C64 Video Capture

My old pc has a, now very old, AIMS Lab Video Highway Xtreme PCI video capture card. But it appears perfectly adequate to capture Commodore 64 video.

The Commodore 64 has a composite video output in addition to separate luma and chroma outputs. The luma/chroma can be fed to the S-video input after adding a 330 Ohm resistor between the chroma output and color input. In theory the luma/chroma signal should result in better picture quality.

Protoype adapter cable with resistor.

The finished product. I used copper ethernet cable which conveniently has 4 signal leads, for composite, luma, chroma and (mono) audio. I wired the ground leads together inside the DIN plug. The resistor is attached to the s-video plug, in heat-shrink tubing.

I have cropped and enlarged some captures in various configurations in the images below; the images link to original screen captures.

S-video

Composite video

Composite video signal fed into the luma input. This image will be familiar to anyone who has ever used the Commodore 64 on a black-and-white tv or monitor. The checkerboard pattern is caused by the signal for the color blue.

S-video with the color pin disconnected. Note that you can still see a faint checkerboard pattern, so there isn't complete separation of color and luminance. Apparently this is a defect in the VIC-II video chip itself.

Another s-video v composite comparison:

s-video

composite

These captures were made from a Commodore 64 rev. B-3; video chip: MOS 6569R5.

Michiel Boland

15 May 2017

Addition

This is what things look like on an oscilloscope.

On the left you see an enlarged part of a screen capture, similar to the ones made above, on the right I have measured part of the top two lines of text.

The blue channel represents the luma signal (brightness), while the purple channel is the chroma (color) signal. The chroma signal appears off whenever the value of a pixel changes, which would explain these odd pink hues in the left picture.

Ideally the chroma signal should have been constant, since we are displaying blue on blue, only with different luma values.

18 Apr 2019