" 16 PHILlPS TECHNICAL REVIEW .' , VOL. 1, No. 1 AN EXPERIMENTAL TELEVISION TRANSMITTER AND RECEIVER By J. VAN DER MARK. Summary. On the occasion of the erection of a television transmitter at the Philips Laboratory, some of the main principles of modern television are discussed. The circuit and components of a modern television transmitter and receiver are described with special reference to the Philips experimental unit. Principle of Television and the light values of all elements are telegraphed simultaneously. In television this simultaneity natur- The human eye is a very complex and sensitive ally cannot be effected, as only one conductor (a'single organ, whose optical mechanism functions briefly carrier wave) is available for aU picture elements, as follows: The crystalline lens of the eye produces so that the separate light values must be tele- an image of the field of view on the retina which graphed in ~uccession. In consequence television is made up of a very large number of minute technique is rather complex. as may be exemplified lightsensitive cells. Each of these cells through by a simple calculation. To obtain a picture of its own nerve filament communicates to the brain satisfactory quality, the area of a picture mea- the stimulus it receives from the amount of light suring 4 X 4.8 in. must be resolved into about falling on it, and from the sum-total of the stimuli 40000 elements, In televising moving pictures, it received by it the brain builds up the 'image seen is necessary, as in cinematography, to send a suf- ,by the eye. ficient number of pictures per second, at least 25, . lp. the. human eye Nature has provided us with in order to produce a connective image on spec- the basic principles of television fully worked out; tator's eye. Thus, only 1f25th of a second is avail- also in televising the area of the picture to be ablè for the transmission of each picture, in other transmitted is' resolved into a large number of words e'ach second the light values of 25 X 40000 = small elements or cells , (fig. 1). Each of these 1,000,000 elements of the picture must be tele- ~' elements is given a number and the light _value ,l graphed 1). of each element is :~elegraphed to the receiver in sequentialorder. Exactly as at the tran~mitter, Conversion of a Picture into a Modulated Radio ...._.., the picture surface at the receiver is also resolved Wave. Resolution of the Picture into Elements or into elements which are numbered in the same Cells way, and each element is given the light value telegraphed for. its particular number. In this way Both at the transmitter and receiver the picture is resolved into a series of elements _or cells by I 2 3 4 5 6 7 89/0 I 2 3 4 5 6 7 8 9 /0 "scanning" it with a beam of electrons furnished 11 12 •• .. .. 11 12 •• .... , by a cathode ray tube. The scanning spot at which the electronic beam strikes the surface of the picture describes it path on this surface of the type shown in fig. 2~ This path is made up of a /5055 Fig. 1.' Principle of television. The surface of the picture to series of nearly horizontal lines packed close be televised .is resolved into a number of small elements which ~Qgether, the beam passing over these lines in 'are numbered in succession.The .brighti-ess of each individual element is telegraphed, At the receiver where the picture surface is subdivided into similar elemei.ts each element is 1) This is aptly brought out by the following example:' For given the brightness transmittcd for its respective number, some years facsimile and picture telegraphy has enabled so that· the received picture exactly reproduces the original. pictures to be transmitted by telegraphy, the. trans- mission of a single picture taking from 10 to 20 seconds or even 'longer. In the Melbourne Air Race in October, the picture reproduced at the receiver i., the same 1934, a film was made of the arrival of the winners at as thát transmitted by the sender. Melbourne and was transmitted to London. This short film, which was on exhibition at London cinemas on the In the eye every element of the picture in the same day already, took a 3/4 minute to project, while the transmitter (cell of retina) hasIts own conductor time of transmission from Australia, was about 6 hours. , In television the snme transmission must be completed (optic nerve filament) to the receiver (brain), in a 3/1 minute. ' ) . , . JANUARY 1936 TELEVISION TRANSMITTER AND' RECEIVER 17 succession, When the beam reaches the end of lines N in the whole picture, Usually rectangular one line, it jumps to the beginning of the next- line, pict_ureswith sides in a ratio of 6: 5 are televised 2). scans it in exactly the, same way and so oil over The total number of elements in the picture is then the whole picture, until it arrives' at the end of the 1.2 N2. The number of picture elements to be last line. Then it flies back to the beginning of the, transmitted per second, which determines the max- ." t ~ "-, _--- ==- <, <, ,_ <, <, , . <, -, I-. <, -, ~ ---= .__.._, =- --.;. ,,- <, 1S054 ' Fig. 2. The numbering in fig. lis replaced by "scanning" tbe picture in a definite .sequence. The path ?f the scanning In t spot shown here determmes the order which the various 'S04.9 elements are telegraphed. Fig. 3. Composition of the two motions of the scanning beam. A deflecting voltage with a saw-tooth time diagram controls' first line and goes through the same sequence of mot~on along each line (left), while a similar voltage (right) operations again. During scanning, the scanning N times slower (N being the number of lines in the picture) controls the beam motion in such a way that it does not beam' measures the brightness of each element incessantly scan the same line hut passes along the N lines of the picture as described in the next section. in successionand then Hiesback toits starting point again. The beam is guided along its scanning path by two imum modulating frequency of the radio wave 'voltages which deflect the beam to varying degrees required for televising the picture, is then ,30 N2 simultáneou~ly, each voltage fluctuating wi1~h with a picture frequency of 25. In this' way very a saw-tooth voltage-time .diagram as' shown in" high' frequencies are soon rêáohed, Since an alter- fig.3. The slow voltage controls the scanning nating current of p cycles per second -already cor- motion in the vertical direction with a frequency responds to 2p alternations per second of light equal to the number of pictures per second, whiÏe , . and dark, the required modulating frequency can the fast voltage controls the motion in the horizon- be halved, so that with 180 lines the maximum , tal direction with a frequency equal to the product modulating is cycles and with of the number of pictures per sècond and the number -frequency 500,000 of lines in the picture. 450 lines 3,000,000 cycles. As already stated, -television signals are trans- , In order to reassemble the picture at the receiver mitted in ,the same way as microphone signals in from the elements in tlie same manner, as it was broadcasting by modulation of a carrier wave resolved at the transmitter, the electron beam whose frequency must be considerably higher than . in the cathode 'ray tube of the receiver must at the modulating frequency. For televising it is every moment- occupy exactly the same position therefore necessary to use a carrier wave in the relative to the picture as the electron beam of the ultra short-wave range between 7 and 5 metres. transmitter, i.e. the scanning seguence in the This very short wave, however, has a particular receiver and transmitter must be completely syn- drawback. Contrary ,to broadcasting waves, of chronised. This is realised by means of two distinct several m in length thes~ waves do.not propa- types of synchronising signals which are ~adiated too gate along the curved surface 'of the earth. They from the transmitter at the end of every line and can therefore only be received within a radius each picture respectively. We shall return to -this point later .. which barely exceeds the' distance at which the transmitting aerial is still in sight of the receiving The modulation of the light value when, scanning aerial ê}. To make this area as large as possible, the a horizontal line may be regarded as equivalent to a resolution into à definite number ~f elements. 2) This is the usual size ratio of sound-film pictures. If the picture is a square and has the same sharpness 3)' Recently these waves have been detected for short inter- both ,hor~zontally and vertically, the number of vals also at greater distances, but reception has been so patchy. that s~tisfa~tory. tra~smission to points :beyond these elements must be equal to the, number of the VIsiblehorizon IS quite Impracticable. , 18 PHILIPS TECHNIÇAL REVIEW VOL. 1, No. i aerial must be suspended from very high masts. emits photoelectrons under the action of the in- In the case of the transmitter at Eindhoven, the cident light and thus looses' a part of its charge, primary aim has not been to obtain a range of this diminution being proportional to the light reception a's large as possible; the aerial has, there- value of the picture at the' respective point.