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VDC VGA Mania   [2013]

VDC VGA Mania Released by :
Akronyme Analogiker

Release Date :
21 September 2013

Type :
C128 Release

User rating:awaiting 8 votes (5 left)   See votestatistics

Credits :
Code .... tokra of Akronyme Analogiker

Download :
http://csdb.dk/getinternalfile.php/120463/vdcvgamania.zip (downloads: 473)
http://www.tokra.de/c128/vdcvgamania.zip (downloads: 361)

Look for downloads on external sites:
 Pokefinder.org


Production Info
Submitted by tokra on 21 September 2013
The idea for this small demo came out of a discussion on the commodore128.org-forum about connecting the C128 to a VGA-monitor in the 80-column mode which the VDC chip produces:

http://www.commodore128.org/index.php?topic=3877

The C128 in 80-column-mode puts out a signal that is pretty much like the old CGA of early IBM PCs. You have a horizontal frequency of about 15.7kHz and a vertical frequency of 60Hz for NTSC and 50Hz for PAL making it possible to display such a signal on a TV relatively easily as well.

Also, the signal is digital (RGBI). You have 8 colors with (RGB) with intensity (I) either turned on or off for a total of 16 colors.

Now, VGA has horizontal frequency of 31.5 kHz (roughly twice as much as the C128) and colors are analog RGB. To be able to display a C128's signal on VGA you need to double the horizontal frequency as well as convert the 16 digital colors to analog values.

The frequency doubling is usually done by a line-doubler like the GBS-8220. This uses digital signal processing to create a completely new picture out of the data it receives.

The color-conversion from digital RGBI to analog RGB can be done in several ways. The easiest solution is to just connect the R, G and B like with this cable:

http://www.ambery.com/db9todbadca.html

You get only 8 out of 16 colors this way and the signal levels will be way to high (RGBI uses 5V-levels, while VGA uses 0.7V). While this works for a first test this is obviously not the desired solution.

An easy and cheap enough solution is a small resistor-network of which several layouts can be found around the internet. A very good explanation can be found at Hydrophilic's site:

https://sites.google.com/site/h2obsession/CBM/C128/rgbi-to-vga

This way you get 16 colors, although you get the color dark-yellow instead of brown. The color brown is a special case, you can read about that in Wikipedia:

http://en.wikipedia.org/wiki/Color_Graphics_Adapter#With_an_RGB..

The most elegant RGBI->RGBA conversion I have seen yet was provided by Richard42 in the commodore128.org-forum. This also includes a fix for the color brown. Read more about this in the forum-thread mentioned at the start.

Now, to get back to what this program actually does: It alleviates the need for the digital signal-processing or line doubler (for which you would need the GBS-8220) by putting the VDC into a mode of nearly 31.5kHz and 60Hz which VGA-monitors will recognize. While you still need the conversion of the color from digital to analog, the VDC-chip will actually provide the signal that is displayed on the VGA-monitor.
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CSDb (Commodore 64 Scene Database) is a website which goal is to gather as much information and material about the scene around the commodore 64 computer - the worlds most popular home computer throughout time. Here you can find almost anything which was ever made for the commodore 64, and more is being added every day. As this website is scene related, you can mostly find demos, music and graphics made by the people who made the scene (the sceners), but you can also find a lot of the old classic games here. Try out the search box in the top right corner, or check out the CSDb main page for the latest additions.
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