Re: Q: Videx Videoterm

["Michael J. Mahon" <mjmahon@aol.com>]

Marc S Ressl wrote:
> Michael J. Mahon wrote:
>
> > Marc S Ressl wrote:
> >
> > > Hello there,
> > >
> > > I am emulating the Videx Videoterm for OpenEmulator (and it is mostly
> > > done!), but I have a problem.
> > >
> > > When scrolling past the last line, there often appear ghosts from other
> > > lines at the last line. This also happens (more rarely) on some other
> > > parts
> > > of the screen.
> > >
> > > I can understand this as the Videx firmware takes quite some time for the
> > > scrolling, but something does not feel right. Is this ghosting visible on
> > > real boards?
> >
> > Marc, what, exactly, does the "ghosting" look like?  Is it a mixture
> > of the previous line with the new (scrolled) one?  Is it other random
> > lines showing up?  If so, do the ghosts show up preferentially at one
> > place, or are they pretty uniformly distributed?
> >
> > It's been years since I looked at the Videoterm code, but if it updates
> > the SRAM asynchronously with 6845 refresh, then line "mixing" can easily
> > occur.  (I don't mean electrically asynchronously, just unsynchronized
> > with the video frame generation.)  This is the result of 6845 video
> > generation proceeding as the buffer contents are moved.
> >
> > If the code makes an attempt to synchronize with the 6845 horizontal or
> > vertical blanking interval (which would account for some slowness), then
> > the emulation of the status reads that permit this synchronization may
> > not be working properly, resulting in unsynchronized text buffer updates
> > and consequent visible line mixing.
> >
> > Some simple test cases with scrolling lines of different contents
> > and/or lengths should case some light on the cause.
> >
> > -michael
> >
> > NadaNet 3.1 for Apple II parallel computing!
> > Home page:  http://home.comcast.net/~mjmahon/
> >
> > "The wastebasket is our most important design
> > tool--and it's seriously underused."
>
>
> Hi Michael!
>
> Thanks a lot for your blessings :-)!
>
> I guess an image is worth a thousand words, so here go two images:
>
> http://openemulator.googlecode.com/files/Videx%201.png
> (here you see the ghosting at the lower right, this happens fairly often,
> several times per second)
>
> http://openemulator.googlecode.com/files/Videx%202.png
> (here you see the ghosting+another effect in the middle of the screen, this
> happens not very often, maybe one time per 2-3 seconds)
>
> With the best wishes,
>
> Marc.-

So the frequently occurring "ghost" line at the lower right is
the line just scrolled off the top of the screen, while the
less frequently occurring ghost looks like lines horizontally
displaced from where they were prior to a few scrolls.

Do these "ghosts" only last one video frame, or do they persist
longer?  (I'm betting only one frame, hence "ghost".)

I'm still guessing that this is a problem with unsynchronized
updated to the card's text buffer.

I've forgotten how the Videoterm scrolls.  Does it actually move
the buffer contents, leaving the 6845 address pointers alone, or
does it treat the text buffer like a circular buffer, updating
the 6845 address registers?

Clearly, something is not happening at the right time in this
process.

-michael

NadaNet 3.1 for Apple II parallel computing!
Home page:  http://home.comcast.net/~mjmahon/

"The wastebasket is our most important design
tool--and it's seriously underused."