CGA

[ana...@zeus.jersey.net]

Someone once told me that you could split and then recomine the output
of a cga card into a TV feed. Is that possible? Also does only know
where I could find plans to build a converter to take VGA output and
convert it to an NTSC signal?

[Gary Tait]

On Tue, 3 Dec 1996 ana...@zeus.jersey.net wrote:

#Someone once told me that you could split and then recomine the output
#of a cga card into a TV feed. Is that possible? Also does only know
#where I could find plans to build a converter to take VGA output and
#convert it to an NTSC signal?

Some CGA cards have a composite colour and mono jack(s) on them.

Gary Tait , VE3VBF
- Please do not quote US prices, they are no good to me ,here in Canada -
- If you must quote prices , please quote prices from dealers in Central-
- to Western Ontario, Canada . (my location is near Kincardine) -

[ana...@zeus.jersey.net]

Someone once told me that you could split and then recomine the output

of a cga card into a TV feed. Is that possible? Also does only know

where I could find plans to build a converter to take VGA output and

[Tom Watson]

Most CGA cards (even the IBM one) have composite video (NTSC) output as a
"RCA jack", as well as the DE-9 pin RGB connector. Note that due to
bandwidth restrictions in NTSC (or any color scheme used for broadcast),
one must use the "CO40" mode (40 column) for the NTSC output. While it
may be close to operational in other modes, any more than 40 columns may
cause false colors, and bluring of the characters. Also note that the
Composite video signal may have sharp edges, which cause higher frequency
artifacts in the signal. If used as input to a standard TV modulator,
they can cause interference to the sound channel (buzz). Sometimes this
can be heard on broadcast signals when they are sloppy.

As for VGA converters, they are usually sold as a seperate box, and get
complex. The prices I have seen are usually over $200. In addition, they
may need to have the scan rates set to match NTSC standards (15734h,
59.93v). If they don't, the converter usually has some memory included as
a frame buffer, and does things like convolution (or some such) to make
the picture look "reasonable".

Building an NTSC "modulator" (given RGB inputs) is not a task for the
squeamish. Things like quadrature modulation, bandwidth limiting, color
matching, and the like crop up at various times. There exist some chips
(somewhere) to do the job, but I don't have the reference at hand. It is
NOT a "simple" one night breadboard task.

Good luck.

p.s. Me, I'd use an older CGA board. It has all the work done. Cost is
about $30 or so these days (when you find them). Less in a surplus store.

--
Tom Watson
t...@3do.com (Home: t...@johana.com)

[John Payson]

In article <tsw-051296...@cypher.3do.com>,

Tom Watson <t...@3do.com> wrote:
>Most CGA cards (even the IBM one) have composite video (NTSC) output as a
>"RCA jack", as well as the DE-9 pin RGB connector. Note that due to
>bandwidth restrictions in NTSC (or any color scheme used for broadcast),
>one must use the "CO40" mode (40 column) for the NTSC output. While it
>may be close to operational in other modes, any more than 40 columns may
>cause false colors, and bluring of the characters. Also note that the
>Composite video signal may have sharp edges, which cause higher frequency
>artifacts in the signal. If used as input to a standard TV modulator,
>they can cause interference to the sound channel (buzz). Sometimes this
>can be heard on broadcast signals when they are sloppy.

I've found that CGA cards fall into three categories:

[1] Some have a really lousy RGB->composite color converter. When bit 2 of
port $3D8 is clear, colored characters will get ugly vertical stripes
though them [modulated at 3.58Mhz]; note that when in "high-bandwidth"
modes [e.g. 80x25 text or 160x100x16 lo-res] the colorburst signal at
the start of each line is invariably botched; if you want color in those
modes, you must set the border to color #6. Alternatively, if bit 2 of
port $3D8 is set, the card will generate monochrome; note that some cards
do better at this than others, but the monochrome is usually of pretty
good quality and often can provide a 16-level grayscale [though not in
color #0-#15 order]

If you are writing your own software, you will probably get the best
results by using 640x200 mode and clearing bit 2 of port $3D8. Working
left to right, illunimated pixels will be [if memory serves], brown,
magenta, green, blue, brown, magenta, etc. and nearby pixels may be
combined to produce colors. For example, illunimating columns 0, 1, 4,
5, 8, etc. would produce a region of orange. Anyone who has done color
on the Apple //e will be familiar with these techniques and while they
may seem strange they actually work quite well; the effective horizontal
resolution will vary from 160x200 to 640x200 depending upon what colors
are being juxtaposed. Generally this mode works best for combining a
160x200 16-color picture with 40-column text [which may be white on black,
brown, dark-magenta, dark-green, dark-blue, or gray; or black on white,
yellow, light-magenta, light-green, light-blue, or gray].

[2] Some have a monochrome RGB->composite color converter. These are gen-
erally decent and operate as if bit 2 of port $3D8 was simply stuck high
all the time.

[3] Some have no composite video output whatsoever.

Note that visual inspection cannot easily tell you into which category a card
falls; even the lack of an RCA jack won't tell you, since some cards have no
such jack but output composite video on pin 7 of the DB9.

>As for VGA converters, they are usually sold as a seperate box, and get
>complex. The prices I have seen are usually over $200. In addition, they
>may need to have the scan rates set to match NTSC standards (15734h,
>59.93v). If they don't, the converter usually has some memory included as
>a frame buffer, and does things like convolution (or some such) to make
>the picture look "reasonable".

There are two types of VGA converters: line converters and frame converters.

A line-converter takes advantage of the fact that the VGA horizontal scan rate
is almost exactly twice as fast as a TV's. If the VGA is programmed for the
correct frame rate, then a stable TV picture may be obtained while only buff-
ering one or two scan lines [buffering two scan lines will allow the converter
to reduce interlace flicker]. Such boxes may work well, but require that the
VGA card in question actually produce a fairly precise 31,468Hz horizontal
signal; some are closer to that than others. If the horizontal signal is not
quite right, many TV sets will display the image fine, but many VCR's will do
a poor job of recording it.

A frame-converter continuously digitizes its input into shared-access memory
while it asynchronously displays the contents of that memory in a precise
NTSC [or PAL] format frame. Note that frame converters will cost more than
line converters, and may have more severe resolution/buffering limitations
[e.g. one frame converter I used required the user to select between 640x400
interlaced and 640x240 non-interlaced [the latter having double-high pixels
which held the "average" of two consecutive scan lines]. On the other hand,
such converters will produce a video signal which is actually within spec
rather than one which is merely close.

--
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