US2048761A - Television system - Google Patents

Description

July 2s, 1936. L, E. Q. WALKER 2,048,761

TELEVISION SYSTEM Filed May 11', 1952 A11/f2 /TUDf Afl/P07005 INVENTOR LLQ, WALKER4 ATTORNEY Patented July 28, 1936 PATENT OFFICE TELEVISION SYSTEM Louis Edward Quintrell Walker, Braintree, England, assignor to Radio Corporation of America, a corporation of Delaware Application May 11, 1932, Serial No. 610,536 In Great Britain May 29, 1931 1 Claim.

The present invention relates to television and like systems and has for its object to provide improved means for maintaining accurate synchronism between transmitting and receiving apparatus.

In any television or like system in which the scene or pictureto be transmitted is scanned or explored and resynthesized by means of rotating drums, mirror wheels, disks or the like, the problem of maintaining accurate synchronism resolves itself into keeping the receiver mechanism rotating at the same speed as the transmitter mechanism.

-In many television systems proposed hitherto synchronism of the .transmitting and receiving mechanisms has been obtained either by transmit-,ting a special synchronizing signal over a land line or some other transmission channel, e. g., a radio channel or by providing at transmitter and receiver accurately manufactured constant frequency devices which are arranged to be started and stopped together and which are intended to be of such accuracy that they will in operation be synchronous.

The present invention provides a considerable practical simplification in methods of `synchronizing television and similar transmitters and receivers and dispenses with the use of more or less expensive master time period devices or of any need to transmit special synchronizing signals, synchronism being obtained instead by utilizing a periodic component which in practice is to be found in the television signals themselves.

The existence of the periodic signal component, which periodic signal component is normally the scan line frequency (though in vpractically all cases harmonics of the frequency exist and may be utilized) will be better understood from the following mathematical explanation:-

The form of signal transmitted by any of the usual known methods of transmission may be repwhere e is the instantaneous value of E. M. F. of the complex signal current.

\ purposes.

(Cl. P18-69.5)

mately periodic having a mean time of recurrence T and the signal can therefore be represented adequately by the series;-v

' the scan line.

According to this invention synchronism between transmitting 'and receiving apparatus in a 10 television picture telegraph or similar facsimile system is obtained by utilizing a periodic component of energy existing in the television or like signals transmitted, said component beingy a function of the scanning line frequency.

Preferably the component actually used is the scan line frequency itself.

It might be thought that under conditions (which incidentally seldom occur in practice) when the television screen at the transmitter is 20 equally illuminated, that there will be no periodic component utilizable for synchronizing This assumption is not correct despite the fact that it appears to be so at rst F sight. Obviously, if the illumination of the screen at the transmitter be unequal the television signals obtained will be of the general wave form shown conventially and graphically in the accompanying Figure 1 and will contain,if`c'ourse,f'wir a component equal to the scan line frequency, which frequency may, for example, be 375 cycles per cent in a practical case. The scan line frequency component is shown in broken lines in the accompanying Figure, 1, the other varying quan.- tities which, when added to the brokenscurve-makenas up the full curve, being the television variations.

If the television system screen in equally illuminated the wave form will be as shown in the accompanying Figure 2. Although this wave form does not include a separate and distinct 375cycle 4o component as does the wa've form of Figure 1,

it may be shown that nevertheless there is present in fact a component of the scan line frequency which component may be utilized as a synchronizing impulse.

Consider the case of a single unidirectional pulse of wave form as shown in the accompanying Figure 3. It is well known that such a wave can be represented by Fourier integral and the transmission of such a pulse is mathematically the equivalent of the transmission of a continuous band of frequencies extending from zero to infinity. If the 4duration of the pulse be 1/375th part of a second the relative magnitudes of the frequency components present in such a pulse will be such that maximum or peak values of amplitudes occur in the region of zero, 375 cycles, 1125 cycles, 1875 cycles, etc. corresponding to frequencies of zero, the scan line frequency and odd harmonics thereof. Accordingly, a train of pulses as shown in Figure 3 (even a relatively short train) tends to contain only isolated finite harmonic components characteristic of a Fourier series wave, these components definitely existing at the scan line frequency and (assuming a square topped pulse) at odd harmonics thereof. Excluding the zero frequency component which, of course, is ineffective either for transmission or synchronizing the maximum and most, useful component is that at the scan line frequency. This distribution 'of energy in television signals tends to lie about the scane line frequency and odd harmonics thereof (where square topped pulses as in Figure 3 are in question) or. in the more usual case, where square topped pulses are not in question, the energy tends to be in bands lying about the scane line frequency and odd and even harmonics thereof.

Although energy is more or less concentrated in nite bands around all the frequencies stated, said energy is mostly concentrated at the scan line frequency and therefore this is the frequency which may in practically all cases be most usefully employed for synchronizing purposes. As the number of scan lines in the picture increases, i. e., as the approach to "steady state conditions becomes greater the bands of energy concentration decrease in width and in normal television conditions where synchronization depends upon the .average effect of a large number of pictures, the bands will in practice be sufficiently narrow to be usefully employable for synchronizing motors driving transmitting and receiving apparatus.

In the accompanying Figure 4 is shown (in full line curve) the voltage generated by picture signals and (the broken line curve)` the 375 cycle component of this voltage is a system in which there are 375 scanning lines per picture.'

T'he accompanying Figure 5 shows the voltage l which isapplied to the television receiver motor for synchronizing this voltage being, as will be seen, the 375 cycle component. Figures 6 to 8 of the accompanying drawing show diagrammatically and schematically television receiving arrangements in accordance with the invention.

Referring to Figure 6 received television signals are applied to the input terminals of a normal thermionic receiving amplier represented in the gure by a single valve V, the plate circuit of which contains a light source LS and a resistance R. 'Ihe light source LS and resistance R will, of course, be traversed by currents proportional to the complex received signal currents and consequent fluctuations in potential across the resistance R are applied by means of a coupling condenser C and resistance CR to the input amer/61 terminals of a further amplier valve V whose anode circuit contains a suitable choke CH. In parallel with the choke is a circuit consisting of a lter Z tuned sharply to the desired harmonic component, e. g., 375 cycles per second, said'lter being in series with the alternating current windings W of a synchronous driving motor driving the receiving apparatus (not shown).

The uctuating anode `current in the second amplifier valve V' sets up a uctuating voltage across the choke CH and causes a large uctuating current to pass through the alternating current winding W of the motor.

In the arrangement shown in Figure 7, TA is a television receivingamplifler of normal design to the input terminals I of which are applied received signals and the output terminals O of which are connected to the normal picture building light source as in the usual way. In parallel across the terminals I is a lter Z which may consist of a series of sharply tuned resonant arms with correct terminating impedances at each end, said filter selecting the scan line frequency and passing it on to an aperiodic amplifier AA whose output is utilized to synchronize the motor driving the receiver scanning system.

In Figure 8 part of the signal input applied at I is fed to a tuned multi-stage amplier MA;V

the grid circuit of each valve in this amplifier being tuned to the scan line frequency (e. g., 375

c. p. s.) and the output being utilized to synimpulses are not of uniform amplitude but varyfrom cycle to cycle with the `result that the receiver motor does not run ata uniformly steady n an be shown that with the arrangement illustrated with the present invention, if the filter signal, supplying said signal to an electro-optical converter, deriving from said signal a constant amplitude wave of a frequency equal to the line frequency, amplifying the derived wave and utilizing said derived wave to synchronize and positlon in space ,the optical eects produced by the electro-optical converter.y

murs EDWARDfQUIN'rRELL WALKER.

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