64 Column Zeddy.... Well maybe...

[Andy Rea]

Okay it's not a joke, when tinkering with my CPLD code, i accidently mixed up the clocks and fed the video shift register with 13Mhz so all the characters came out half the normal width with space inbetween, looked kinda odd but hey oh...

So the idea, Borrowed off PokeMon is to created a modified Zeedy Rom that does 64 columns. What Would one need to do, can't be that hard right

First off we would need to alter the value(s) loaded into the Refresh register during display generation to allow 32 more NOPS before the int is generated by A6 going low during refresh.
Secondly the PRINT, print AT, PLOT/ UNPLOT routine(s) would also need to be modified.

hopefully this can be achieved easily by just changing a few bytes, like the test in the print routine that 'CP $21' to see if the end of the line has been reached, would simply change to CP $41, probably the same can be said of the plot/unplot range tests.

i also see no reason why and extended version of WRX could not be used.

Regards Andy

[Andy Rea]

With a bit more thinking about it...

running the Z80 at a constant 6.5Mhz and the shift register at 13Mhz, we will also need to patch the load/save routine bu doubling the timing constants.

Andy

[sirmorris]

How would the TVs handle that pixel clock? I s'pose modern tvs might have an edge here?

[Andy Rea]

Dunno, probably won't if it's an old un, on my CCTV monitors it was ok, might be a case of suck it and see.

I was more wodering how extensive the modifications to the rom would need to be.

Andy

[PokeMon]

First off we would need to alter the value(s) loaded into the Refresh register during display generation to allow 32 more NOPS before the int is generated by A6 going low during refresh.

Nice to hear. As I already have other things to do, working on a low cost memory board with full 56kByte support. And I mean what I say.
Without any internal modifications. Unfortunately I killed my memory expansion accidently and now has to realize what was in my mind for some later period.

Keep in mind to test with maybe 13.5 MHz instead of 13.0 MHz as this is an standard sampling frequency for digial video (SD, 720i/p) which is used by modern LCD or Plasma TVs. I guess it could result in a better picture quality (sharpness). So when you nearly finished you maybe could give a try.

[Andy Rea]

13Mhz, 13.5Mhz makes no difference to me, i'll actually be using a 53.2Mhz divided down which will give 13.3Mhz, but 27Mhz oscilator modules are quite easily available, so no reason why 13.5 can't be used.

Andy

[PokeMon]

Anyway I am not sure if it makes a remarkable difference in the picture.
It's only a guess or a little bit more (a feeling).

[siggi]

Ähem: a 64 char/line exists already as ANSI-mode of Z88DK (using HRG). Since the screen does not get bigger, the characters have to be smaller. The 64 chars per line are readable, but IMHO not very good.
So what would be the advantage of a "standard 64 char/line mode"? Smaller pixels? Would the characters be better readable than currently with Z88DK?

Siggi

[PokeMon]

Ähem: a 64 char/line exists already as ANSI-mode of Z88DK (using HRG). Since the screen does not get bigger, the characters have to be smaller. The 64 chars per line are readable, but IMHO not very good.

This depends on implementation and how video signal is processed. It's not a good idea to use the RF modulator because the TV channel is limited in bandwith but not the composite video signal itself. And it depends on the screen / TV. Modern displays with 720p resolution can display the 512 ZX pixel (doubled from 256) very well if the composed signal is good enough.

[Andy Rea]

With a good display, yes much better than a 4 pixel wide char set, characters remain the same height but half the width, and because the Zeddy is running at 2X always you get a faster machine.

Maybe...

just been looking at the assembly listing for the standard 81' rom, seeing what might need changing.... kinda gonna be a bit difficult to test as i can't use EightyOne.... so it's gonna be a bit of trial and eror on some real hardware.....trouble is i only have one zeddy that can do it, and i aint upsetting that one, so i'll just have to build another

Andy