US2191796A - Television combining amplifier - Google Patents

Description

Feb. 2?, W46, M. LEWIS TELEVISION COMBINING AMPLIFIER Origmal Filed Oct. 5, 1954 INVENTQR Harold M Lew/Is 8W ATTORNEY Patented Feb. 27, 1940 TELEVISION COMBINING Hamid M. Lewis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela- Original application October 5, 1934, Serial No.

This invention relates to amplifiers for combining and amplifying a plurality of signals including signal components having frequencies aggregating a wide frequency band.

This application is a division of applicant's copendlng application, Serial No. 747,070, filed October 5, 1934.

It is an object of the present invention to provide an improved amplifier for combining and amplifying a plurality of signals, which may comprise television signal components, including frequencies aggregating a wide frequency band, for developing a resultant signal representing the sum of the combined signals.

In accordance with the present invention, there is provided an amplifier which comprises a plurality of vacuum tubes. Means are provided for individually applying to the input electrodes of the several tubes a plurality of signals, which may represent television signal components, in-

cluding frequencies aggregating a wide frequency band. A'common output circuit is provided for all of the tubes comprising an electric wave filter having a uniform impedance over the wide frequency band. The output impedance of the filter is of a value which is low relative to the anodecathode impedance of each of the tubes. With this arrangement, the signal voltage developed across the common output filter impedance by each of the tubes is substantially independent of, and unaffected by, variations of the impedances of the other tubes. Therefore, the instantaneous voltage developed across the output filter impedance is the sum of the instantaneous signal output voltages of all of the tubes.

For a better understanding of the present 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 theappended claims.

The single figure of drawing is a circuit digram of a television transmitter including a combining amplifier embodying the present invention.

Referring now more particularly to the system.

Divided and this application June 29, 1938, Serial No. 216,423

4 Claims. (c1. its-7.1)

For the purpose of combining and amplifying component modulation-frequency voltages representing the varlous components of the television signal, there is provided an amplifier system comprising five vacuum-tube amplifiers 61, 68, 69,

70, and ii, each having a separate component modulation-frequency band suppliedto its input. The output circuits of these several tubes have in common as their load circuit a low pass filter unit composed of series inductance arm l3, shunt condenser arms 16 and i1 and terminating resistors M and 15. This filter develops and transmits signals of all frequencies ,from 24- 813,000 cycles to the grid circuit of a vacuumtube repeater 12 via condenser 18 and potentiometer 19. The shunt condensers I6 and 11 represent or include the output tube capacltances of vacuum tubes 61, 68, 69, I0, and H and the input capacitance of the following tube 12. For the wide frequency band to be passed, the mentioned inherent tube capacitances alone general ly sumce for elements 16 and I1 without the necessity for supplemental physical condensers.

Since the branch 18,19 excludes direct current, the susceptance of condenser 18 is made large compared with the conductance of resistor 19 to ensure passing without attenuation all frequencies as low as the picture frequency of 24 cycles per second. If the anode-cathode impedance of each of the several tubes 61-11, in-

elusive, is high compared with that of the filter,

as is the case if tubes of the pentode type are used, ,as shown, the signal voltage developed across the filter and passed on to the input of tube 12 will be practically the sum of the instantaneous signal outputs of the several tubes.

The output circuit of tube 12 extends to a lowpass filter comprising a series inductance arm the tube 12 across this filter are, therefore, ap-

plied in like phase to the anodes of the tubes of the modulator I. In efiYect/the signal voltage across output terminating resistor 82 aidsor op-, poses the anode voltage supply of the modulator 1 in accordance with the instantaneous values of the modulation-frequency signals. For this type of modulation, the adjustments of the modulator I are made such that the radio-frequency output of the modulator 1, as applied to the input of the amplifier '8, varies linearly with the anode voltage and, hence, the amplitude of 68 carrier-frequency voltage applied to the amplifier 8 is caused to vary directly as the modulation voltage output of the amplifier I2 The filter 8084 is designed, in a manner understood, to simulate a pure resistance to the extent that all frequencies within its pass band (as 24-813,000 cycles) are equally attenuated, Additionally, the design is such that the phase shift varies linearly with frequency. Thus the output voltage wave form is a replica of the input wave form. Likewise the action is similar to that of a resistance-coupled amplifier at low frequencies, in that the signal wave form as applied to the input of tube I2 and as developed at the associated low-pass filter output is oppositely poled.

the grid of the tube I2. This requires positive poling at the grids of tubes 61, 68, and 69.

For generating the several component frequency bands of the modulation frequencies, five circuit sections are required. The top section starts with a photocell 56 which is provided with a unidirectional voltage source 51 and a load in the form of a low-pass filter unit 85 having a cutoff frequency at 800,000 cycles. The photocell 50 represents a source of, video-frequency signalswhich, in the well-known manner, vary in accordance with the values of light in successive elemental areas of a scene to be transmitted. For example, the photocell may be employed in a conventional mechanical scanning system, in which a scanning light beam is focused on the photocell to develop the video-frequency signals, including a: source of light focused in a horizontal line and passing through a vertically moving film carrying the scenes, with a rotary mirrordrum and mask arrangement operating to focus successive points of the horizontal line on the photocell. With the filter 85 properly designed, its inductance tunes with the input and output capacitances in series to give a resonant peak near the upper frequency cutoff. and thus to raise the gain' at the high-frequency end of the band. The employment of properly chosen terminating resistances then acts to level the response curve of the filter to give a uniform response up to the cutofl frequency. The'shunt capacitances of tube capacitances in thisv way, much greater uniformity in gain over the band can be obtained.

The output voltage of the filter 05 is applied to two amplifiers: the one, including tubes and 90, constituting a vision-frequency amplifier of 24-'800,000 cycles and the other, including tubes 94 and 95, serving as the direct "current (that is, -24 cycles) background-illumination amplifier. The voltage as applied to the input of tube 881s via a blocking condenser 86 and a grid-leak resistor 81. The condenser 06 is chosen to have a large capacitance so that its reactance at the lowest frequency, 24 cycles, is small compared with the resistance of resistor 81 (that is, a low time constant is employed) and so .that all freenemies in the band 24400000 cycles areapplied effectively without attenuation to the input of the tube 88. The coupling filter 00, tube 00, and its output filter 9| complete the system for applying the vision-frequency components, amplified and undistorted, via the condenser 02 and potentiometer 93 to the grid of the amplifier tube 61. Adjustment of the potentiometer 03 serves to set the level of modulation as far as the vision-frequency components are concerned.

The poling of battery 51 is such that, when light strikes on photocell 56, the voltage drop across the filter 85 is applied negatively to the control grid of the tube 88. Simultaneously, the

grid of the tube 90 becomes more positive and the grid of the tube 61 becomes more negative. This is the proper polingiwhen followed on through tube I2 to modulator I as previously described) to cause the carrier-wave amplitude to decrease. Darkness at photocell 56, conversely, causes an increase in amplitude of the carrier wave. Hence, the poling shown is such as to give negative" modulation of the carrier wave by the vision-frequency components.

The voltage output of filter is also applied to thegrid of the tube 5| which with tube 95 and interstage coupling resistor 05 operate as a direct current amplifier to develop across resistor 98 in the output circuit of tube 95 a voltage proportionaltothe general background illumination of the picture. Condensers 91 and 99 shunting resistors 96 and 98, respectively, prevent the development of voltage of higher frequencies since only 0-24 cycles need be passed by this amplifier.

Unit I! is an intermediate-frequency generator for exciting the grid of a modulator tube Ill. The space current circuit from the battery to the anode of tube I00 has in series with it the resistor 98 across which the background-illumination potentials are developed. The intermediate-frequency carrier wave is applied by tube I00 to the input circuit I02, I03 of the amplifier tube through a tuned transformer IOI. The modulator tube I00 is adjusted so that the carrierwave amplitude,as applied to the grid of the amplifier I0, varies directly as the anode voltage of tube I00. Thus the carrier-wave amplitude varies directly with the background-illumination voltages developed across resistor 00. The poling is such that darkness on photocell results in an. increase in the carrier amplitude to provide negative modulation of the carrier wave. The amplitude of the background-illumination modulated intermediate-frequency carrier wave applied to the grid of the amplifier I0 to modulate the transmission carrier wave is set by adjustment of potentiometer I03.

The arrangement of the channel for audiofrequency modulation component is, briefly, like that for the background-illumination component. A photocell 04 is operated by light varying in accordance with the sound signal to be transmitted, as by a sound record on film, and develops corresponding audio-frequency voltages across a resistor I04 in its output circuit for application to an amplifier tube I05. Unit 2| is an intermediate-frequency carrier-wave generator (807,000 cycles) for exciting the grid of modulator tube I01. The output of tube I05 is coupled by low-pass filter I06 (having a cutoff at 6000 cycles to develop the audio-frequency band of 50-6000 cycles, approximately) to the anode of modulator I0I. The resulting sound-modulated carrier is applied to the grid circuit of amplifier tube II through a transformer I00 tuned to the intermediate. frequency.

The line-frequency impulse generator comprises a voltage source I09 which charges a condenser Hll through a constant current tube l l l. A short-circuiting tube H2, including a resistor i H in its output circuit and regenerated by a tube H3 including a resistor H5 in its load circuit, serves rapidly to discharge the condenser HG whenever a predetermined voltage across the condenser Hi! has been reached. The voltage across the condenser H0 is of saw-tooth wave form, while the current through the condenser H8 and anode-cathode circuit of the short-circuiting tube H2 is of impulse wave form. Hence, the voltage across the resistorv i It is also of impulse wave form, the peaks being poled negative with respect to ground, and the voltage across resistor H5 in the output circuit of the feed-back or reversing tube H3 is likewise of impulse wave form, the impulse peaks being oppositely poled, that is, positive relative to ground. The line-frequency impulse peaks developed across the resistor l l5 are applied to the control grid of the amplifier 63 in amplitude set by a potentiometer H6 so that the grid of the amplifier 68 is highly positive during each line-frequency impulse peak. By following this transmission through to its efiect on modulator 77, it will be clear that the carrier-wave amplitude is increased to represent each line-frequency impulse peak and to provide, according to definition, negative impulse modulation at the line frequency. The setting of the frequency of the line-frequency impulse to its approximate value is made by adjusting the bias on the control grid of tube ill.

The generator 65 is a source of voltage of line frequency as determined by the scanning mechanism and this voltage is applied to the grid of shorting tube M2, via a resistor-condenser connection, to pull into step and hold in synchronism, at the correct line frequency, the impulse generator. The phase or timing of the line-frequency impulses can be controlled so that they coincide with the initiation of the line-scanning intervals by setting the bias of the grid of tube Hi. In practice, the control voltage from the synchronizing unit 65 holds the impu se generator in synchronism over a wide range of bias adjustments on the grid of the charging tube I ll so that considerable phase control to provide coincidence of line-frequency impulses and initiation of the linescanning interval can be had in this way.

The picture-frequency impulse generator is illustrated as entirely similar to that just described for the line-frequency generator. The constants of the circuit are chosen for efiicient generation of the lower frequency of 24 cycles. The elements are, therefore, similarly numbered, the numbers being primed. The control of the impulse generator is by the synchronizing generator $6 and the poling is such that negative" impulse modulation of the carrier wave results. The amplitude of the impulse as impressed on amplifier 89, that is, the control of the percentage modulation, is set by adjustment of a potentiometer ill.

The several amplifiers 61, 68, 69, T0, and H are designed and operatedv as linear amplifiers. Since amplifiers 68 and 89 are repeaters of impulse wave forms which in each case are applied with positive poling of the impu se peaks to their control grids, these amplifiers are preferably operated with large negative control-grid biases. The amplifier 112 must likewise be carefully designed'to prevent overload, especially where the instantaneous values of line-frequency and pic faithfully the modulation-frequency components,

must be designed and adjusted so that the amplitude of the carrier wave developed in its output circuit increases linearly with the modulation voltage applied from the amplifier 12, even during the interval in which the lineand picture-impulse peaks add to give the maximum increase in carrier amplitude. The bias adjustment of the amplifier I2 may bemade such that it is a linear amplifier for all voltages applied to its grid up to a predetermined negative maximum equal to the amplitude of a lineor picture-impulse peak. For such adjustment, the amplifier 12 will overload to suppress any line-impulse peaks which ride above the picture-impulse peak.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a television system, a combining amplifier comprising a first signal-translating channel including a vacuum tube, means for applying to the input electrode of said tube a signal voltage representing a component of a television signal aggregating a wide frequency band, a second siga component of a television signal aggregating a wide frequency band, a common output circuit for both of said tubes comprising an electric wave filterhaving a substantially uniform impedance at all frequencies in said wide bands, said tubes being connected in parallel across said common output circuit, said impedance being of a value which is low relative to the anode-cathode impedance of each of said tubes, whereby the voltage developed across said impedance by each of said tubes is substantially independent of, and unaffected by, variations of the impedances of said tubes and whereby the instantaneous voltage developed across said output impedance is the sum of the instantaneous voltage outputs of both of said tubes.

2. In a television system, a combining amplifier for separately amplifying and combining voltages representing components of a television signal having frequenciesaggregating a widefrequency band, comprising a plurality of vacuum tubes, means for applying said voltages to the value which is low relative to the anode-cathode impedance of each of said tubes, whereby the voltage developed across said impedance by each of said tubes is substantially independent of and unaffected by,-v a'riations of the impedances of said tubes arid whereby the instantaneous voltage de-.

veloped across said output impedance is the sum of the'instantaneous voltage outputs of all of said tubes.

3. In a television system, a combining amplifier for combining and amplifying voltages from a plurality of sources and representing components of a television signal having frequencies aggregating a wide frequency band, comprising a plurality of vacuum tubes having separate input circuits for individually applying said voltages thereto, and a common output circuit for all of said tubes comprising an electric wave filter having a uniform output impedance for all frequencies in said 'wide band, said tubes being connected in parallel across said common output circuit, said impedance being of a value which is low relative to the anode-cathode impedance of each of said tubes, whereby the voltage developed across said impedance by each of said tubes is substantially independent of, and unafiected by, variations of the impedances of said tubes and whereby the instantaneous voltage'developed across said output impedance is the sum of the instantaneous voltage outputs of all of said tubes.

l. In a television system, an amplifier for combining and amplifying voltages plurality of sources, said voltages representing from a components of a television signal and having frequencies aggregating a, wide frequency band, comprising a plurality of vacuum tubes of the multigrid type each including an anode, a cathode, a control grid, and a screen grid, an input circuit for each of said tubes, means for applying said voltages individually to said input circuits, an output load circuit common to the anode-cathode elements of all of said tubes, said anode-cathode elements being connected in parallel across said common output circuit, said output circuit comprising a filter structure to provide a substantially uniform impedance at frequencies throughout said wide band, and means for applyin unidirectional operating potentials to the elements of said tubes to adjust the anode-cathode I impedance of each of said tubes to a high value relative to said impedance of said output circuit,'whereby the instantaneous value of the voltage developed across-said output circuit varies in accordance with the instantaneous sum of the several input voltages.

HAROLD M. LEWIS.

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