|How-To: use a C64 a PC tv-card|
there is a transition going on that classic CRT disappear due to the
rise of flat panel LCD displays. If you ever tried to connect a C64 to
such a new generation display you might have noticed that sometimes
there are problems with visual quality of the representation of the C64
signal, whereas normal TV looks good. This is because the filtering
algorithms in modern LCD-TVs are heavily optimized for standard TV
signals. On most LCD TVs you can not disable this filters to show the
input signal 'as is'. So you might test a lot of available LCD-TVs if
they represent the C64 signal correctly. Another idea is to use a PC
with a tv card because at this point you have full control over the
filters, though the disadvantage is you always need the PC. But then
again most people that have a C64 have a PC too anyway...
The following will part will focus on PAL and a BT878 based tv card. If you are a little bit into the topic you could extrapolate some details to NTSC and other tv cards...
How a normal BT878 card works with a tv signal:
If you use a tv application without filters for a tv pal input signal the behaviour of the tv card goes like this:
The card decodes the first half image and the application transfers this to the even lines of a image buffer in full PAL resolution, then the tv card decodes the second half image and the application transfers this to the odd lines of the full resolution PAL image buffer. Then the application shows the full resolution buffer. The result is a full resolution PAL image at 25 Hz.
Now the C64 delivers an progressive signal, this means that the images should be shown at 50 Hz. So what happens if you just use a standard tv application and plug a C64 at the tv card?
Well the C64 signal gets splitted, one progressive image gets transfered to even lines and the next to odd lines and displayed at 25 Hz. The result is that you get always two following images mixed together with strong grid artifacts on horizontal movement. And it is displayed at 25 Hz which results in stuttering.
The solution would be another approach of displaying. Each single image the tv card gets needs to be displayed at a 4:3 aspect ratio at 50 Hz. After a while of searching for such application I stumbled upon DScaler. This is a application with alot of different deinterlacing filters, but one does the job. It is called 'Old Game'.
Now the grid artifacts are gone and there is no permanent stuttering because of the 50 Hz -> 25 Hz.
Though now the next problem arises. The typical PC refresh rate of TFTs is 60 Hz or 75 Hz. The visible effect is that every 5 th image from the C64 gets displayed two times at 60 Hz TFT refresh rate. So there is a 25 Hz drop 10 times a second.
The solution for this is to set your TFT to 50 Hz. While a lot TFTs do not mention to support lower refresh rates interestingly most TFTs do. The bigger problem is often to persuade the graphics cards driver to add a non standard 50 Hz display mode.
By combining both you get a very good representation of the original C64 signal. (Don't forget to VSync the video overlay to the TFT)
- Enable the filter 'Temporal Noise Reduction' in DScaler that will reduce brightness variations between subsequent frames. (Once a friend said, that the quality is so good that it doesn't represent his classic C64 experience anymore ;-) )
- The advantage of TFTs when displaying interlaced FLI graphics: Interlaced FLI graphics is often used to mix different colors together to get a new color. To reduce flickering usually colors of the same brightness are used. While a classic CRT has a brightness degradation at a certain spot after the raster beam hit it until the next frame on a TFT the brightness stays. Thus it is more influenced by the speed of the TFT when changing from color A to color B on a line of equal brightness. So overall flickering drops to nearly unnoticable when showing IFLI pictures.
- Again IFLI: If you disable filter 'old game', the first image will be put in the even and the second in the odd lines. Effectivly on IFLI this means splitting the two pictures to even and odd lines - doubling vertical resolution and show it at 25 Hz without any flickering.
- Quite high effort because you need a running PC for this
- While the overlay can be VSync'ed to the TFT output the C64 input signal can not so easily. As the 50 Hz of the C64 and the 50 Hz of the TFT are not exactly identical because of different clock sources there is sometimes a skipped or doubled frame.
Michael Huth 07 Sept. 2008
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