Re: Apple ][ to VGA monitor adapter board - looking for best color decoding method

[ferdimh@gmx.de]

On 14 Apr., 18:03, MdntTrain <j...@cimmeri.com> wrote:
> On Apr 14, 7:05 am, ferd...@gmx.de wrote:
>
> > The pics are here:http://ferdimh.homelinux.net/apple2/apple2vga/pics/
>
> > Ferdinand
>
> Outstanding work, Ferdinand.
>
> For maximum flexibility in connecting to other monitors and TVs, I'd
> really like to see the line doubler portion be an external box that
> could be used optionally, and with either the II, IIe, or IIgs.   Then
> each machine would have its own version of a RGB/YUV card that could
> either be used directly, or with the scan doubler unit.
>
This is a nice idea, but it won't work in the way my card works.
The 1-bit video stream of the ][ / //e is double-scanned, and the RGB
conversion is done afterwards. This saves an ADC, some sort of clock
regeneration, and lowers the data rate, so that normal RAM can be
used. It also simplifies the scanning counters, because they only have
to be 7 bits wide (912 pixels per line packed in 8-bit groups=114
bytes).
RAM size is not of any concern anymore. Because the clock is taken
from the Apple directly, it is in exact sync and is able to reproduce
the crisp 80-col text. An external scan doubler which is not in sync
with the Apple clock will just produce aliasing artifacts.
Compatibility with the //gs is another problem, because the GS uses 2
slightly different dot clocks. If the scan doubler works perfectly in
one mode, it won't in the other.
But using the RGB decoder without the scan doubler is possible, it
just needs to be modified to run on half the clock (need to change two
inductors, three caps and maybe two resistors). So it might be an
option to make an YUV/RGB card w/o scan doubling.
I just wouldn't know any uses for this card: there are little 15kHz
analog RGB monitors available, and if, they usually also have a
composite video input. YUV inputs on TV sets might be worth it though.
Using a common VGA CRT monitor seems to be well worth the extra scan-
doubling effort (which isn't that high anyway - about half of the card
cares about the color decoding) - But don't forget: The timing is non-
standard!
With a 60Hz apple it is almost exactly 640x480@60Hz, but with a
different dot clock and with a bunch of lines unused. So with an LCD
you will have - if it can be adjusted to the dot clock at all - part
of the screen unused. The LCD I tested can not be adjusted to the
correct dot clock, the display in 80-col mode is ok, but not perfect.
I'll put a photo of it to the link above.
With a 50Hz apple (like I have) the timing doesn't exist in any VGA
timing standard. I found out that my ATI graphics card outputs a very
similar timing if the TV out is turned on. I've seen this timing work
on every CRT monitor I had, but it has only worked on one LCD (the
newest - 2 months old) of three I tested so far. The 50Hz refresh is
somewhat ugly on a CRT screen however, but not very objectionable with
white-on-black text.

Ferdinand