Hi Andrew,
Thanks a lot for your detailed post, which of course gives me a lot to think about!
Andrew Davie wrote:
There are standards (32 lines springs to mind) which would make your life easier. More particularly, there's software that converts from video format into those standards. I'm not sure 240 lines is a standard, but in any case the software I'm referencing is "Video2NBTV" by Gary. It would make your life easier to consider these pre-existing formats. You also have chosen a completely different aspect ratio to the standards I know. So again - why be different?
I am well aware of existing standards in the mechanical TV community. A couple of years ago, my students and me built a 32 line Baird Televisor from scratch in a four day workshop, and we used "Video2NBTV" for encoding. For this project, however, I might not need an encoder at all, since I plan to build not only the receiver, but also the camera to have live transmissions from one rotating disk to the other.
I was looking at the size of your disc (!) and thinking "gosh, that's going to be dangerous!" - you haven't mentioned a frame rate. Although you've massively increased the scanlines over a "standard" NBTVA format (which is 32 lines), you're actually displaying a tiny image. 32 scanlines gives suprisingly good pictures, done right. Anyway, frame rate will determine how fast the disc has to spin (currently 12.5Hz = 750rpm). I've done lots of playing by using an audio editor to "speed up" some of the videos I play, and to some degree they look "better" because they're (slightly) less flickery, but ultimately you're affected by the amount of light that gets through any hole.
If you're increasing the scanlines then you're decreasing the hole diameter (for a given disc size) so that's a consideration - are your holes big enough to allow sufficient light through. You may have to consider an incredibly bright lights source. Let's assume you are going with the 12Hz frame rate, then you have your outer edge travelling 4,680.973 mm (circumerence) * 12.5 (frame rate) mm/s = I make that 58 m/s. I don't do 3-digit precision so to the meter is close enough
. That's hefty, and probably dangerous. I am unsure if you will have issues, but I expect a huge motor will be required!
Yes, very dangerous indeed! I calculated the rim speed for the 1500mm disk at 253 km/h (157 mph) (900 rpm for 15 fr/sec). this is about the same speed as a 12,000 rpm angle grinder with a 125mm disk....
But of course I will take all the necessary precautions and build a solid case around the Nipkow disk. I am already collaborating with a mechanical engineer to make sure we choose the right materials and technology for achieving this safely.
About the scan lines: increasing the number from 32 up to something around 200 kind of is the point of my whole endeavour, since I want to create a mechanical TV system that (at least in theory) could have existed in the 1950s or '60s. And the fact I really like about the whole Nipkow principle is that you can only increase resolution by making the disk much bigger!
I am also working with an electrical engineer for motor control, we are currently considering a 1.5 kW motor, that would still need about a minute to bring the disk up to speed.
The remaining brightness with 0.25mm holes is of course to be considered, I'll try to calculate that.
I would assume if the motor isn't noisy, the spinning disk probably will be!
I'm afraid so.I read that 240 frames at 900 rpm will give you a siren effect with a 3,600 Hz sound. Do you think that the small size of the holes will keep the volume tolerable?
I suggest you slightly enlarge your holes (just a fraction) to give a bit of overlap between them - and consider the shape. Round holes, diamond holes, square holes - they all have characteristics which affect the picture.
I'll have to see if a different shape than round can be achieved with these small diameters (I read that square holes would give you about 25% more brightness). Funny enough, I also considered the holes to be overlapping, would that help with the visibility of scanlines?
Now a general comment - I understand you're exploring technology and alternate ways of showing images, particularly related to TV. One alternate that I haven't seen anywhere is to place the holes on a very long looped belt and pull that through in front of the light source at high speed. You could get a much bigger picture, but with a really weird mechanism. The picture would be totally rectangular rather than arced. The technological challenge is pulling a long (very) belt efficiently and somehow managing it once it's passed the viewing area. If the image was (say) 10cm high, and we had 32 scanlines, then the belt would be 320cm long. That seems manageable and already you have an image that's way bigger than any Nipkow disk. You'd have to pull it through at 10*32*12.5 cm/s which is 40m/s and although do-able, quite the technical challenge I think. Anyway, just throwing that one out there.
Great idea! I also read about the belt concept. It reminds me a little bit of the Tefifon, an obscure audio tape format from the '50s that used a plastic strip with a mechanical groove instead of magnetic tape, kind of like a vinyl record morphed into an audio cassette (
https://en.wikipedia.org/wiki/Tefifon)
With this project however, I really want to stick with the Nipkow Disk, because I really like the simplicity and technical transparency of this historically very first idea of how to break up a moving image into lines and frames.
Back to your concept - in summary
* wow, that's a big disk
* holes may be too small at 240 line resolution to actually see an image?
* format you choose is non-standard, requiring lots more work to create content
* picture is tiny!
* frame rate is undefined, but affects heavily the practicality of using that big disk.
Thanks for summarizing your concerns, all of them valid of course. As mentioned above, having a huge disk with a tiny but very high resolution image is kind of the raison d'être for the whole project (stupid, I know).
best,
geb