"Members of the Royal Institution and other visitors to a laboratory in an upper room in Frith-street, Soho, on Tuesday saw a demonstration of apparatus invented by Mr. J. L. Baird, who claims to have solved the problem of television." From an article on page 9 in the London Times for Thursday, January 28, 1926.
"It [Italy] possesses a Government under the commanding leadership of Signor Mussolini which does not shrink from the logical consequences of economic facts and which has the courage to impose the financial remedies required to secure and to stabilise the national recovery." From a speech by Winston Churchill, Chancellor of the Exchequer, announcing that Britain was granting Italy a loan of 80 million £ ($400 million) at 5% interest per annum and maturing in 1987. Reported on page 1 of the same issue of the London Times.
he picture on the right shows what a Nipkow disk looks like. It's a thin, typically metallic, disk, perhaps .02 inches thick, and some 30 or more inches in diameter. To reduce its mass, it might have spokes. It has a series of small holes, in this case 10, but more usually from 30 to 240, arranged on a spiral. The angular spacing of all the holes is the same.
The Nipkow disk takes its name from its inventor, Paul Nipkow, who invented it in 1884 as a means of sending pictures over telegraph wires. Nipkow never created a working model of his invention.
To see how this extremely simple device can be used to create television
images we need to break any television system into its elementary components.
These are tabulated in the left column of the following table. The right hand
column tabulates the devices used to implement these functions in television
systems of the mid 1920's.
|Functions of Any Television System||Implementing Device (1920's)|
|1||Imaging the three dimensional scene onto a flat plane.||Camera lens|
|2||Sequentially sampling the brightness of the image.||Nipkow disk|
|3||Converting the brightness samples to electrical currents.||Photocell|
|4||Transmitting the electrical currents to a receiver.||Radio wave|
|5||Converting the electrical currents to brightness.||Glow lamp|
|6||Spatially distributing the brightness samples in the same way they were created.||Nipkow disk|
If we place a lens between the subject and the Nipkow disk, we can image the subject onto an area on the surface of the disk near its top as shown in figure 2. If we rotate the disk counterclockwise, the holes in the disk will scan horizontally across the image in concentric circular arcs moving progressively from the top to the bottom of the image as shown in figure 3. In an alternate version, the image can be placed on the left side of the disk. In this case the scan lines are vertical and the frame is scanned from left to right.
Item 3 in the table is a device for converting the varying elementary brightness of the scanned image into a correspondingly varying electrical current. In the early days of television, the preferred device for this function was a selenium photocell.
The photoconductive property of selenium was discovered by Willoughby Smith (Fig. 4). Smith's profession was the design of undersea telegraph cables. He reported his discovery that the electrical conductivity of selenium was dependent on the intensity of light impinging on it to the British Society of Telegraphic Engineers in a letter dated February 4,1873. By the 1920's selenium photocells were readily available.
From Television, by R.W. Burns.
Ironical Chronicle graphic.
Ironical Chronicle graphic.
Fig. 5 shows the arrangement of the elements of a system for televising an image as implemented in the mid 1920's. The scene to be televised is imaged onto a portion of the disk by the lens. This image is scanned by the holes in the Nipkow disk. By connecting a battery in series with the photocell, the variations in the brightness of the light falling on the photocell are converted to an electrical current varying in proportion to the brightness of each spot on the image as it is scanned by the disk.
At the time that Nipkow invented his disk, the only known means for carrying these current variations to a remote receiver were electrical wires. However, on November 11, 1886, Heinrich Hertz had experimentally verified James Clerk Maxwell's prediction of the possibility of radio waves. Hertz's laboratory experiments were reduced to practical radio communication by Guglielmo Marconi in 1901. Thus, by the mid 1920's, radio waves as a means for carrying the video signal to a remote receiver were available.
Fig. 6 shows the system for reassembling the image at the receiver. The receiving system is identical to that of the originating system except for the element which converts the video currents back to brightness variations and the fact that the reconstructed image is now viewed through the lens.
From Television, by R.W. Burns.
The glow lamp in Fig. 6 is a kind of lamp whose brightness can respond instantly to changes in current. In the light sources used at the time, it consisted of two electrodes in a glass envelope containing an ionizable gas. The amplified signal from a radio receiver tuned to the television transmitter's frequency is applied to the terminals of the glow lamp. If the Nipkow disk at the receiving end is synchronized with the one at the transmitting end and if both disks are spun fast enough, the viewer's persistence of vision will create the illusion of a complete image, including its movement.
In motion picture projection, the standard frame rate is 24 frames per second. In the Nipkow disk-based system, a frame corresponds to one rotation of the disk. Thus, to duplicate the motion picture standard frame rate, the disk would have to rotate at 1440 revolutions per minute.
The early Nipkow television systems raised issues in communication theory which had not previously been considered. The most fundamental was probably the relationship between the resolution of the image and the bandwidth of the radio signal which carried the video information. High resolution meant a large bandwidth and a large number of scan lines. In the Nipkow system, the number of scan lines is equal to the number of holes in the disk.
In the earliest experiments, ten hole disks were used. In its highest form of development, reached in the mid 1930's, disks with 240 holes were used. Thereafter, all-electronic systems, which had been under development since the late 1920's, made the mechanical Nipkow disk scanner obsolete. The standard television signal broadcast in the US since the 1950's has been a 525 line system operating at 30 frames per second.
The first public demonstration of a television system was given by John L. Baird in London on Tuesday, January 26, 1926. A report of it appeared on page 9 of the London Times two days later.
It used a vertical scanning Nipkow disk.
The following two links present a human interest side of this story of the early days of television.