[Coco] again: CoCo RGB to VGA conversion

[John Kowalski]

This is a late reply as I do not have regular access to email at the moment,
sorry!

>>I am fairly certain that the coco is NOT interlaced. There
>>is only 1 field.
>
>Maybe someone can clarify this. The coco sends a frame 60 times per second.
>Each frame is overlayed over each other the same. What Sockmaster has found
>is that he can fool the 1986 GIME into dropping down a scanline, into the
>black region. By alternating 2 frames of data with this scanline skip, he
>simulates an interlaced VGA resolution display (15Khz still). (He write a
>terminal program that utilizes this mode called Twilight Term). This is why
>I thought a de-interlacer circuit would then convert it to a nice
>flickerfree VGA 31Khz output.
>
>Maybe Sockmaster can clarify?

What Nick is saying is that although the CoCo does not normally support
interlaced video, it *is* possible to generate interlaced video (with the
1986 GIME in any case) in software.

Now, even though the 1987 version of the GIME can't do this, it would still
be possible to build a CoCoRGB-to-VGA converter that would *ALLOW* the CoCo
to generate and display 640x450 resolution graphics (regardless of which
version GIME your CoCo3 has).

All the CoCo would have to do is keep two 640x200 or 640x225 images in
memory and alternate between them every field in software.  The CoCo may
either detect an "even/odd" signal from the converter, or output one so that
the RGB-to-VGA device would know which field was even lines and which one
was odd.
The RGB-to-VGA converter could then combine both fields to create a solid,
uninterlaced VGA signal.

The result is:
- for all existing software, you will get the expected blocky 640x200
resolution.
- for software/drivers specifically written to support it, you may have up
to 640x450 resolution available on your CoCo.

PS: The released version of Twilight Term does not support interlaced video.
There is a much older, unreleased, experimental version of Twilight Term
that does.  It ran in 640x450 resolution and had an exceptionally large 80
by 56 text display.  I only made this version to test the feasability of
interlaced video on the CoCo3.

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/