Re: Using IIgs with VGA monitor - how to go?

[mjmahon@aol.com (Michael J. Mahon)]

Scott Alfter wrote:

>In article <9G5ja.33187$7Im.15777@news04.bloor.is.net.cable.rogers.com>,
>Glenn Jones <rg.jones@rogers.com> wrote:
>>"Volkmar Friauf" <vf@apple2.com> wrote in message
>>news:b6hisl$5a4p8$1@ID-60882.news.dfncis.de...
>>> recently, picture quality of my IIgs Color monitor is degrading rapidly,
>>> so I've been looking for an alternative. Since I have a fine VGA display
>>> for my PC, I am looking for a way how to use that display with my IIgs
>>> (possibly with a monitor switch so I easily switch between IIgs and PC).
>>
>>Slightly off topic ....
>>
>>I have a laptop that has s-video in and s-video out.. It also came with a
>>s-video to composite conversion cable.
>>
>>I thought what the hay... so I conected up the cable...fired up MGI
>>Videowave and there in the preview window on the LCD screen was the IIGS
>>display.
>>
>>Monochrome looks great .... colour on the other hand was pretty bad... by
>>toning down the brightness and colour level I could make the Desktop useable
>>but with very little colour.
>
>If there's a way to extract S-video from a IIGS, that might be doable.  I
>don't have a schematic for it, but somewhere in the process of converting
>analog RGB video to composite video, the Y and C signals that are mashed
>together to form a composite signal must be present.
>
>Start with the four signals on the RGB connector: red, green, blue, and
>composite sync.  First, the video hardware converts RGB to YCbCr. If you
>mix Y with sync, you could take these three signals and get the same
>component video that your DVD player produces.

And for the best quality, this is where you should stop and connect
to a high resolution video monitor.  All component video signals are
baseband, and no intrinsic bandwidth limits apply (though a monitor
may limit them).

>  Combine Cb and Cr and you
>get C, which is one of the two signals in an S-video connection (the
>previously-mentioned Y/sync combo is the other signal).

And you do this by using the Cb and Cr baseband signals to modulate
a chroma subcarrier at 3.58MHz, so that their bandwidth is intrinsically
limited.  This is the end of high-definition color information.  (Though
the baseband Y signal will still make crisp monochrome.)

>  Combine the two
>S-video signals together and you get composite video.

And also require a very high quality filter to remove the modulated
chroma subcarrier from the luminance signal without seriously
limiting its bandwidth.  Practically speaking, there are no such
designs, since composite video is "known" to be band-limited to
4MHz to allow for the 4.5MHz sound subcarrier in broadcast TV.

>I don't have a IIGS schematic, but if these signals are available at some
>point where they can be tapped off and run to a connector, you could get
>S-video to your other computer's TV-in jack; this would be of much better
>quality than composite video.

For monochrome signals, but color information is still band-limited.

>  Another option (probably easier than hacking
>at the motherboard) would be to build something that plugged into the RGB
>output jack and provided component and S-video outputs.  If somebody makes a
>13" TV with component and/or S-video input, this could be a suitable monitor
>replacement.  (Component would be best, but S-video should be pretty
>good...when I switched my TiVo from composite to S-video, I saw a big
>improvement in the clarity of its menus.)

Yes, the separate, higher bandwidth luminance signal will sharpen
things up a lot, but the chroma information is still quire smeary.
This is clear for any fine detail in color, such as colored text.

>(FWIW, there's already a product on the market (the Video Turtle) that
>converts RGB to S-video.  At $150 (according to the information I found),
>it's still fairly spendy.)

Conversion of RGB to component video should be almost a passive
circuit, requiring buffer video amplifiers at most.  The cost should be
under $10, which means it ought to be available for $50 if anyone
builds it.

-michael

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