VGA or not VGA .. that is the question !

[lezanderson]

Recently i've seen and read quite a lot of stuff about people using PAL/NTSC to VGA converters with homebrew micros and stuff. The reason for this I'm not quite sure, but it seems if though they want their homebrew system to be like a 'Modern' PC.. Personally I think that all homebrew systems should use a TV output as that's part of the charm and gives a true 'Retro' fell. Also practically no homebrew machine uses a screen resolution which warrants a Monitor output. I have an old (15yrs old) JVC CRT TV and the picture quality is excellent and can rival most monitors ! My JVC TV could easily handle 512 x 256 or even 640 x 256 quite easily (perhaps even 800 x 512) , and lets face it everyone has a TV in the house or can buy one very cheaply.

You only need look at the GR8BIT to see the paradox of using a PAL/NTSC to VGA convert an old Retro system (MSX ) so it can use a modern monitor ! The GR8BIT uses a V9938 then a VGA convert board to give VGA output But Why would you want a MSX, using a V938 machine to emulate a PC ??

So you still want VGA output ... OK perhaps you're right... Then why not just use a V9990 it's a great ( inexpensive) MSX compatible video chip... giving VGA output and some powerful screen modes... Max screen resolution being 1024 x 424 (undocumented mode). I could never understand why the GR8BIT didn't use it...bizarre !

The only drawback with the V9990 is it's surface mount and you'll have to design your own Video card . Though you have got a template to look at GFX900 MSX video card and the Technbytes V9990.


The V9990 supports the following resolutions:
mode resolution remark
P1 256×212(424) dualplane patternmode (2 independent planes)
P2 512×212(424) patternmode
B0 192×240(480) undocumented overscan mode (no borders)
B1 256×212(424)
B2 384×240(480) overscan (no borders)
B3 512×212(424)
B4 768×240(480) overscan (no borders)
B5 640×400
B6 640×480 needs a 25.2MHz oscillator
B7 1024×212(424) undocumented mode


Other features of the V9990:

512kB VRAM (4 times as much as on the standard MSX2, so 4 times as much pages!)
A lot faster than V9938/58
Dualplane mode P1: two planes that can scroll independently, with 4 palettes of 16 colours out of 32768 per plane
125 sprites in P-modes, with upto 16 sprites per horizontal line. Any 16×16 block can be used as sprites, with the use of all colours out of one of the four palettes of 16 colours out of 32768
More interrupt-types
Easier programming
Posibility to use larger image size than display size
Interlacing is done by just doubling the picture-size, not by splitting odd-lines and even-lines to different pages, but just the uppper and lower half of the screen
Good priority-support for sprites

[1024MAK]

Ahh! But I think it is because they either have suplus 4:3 VGA LCD VDU's or want to use the same VGA LCD VDU with their retro computer(s) and a modern PC to save space...

I use a Sharp Aquos TV and that works with all the 8 bit and 16 bit retro computers that I have tried (once I worked out the correct settings). Although when using MTX computers, the colours are a bit dull compared to my other retro computers.

Mark

[lezanderson]

Also in RUSSIA I believe they use the SECAM TV standard, as does FRANCE.. so that's probably why the GR8BIT uses a TV to VGA adaptor ?

[Martin A]

Although "proper" VGA resolution is nice. It's not overly retro friendly once you move away from character based displays. After all, the 80 column text from the Rememorizor is somewhat sharper than the 80 column output from the V9958 board in text 2 mode.

There is an issue with the Z80 (or any of the 8 bit CPU's) and higher resolution bitmap modes, as the amount of data that needs to be transferred, tests both the chip's bandwidth and memory map. A TV might well be able to cope with 640 x480, But that resolution in 256 colours needs 300k memory. Just clearing the screen on a 4mhz Z80 is going to take time.

For the output from the V9958 we're using SCART, which although rare on monitors is pretty common on flat screen TVs.

Even on the V9958, 256 x 192 in 256 colours is 48k. With the time it take to transfer data to the VDP we won't be seeing any real time video playback!