Re: C128D to SVGA

From: Nicolas Welte (
Date: 2001-08-15 23:23:04

Marko Mäkelä wrote:
> If the IC markings on the device are badly worn (like those on my box, I
> guess they have a bad conveyor belt at the factory), then you cannot
> easily reverse engineer the scan doubler circuit to add an RGB input.
> The only viable solution would be to build an NTSC/PAL encoder for the RGB
> signal.  There are one-chip solutions from some manufacturers.  A friend
> of mine (who also got the Redant box) is going to build something.
> Hopefully the picture quality won't be too bad when using S-video.

I have a schematic of a (very low quality) scan doubler with built-in tv
tuner and a composite input. It's okay to get the general idea how these
devices work: there is a digital video processor that is able to
digitize one of it's video inputs (either composite or s-video), a dual
ported line buffer to store a scan line, and a video DAC to generate the
RGB output from the line buffer's contents. The timing (aka scan
doubling) is controlled by a custom programmed IC (so goodbye to
cloning) and the whole control of the device is done with a 8031 type
MCU with internal firmware (so also goodbye to cloning). The most
interesting thing that I could see from the schematics is the way that
the scan line is stored in the buffer: it is a 16 bit YUV format, with
8bit color and 8 bit luminance information. It would be pretty easy to
interface the 4 bit RGB output of the C128 here to get the correct
colors, but not in this box here, I'm sure. I got this box very cheaply
and was very upset about the low resolution it produced with the
composite output. A 40 column screen was almost unreadable. In the
schematics I found an interface header for an optional teletext module
that offered s-video (not available externally), but this input was only
active if the teletext chip was detected on the I2C bus. I played with
the idea to simulate the I2C registers of this chip with my C64 (got
this idea during the I2C thread on this list), but luckily I found that
my other (good quality) scan converter has the teletext option with
exactly the right chip, so I interfaced this module into the other box
and connected an external s-video input. Well, it turned out that the
output was as unreadable as with the composite input :-( I ended up
selling the box to my (then) girl friend very cheaply just for watching
TV :-) BTW, this box is the well-known Proview TV-box that was available
for many years from Conrad Electronics in Germany. I never read a bad
review about it, but I heard complaints from its users. Since these
boxes can be had very cheaply on online auctions sometimes, I decided to
get one just to have myself a look at it. I payed about EUR 50 for my
first box, a good one with TV tuner, teletext module, s-video, composite
and RGB-passthrough, and maybe EUR 20 for the bad one :-) 

I'm also still playing with the idea to use an Amiga
scandoubler/flickerfixer to double the C128 RGB output. Thanks to Pasi's
document on hacking a C= 2320 into an A500 I'm almost sure it should
work. That type of cards take a 18bit digital RGB input and a clock
signal and put it out scandoubled as analog RGB.


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