US2391090A - Monitoring device for television systems - Google Patents

Description

Dec. 18, 1945. T. T. GOLDSMITH, JR 2,391,090

MONITORING DEVICE FOR TELEVISION SYSTEMS Original Filed June 21, 1941 HTTURNEY tube f uomoamanavrosroa I Thomas '1'. commits; In, Oedar em, hi. 3-. lnortodllennnumonthbmtcrinlna,

,NJ-i a motion application June 21, 1941, Serial No.

Divided and this application l lll aisiasuuna'mm;

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This invention relatesto a device for monitor- 7 ing received television signals. In carrying out the invention a cathode-ray tube is provided for receiving a television picture signal and a vacuum tube is connected in parallel therewith so thats composite mixture of the incoming picture signal and a local blanking signal can be monitored on an oscillograph.

This is a divislon of my application Serial No. 399,055,:iiled June 21, 1941, now Patent No. 2,373,114, granted April 10, 1945. I

The invention may be understood from the description in connection with the accompanying drawing, which shows how various units may be connected and is in part a diagram of connections ticed'.

In the drawing, reference character I indicates a television receiver for an input signal through connection 2 either to a transmission line or through the air. The receiver delivers a composite television signal 3 that is delivered to a one-stage amplifier 4 which inverts the picture signal. This inverted signal is passed through a direct current inserter I, to be described in detail later, which controls the reference level from which the modulation occurs and yields a wave form which is opposite in polarity to the conventional wave form 3. This latter wave form is delivered by means of connection I to the grid 48 of the bright cathode-ray tube 8, thus providing the grid modulation to this tube 8 for producing a bright reversed polarity television picture on the screen 01 this tube.

The television receiver I delivers a horizontal sawtooth wave form by means of connection I I to a horizontal deflection I and to the horizontal blanking generator I2. The receiver I also delivers a vertical sawtooth wave form by means of connection It to the tube 8, and to the vertical blanking generator l4. The horizontal blanking generator I2 and the verticalblanking generator I4 may consist, for example, of circuits oi the type described in the Campbell Patent 2,207,048.

The square wave form blanking pulses from generators I2 and I4 are suitably mixed in the blanking mixer I1 and after passa e through a direct current inserter 25 the blanking signals are applied by means of connection 26 to the cathode 41 of the cathode-ray tube 8.

As described in the parent application, the

composite television picture signal in negative or reversed polarity is applied to lead 40 and is illustrating how the invention may be pracof Delaware August passed through thecondenserfl to a diode tube 42 which is' so connected as to provide direct plate of cathode-ray current insertion tocontrol the pedestal level of the television picture signal atfa fixed voltage relation for the grid 48 of the cathode-ray tube 8. The resistor 44 between tube 42 and lead .I is

of low resistance value, yet sumciently high in resistance so that the synchronizing peaks on the composite wave form are practicallyignored;

consequently the signal appears on the plate 43 1 of the diode 42 as though the pedestal level were essentially flat from the beginning to the end of the blanking interval. A high resistance leak 45 is placed across the diode 42 so that the voltages may return to normal after momentary unusualsurges. The condenser 4I tends to apply signals to the plate 43 which go positive and negative with respect to a predetermined voltage.

However, upon an attempt to drive the plate 43 positive with respect to the cathode 4B, the tube 42 will conduct current and establish a bias voltage on the condenser 4| so that no subsequent signals can go more positive than the cathode 46. The reference voltage level is established in this way, and the television signals are maintained always negative, except for the synchronizing peaks, and the pedestal level is caused to assume a fixed voltage to maintain the proper brilliance setting of the grid 48 in the cathode-ray tube 8'. In this way the receiver automatically varies the absolute brilliance of the received picture when the transmitter sends one line, for example, which is essentially bright on a standard picture of positive polarity and a subsequent line which is essentially dark in a standard picture of positiv polarity.'

It is to be remembered that the cathode-ray tube 8' reproduces the television picture in negative polarity and therefore this direct current inserting diode 4| accomplishes the result of maintaining a direct current level formaximum white with means for modulating always toward black from this level, whereas the conventional tube with a picture of positive polarity has a' It is'employed to prevent blanking slim-1 voltage across the potentiometer II. The high resistance I8 shunts the diode and enables the circuit to return to its operating range automatically after an abnormal surge may have appeared. By means of the potentiometer II the absolute value of the direct current voltage may be set independent of incoming signals and this potentiometer serves as the brightness control of the cathode-ray tube 8. The vacuum tube II is connected as a cathode iollower stage to pass the blanking signals without inversion of polarity but with reduction of source impedance. The blanking signals are applied from the direct current inserting diode is to the grid 58 of this tube. The cathode 80 is connected directly to the cathode 41 of the cathode-ray tube 8. The resistance II in the cathode circuit of tube 58 is of low value so as to transmit the blanking wave forms without distortion and so that changes in current in the cathode-ray tube 8 will not cause an appreciable change in the direct current level control circuit.

A vacuum tube In is connected in-parallel with the cathode-ray tube 8 for the purpose of proirom app viding monitoring of thecombination signal resulting from the two signals applied, respectively, to thecathode 41 and the grid 48 of the cathoderay tube 8. The combination signal appears on the cathode-ray tube screen of tube 8 as changes in brilliance, but for adequate control of these signals it is desirable to produce and monitor the resulting wave form of the combination. Therefore the grid 1| of vacuum tube III is connected directly to the grid 48 of the cathode-ray tube 8. The cathode 12 of tube I is connected directly to the cathode ll of tube I. The plate I: of tube It thus carries a current which is characteristic of both of the signals that are applied to tube 8. A voltage develops across the plate load resistor 18 which represents this combined signal. It may be applied by means of a selector switch 18 to the vertical deflection system Vor y-axis terminals of the oscillograph ll while a sawtooth wave of proper frequency is applied to the H or :c-axis terminals in the usual way so that the wave form will appear on the screen 18 of the oscillograph for monitoring purposes. The selector switch I. also provides for monitoring the signal through the connection that appears upon the cathodes 41 and 12 of tubes 8 and 1c and for monitoring the signal through the connection 8| that appears upon the grids. 48 and II or these tubes, thus providing monitoring of any one oi. these signals independently. The switch 18 may also be used to monitor other points in the circuit ii desired by use of the lead 82.

What is claimed is:

1. In a television system, a cathode-ray tube and a vacuum tube, means to apply a television picture signal tothe control grids of each of said tubes. means to apply blanking signals to the cathodes of said tubes, and means for monitoring a composite mixture of said signals.

2. In a television system, a cathode-ray tube and a vacuum tube, means to apply a television picture signal to the control grids of each of said tubes, means to apply blanking signals to the cathodes of said tubes, and an oscillograph for monitoring a composite mixture of said signals.

THOMAS T. GOLDSMITH, JR.

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