In the early 1920s the Scottish engineer John Logie Baird is credited with inventing the first practical television system. Many inventors had worked on the idea at various times, but Baird gave the first public demonstration of a practical system on March the 25th 1925 at Selfridge's department store in London.
The machine was crude with very poor contrast, capable only of transmitting the silhouette of an object moved in front of the camera. Baird quickly improved the system, and by October that year he was able to demonstrate in his laboratory the transmission of greyscale images
The first pictures were vertically scanned at 5 fps (frames per second) and with a resolution of 30 lines (the 30 lines was to become the standard for Baird's mechanical television system and broadcasts). In January 1926 he demonstrated this system publicly with an increased framerate of 12.5 fps.
The main component of the televisor is the Nipkow disc. Invented by Paul Julius Gottlieb Nipkow in 1883. The Nipkow disc is simply a solid disc with a spiral of holes around the outside. As the holes pass in front of a modulated light source it alows the light to pass through along one each of the picture. The modulated light source in the televisors of the 1920s was a neon tube, driven by a valve amplifier.
The Baird standard for mechanical television transmissions was 30 lines at 12.5 frames per second. Other countries such as America and Germany also made transmissions, everyone using different standards. The modern day Narrow Bandwidth Television Association (NBTVA) have decided on a standard of 32 lines, at 12.5 frames per second.
The Narrow Bandwidth Television Association (NBTVA) is an international association, based in the UK, but with members all over the world. They are dedicated to the early history of television and all sorts of experiments and activities surrounding mechanical and early electronic television. NBTVA members have even developed ways of recording colour images and playing them on a mechanical monitor, and mechanical televison in 3d (Baird was the first to display moving colour images and 3d television in 1928 and also that year made the first transatlantic television transmission between London and New York!)
The NBTVA holds an annual convention and AGM. Click here for some of my photos from past NBTVA Conventions.
My televisor (now dismantled) was a Nipkow disc type machine like Baird's displays. The frame was constructed from Meccano, and the electronics based on circuits designed by NBTVA member Peter Smith and published on the NBTVA website. These circuits control the modulated light source (an array of ultra bright light emitting diodes) and the speed of the motor, by synchronising it with pulses on the incoming television signal.
In the NBTVA standard, the incoming signal contains 31 sync pulses (one for each line, except for the first of the frame, this is intended to provide frame synchronisation) these are stripped from the video signal and fed to another circuit which compares them with pulses being received by an infra red emitter and detector positioned either side of a slotted wheel with 31 holes (and one blank to correspond to the missing frame pulse). A phase locked loop circuit compares the two pulse trains and speeds up or slows down the motor until the phase of the signals is the same, thereby locking the speed correctly so that the picture does not 'roll'.
My televisor had a rotating ring which allowed the detector apparatus around a small slotted wheel connected to the main shaft. This addition made it possible to carefully rotate the sync position to adjust the framing of the picture in the viewing aperture.
My oscilloscope driver
This project is an all electronic device to display NBTVA standard video signals on a CRT oscilloscope with beam modulation input. It contains a sync separator and 'missing pulse detector' to synchronise to the incoming video signal; and a counter circuit that generates the X and Y deflection digitally with a pair of R2-R ladder DACs.