DIGITAL TELEVISION TRANSMITTER

Abstract: A UHF transmitter for digital TV signals includes, for example, twelve silicon carbide transistor RF output power amplifier panels (A1...A12) driven in parallel by two silicon carbide amplifier driver panels (D1, D2) and where each panel (D1, D2, A1...A12) is comprised of two modules (54, 56) each further including a 180 DEG signal splitter (94, 96), a pair of quadraphase signal splitters (98, 100 and 102, 104) and combiners (106, 108 and 110, 112) a 180 DEG signal combiner (114, 116), and five RF amplifier pallets (58...66, 68...76) which themselves are comprised of two silicon carbide transistors (124, 126) located on a common flange (128) and operated in push-pull relationship so as to suppress even order distortion products and reduce sensitivity to ground currents. A monitoring system (14), including a touch screen control display and control panel (20) senses the status of all assemblies so as to allow an operator to control the transmitter and inquire about the status of the various components and power supplies, and to replace one or more units (D1, D2, A1...A12) as required without the need for turning the transmitter off.

Description: DIGITAL TELEVISION TRANSMITTER Background of the Invention Field of the Invention This invention relates generally to television broadcast transmitters and more particularly to a UHF digital television transmitter. Description of Related Art Beginning in 1997 and extending through the year 2112, U.S. television broadcast stations will be converting their conventional analog NTSC modulated TV signals to digital 8VSB (vestigial sideband) modulated TV signals. The conversions will take place in the present UHF television band. The existing VHF channels will be returned to the FCC for other services. The 8VSB modulated signal has a higher immunity to amplitude modulated noise and multi-path as opposed to the present NTSC system. Additionally, by compressing and randomizing the data, high quality pictures can be transmitted in the same 6MHz channel . This type of modulation is generally known and is discussed in detail in the ATSC Digital Television Standard Report, dated September, 1995, at pp. 46-67. An 8VSB signal, however, differs from the present NTSC signal. For example, the peak to average power ratio of an 8VSB modulated signal is much higher. The peak to average power ratio is approximately 7db for an 8VSB signal. In addition, the flat band-limited white noise-like spectrum of the 8VSB signal will degrade rapidly if it is passed through a non-linear system. In state of the art power amplifiers, typically silicon power amplifiers, the relatively high peak power drives such amplifiers into saturation and causes distortion accompanied by spectral regrowth. This spectral regrowth decreases the receiving range of the transmitted signal and interferes with adjacent TV channels. Accordingly, broadcast television stations will require a transmitter which can transmit an 8VSB signal without introducing appreciable signal distortion in the process and thus a linear transmitter with high peak power to average power capability will be necessary. Also, such transmitters should be of such a size so as to not force the station to make major modifications to their facilities. Presently there are two types of UHF television broadcast transmitters which can transmit 8VSB digital television signals efficiently. These include the inductive output tube (IOT) transmitter and the silicon solid state transmitter. The IOT is a klystron tube which utilizes a tunable input and output cavity to resonate on the assigned television channel. However, the IOT requires relatively high voltage power supply units and heat exchangers in addition to the RF amplifiers. This ancillary equipment requires a relatively large amount of floor space and decreases system reliability. In addition, the life of an IOT tube is less than 20,000 hours. The solid state silicon transmitter, on the other hand, has more instantaneous bandwidth than the IOT, but silicon solid state transmitters are much larger in size because of the low output power of each transistor. Also, a silicon transmitter is much more expensive than the tube transmitter. Summary Accordingly, it is an object of the present invention to provide an improvement in television broadcast transmitters . It is a further object of the invention to provide a solid state television broadcast transmitter. It is another object of the invention to provide a solid state UHF digital television broadcast transmitter. It is still another object of the invention to provide a television broadcast transmitter that can adequately handle the high peak power demanded by 8VSB modulation and provide a minimum amount of distortion irrespective of the RF output power level. Still another object of the invention is to provide a television broadcast transmitter which is less than one half the size of existing transmitters for the same output power level . Still a further object of the invention is to provide a digital television transmitter which is configured to permit a station engineer to maintain and service the transmitter while it is on the air. Briefly, the foregoing and other objects are achieved by a UHF transmitter which uses silicon carbide transistors. In its preferred embodiment, the transmitter is comprised of twelve silicon carbide transistor RF power panels driven in parallel by two silicon carbide driver panels and where each panel is comprised of two modules, each module contains a pair of quadraphase signal splitters and combiners, and five RF pallets which themselves are comprised of two silicon carbide transistors located on a common flange and operated in push-pull relationship so as to suppress even order distortion products and reduce sensitivity to ground currents. Any of the panels can be replaced while the transmitter is in operation. A monitoring system, including a touch screen control display and control panel senses the status of all of the silicon carbide transistors and assemblies so as to allow an operator to control the transmitter and inquire about the status of the various components and power supplies, and to replace one or more units as needed without the need for turning the transmitter off. Further scope of applicability . of the present invention will become apparent from the following detailed description. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are provided by way of illustration only, inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. Brief Description of the Drawings The present invention will be more fully understood when considered together with the accompanying drawings which are provided by way of illustration only and thus are not limitative of the invention, and wherein: Figure 1 is a front elevational view generally illustrative of the relative positions of assemblies arranged in two side-by-side cabinets; Figure 2 is an electrical block diagram illustrative of the preferred embodiment of the modular type units included in the subject invention; Figure 3 is an electrical block diagram illustrative of each power amplifier panel component shown in Figure 2 ; Figure 4 is an electrical schematic diagram illustrative of one of the RF pallets included in each panel shown in Figure 3 ; and Figures 5A-5-K are illustrative of various touchscreen control displays generated on the monitor shown in Figure 1. Detailed Description Of The Invention Referring now to the drawings and more particularly to Figure 1, shown thereat are two equipment cabinets 10 and 12. Cabinet 10 comprises a control cabinet and has for its purpose housing a control and display panel 14, a space 16 for housing a high definition television (HDTV) modulator and exciter shown in Figure 2, and a space 18 for housing a plurality of redundant preamplifiers and an automatic gain unit (AGC) . This equipment comprises original equipment manufacturer (OEM) type apparatus and is responsive to a digital TV signal data transport located in a studio, not shown. The control and display panel 14 includes a touch-screen control monitor 20 in addition to a set of manually operable control switches and lights 22, 24, 26 and 28, one of which, switch 28, comprises a primary on-off power switch 28. The cabinet 12 is used to support and house fourteen identical high power silicon carbide transistor panels consisting of two driver panels Dl and D2 and twelve output amplifier panels Al , A2 ... All, A12, and which are further shown in Figures 2, 3 and 4, and which will be subsequently described. Below the panels are located a plurality, typically three identical blower assemblies 30 1# 30 2 and 3O3 which operate to supply coolant air to the underside of the panels Dl, D2, Al, ... A12. The area below the blowers 30η_, 302 and 3O3 comprises the location where a plurality of separately replaceable power supplies are located. As shown, four racks 32^, 322, 323 and 324 each containing four 60V drain bias voltage ( 33) power supplies 34^ ... 34 4 , 36η_ ••• 364, 38χ ••• 384 and 40τ_ ... 40 4 . Above the power supply racks 32 ] _ and 32 2 are two multi-volt supply racks 42 and 44, which generate a gate bias voltage (-Vgg) as well as a 4-5VDC, +15VDC and -15VDC supply voltages. The twelve RF power amplifier panels Al , A2 ... All, A12, are combined to typically provide at least 4 kilowatts (KW) of average power and at least 20K of peak power. Such a combination is shown in Figure 2. Typical television broadcast stations, however, will require between 4 to 16KW of average output power. Accordingly two to four HPA cabinets 12, such as shown in Figure 1, can further be combined to provide the 4 to 16KW of RF output power. High power, low loss combiners, not shown, are located in the rear of the HPA cabinet 12. The combiner construction allows any of the RF driver and amplifier panels Aη_ ... A ] _2 and Dl, D2 to be removed and reinserted while the transmitter is operating. The driver and output amplifier circuitry include the use of silicon carbide transistors which, due to their high output power capabilities, require less transistors for a given output power level than a silicon transistor assembly. This being the case, complex cooling hardware is not required because the transistors can operate at relatively higher temperatures. Accordingly, since the television transmitter in accordance with this invention will require less hardware than its silicon counterpart, its size can be less than one half the size of a silicon transmitter of comparable output power. Referring now to Figure 2, shown thereat is an electrical block diagram depicting the preferred embodiment of the subject invention and which includes, as shown in Figure 1, two driver panels Dl, D2 and twelve high power output amplifier panels Al ... A12. As shown, a television signal in the form of a digital data stream to be transmitted is first fed into an input section consisting of a high definition television (HDTV) modulator, exciter and preamp unit 46. This equipment comprises conventional state of the art apparatus which can be obtained from any number of sources . The signal outputted from the input section is fed through an automatic gain control (AGC) unit 48, also of well known construction. From there, the digital TV signal is fed to a two way power splitter 50, where one half of the input power is fed to one half of either driver panel Dl or D2 , while the other half is fed to the other half of either driver panel Dl or D2 through a 90° phase shifter 52. It is to be noted that each of the fourteen panels Dl, D2 , and Al, A2 , ... A12 are of identical construction, and are comprised of two identical modules 54 and 56, which are shown in greater detail in Figure 3. As shown, module 54 is comprised of an amplifier driver 58 and four power output amplifiers 60, 62, 64 and 66 and are referred to as pallets. In a like manner, module 56 is comprised of a driver pallet 68 and four power output amplifier pallets 70, 72, 74 and 76. This arrangement is repeated throughout the system, with a driver Dl or D2 being used in connection with amplifier panels Al ... A12. Further as shown, panels Al ... A6 are connected in one parallel output amplifier set, while amplifier panels A7 ... A12 are connected in a second parallel output amplifier set. It is to be noted that only one driver panel Dl or D2 is utilized at any one time. This provides an element of redundancy. As shown in Figure 2, double pole two-position switches 78 and 82 are respectively located on the input side of the drivers, while a like pair of double pole two-position switches 82 and 84 are located on the output side of drivers. The switch positions couple one half of a digital TV input signal to be transmitted through modules 54 and 56 of the same driver panels Dl or D2. The outputs of the power amplifier panels Al ... A12, are combined at a power combining node 86, with the output from panels Al ... A6 being first being shifted by 90° by a phase shifter 88. The quadrature two phase shifters 52 and 88 result in quadrature splitting and combining in order to minimize the generation of third order intermodulation sum products while providing two mutually in phase signals at node 86. Less filtering is required when such products are suppressed. Additionally, quadraphase combining reduces amplifier sensitivity to VSWR variations. The power appearing at node 86 is then fed to an antenna, not shown, or further combined with one or more power configurations as shown in Figure 2, as noted above, for providing, for example, 4 to 16K of RF output power. Referring now to Figure 3, shown thereat is a block diagram which is illustrative of one of the panels Dl, D2, Al, A2 , ... A12. It is to be noted that in each instance, RF power is again split and recombined, using quadraphase combining techniques. Although not shown previously, the driver pallets 58 and 68 are proceeded by respective RF switches 90 and 92. The output of the drivers 58 and 68 are also now fed to respective 180° power splitters 94 and 96, with the power splitter 94 being connected to a first pair of quadrature power splitters 98 and 100. The other 180° power splitter 96 is likewise fed to a second pair of quadrature power splitters 102 and 104. Pairs of outputs from the RF amplifier pallets 60, 62 ... 74, 76 are fed to respective quadrature combiners 106, 108, 110 and 112 and then to 180° combiners 114 and 116. Combiner 114 provides a first of two high power RF outputs on circuit lead 118, while the second 180° combiner 116 receives the outputs of quadrature combiners 110 and 112 to provide a second high power RF output on circuit lead 120. With an arrangement such as shown, the overall output of any RF panel Al ... A12 can be in the order of 400 watts of average power. Again, quadraphase combining minimizes third order intermodulation sum products and sensitivity to VSWR variations. Referring now to Figure 4, each RF pallet 58, ... 76 shown in Figure 3 contains a UHF silicon carbide static induction transistor (SIT) package 122 consisting of two silicon carbide transistors 124 and 126, shown schematically by reference numerals, mounted on a common substrate and cooling flange 128 and are operated in push-pull relationship so as to suppress even order distortion products and reduce sensitivity to ground currents. The silicon carbide transistors 124 and 126 typically comprise a semiconductor structure known as a "Gemini package" manufactured by the Kyocera Corp . Each of the transistors 124 and 126 is comprised of 12 cells, not shown, and can typically deliver 55 watts of average output power when the signal applied thereto is 8VSB modulated. The transistors 124 and 126 are biased for class AB operation when operated with a drain voltage +Vdd of 60 volts. The spectral regrowth has been found to be better than -28dBc under such bias conditions. It is to be noted that 55 watts of average output power with -28dBc of spectral regrowth cannot be achieved with a single silicon transistor. The linearity and output power of the silicon carbide transistor utilized herein meets these parameters with potential for even better results. Adding additional cells to each transistor would increase the output power of the respective transistors 124 and 126 and the spectral regrowth should scale accordingly. As further shown in Figure 4, the input signal is applied to the pair of push-pull transistors 124 and 126 by way of a quarter wavelength (λ/4) piece of 50 ohm coaxial cable 130 and an input impedance matching network 132 comprised of, among other things, a pair of variable capacitors 134 and 136 mounted on respective pairs of microwave stripline conductor elements 138, 140 and 142, 144. Fixed capacitors 146 and 148 are connected between stripline elements 138 and 142 and 140 and 144, respectively. A third fixed capacitor 150 is coupled in parallel with the variable capacitor 136 across stripline elements 142 and 144. The silicon carbide transistors 124 and 126 receive a gate bias voltage -Vgg which is applied by two separate gate bias networks 152 and 154. Both bias networks 152 and 154 are of identical construction and include a fixed resistor 156 coupled between a source of -Vgg, not shown, and a voltage divider node 158 to which is connected a variable resistor 160 returned to ground. A bias voltage appearing at circuit node 158 is coupled to the respective transistors 124 and 126 via LC filter circuitry including a pair of series connected inductors 162 and 164, and three fixed capacitors 166, 168 and 170 connected to ground. On the other side of the silicon carbide transistors 124 and 128 are two identical drain bias networks 172 and 174 which connect to a source of drain bias voltage +Vdd, also not shown. Both networks 172 and 174 are similar to the gate bias networks 152 and 154 in that they include filter circuits consisting of a pair of series connected inductors 176 and 178 and three grounded fixed capacitors 180, 182 and 184. Now, however, the drain bias networks 172 and 174 will also include fuse devices 186. Further as shown, the +Vdd voltage is applied to the transistors 124 and 126 through respective stripline conductor members 188 and 190 forming a portion of an output impedance matching network 192. The network 192 is identical to the input impedance matching network 132 in that it includes in addition to the stripline members 188 and 190, a second pair of stripline members 194 and 196 which are coupled together by a pair of fixed capacitors 198 and 200. As before, a pair of variable capacitors 202 and 204 are connected across pairs of stripline elements 188, 190 and 194, 196. A fixed capacitor 206 is also connected between stripline elements 188 and 190 in parallel with the variable capacitor 202. Finally, the RF output signal from the two silicon carbide transistors 124 and 126 is coupled from the output impedance matching network 192 by means of a quarter wavelength (λ/4) piece of 25 ohm coaxial cable 208 and is fed, for example, to one of the quadrature combiners 106, 108, 110 and 112 shown in Figure 3. In order to allow a station engineer to maintain and service the digital television transmitter in accordance with this invention, the transmitter includes a system which monitors the status of all the silicon carbide transistors and assemblies in the transmitter as will now be shown. As noted above, the control cabinet 10 shown in Figure 1 includes a touch screen interface including a control monitor 20 which permits the operator to control the transmitter and inquire about its operating status. Referring now to Figures 5A-5K, shown thereat are some of the displays which can be generated on the monitor 20. The display 210 depicted in Figure 5A appears on the screen of the monitor 20 when the transmitter is initially turned on via the power switch 28. It essentially portrays a front planar view of both cabinets 10 and 12 and the respective apparatus located therein. The display 210, however, additionally depicts a set of generally rectangular operator control push-buttons 212 which can be touched by an operator. As shown in Figure 5A, a "log in" element 213 supplies the operator with a lighted prompt which when touched, causes a display 214, as shown in Figure 5B, to be generated. This display 214 resembles a typewriter keyboard 216 so that "log in" can be accomplished by touching selected keys on the screen of the monitor 20. Once "log in" has taken place, one of three possible displays, 218, 220 and 222, appears as shown in Figures 5C-5E. Display 218 indicates that all parts of the transmitter are functioning satisfactorily and further providing six operator selection prompts 219, such as "transmit on/off", "preamp select", "driver select", etc. If display 220 (Figure 5D) appears, it would indicate that RF output panel All is operating less than satisfactorily and provides a cautionary indication. If, however, display 222 is generated as shown in Figure 5E, it indicates, for example, that panel A10 is defective, needing replacement. Furthermore, by touching one of the designated panel areas 223 of displays 218, 220 and 222 as shown in Figures 5C-5E, respective corresponding panel displays 224, 226 and 228 as shown in Figures 5F-5H, will appear on the screen of the monitor 20. Display 224, for example, represents the condition indicated in Figure 5C, where all units are properly operating, display 226 corresponds to the condition where a caution condition exists for panel All and which corresponds to display 220 shown in Figure 5D, and display 228 corresponds to the defective panel A10, previously represented in Figure 5E . Figures 51 and 5J depict respective displays 230 and 232 where, for example, the driver panel Dl was touched or an output panel All was touched. As shown in Figure 51, the ten transistor pallets 58, 60 ... 74, 76 are depicted on the screen with an indication of their respective operability and current presently being drawn thereby. The display, moreover, provides for additional panel selection at 231 as well as providing an indication of the power supply voltages being supplied at 233. The display 230 as shown in Figure 51 is indicative of acceptable operation. On the other hand, display 232 as shown in Figure 5J indicates that output amplifier panel All has a defective pallet 66 (Fig. 3) . Another type of display is shown by reference numeral 234 in Figure 5K. What is depicted there is an indication of the respective currents being drawn by all of the ten pallets 58...66 and 68...76 (Figure 3) included in both the driver panels Dl, D2 and the 12 output amplifier panels A1-A12 shown in Figure 2. Thus what has been shown and described is a silicon carbide based UHF digital television transmitter having the following unique features: (1) the transmitter is less than one half the size of its silicon transistor or tube counterpart;- (2) the transmitter's output power amplifiers introduce a minimum amount of distortion to the 8VSB digital television signal irrespective of the RF output power level; and (3) the transmitter's cooling, power supplies and monitoring systems are completely self- contained and on-line maintainable by a single operator. Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention are herein meant to be included.

Claims: CLAIMS 1. A UHF television broadcast transmitter, for transmitting a digital television signal comprising: a power splitter 50 coupled to a television signal to be transmitted; at least two driver panels Dl, D2 selectively coupled one at a time to the power splitter 50 and including a pair of identical silicon carbide transistor amplifier modules 54, 56, said modules 54, 56 being coupled to the power splitter 50 in quadrature phase relationship; at least two sets of parallel power amplifier panels A1...A6, A7...A12 selectively coupled to a respective amplifier module 54, 56 of a selected driver panel Dl, D2 , each of said power amplifier panels A1...A12 also including a pair of identical silicon carbide transistor amplifier modules 54, 56 connected together in parallel; and means 86, 88 for combining the respective outputs of said two sets of power amplifier panels A1...A6, A7...A12 in quadrature phase relationship for providing an amplified in phase composite output signal of said television signal. 2. A television broadcast transmitter according to claim 1 and additionally including switch means 78, 82 and 80, 84 located on both an input side and an output side of said at least two driver panels Dl, D2 for selectively coupling said television signal from said power splitter 50 through only one of said driver panels to said power amplifier panels A1...A12 at any one time. 3. A television broadcast transmitter according to claim 2 wherein said amplifier modules 54, 56 of said power amplifier panels A1...A12 and said amplifier modules 54, 56 of said driver panels Dl, D2 comprise like modules. 4. A television broadcast transmitter according to claim 3 wherein each of said modules 54, 56 is comprised of at least one other power splitter 98, 102 coupling a portion of said television signal to a pair of silicon carbide transistor amplifier pallets 60, 62 and 70, 72 in quadrature phase relationship and at least one other power combiner 106, 110 coupled to said pair of silicon carbide transistor amplifier pallets 60, 62 and 70, 72 in phase quadrature for combining respective outputs of said pair of pallets 60, 62 and 70, 72. 5. A television broadcast transmitter according to claim 4 wherein each of said silicon carbide transistor amplifier pallets 60, 62 and 70, 72 includes two silicon carbide transistors 124, 126 connected together in a predetermined circuit relationship. 6. A television broadcast transmitter according to claim 5 wherein said predetermined circuit relationship of said transistors 124, 126 comprises a push-pull circuit relationship. 7. A television broadcast transmitter according to claim 6 wherein said two silicon carbide transistors 124, 126 are part of a common semiconductor package 122. 8. A television broadcast transmitter according to claim 7 wherein said two silicon carbide transistors 124, 126 are mounted on a common cooling flange 128. 9. A television broadcast transmitter according to claim 4 wherein each of said modules 54, 56 additionally includes another power splitter 100, 104 coupling a portion of said television signal to a second pair of silicon carbide transistor amplifier pallets 64, 66 and 74, 76 in quadrature phase relationship and another power combiner 108, 112 coupled to said second pair of silicon carbide transistor amplifier pallets 64, 66 and 74, 76 for combining respective outputs of said second pair of silicon carbide transistor amplifier pallets 64, 66 and 74, 76. 10. A television broadcast transmitter according to claim 9 wherein each of said modules 54, 56 additionally includes a silicon carbide transistor driver amplifier pallet 58, 68 coupled to said one other 98, 102 and said another power splitter 100, 104. 11. A television broadcast transmitter according to claim 10 wherein each said modules 54, 56 additionally include yet another power splitter 94, 96 coupled between said driver amplifier pallet 58, 68 and said one other 98, 102 and said another power splitter 100, 104 for coupling said portion of said television signal from said driver amplifier pallet 58, 68 in a mutually opposite phase relationship to said power splitters 98, 100 and 102,104, and yet another power combiner 114, 116 coupled to said one other power combiner 106, 110 and said another power combiner 108, 112 for recombining mutually opposite phase relationship signal to an in-phase composite amplified television signal. 12. A television broadcast transmitter according to claim 11 wherein said one other power splitter 98, 102 and said another power splitter 100, 104 comprise phase quadrature signal splitters and said one other power combiner 106, 110 and said another power combiner 108, 112 comprises phase quadrature signal combiners. 13. A television broadcast transmitter according to claim 12 wherein said yet another power splitter 94, 96 comprises a 180° signal splitter and said yet another power combiner 114, 116 comprises a 180° signal combiner. 14. A television broadcast transmitter according to claim 11 wherein each of said modules 54, 56 includes an RF 90, 92 switch located between one module 54, 56 of said driver panel Dl, D2 and said driver amplifier pallet 58, 68. 15. A television broadcast transmitter according to claim 11 wherein all of said silicon carbide transistor amplifier pallets 58...76 include a pair of silicon carbide transistors 124, 126. 16. A television broadcast transmitter according to claim 15 wherein said pair of silicon carbide transistors 124, 126 are connected in push-pull circuit relationship. 17. A television broadcast transmitter according to claim 1 wherein said driver panels Dl, D2 and said power amplifier panels Al ...A12 comprise a set of like removable panels located side by side in an equipment cabinet 12. 18. A television broadcast transmitter according to claim 17 wherein said equipment cabinet 12 also houses air cooling means 30 for said panels Dl , D2 , Al...A12. 19. A television broadcast transmitter according to claim 17 wherein said equipment cabinet 12 also houses a plurality of power supplies 34, 36, 38, 40, 42, 44 for said panels Dl, D2 , A1...A12. 20. A television broadcast transmitter according to claim 1 and additionally including means 14 for controlling and monitoring the operation of said driver panels Dl, D2 and said modules 54, 56 thereof and said power amplifier panels A1...A12 and said modules 54, 56 thereof. 21. A television broadcast transmitter according to claim 20 wherein said means 14 for controlling and monitoring includes means 20 for generating a visual display . 22. A television broadcast transmitter according to claim 21 wherein said means for generating a visual display comprises a touch screen display and control panel 20.

Inventors: DAVIS CARLTON DANIEL, HAWKINS JACK JEROME, LONG ROBERT KENT, DAVIS, CARLTON, DANIEL, HAWKINS, JACK, JEROME, LONG, ROBERT, KENT
Applicants: NORTHROP GRUMMAN CORP, NORTHROP GRUMMAN CORPORATION
Application year: 1998
Main classification: H04N5
IPC codes: H04N5