July 13, 2017 by Vincent Portet
This series will give you all the required information to build or tweak a mechanical narrow band television.
Editor’s Note: This is part of a several part project by Vincent Portet. If you would like to see a specific chapter, refer to the table of contents below.
How to Build a Color Mechanical Television, Part 1
Color Mechanical Television Part 2: Choosing Components
Color Mechanical Television Part 3: Circuit Construction
Color Mechanical Television Part 4: Color Systems
Color Mechanical Television Part 5: Software Tools
Color Mechanical Television Part 6: Tuning Procedure and Calibration
This series will give all the required information to build or tweak a mechanical narrow band television similar to the prototype shown in this Youtube video. In part 1, we will focus on some of the core concepts. Later, we will get into building our own color mechanical TV!
Building and tweaking this kind of mechanical and electronic hardware requires some experience and a lot of patience. Electromechanical parts (speed response vs torque of the available motor, disc inertia, and size) have an impact on the global system behavior so the prototype will not be “plug-and-play”. A lot of adjustment means have been implemented in the schematics to face these kinds of issues. However, it remains possible that you will have to modify some values/components to match with your mechanical configuration, especially in the motor synchronization system (as in all other kinds of mechanical TV designs). If experienced in homebrew design but not familiar at all with NBTV and/or mechanical television, I recommend checking out the Early Television Museum website.
The design proposed in this document is modular so that it is possible to choose some parts from it, if not all of them, according to one's preferences:
In this case, where you don’t use the color decoder, you will at least have to insert this filter between the computer audio channel output and the audio amplifier input to suppress the color carrier superimposed by default by the software converter. (If the amplifier has no volume control, insert a 10K potentiometer - as a voltage divider - between the filter output and the amplifier input)
Experimenters willing to build their own Nipkow’s disc can use this helping SW tool.
The electronics and the video generation software tools are designed to be compatible with 30-line and 60-line discs while reserving the first line for a HSYNC signal. In other words, the first line sector is present on the disc (and the corresponding hole can exist) but is it always black, bringing the effective resolutions at 29 and 59 lines. The rasterization is horizontal like in classical video (unless the original video has already been rotated before creating the NBTV video), and its speed is 12.5 frames per second.
The prototype shown in the Youtube video is based on a Nipkow’s disc spinning at 12.5 turns per second. It is relatively small, especially if dealing with 60 lines. This experience helped to conclude that such a small disc featuring so many lines can only be built with advanced machining equipment. Even a printed template and a careful driller will not succeed in reaching a satisfying result. This is why generated pictures shown in the video are disappointing. Handmade discs should be 30-line discs with a diameter close to what can be seen in the video (27 cm) or 60-line discs, two times bigger if targeting an aspect ratio close to 1:1 and a reasonable picture size.
The original video file to be processed by supplied software (detailed further) must have the same aspect ratio than the final picture to be created on the disc (directly depending on disc geometry). The video converting program will adapt the entire original images to fit in an NBTV video at 57x59 pixels or 121x29 pixels for 30-line variant. In other words, pixels may not be "squares" depending on what you do: With 60 lines, the effective pixel shape proportions are almost the same than the disc-created picture proportions, while with 30 lines, the ratio height/width is quadruple. (Pixels are in fact more similar to small bars perpendicular to lines, like in the most current cases of 30-line television systems. Ideally, the disc holes should have the same proportions and shapes but this is not possible).
In the next installment, we'll talk about the components used to make it and design some circuits! If you want to check out another mechanical TV design, be sure to look at Robin Mitchell's Baird Mechanical Television tutorial!
