How television works 

8 September 2023 tbs.pm/79628

Ally Pally

The London Television Station, Alexandra Palace. The mast carries two separate aerials: (above) vision (below) accompanying sound

 

From the BBC Annual for 1937

 

The transmission of television pictures is closely allied to the broadcasting of sound, but there are certain fundamental differences in technique.

In order to appreciate the points of divergence, it is desirable to consider the differences which exist between the two senses to which these arts make their respective appeal.

The sense of hearing, to which sound broadcasting is directed, involves certain considerations which are better appreciated from a brief examination of the properties of the ear. The ear may be said to be essentially a single-channel device, having but one ear-drum, and one connexion to the brain through the aural nerve. In consequence, the ear does not convey separately a number of sounds that strike it simultaneously, it conveys them as a mixture.

From this fact, it is clear that any number of simultaneous sounds will resolve themselves into a single pressure wave consisting of a mixture of them all, and this mixture may be picked up and converted into a single electrical waveform by a single microphone. The resulting waveform, however complex it may be, can be passed through a single amplifier, and broadcast from a single transmitter.

Turning to the sense of sight, to which television makes its appeal, a very different state of affairs is disclosed, and in order to appreciate the precise problems involved, it is necessary to consider, briefly, the action of the eye itself.

 

A clown plays piano whilst a man plays violin

Noni in ‘Starlight’ programme

 

An image of the scene being observed is cast by the lens, or cornea, on the light-sensitive plate, or retina, at the back of the eye. The retina consists of a multitude of tiny elements, each separate and distinct from its fellows. Each element is separately connected to the brain by an individual fibre in the optic nerve, and there are probably something like one million such elements in the average human eye.

The eye is therefore an exceedingly complex organ, and exercises keen discrimination, picking out minute details of the scene before it, and conveying a perception of each detail separately to the brain. It does not mix the details up, otherwise the viewer would perceive only an unintelligible blur.

To satisfy the demands of the eye for the purposes of television transmission, it is therefore useless to send a mixture of detail. All details of a scene must be transmitted separately and simultaneously, and to present a received picture of good definition something approaching a quarter of a million separate details, must be transmitted.

 

A man sits; another man stands at a blackboard

Ian Hay and Will Hay, a comedy dialogue in ‘Picture Page’ programme

 

Happily, there exists a device known as the photo-electric cell, which reacts to light in the same manner as the microphone does to sound, and is, in fact, equivalent to one of the elements of the retina. The whole process of television is based upon its use; but to send, in a straightforward manner, a quarter of a million details would necessitate a quarter of a million photo-cells, a quarter of a million radio transmitters, and a like number of receivers at the other end. As this is clearly impossible, an expedient has to be adopted in order to place the transmission within the bounds of possibility.

The expedient, or trick, which is used takes the form of a process known as ‘scanning’, which consists in dissecting the picture into a large number of minute pieces, like the pieces of a jig-saw puzzle, each piece being sent out separately and the whole reassembled at the receiver with such rapidity that the eye is deceived.

The actual process is more analogous to dictating a book over the telephone. Here the problem is much the same. It might be possible to pronounce all the words on a page simultaneously, but little intelligence would be conveyed, and in consequence it is necessary to dictate word by word, line by line, and page by page, telling the recipient when to start a new line and a new page.

A man and a woman look over a script

ANNOUNCING A TELEVISION PROGRAMME

In the process of scanning, the picture to be transmitted is divided up into a great many lines, the detail which occurs along each line being dictated by the scanning apparatus, and faithfully transcribed by the receiver. The scanner also informs the receiver when to start a new line, and a new page or frame, by means of ‘synchronizing signals,’ which are known as the ‘line’ and ‘frame’ synchronizing signals respectively.
Each frame represents a complete picture, and if successive frames are built up rapidly enough, the property of ‘persistence of vision’ deceives the eye into thinking it is seeing a continuous picture. The cinematograph relies upon the latter property in projecting upon the screen a rapid succession of photographic images, the movement in each of which is slightly more advanced than in the preceding one, so that an illusion of continuous motion is produced.

It is clear that sufficient frames must be transmitted each second to produce the desired illusion, and to avoid the impression of flicker. The Television Advisory Committee laid it down that a minimum of 25 frames per second should be employed at the London Television Station. It was further laid down that a minimum of 240 lines per picture should be used.

Those familiar with process blocks used in printing know that the greater number of picture points to the inch, the better the picture. So in television, the greater the number of lines, the better the definition of the picture, provided that full advantage can be taken of the greater number of lines in the reproduction. The technical difficulty of transmission, however, increases very rapidly with the number of lines and frames, so that the actual choice of standards is to a great extent influenced by practical considerations. The standards adopted were:

  • Baird Television, Ltd. – 240 lines, 25 frames per second, sequentially scanned.
  • Marconi-E.M.I. – 405 lines, 50 frames interlaced, giving 25 complete picture frames per second.

The process of interlacing consists in scanning all odd lines first, subsequently returning and scanning the even lines required to complete the frame; and it is claimed that the apparent flicker frequency is doubled without materially increasing the technical difficulties of transmission. Flicker is said by this method to be reduced to imperceptibility.

 

A ticked-off woman sits with her arms folded

AN UNRETOUCHED PHOTOGRAPH OF THE IMAGE ON THE SCREEN OF A TELEVISION RECEIVER.

 

Dissecting the Picture

 

MECHANICAL METHODS

Many methods have been employed, from time to time, in the dissection of the picture Many methods have been employed, from prior to transmission, and one of the earliest made use of the Nipkow disk, invented many years ago and named after its inventor. This device consists of a circular metal disk, pivoted at its centre, and drilled with a series of holes on a spiral path. Upon rotating the disk, these holes scan an area. Reference to Fig. I will make this clear.

 

Drawing of a Nipkow disk

FIG I. NIPKOW DISK

 

Sundry variations of this arrangement have appeared, such as mirror drums, disks carrying a spiral of lenses in place of the small holes, etc., and, while these devices are more efficient in the transmission of light, they have dropped out of use as they cannot be constructed with sufficient accuracy to be suitable for high-definition television.

The original Nipkow disk alone survives, and this type of disk, in a modern form, running at high speed in an evacuated casing, is utilized by Messrs. Baird for transmission method, which is applicable to close-up and by all their processes. In the spotlight three-quarter length pictures of one or two caused to traverse the subject in a series of persons only, a projected spot of light is lines and frames in the usual manner. Light reflected from the subject falls upon photo-electric cells, the resultant current variations from which constitute the vision signal.

Standard motion-picture films are scanned in a similar manner by passing the flying spot of light through the film and so to a photo-cell.

The intermediate film process is applicable to more extensive scenes to be televised from the studio, and involves the recording on film of a photographic image of the scene by means of an ordinary motion-picture camera. Details are given elsewhere; here it is enough to say that immediately after exposure the film is rapidly developed, washed, fixed, and scanned by means of a Nipkow disk while it is still wet.

 

Soldiers with heavy cannon

Anti-Aircraft Defence Display (12 December 1936)

 

ELECTRONIC METHODS

The Baird system installed an ‘Electron’ camera and with this, as indeed with the Marconi-E.M.I. system, the process is entirely different in that scanning is carried out by purely electronic means, without the use of any moving parts.

In the Marconi-E.M.I. system the ‘Emitron’ is used, a device which has been described with some accuracy as the ‘electric eye’. A lens, representing the cornea, casts an image of the scene to be transmitted upon a specially prepared plate which is a close imitation of the retina and which is placed at the wide end of a cathode-ray tube. This plate, which is made of insulating material, is covered with tiny photo-sensitive nodules, very close together but each separated from its fellows. When the optical image falls upon this plate or ‘mosaic’, photo-emission takes place, and the elements acquire a charge of electricity which is greater or less in proportion to the amount of light falling upon them, so that the plate has upon it a faithful electrical picture of the original scene.

 

Dancers

The Vic-Wells Ballet Company in ‘Façade’ (8 December 1936)

 

In the tube is also a device known as an ‘electron gun’, which shoots a stream of electrons, or particles of negative electricity, like bullets from a machine-gun. The gun is aimed at the mosaic, and its aim is varied by application of suitable electric fields so that the point where the beam impinges on the mosaic is moved across the surface in a series of lines. By this means the whole of the surface of the mosaic is explored line by line, and the charges that have accumulated on it are carried away by the electron stream, which acts as a weightless brush, and form the vision signal to be transmitted. After the passage of the beam the mosaic is left wiped clean like a slate, and ready to receive the next series of impressions. The ‘Emitron’ is mounted in a portable camera similar to a motion-picture camera, and several such cameras may be used in the presentation of studio and outside scenes, or in conjunction with a projector for reproduction of standard film.

The amount of information which has to be transmitted to make up a good picture is very great, and it may be said that one picture signal contains as much information as would be contained by about 1000 simultaneous telephone calls. Speaking electrically, this implies the necessity of transmitting a band of frequencies, or electrical vibrations, ranging from 1 vibration in many seconds to about 2,500,000 vibrations per second. Compared with the frequency range required for the transmission of sound, this band is enormous; for good sound can be transmitted embracing the band of frequencies 30 to 10,000 vibrations per second only.

 

A woman and a pantomime horse

Florence Mayo and Felix the Horse in ‘Animals All’ programme (22 December 1936)

 

Transmitting the Picture

 

Close-up of one of the 'spikes' on the Ally Pally mast

A DETAIL OF THE AERIAL SYSTEM

The radio transmitter used for television has much in common with transmitters used for sound broadcasting, in that the signals are sent out superimposed on a carrier wave. The carrier wave may aptly be likened to one of those endless belt conveyors which are installed beneath the counter in some warehouses. The conveyor rolls along silently by itself, conveying nothing, until an assistant at one end of the counter wraps up a parcel of goods and throws it on to the belt. The parcels are carried down to the other end of the counter and deposited in a receptacle where the customer waits.

In sound broadcasting the parcels are speech or music, the assistant is the artist in the studio, the carrier wave of the radio transmitter is the conveyor, and the customer is the listener. Similarly, in television, the parcels are bits of picture to be conveyed to the viewer by the carrier wave which forms the conveying link through the ether.

Remembering the great band of frequencies to be transmitted, it will be appreciated that the parcels come in almost overwhelming numbers and at a great rate. Consequently, the conveyor must move very swiftly, or one parcel will jostle another and the whole system become clogged, so that many of the parcels will be unable to get on the conveyor at all.

 

Two men stand at a bank of controls and dials

Marconi-E.M.I. Control Room showing (left) Pulse Generators, and (right) ‘Emitron’ Camera-Amplifiers

 

A man works at a machine covered in gauges

The Baird Teleciné Scanning Apparatus

 

Turning from the analogy, the carrier wave must move rapidly – in electrical parlance it must have a very high frequency of vibration. As a result of this fact, the carrier wave frequency employed for vision at the London Television Station has a frequency of vibration of no less than 45,000,000 a second, which corresponds to a wavelength of about 6.7 metres.

It is this enforced use of the ultra-short-wavelength that limits the range of the station at present to a radius of about 30 miles under ordinary conditions. Reports of more distant reception have, however, been published, and it is hoped that the range may ultimately be substantially increased.

 

Banks of consoles covered in gauges

THE SOUND TRANSMITTER

 

Reproducing the Picture

 

The signals emanating from the transmitting aerial are picked up by a receiving aerial and passed to a receiver, which functions in a manner very similar to a sound receiver, except that the output is not passed to a loudspeaker, but to a reproducing scanner instead.

A number of types of reproducer have been used, but the present-day tendency is to employ a cathode-ray tube. This consists of an evacuated glass vessel having a bulb at one end and a narrow neck at the other. An electron gun is situated at the narrow end, trained upon a chemically coated screen at the bulbous end. This screen is composed of a thin, practically transparent layer of a mineral salt, which is deposited upon the glass, and which possesses the property of fluorescence. A fluorescent material is one which glows and emits light when it is subject to electronic bombardment, the brightness of the glow depending upon the number of electrons striking it in a given time.

 

An orchestra plays in front of a huge glass-fronted cabinet

The Baird Intermediate Film Scanner

 

A camera points at a man playing the violin

The Marconi-E.M.I. studio showing ‘Emitron’ Cameras

 

As in the ‘Emitron’, the aim of the gun is varied by suitably applied electrical fields, so that the point of impact of the electrons on the screen is caused to trace a series of lines side by side across the screen. By means of the synchronizing signals previously referred to, the movement of the spot of light on the receiving screen is made to follow exactly the movement of the scanning aperture or beam at the transmitting station. The ‘line’ synchronizing impulse tells the receiver cathode beam when to start a new line, while the ‘frame’ synchronizing impulse tells it when to start a new frame.

At the same time the number of electrons fired by the gun is controlled by the intensity of the electric currents coming from the transmitting scanner and being transmitted, so that a particular area of the receiving screen is either bright or dim, according to whether the particular portion of the subject being scanned is light or dark.

 

A small orchestra

The B.B.C. Television Orchestra

 

In this way an image is built up at the receiver which is a faithful reproduction of the scene exposed to the television camera in the studio. Pictures in the home are at present limited to a size of about 12 in. by 10 in., where cathode-ray tubes are employed, owing to the impracticability of constructing larger tubes which will stand up to the pressure of the atmosphere. It is to be hoped, however, that means of enlarging the picture will be forthcoming in due course, and a great deal of research work is at present being directed to this end.

 

A woman sings into a camera

Sophie Tucker in ‘Starlight’ programme

 

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