by Andrew Davie » Thu Jan 25, 2007 12:24 am
On reflection, a standard Nipkow disk could be used, with a secondary ‘colour filter’ disk spinning behind the Nipkow but in front of the light source (which should be white, of course). The colour filter disk would colour successive scan line ‘holes’ red, green, blue, red, green, blue – and be spinning such that for every revolution of the colour disk, the Nipkow disk would spin three times. Thus the colour filter disk would have three sections – one colouring individual Nipkow holes in the order red/green/blue, the next green/blue/red, and the next blue/red/green. It would consist of very finely spaced colour filters, in three distinct pie-sections. The spacing, of course, the same as that of the apertures in the Nipkow disk.
To be clear, consider the first hole in the Nipkow disk. As the hole scans the image area, the colour filter is red. As the second hole scans the area its colour filter is green. As the third hole… blue. 4th hole… red. 5th hole, green… etc. Now the next time we see the first hole (a complete rotation of the Nipkow disk) the colour filter for the first hole is showing green, and the 2nd showing blue… etc. The third rotation of the Nipkow disk, the first hole colour filter is blue.. etc. So after every three rotations of Nipkow disk, we have scanned red/green/blue for the first hole, green/blue/red for the second, blue/red/green for the third… and the net effect of this (scanning with three colour filters) is that we see a greyscale image (ie: normal NBTV). If, however, we have also changed the actual video data so that for the first frame for the first hole we’re showing the red components, and for the second hole the green components, and for the third hole the blue (4th red, 5th green, 6th blue etc), and then for the second frame for the first hole we’re showing green, for the second hole blue, for the third hole red components, etc… then we get a full-colour image using a system I called ‘interleaved chronocolour’ on the game system.
The above mechanical system would be a reasonable analogue equivalent of what we did on the video game system to get colour display on a B&W system, and my experience is that this does in fact provide quite good colour reproduction. The nice thing is that you don’t have to increase the frame rate of the display signal – you just mix the red/green/blue components on a per-scanline basis. It still gives reasonable black and white images, because the colour sub-components are mixed in a ‘rolling’ fashion and alternate on a three-frame basis.
A mechanical TV would be much better than the game system, in that the game system only had single intensity (on or off) red/green/blue, whereas the mechanical system would have the graduated intensity available to the normal mono display.