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Dead medium: Baird Mechanical Television, Part One: Technical Introduction
From: house127@teleport.com Trevor Blake
Source(s): 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 TELEVISON (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 TELEVISON (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

1881: 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 1884. 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@teleport.com - http://www.teleport.com/~house127