NTSC only TV missing any such magic will certainly only be looking for colorburst.)
I’m skeptical the IBM 5153 monitor will sync to a PAL frequency input, I suspect its less forgiving than your average TV set, but that’s just a gut feeling. Generalizing a bit further my rough guess is anything made for the US market is going to be pickier than TVs sold in Europe or Asia, because the Americas are pretty much an NTSC monoculture, while internationally multi-standard support was far more useful.
I’m skeptical the IBM 5153 monitor will sync to a PAL frequency input, I suspect its less forgiving than your average TV set, but that’s just a gut feeling. Generalizing a bit further my rough guess is anything made for the US market is going to be pickier than TVs sold in Europe or Asia, because the Americas are pretty much an NTSC monoculture, while internationally multi-standard support was far more useful.