Re: TI-99/4A Screen Resolution Question
- From: briantwcries@xxxxxxxxx
- Date: Sat, 17 May 2008 04:20:41 -0700 (PDT)
On Apr 19, 3:53 pm, Michael Zapf <newsm...@xxxxxxxxx> wrote:
Math1723 schrieb:
I guess the emulator was made by Raphael Nabet for the MESS team. He
might be on this group and could help answer the 280x210 question.
I'd like to hear Mr. Nabet's response. I'm wondering if CALL SCREEN()
fills the entire bordered area of 284 x 243 (active display plus
borders), but the M.E.S.S. emulator crops the this to 280 x 210. It
would be very hard to tell, unless someone with an actual TI can shift
the monitor's center to the max and see if he can count the pixels.
The last person fumbling around with these settings was perhaps me ...
(if you are talking about the recent version of MESS).
I had to put quite some work into the TI emulation of Raphael to get it
working again, after the other MESS maintainers changed code in the core
- and broke the TI emulation in several places. One change affected the
video display with the 9918/9, 9928/9 - for some versions, no border was
displayed at all.
As I did not have enough background information I tried a setting which
seemed plausible to me, making the screen look like in prior-0.97
versions. So you probably should not put too much trust into the values
found in ti99_4x.c. I remember that I did not have to change many
values; I think it were some zeros that had to be replaced with other
values in the screen profiles.
Michael
The fact that the NTSC and PAL versions of the TI output more than 192
horizontal lines had nothing to do with TVs cropping the image. The
9918/29's addressable area is 256x192, and both NTSC and PAL have more
than 192 horizontal lines. In NTSC, there are 525 lines interlaced.
That means 262.5 lines non-interlaced, which is exactly what the NTSC
model put out. In PAL, you have a 625i image, and the TI put out 312.5
lines out, non-interlaced.
Because there was a border above and below the addressable area, the
pixels weren't square. That means that if you draw a circle on the TI,
you'd come out with an ellipse on your TV / monitor. This was bad
enough in NTSC, where 71% of the vertical was adressable (192 of
262.5), but much worse in PAL, where the borders were only 61% of the
vertical was addressable, leaving a large background colour strip
above and below the image, constituting 39%!
If TI had simply put out 192 lines, this wouldn't have been compatible
with standard TV sets, so there was a need for a workaround. Emulators
don't need this workaround, and that makes them inaccurate in terms of
rendering. A circle drawn comes out as a circle in MESS.
That addresses the vertical business. As for the horizontal, yes, I
agree with the comment above that TI probably added some extra pixels
per line to get around TV sets cropping the sides. Since the
horizontal resolution on analogue TVs isn't fixed, it was completely
up to TI to decide how many pixels to add per line. The vertical
resolution, as mentioned above, is a constant in NTSC and PAL, and
it's not something you get around easily.
One digression, though, as if this weren't confusing enough: The PAL
TIs put out 312.5 lines 50 times per second. This is equivalent to 25
frames with both fields of each frame being drawn in exactly the same
place. This is NOT standard timing, and causes problems sometimes -
for example, if you tried to record the output to VHS, the recorder
would have trouble locking onto the signal or recording any colour.
I've been told that the NTSC version of the console put out a standard
NTSC interlaced signal, but I don't believe this. I've never had the
opportunity to own and NTSC console. I wish I could get my hands on
one.
.
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