US1871266A - Electrooptical transmission - Google Patents

Description

ELECTROOPTIGAL TRANSMI SS ION Filed April 8, 1930 Il RE QUE N DIY |0000 20000 30000 40000 VVE/WUR f.' GRAY Patented Aug. 9, 1932 narran stares rArsNr esterna FRANK GRAY, or NRW YORK, N. Y., AssreNoR-.ro BELL TELEPHONE LABORATORIES, :i

INCORPORATED, or NRW Yoan, N. Y., A' CORPORATION OF New .YORK

ELECTRGOPTICAL TRANSMISS'IQN Appiication inea April s,

This invention relates to electro-optical transmission and morel particularly to apparatus for reproducing an image.

At receiving stations of certain image producing systems heretofore proposed, image currents are supplied to a glow lamp containing a charge of pure gas of the helium group, for example, neon. The incoming currents modify the intensity of the light radiated by the lamp, which is viewed through a suitably apertured scanning member which is driven in synchronism with a scanning member at the transmitting station.

Considerable difliculty has been experienced in producing a well defined image at receiving stations which operate in the manner described above. In general the image is either blurred, ill-defined or distorted, be-

cause the intensity of the light supplied by theV 2o lamp fails to vary in accordance with, or synchronously with, the variations of the image currents, or the intensity is not reduced to the proper value when reduced current is supplied to the lamp.

The undesired conditions noted above are due to the fact that the lamp, because of undesired resistance and reactance characteristics, discriminates in favor of current components of certain Jfrequencies lying Within the frequency range used in television, and

certain other signal systems; that it introduces a phase shift of the current components with respect to the corresponding applied Y voltages which is not proportional to frequency, and that there is a time lag of the light vwith respect to the current flowing through it which is evidenced by the presence of an after-glow. v

An object of the invention i'sto overcome or minimize the disadvantages noted above, in a signalling-system.including glow lamps. Another object of the invention is to improve the quality of the image produced at a television receiving station employing glow lamps.` i

Still another object is to4 provide means at a television receiving station to produce a sharply defined image of the subject scanned at a transmitting station.

l1930. ser'iarNo. 42,4251.

A further `Object is to Vimprove the fre,- quenoy characteristics ofglow lamps;

An additional obj ect is to prevent light lag effects and particularly after-glow in the operation of a glow discharge lamp.

In one preferred embodiment of the invention, the above noted objects may be accomplished by providing a receiver comprising an amplifier for supplying image currents to a glow lamp having a gas charo'e comprising 60 amixture of inert gas and a small percentage of active gas.

Because of the use of the active gas, the resistance of the lamp is maintained substantially constant over the television frequency range, its reactance varies approximately linearly with frequency, Vand its impedance is lower than the impedance of a lamp charged with the same practically pure inert gas. The active gas also operates to substantially eliminate the light lag with change of current iowing through the lamp.

rThe present invention also contemplates including the glow lamp in an amplifier output circuit having an impedance which is con-v stant over the television range and which is high compared with the impedance Vof the lamp, whereby the effect of the `impedance variations of thelamp, uponr the amplitudes of the different frequency com onents of the 8o image current wave, is rendere substantially negligible. n V

A more detailed description ofthe invention Vfollows and is illustrated in tliefaccompanying drawing.

1 s Fig. 11s a schematic diagram of a receiving circuit embodying the invention and Figs.2 and 3 illustrate curves, used to explain the invention. i Y Referring tOFig. l, there is shown a cil- 9'0 cuit l for applying the incoming image currents to theinput circuit of an amplifier.

Obviously the circuit 1, may be the termina-v tion of a line over which the image current-s are transmitted or the output circuit of a hiOh frequency terminal apparatus, vif the image currents are'transmitted as .modulailziois of a carrier wave over a lineor radio The image currents are impressed upon the 1b@ thev changing-amplitudes of .the amplified im# input-electrodes of the space discharge tube amplifier 2, the output circuit Q of which includes a glow discharge lamp 3.

The input circuit of the amplifier includes a battery 4, for impressing a biasing potential upon its grid 5, and a resistance 6, for im- Vproving the operation of the tube 2, aS an amplifier. Battery 7, supplies space current to the anode-cathode path of the tube 2, through' Vthe discl'iar,e` path of the 74Vglow lamp 3. Y v

The intensity of the light produced by the lamp 8, is caused to vary in'accordance with age current supplied by the tube 2.

`Av v.In television systemsvthe glow area vof the lamprS is viewed through the apertures of a scanning member, which is driven -in syn- -chronism with a similar member at the transmitting station. Y The scanning members areoperatedat such a rate of speed that an image of the subject, scanned at the transmitter, is completely repeated atrthe receiving station in a period corresponding to the persistance of vision, i. e., from le to ,La of 'f va second. The frequency range of the image currents is fromV very near zerol :to from 10,000 to 50,000cycles .per second, depending i upon 'the size of the image, the scanning peri- .,od and the vresolution desired. Examples of `television systems in which the circuit of Fig. 1 may be employed, or in which the glow lamps .of this invention may be substituted forthe ones shown, are those-disclosed in application of F. Gray Serial No. 227,649

led October V21 1927 and application of J. T.F.'-Iorton SerialNo. 230,418 filed November 2, 1927. y Y The glow lamp 3, is provided with a gas charge comprising an inert gas and a small quantity of active gas. For example, the Charge may comprise 99 to 90 per cent neon *Y and 1 to A10 per cent hydrogen.

With regard to the alternatingcurrent componentsl which flow through the glow lamp j8,the circuit of Fig. l'may be considered as though the amplier tube 2, were a source for supplying alternating current at Y an E. M. F.=E and had anA interna-l resist-- ance-1K0. That is, the internal output resistance of the tube-2, is equal to Ro'ohms.

` The lamp 8, maybe regarded" as an impedance Z, comprising a resistance R, and a reactance X.

' The absolute value of theimpedance Z Vx/RZ-l-Xz.

Interms of the voltage `drop"V across thev lamp, the current'through it at any frequenand =thephase angle between current and voltage is =arctan alf R Y In the operation of the circuit of Fig. 1, current of any frequency flowing through the lamp 2, is

Y (R R02 jl- X 2 and the phase angle between the current and E. M. F. is:

1 X @FMP im For small values of i X Y I RQ'p-l-R kthe phase .angle is approximately .@ZR-PR j' Referring now to Fig. V2 there are shown vtwo sets of curves. Oneset, A, B and C relates to a discharge lamp charged with pure neon, and the other set, D, E and F to the same lamp, provided with a gas chargecomprising'90% neon and 10% hydrogen by volume and at a pressureY of a few millimeters of mercury.

the pure neon lamp varies from about 1,800@

at approximately zero frequency `to about 5,300@ at 40,000 cycles.

Curve B indicates that the reactance X of the pure neon lamp `varies from about 400w at substantially zero frequency to about 5,500@ at 40,000 cycles.

Curve C, which relates to a 3,000w circuit including the pure vneon lamp, indicates that the total impedance of the circuit varies from 4,800w at'approximately zero frel stantially Zero frequency `to 4,700@ at 40,000

cycles. Y y

Fig. Bincludes curves .Grand H which indijcate the Vphase V'angle v between the applied i F. and current,v over the Zero to 40,000

Ycycle range, for the itwo lamps referred to above. Y

As shown by curve G thefphase shift inthe 'pure neon lamp varies from 0% for direct current to about-42% at 22,500 cycles and 35% at 40,000 cycles. Y V` According to curve A the resistance R of For the lamp provided with a gas charge including 10% hydrogen, curve H Vindicates that the phase shift of the applied current Vwith respect to the applied E. M. F. varied from zero, for direct current, to 241/2% at 35,000 cycles and 23% at 40,000 cycles.

Thus, the presence of the active gas lowers the impedance of the lamp and makes Vit a smaller part'ofV the total Vimpedance of any circuit in which it is used, and the resistance `of the lamp is made more nearly constant over Vthe television range. Both of these factors result in a more uniform current response at the various frequencies. By lowering the reactance of the lamp, the active gas renders the phase shift more nearly proportional to frequency and, in a circuit of reasonably high impedance, gives a phase shift which is practically proportional to the frequency over the complete range. The curves show that this proportionality holds up to about 35,000 cycles for the improved lamps herein described when included ina 3,000wcircuit.

While many advantages result from the use of a small percentage ofactive gas in a charge of inert gas, it will be apparent that to secure the best results, it is necessary to use the lamp in a circuit having an impedance which is high compared with the impedance of the lamp.

Within wide limits, the impedance of the Vlamp is improved as the percentage of active gas is increased. However, since the presence of the active gas serves to reduce the brightness of the light produced, there is a maxiinum limit which should not be exceeded.

Under present conditions in television, not more than 5% of hydrogen should bevused. As a matter of fact, the advantages of this in vention are completely realized in a neon lamp having a hydrogen content of l or 2%.

In addition to improving the impedance characteristics of the lamp, the active gas effects a result of major importance in electrootpical systems in that it prevents after-glow, i. e. the production of a glow within the lamp after the flow of image current through the `lamp has ceased'. The production of afterglow may be explained lon the theory that, when an electron collides with an inert gas atom, the electron bounces i away from the atom with practically no loss of energy. ln a pure inert gas, electrons can thus lose their energy only very slowly andare consequently slow in recombining with positive ions. This residual ionization, or time lag in change of ionization, causes after-glow which makes the lamp act somewhat as an inductive rea'ctance in an alternatingcurrent circuit and-causes the image to be blurred.

However, when an electron collides with an active gas molecule it bounces awaytherefrom with very little of its initial energy. The presence of a small percentage of active gas in an inert gas, therefore, causes the electrons to lose theirfenergy more rapidly than in a pure inert gas and causes them to recombine more quickly with the gas molecules, whereby after-glow is substantially obviated.

Quite independent of the above or any theory as to the cause of after-glow in the operation of glow lamps includuing a pure inert gas charge, it has been determinedthat this undesirable condition can be obviated by adding a small percentage of active gas to an inert gas charge.

There is evidence also that a lag in ionization occurs when current is applied or increased, and that this effect is reducedin the improved lamp herein disclosed.`

The present invention provides a glow lamp having an inert gas charge to which active gas is added for the purpose of determining and controlling the response of the lamp when it is producing light i. e., under operating conditions; whereas heretofore active gas was introduced into a glowV lamp for the sole purpose of lowering its starting or break down voltage. l

lihile a definite circuit arrangement has been described and certain speciic details have been given, it is to be understood that this was done merely for the purpose of completely and clearly disclosing the principles and nature of the invention. lts novel features will be defined in the attached claims.`

Obviousy the present invention is applicable to sound recording, in which the distortion produced by the ordinary glow lamps including a pure inert gas charge, is applicable. The invention is of particular value for such use when the frequency range is wide, as in high quality recording.

l/Vhat is claimed is:

prising a circuit,means for producing` image itu currents in said circuit, and included in said circuit a glow discharge lamp having means for maintaining its resistance substantially constant over the frequency range involved and its reactance varying approximately linearly with frequency.

2. An electro-optical receiver lcomprising a glow lamp, a circuit including said lamp and having high impedance compared therewith, and means for maintaining substantially constant the resistance of said lamp over the frequency range involved.

3. A television receivercomprising a glow lamp, means for repeating image currents in a circuit including said lamp and having high impedance compared therewith, and means Lfor causing the reactance of said lamp to vary proportionally with frequency over the frequency range involved.

4. A television receiver comprising a glow lamp, means for producing image currents in a circuit including said lamp and having high impedance compared therewith, said elow lamp including a charge comprisingan inert lamp, means for repeating image currents in a circuit which includes said lamp and has high impedance compared therewith, and means for causing said lamp to have a resistance which is substantially constant, a react-k anc-e variable substantially linearly with frequency and a phase shift proportional to frequency for currents in the frequency range involved. f

6. An electro-optical system comprising a glow lamp, means for supplying .to said .lamp current extending over a wide frequency range, and means for causing the reactance of said lamp to vary substantially linearly with frequency for currents within said range. Y

7. An electro-opticalsystem comprising a glow lamp, means for supplying to said lamp current extending over a frequency' range including frequencies above 10,000 cycles per second, and means for causing the reactance of said lamp to vary substantially linearly with frequency for the current supplied by said means.

8. An electropptical system comprising a glow lamp, means for supplying to said lamp current of a frequency range from below Y10,000 cycles per second'to at least 20,000 cycles per second, and means for causing the reactance of said lamp'to vary substantially linearly with frequency for the current supplied by said means. y

9. An electro-optical system including a glow lamp, and means for causing the reactance of said lamp to vary linearly with frequencywhen supplied with currenthaving frequencies extending over a wide range.

10. An electro-optical system including a. glow lamp, and means for causing the reactance of said lamp to produce a phase shift proportional to frequency for current supplied thereto, having frequencies extending over a wide range.

11. A circuit, a glow discharge Vlamp in said circuit and means for impressing upon said circuit signaling potentials having a. wide range of frequency components for setting up light variations corresponding to said signaling potentials, said lamp having a gaseous filling comprising an inert gas and a small percentage of active gas, se that the combined impedance ofvsaid circuiti and said lamp is substantially constant over said range of frequencies.

12. A television system comprising a space discharge amplier, means for impressing an image current band on said amplifier, a glow discharge lamp included in the output circuit of said amplifier, said lamp'having agaseous lilling of neon and less than 10% of hydrogen,

Ithe combined impedance of saidoutput circuit and lamp being substantially constant over said image band range.

' lf3.V Thecombination with4 a glow lamp-hav-` ing argascharge' of inert gas and a small T70 amount of active gas, and means vforimpressing upon-a-circuitincludingsaid lamp Ya variable' electromotive Vforce having freqeuncy components extending over awide frequency range, whereby the Vimpedance of said com- 4 bined circuit andlamp is substantially constant and the response of said lamp'isv substantially proportional at each instant to the Aamplitude of the' impressed'electr'omotive 8,0

taining a gaseous'filling comprising inert vgas so and an active gas, the proportion of the latter by volume Ybeing of the order of 10% or less, means for impressing uponsaid lamp variable potentials having frequency components extending over a frequency range of at 9,5

least live thousand cycles per second.

16. The combination lof a. glow discharge lamp containing inert andY active gas, theportion of active gas by volume being of the order of 10%l or less, means for impressing a steady biasing potential upon said lamp and means for impressing upon said lamp variable potentials having frequency components extending over a range of .at least twenty thousand cycles per'second.

17. The combination with 'a'glow discharge lamp havinga gaseous filling comprising inert gas andhydrogen, the portionof hydrogen by volume being 10% or less, and means for impressing vupon said lamp variable. potentials comprising a wide range of frequency components.

18. The combin ation with a glow discharge lamp containing a gaseous filling comprising inert gas and hydrogen, the vproportion of the latter by volume being 5% or less, and means for impressing upon said lamp variable potentialscomprising a wide range of frequency components.

19. Means for producing atelevision image .1,20

comprising a glow discharge'lamp having a gaseous filling of inert gas and active gas, the proportion by volume of the Vlatter being of thev order of 10% orl'ess,Y and means yfor impressing upon said lamp imagey potentials having frequency components extending 4over a range of twenty thousand cycles per second or more. Y

ln witness whereof, I'hereunto' subscribe myname this 7 th day of April, 193.0. l i

' f VFl'tANK GRAY.

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