Re: Q: Videx Videoterm

[mressl@macgui.com (Marc S Ressl)]

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.-