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**Dead medium: Baird Mechanical Television, Part One: Technical Introduction**

From: house127_AT_teleport.com (Trevor Blake)

Sources:

BOOKS Manly, Harold: DRAKE'S RADIO ENCYCLOPEDIA (Drank & Co. 1927) Ghirardi, Alfred: RADIO PHYSICS COURSE (Radio & Technical Pub. 1933) Zworkin, Y. K. and Morton, G. A.: TELEVISION (John Wiley 1940) Goldstein, Norm: THE HISTORY OF TELEVISION (Portland House 1991) Kisseloff, Jeff: THE BOX (Viking 1995) Ritchie, Michael: PLEASE STAND BY (Overlook Press 1994) Winship, Michael: TELEVISION (Random House 1988) Yanczer, Peter: THE MECHANICS OF TELEVISION (Peter Yanczer 1987) (Peter Yanczer, 835 Bricken Pl., St. Louis MO 63122 USA)

MAGAZINES Popular Science, March 1932 Mechanics and Handicraft, Vol. 1 #1, Winter 1933 Television: Journal of the Royal Television Society, April 1995

VIDEO The Race for Television, BBC

INTERNET The efficiency of on-line search engines and the shifting nature of the Internet make long and comprehensive lists of URLs both unnecessary and inaccurate. A search for 'John Logie Baird' or 'mechanical television' should turn up several interesting sources. Only two are listed here.

http://www.teleport.com/~house127/lobby/mechtele.html This article, including illustrations.

ftp://ftp.teleport.com/pub/users/house127/avdept/mechtele. zip

A lengthy thread from alt.technology.obsolete on mechanical television, as well as one or two pieces of e- mail on the subject. Compressed using pkzip.

VISIONEER: JOHN LOGIE BAIRD AND MECHANICAL TELEVISION

by Trevor Blake 

Part 01: TECHNICAL INTRODUCTION Part 02: JOHN LOGIE BAIRD Part 03: OTHER COUNTRIES, OTHER SYSTEMS


 * PART ONE: TECHNICAL INTRODUCTION**

The discovery leading to the possibility of mechanical television was an accident. While laying the first trans-Atlantic cable, a worker noticed that some of his tools were glowing. An analysis of the metal revealed a concentration of selenium, the metal used soon after in the earliest photoelectric cells. Selford Bidwell used a photoelectric cell to transmit an image electronically in : over the course of several minutes, a two-inch square image could be sent via telegraph lines.

Three years later, Paul Nipkow was granted a German patent for the **Nipkow disk** == a complete and functional television system in. The development of the neon tube in 1910 furthered mechanical television.

Film achieves the illusion of motion by taking advantage of the persistence of vision: still images in a fixed location which are refreshed at a rate of sixteen times per second (or more) are interpreted by the human mind as moving images. Television achieves the illusion of motion in a similar but unique fashion. Rather than refresh the entire image at once, as film does with each cell that passes in front of the projector's light, television refreshes an image one line at a time in a scanning process. Within the cathode ray tube, an electron gun scans a single line of an image from one side to the other, then scans the line underneath it, until it has scanned an entire image.

The Nipkow disk is an earlier, mechanical means of achieving the same side-to-side, top-to-bottom scan process. It consists of a disk that rotates on its axis. A series of evenly spaced, uniformly sized holes are cut into the disk, spiraling in toward the center. The disk is housed in a box with a small viewing window: the outermost hole of the disk will form the outermost scan line visible in the viewing window, and each additional hole will form additional scan lines.

The rotation of the disk as seen through the viewing window provides scanning from side to side, and the spiral placement of the holes provides scanning from outermost to innermost scan line. A light source which can be varied in intensity is placed on the opposite side of the disk behind the viewing window. As the light flickers and the disk rotates, television is achieved.

Mechanical television cameras and receivers alike use the Nipkow disk, but where the receiver uses a flickering light to produce an image, the camera uses a photosensitive cell to generate an image. The rotation of the disks is synchronized by part of the transmission signal (which has included radio, short wave and telephone) or direct wiring. The disks rotate at around 900 rpm and initially produced television two inches square.

The earliest mechanical televisions offered between 16 and 24 lines of resolution. By the late 1920s, they offered between 48 and 60 lines. Double and triple spirals of scanning holes were used, as well as scanning drums and belts. Lenses were fixed in the scan holes to project the image onto a larger screen (up to 8 inches in some cases).

Mechanical television cameras were synchronized with film projectors, allowing the transmission of film. Studio B of the BBC used a hybrid of this system: the subject was filmed, the film was instantly processed and then scanned for transmission. There was a delay of around one minute between event and transmission as the film developed.

The light required for mechanical television is intense, so much so it was nearly impossible to perform while being televised. The flying spot camera was one solution to this problem: an additional scanning disk, synchronized to the camera, cast a brilliant light on the subject in the same spot they were being scanned. The rest of the studio, including the control room, was kept in complete darkness. Another solution to this problem was the use of multiple arrays of concave lenses to focus light into the camera more efficiently.

Trevor Blake 127 House - An Independent Archive of Systematic Ideology P.O. Box 2321 Portland OR 97208-2321 USA - (503) 635-1796 house127_AT_teleport.com - http://www.teleport.com/~house127 Type in the content of your new page here.