Unique scanning disc system.

[Stephen]

I knew that John Logie Baird invented the lens-type scanning disc, as shown in British Patent GB230,576, filed on 29 December 1923. This sort of lens disc requires a separate lens for each scanning line. Although it is most efficient because it may concentrate all of the light that each lens gathers from the modulated light source into the viewed image, it can be bulkly, difficult to adjust and have balance problems if the lenses have the ordinary single spiral arrangement.

I just realised that another early patent by Mr. Baird, GB265,640, filed 6 August 1925, describes a means to greatly simplify a lens disc scanning system so that far few lenses need be used and they may be arranged in a simple circle rather than a spiral. In this patent, amongst other things, Mr. Baird suggests using one lens disc with a few lenses arranged about a circle for vertical scanning and another disc with a few more lenses arranged about a circle for horizontal scanning, with the discs mounted in tandem. Thus, one disc might have eight lenses for shifting the light from a modulated light source vertically at the line rate and another with eight lenses for shifting the light horizontally at the frame rate.

Thus, in the case of the 32 line system, the vertical scanning disc would rotate at 3000 rpm whilst the horizontal scanning disc would rotate at a leisurely 93.75 rpm. There is no need for a spiral arrangement of the lenses, since they individually displace the image in their respective planes. Even fewer than eight lenses per disc would be possible. I only suggest eight lenses because the fewer the number of lenses per disc the greater the arc through which each lens scans, leading to a greater degree of keystone distortion in the reproduced image.

This would be an interesting configuration to implement. Mr. Baird also suggest substituting a polyhedral mirror for one of the lens discs. I have posted this patent at http://www.taswegian.com/NBTV/images/GB265640A.pdf for reference.

[Jim Wood]

That idea has some definite merit. It might be lots easier to get good precision in aligning the lenses (or holes, for that matter) if they are in a circle.

However, think of a disc of lenses revolving at 8000 r.p.m. I, for one, would stand well clear, and to the side, of such a device.

Also, Doug Pitt explained to me once that the ratio between the number of lenses in each case needs to conform to the aspect ratio of the image. Of course he was talking to me about mirrors, as I wanted to affix flat mirrors to the outside of two styrofoam blocks and revolve them at right angles to one another. Not quite sure how this works out mathematically.

An idea I had some time ago was to use two discs with radial slots. These would overlap with minimum clearance, one providing the vertical scan, the other the horizontal. Of course the intersection of the slots would make a square aperture only at one point in the picture; it would get parallelogram-ish elsewhere.

[Stephen]

Another advantage to having both discs with so few lenses is that the lens discs may be very small in diameter. Consequenty, the camera and display may be very compact and the centrifugal forces that the lenses in the 3000 rpm lens disc generate upon the disc will not be nearly as great as an ordinary lens disc rotating at that speed.

Doug was right about the twin mirror drum system. You would need to use mirror drums with mirror counts in proportion to the aspect ratio of the picture. Although Mr. Baird does not address the issue in the patent, this may be true with the twin lens disc system as well. If this is true, then you would want to have one 8 lens disc that revolves at 3000 rpm and either a 6 lens disc that revolves at 125 rpm or a 12 lens disc that revolves at 62.5 rpm.

However, I am not entirely sure if the ratio of the number of lenses in each disc has to reflect the aspect ratio of the picture with the twin lens disc system. For instance, if the focal lengths and object/image distances for the lenses in each disc are such that they provide different magnifications it would seem that they would provide different displacements per angle of revolution as well. If this is the case, it would seem that you could use these variables to control aspect ratio instead of the ratio of the number of lenses in each disc. I shall have to think this through when I have a chance.

[Stephen]

By the way, Jim, your suggestion for using two radially slotted discs should work fine. John Logie Baird mentions such a system as already known in his British Patent GB289,307, filed 15 October 1926. This patent is interesting in that it is an omnibus disclosure that discusses and shows almost every conceivable form of scanning, including interlaced, which Mr. Baird calls "intercalated" on page 2, column 1, lines 6 through 15, as well as sequential right-left, vertical-lateral, and zig-zag. In fact, Mr. Baird describes a zig-zag scanning system using a pair of slotted discs in connection with Figure 5.

I have posted this patent at http://www.taswegian.com/NBTV/images/GB289307A.pdf for reference.