US2253313A - Signal-translating channel - Google Patents

Description

Filed Aug. 9, 1939 Sharp Gmo" Tube 3 Remon Cutoff Tu C F MODULATING FlG.l.

TIMI NG- GENERATOR ...2.50 .:nSQ

Negative Grid Volts 4.. Vu F Patented ug. 19., 1941 UNITED STATES PATENT voFFl'cE John C. Wilson, Bayside, N.

Y. assignor to Hazel-` tine Corporation, a corporation of Delaware f Application August 9, 1939, Serial No. 289,116

` s omnia. (ci. 17a- 7.1)

This invention relates to signal-translating channels comprising a plurality of stages each having a vacuum'tube adapted to be operated over a wide input-amplitude range of signals and adapted to translate the 'signals with substantially no distortion. i

It is customary to operate lvacuum-tube ampliflers only over-the most linear portion of their input voltage-output current characteristic in order/to obtain as distortionless transmission of the translated signal as possible. I-owever, in order to make the most economical use of a given tube, it is desirable to operate the tube over as wide a portion of its characteristic as possible. It is well known that, ii a tube is operated over a nonlinear portion of its characteristic, a distortion of the reproduced signal will generally occur. Various arrangements have been proposed for compensating such distortion but such arrangements do not provide satisfactory operating conditions in some circumstances or, if satisfactory operating conditions are provided,

may involve expensive or complicated addi-A tional apparatus.

It has been observed in many television receivers, which usually comprise a large number of conventional amplifier 'stages coupled in casresults a crushing or amplitude compression of both black and white shades in the reproduced picture while there results a corresponding arn-v plitude expansion of intermediate-shade values. This, in general, isan undesirable result and is usually produced because of operation of the in the signal-translating channel, and there is providedl means for applying the signal translated by said channel to thestages so that corresponding portions of the amplitude range ofthe signal fall on geometrically@corresponding portions of lcade in the signal-translating channel, that there tubes over a substantial nonlinear portion of v their characteristic in the region of cutoff. Since the translated signal is usually applied to successive tubes with opposite polarities, the limiting portions of the signal-amplitude` range are alternately subjected to.distortions which may be small in anyone tube, but which are cumulative throughout the ,signal-translating channel, thereby producing the undesired result mentioned above.

It is an object of the invention, therefore, to provide a novel signal-translating channel in which one or more of the above-mentioned dis-v advantages is eliminated.

' It is another 'object of the invention to provide la novel signal-translating channel utilizing only conventional vacuum `tubes operating over wide portions' of their signal-input amplitude ranges and yet eliminating' all of the abovementioned disadvantages. I

Figs. 3 and 4 are graphs utilized to explain the v:ses

the respective characteristics with opposite polarities, whereby signals translated through the channel are substantially undistorted by translation through the stages.

In accordance with a preferred embodiment of the invention, a stage comprising a conventional vacuum tube having a sharp cutoi characteristic is paired with a stage utilizing a tube of the remote cutoif type -in order to provide the desired compensation as between the two stages.

For a vbetter understanding of the invention, together with other and further objects thereof,

reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. 1 is a circuit diagram, partly schematic, of certain essentials of a television transmitting system embodying the present invention; Fig. 2 is a graph illustrating e certain of the disadvantages present in conventional arrangements of the prior art utilizing similar vacuum tubes in successive stages; while operation of the present invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown a schematic circuit diagram of a-television transmitting system comprising a camera or signal generator Ill which may include the usual cathoderay signal-generating tube and scanning and wave-shaping apparatus. Connected in cascade to the output circuit of signal generator I0,- in the order named, are video-frequency ampliflersll, I2, I3, and I4, a carrier-frequency modulating and amplifying apparatus I0. and a radiating antenna system I1, I8, the general arrangement and the units.

shownschematically being in accordance with conventional practice. A timing generator I is provided which may comprise field-frequency and line-frequency synchronizing-signal generators and blanking or pedestal generators/also in accordance with conventional practice.

For 'the purpose of providing distortionless transmission, as between amplifier stages I3 and I4 of the transmitter of Fig. 1, there is included in stage I3 a vacuum-tube amplifier 2| which may be of the conventional sharp cutoff type. The signal input to tube 2| is stabilized at a predetermined shade level of the transmitted signal by means of a series coupling condenser 22 and a shunt load resistor 23 across which is coupled a direct current reinserting or stabilizing diode 2d. A suitable bias is provided by resistor I9 shunted by condenser 2t for high-frequency signals, in order properly to relate the stabilized signal to the input voltage-output current characteristic of tube 2l. Stage Ill of the transmitter comprises a vacuum-tube amplifier 26 which may be of the variable-mu or remote cutoff type. In order to stabilize .the translated signal for operation over a predetermined portion of the characteristic of amplifier 26, there is provided a series coupling condenser 21 and a shunt load resistor 28 across which is connected a diode 29 adapted to provide direct current reinsertion or stabilization.

Neglecting for the moment the details of operation of the present invention, the system just described comprises the essential elements of a television transmitting system of conventional construction rendering a detailed description of the general system and its operation unnecessary. Briefly, however, the scene to be transtransmitted through amplifiers l2, I3, and Ifl to the carrier-frequency modulating and amplifying unit I6. synchronizing Asignals are provided by timing generator I5 for the scanning Y apparatus of signal generator I@ as well as conventional blanking or pedestal pulses which are r applied to video-irequency amplifier II, while synchronizing signals for modulating the carrier signal are applied to carrier-frequency modulating and amplifying unit I6, all in a conventional manner.

Reference. is now made to Fig. 2 in order to explain a defect which is inherent in signaltranslating channels utilizing similar tubes of conventional design and operated over a curved portion of their input voltageoutput current characteristics. Curve A of Fig. 2 represents such a conventional characteristic and it will be assumed for the purposes of explanation that there is applied to the input electrodes of the tube a television signal voltage representing uniformly stepped shade' values as shown by curve C, on which point B represents the signal-amplitude level corresponding to black in the transmitted image, while the point W represents the white level. In conventional signal-translating channels, the output from a tube having the characteristic of curve A is applied with reversed polarity to the input electrodes of the succeeding tube which usually has an identical or similar grid voltage-output current characteristic. An

the output from' the tube having the characteristic of curve A is applied with reversed polarity, is represented by curve D of the drawing, while curve E represents the output signal from the tube having the characteristics D, the co-ordinates of curve D being interchanged with respect to those of curve A for convenience of plotting. The amplitude levels of the translated signals corresponding to black' and white are indicated by the points B and W, respectively, throughout the path of the translated signals. It is thus seen that the signal C, representing uniformly stepped shade values, .has been ampliiied and distorted in. such manner that both the white and black shades of the signal have been crushed or compressed with reference to the intermediate shade Values. This is an ei'ect which is readily noticeable in many television signal-translating apparatus.

Reference is made to the curves of Figs. 3 and 4 for an explanation of the application of the present invention to this problem. In Fig. 3, curve A represents a characteristic which is identical with characteristic A of Fig. 2. Curve A' represents the characteristic of a tube which, it has been discovered, is effective exactly to compensate for the distortion produced in a tube having the characteristic A when operated over the nonlinear portion of its characteristic. As stated above, curve A is substantially the characteristic provided by a conventional vacuum tube of the sharp cutoff type, while curve A' is a characteristic which is'typical of that provided by a commercial tube of the variable-mu or remote cuto type.Y The criterion for distortionless transmission is that curve A shall be geometrically similar over its operating range to curve A' when plotted with interchanged coordinates. This is more clearly illustrated in Fig. 4 wherein curveA is identical with that of curve A of Fig. 3 and curve A has 4been plotted adjacent curve AV but with 'interchanged coordinates. Under these conditions, it is seen that curve A is geometrically similar to curve A' over the operating range of the applied signal. For the purposes of illustration, it is again assumed that a signal represented by curve C is Aapplied tothe input circuit of the tube having the characteristic A of Fig. 4 and that the output of this tube is coupled with reversed polarity to the input circuit of the tube having the characteristic A', the signals being stabilized at the input electrodes of the two tubes to operate the two tubes over substantial portions of their respective characteristics. Under these conditions, it is seen that the signal output E of the tube having the characteristic A' is undistorted with respect to the signal input C.

Referring now more particularly to the operation of the system of Flg. 1, it will be assumedY that the signal output of video-frequency ampliiier I2 is poled. with the amplitude level thereof corresponding to black negative; that the signal is stabilized at such black level by means of diode reinserter 24 and its load resistor 23; and that the necessary bias is developed across resistor I9. Tube 2l may have the characteristic of curve A of Fig. 4 and, for the purposes of illustration, the input signal thereto may be that of curve C of Fig. 4. Tube 2I is thus so biased as to be operated over a. predetermined portion of its grid voltage-output current characteristic including a substantial nonlinear portion in the anode current cutol' region with the level corresponding to black stabilized at the limit of this nonlinear portion. This signal isl amplied in the usual manner by means of amplifier 2l and f changed coordinates and the signal input to tube 20 is poled with the amplitude level correspond` ing to white negative and the translated signal is, therefore, applied to amplifier 26 with a polarity opposite to that of the :signal applied vto.

amplifier 2l. Diode reinserter 29 together with its load resistor 28 is effective to stabilize the signal for operation over the indicated'portion of the curve A'. Therefore, sincethe stages Il and It are coupled in cascade in the signal-translating channel, corresponding portions of the amplitude range of the signal input to amplifier 2B fall on portions ofthe characteristic A' geol metrically corresponding to the portions of characteristic A on which i'all correspondingportions of the amplitude range of the signal input to amplifier 2| but with opposite polarity. Signals translated through the channels are, therefore, as indicated by curve E', substantially undistorted with respect tothe signal input E by translation through the two stages Il and Il. Video-frequency stages H and l2 may be of a conventional type or may correspond respectively to stages Iland Il. While the inventionv has been described with reference4 to a television signal transmitter,` it will be understood that the invention is also applicable to any type of signaltranslating channel and is particularly useful in the signal-translating channel of a television receiver. It has been found that, by utilizing tubes of the sharp cutoif type in alternate signaltranslating stages of a television receiver and utilizing tubes of the remote cutoii.' type in the remaining signal-translating stages of the receiver, the above-mentioned eifect of crushing of yblack and white shade components in the translated picture is substantially eliminated. That is, it has been found to be expedient in television signal-translating apparatus vto provide a video-frequency signal-translating channel comprising a first groupof repeater stages, similar to stage'lt, including vacuum tubes having grid voltage-outputcurrent characteristics over predetermined ranges of operation including substantially nonlinear portions, and to provide a second and substantially equal group of repeater stages, similar to stage I4. including vacuum tubes having given grid voltage-output current characteristics over predetermined ranges of operation which are geometrically similar, respectively, to those of tubes of the first group when plotted with interchanged coordinates. In such an arrangement, `the stages are coupled in cascade in theV signal-translating channel and means are provided for applying-the signal translated by the channel to the stages so that corresponding portions of the amplitude range of the translated signal fall on geometrically corresponding portions of their respective characteristics and with opposite polarities as between the rst and second groups of stages.

By this arrangement, signals translated through the channel are substantially undistortedy by translation through the channel.

While `there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that variouschanges and modifications may be made therein without departing from the invention, and it istherefore, aimed in the appendedl claims to cover all such changes` and modifications as fall within the true Spirit ordinates,`said stages being coupled in cascadel in saidchannel, and means for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude v range of said signal fall on geometrically corresponding portions of their respective characteristics with opposite polarities, lwhereby signals translated `through said channel are substantially undistorted by translation through said stages. Y,

2. A signal-translating channel comprising, a rst repeater stage including a vacuum tube having a given grid voltage-output current characteristic over a predetermined range of operation'including a substantial nonlinear portion .in the anode-current cutoff region of the characteristic, a second repeater stage including a vacuum tube having a given grid voltage-output current characteristic over a predetermined range of operation geometrically similar to that of said v tube of said first stage when plotted with interchanged co-ordinates, said stages being coupled in cascade in said channel, and means for applying the .signal translated by said channel to said stages so that corresponding portions of the amplitude range of said signal fall on geometrically 4corresponding portions of their respective characteristics with opposite polarities, whereby signals translated through said channel are substantially undistorted by translation through said stages. l

Y 3. A signal-translating'channel comprising. a`

output current' characteristic `over a predetermined range of operation geometrically similar to that of said tube of said first stage when plotted l with interchanged co-ordinates, said stages being coupled in cascade in said channel, and means for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range of said signal fall on'geometrically corresponding portions of their respective characteristics with opposite polarities, whereby signals translated through said channel are substantially undistorted by translation through said stages. `f

4. A signal-translating channel comprising, a

first repeater stage includingfa vacuum tube of the sharp cutoif type having a given grid voltage-output current characteristic over a predetermined range of operation including a substantial nonlinear portion nearer its cutoff region, a second repeater stage including a vacuum tube of the remote cutoi type having a given grid 'voltage-output current characteristic over a predetermined range of operation geometrically similar to that of said tube of said rst stage when plotted with interchanged co-ordinates, said stages being coupled in cascade in said channel, and vmeans for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range o! said signal fall on geometrically corresponding portions oi their respective characteristics with opposite polarities, whereby signals translated through said channel are substantially undistorted by translation through said stages.

5. In a television signal-translating apparatus, a signal-translating channel comprisin a ilrst group of repeater stages including vacuum tubes having given grid voltage-output current characteristics over predetermined ranges of operation including substantial nonlinear portions, a second and substantially equal group of repeater stages including vacuum tubes having given grid voltage-output current characteristics over predetermined ranges of operation which are geometrically similar, respectively, to those of tubes of said iirst group when plotted with interchanged co-ordinates, said stages coupled in cascade in said channel, and means for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range of said signal fall on geometrically corresponding portions of their respective being characteristics and with opposite polarities as betially equal group of repeater stages including vacuum tubes of the remote cutoff type having grid voltage-output current characteristics over predetermined ranges of operations geometrically similar, respectively, to those oftubes of said first group when plotted with interchanged coordinates, said stages being coupled in cascade aussie in said channel, and means for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range of said signal fall on geometrically corresponding portionsrof their respective characteristics and with opposite polarities as between the tubes of said two groups, whereby signals transs lated through said channel are substantially undistorted by translation through said stages.

7. In a television signal-translating system, a video-frequency signal-translating channel comprising, a first repeater stage including a vacuum tubehaving a given grid voltage-output current characteristic over a predetermined range of operation including a substantial nonlinear portion, a" second repeater stage including a vacuum tube having a given grid voltage-output current characteristic over a' predetermined range of operation geometrically similar to that of said tube of said iirst, stage when plotted with interchanged co-ordinates, said stages being coupled in cascade in said channel, and means including signalstabilizing means for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range of said signal fall on geometrically corresponding portions of their respective characteristics with opposite polarities, whereby signals translated through said channel are substantially undistorted by translation through said stages.

8. In a television signal-translating system, a video-frequency signal-translating channel comprising, a first repeater stage including a vacuum tube having a given grid voltage-output current characteristic over a predetermined range of operation includlng a substantial nonlinear portion, a second repeater stage including a vacuum tube having a given grid voltage-output current characteristic over a predetermined range of operation geometrically similar to that of said tube of said iirst stage whenplotted with interchanged co-ordinates, said stages being coupled in cascade in said channel, and means including signalstabilizing means for stabilizing the black level of said signal at that limit of therange of said ilrst stage which is also the limit of said nonlinear portion and for applying the signal translated by said channel to said stages so that corresponding portions of the amplitude range' of said signal iall on geometrically corresponding portions of their respective characteristics with opposite polarities, whereby signals translated through said channel are substantially undistorted by translation through said stages.

' JOHN C. WILSON.

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