US2315388A

US2315388A - Television system

Info

Publication number: US2315388A
Application number: US367888A
Authority: US
Inventors: Alda V Bedford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-11-30
Filing date: 1940-11-30
Publication date: 1943-03-30
Anticipated expiration: 1960-03-30

Description

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Patented Mar. 30, 1943 TELEVISION SYSTEM Alda V. Bedford, C'ollingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1940, Serial No. 367,888

8 Claims.

My invention relates to picture transmission and particularly to an improved method of and means for producing a signal having recurring substantially noise-free pulses which go to a predetermined level such as black.

It has been the practice in the television art for some time to produce a signal in which, at the end of each scanning line, there occurs a. signal pulse which goes to a definite voltage level such as a voltage representing black or blacker than black in a picture whereby the direct current component of the signal may be lost and later automatically reinserted.

As described and claimed in Ray D. Kells application Serial No. 292,790, filed August 31, 1939, for an improvement in Television apparatus, the signal output of a pickup tube of the low velocity beam type may be made to go to such a voltage level by blocking the scanning beam periodically. Specifically, this causes the signal to go to a voltage representing black on the mosaic screen of the pickup tube. The synchronizing pulses are later so mixed with the signal that they are added to this black level.

It has been found that in the above-described system the signal output of the pickup tube con tains noise during the scanning beam cut-01f. This may be amplifier noise such as caused by thermal agitation and shot efiect or it may be stray pickup such as pickup from the deflecting circuits. This noise voltage may be a substantial percentage of the synchronizing pulse voltage, as much as forty percent, for example, with the result that the ability of the signal to synchronize the receivers is impaired.

An object of the invention is to provide an improved means f or and method of producing a television signal.

A further object of the invention is to provide an improved system and method for producing a television signal having substantially noise free synchronizing pulses.

A still further object of the invention is to provide an improved means for and method of producing in a television signal recurring pulses which are caused to reach a predetermined level with respect to black in the picture whereby these pulses may later be utilized for direct current reinsertion.

In a preferred embodiment of the invention, the signal output of a pickup tube of the low velocity beam type is caused to go to black level periodically by blocking the scanning beam as described in the above-identified Kell application. To remove the noise" that is present on this black level, blanking pulses are mixed with the signal output so as to make the noise appear on top of the blanking pulses.

The blanking pulses are then clipped off at a definite level with respect to black by means of a clipping tube which has its clipping level controlled by the blanking pulses themselves. This removes the undesired noise and, at the same time, sets the level of the blanking pulses so that they may later function to reinsert lost D.-C. and/or low frequency components.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure l is a circuit diagram of one embodiment of my invention as applied to a television transmitted,

Figures 2A, 2B, 2C and'2D are curves which are referred to in explaining the operation of Fig. 1,

Figure 3 is a group of curves which illustrate the operation of the clipping tube in Fig. 1, and

Figure 4 is a circuit diagram illustrating another embodiment of the invention.

Referring to Fig. l, the invention is shown applied to a television transmitter comprising a low velocity beam pickup tube indicated at M, which has a control electrode 1 I. A pulse generator [2 produces synchronizing pulses and blanking pulses.

The synchronizing pulses are supplied over a conductor I3 to the pickup tube It] for causing the low velocity scanning beam to scan a mosaic screen (not shown) and thereby produce picture signals.

At the end of each scanning line or horizontal deflection and at the end of each picture frame or vertical deflection a negative blanking pulse is applied to the control electrode II with sufficient amplitude to block the scanning beam. These blanking pulses are shown in Fig. 2B.

As a result, the signal output of the pickup tube In is like that shown in Fig. 2A where picture signals produced during a line scannin, are indicated at 16. The important characteristic of the output signal is that at the end of each scanning line the signal goes to a value representing black in the picture or to a fixed amount beyond black in the event that a small fixed amount of illumination on the mosaic screen has been provided for set-up as described by Kell, this result being obtained because of the characteristics of the low velocity beam type of pickup tube as explained in the said Kell application. The particular pickup tube which is used in the equipment being described preferably is the same as the one shown in Kells above-identified application.

It will be noted, however, that in Fig, 2A the signal during the black level periods is not a fixed voltage represented by a horizontal line but, instead, is an irregular voltage because of the noise previously mentioned. This noise is indicated at IT.

The output signal shown in Fig. 2A is amplified by a preamplifier 2| and supplied to an ainplifier tube 22 having a plate resistor 23.

Blanking pulses are mixed with the output signal by applying them to an amplifier tube 24 which also has resistor 23 as its plate resistor. Thus the two signals are added to produce the signal shown in Fig. 20 which is applied to the clipper tube 26. It will be noted that the noise is now so located with respect to the picture signal that if a substantial part of a blanking pulse is clipped off, the noise is removed from the combined signal.

In accordance with my invention, and as indicated in Fig. 2C by the legend Clipping Level, the blanking pulses are clipped off at a fixed level with respect to black by means of the clipping tube 26 which has the grid bias applied thereby controlled in accordance with the height of the blanking pulses. This is accomplished by means of a direct current reinserting circuit comprising, in the example of Fig. 1, a diode 3| connected between the grid of tube 26 and ground through a biasing or voltage level setting source. This source in the example illustrated is a battery 32 shunted by a potentiometer resistor 33 which has a variable tap 40 thereon.

The signal is applied to the control grid of tube 26 with the blanking pulses of negative polarity as indicated at 30. These pulses cause current to flow periodically through the diode 3! whereby a direct current charge is applied to the coupling condenser 36. The amount of this charge depends upon the height of a blanking pulse as measured from the alternating current axis of the combined signal. The mean value of the noise on the top of a blanking pulse is effectively the top of the pulse since the time con- .stants of the D.-C. inserting circuit are such that the comparatively high frequency noise does not affect the D.-C. insertion.

At the end of each blanking pulse period, the condenser 36 discharges a slight amount through the grid circuit of the tube 26 which circuit includes a grid resistor 31 and a biasing battery 38, the comparatively slow time constant of the discharge circuit being such that the D.-C. bias voltage across the condenser 36 and appearing on the grid of tube 26 will vary in accordance with the said heighth of the blanking pulses. The operation of D.-C. reinserting circuits of this type is well known and is described in Willans et a1. Patent 2,194,514.

The tube 26 is operated with sufiicient negative bias on its control grid to make it clip off the pulses at or slightly beyond black whereby the undesired noise signal is removed. The circuit operation is further illustrated in Fig. 3, the curves showing how the tube 26 and the D.-C. insertion circuit function to remove noise and set the pulses at a fixed level slightly beyond black. The figure is largely self-explanatory but attention may be directed to the fact that by adjusting the level setting voltage (i. e., by moving tap 40 of Fig. 1) it is possible to clip off the pulses either at black level itself or at any desired level beyond black.

From the foregoing, it will be seen that the output of tube 26 is a signal having pulses which may be utilized for D.-C. insertion at a later point in the system after the signal has passed through an A.-C. amplifier, for example. Fur thermore, such D.-C. insertion may be made with great accuracy because the pulses have been clipped at a level determined exactly by the black level of the picture signal itself.

After the clipping action by the tube 26, synchronizing pulses are added to the tops of the blanking pulses in the usual manner for transmission to the receivers, the tops of these pulses being commonly referred to as pedestals.

The D -C. insertion circuit for the system of Fig. 1 need not be the specific one described. Other types, such as the so-called keyed" circuits, may be employed, the D.-C. insertion circuit shown in Fig. 4 being an example of a keyed circuit.

In Fig. 4 there is shown an application of the invention for the transmission of pictures from moving picture film. In Figs. 1 and 4, like parts are indicated by the same reference characters.

In the system of Fig. 4, it is assumed that a moving picture projector of the intermittent type is used to project the film images upon the screen of the pickup tube. Each time an image is projected upon this screen, there is produced a rather large pulse in the output signal as indicated by the dotted line pulse 46 in Fig. 2A, It will be apparent that since this pulse exceeds the blanking pulses in amplitude, it will control the action of the D.C. reinsertion circuit to a large extent unless precautions are taken to prevent this from happening.

In Fig. 4 the D.C. reinsertion circuit includes a triode 41 in place of the diode shown in Fig. l. and this triode is rendered inelTective to pass current for the duration of the pulse 46 by means of a keying circuit. The keying circuit comprises a keying pulse generator 48 which supplies negative keying pulses 49 shown in Fig. 2D) to the grid of tube 41 during the presence of pulses 46 whereby tube 41 is blocked periodically. This blocking occurs at the rate of 60 times per second in the usual system.

The keying pulse generator 48 may be of any suitable type such as one including a blocking oscillator or a multi-vibrator and suitable clipping or shaping circuits. It may be synchronized by means of synchronizing or blanking pulses supplied from the generator l2 over a conductor 5 I.

The level setting voltage is adjusted as in the other embodiment by moving the tap 40.

I claim as my invention:

1. In a television system, the method which comprises producting a composite signal including picture signals and periodically recurring electrical pulses which go periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, producing blanking pulses having a fixed amplitude, so adding said composite signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black" region, and clipping off said blanking pulses at a level determined by the height of said blanking pulses with respect to the alternating current axis of the signal.

2. In a television system, the method which comprises producing a. composite signal including picture signals and periodically recurring electrical pulses which go periodically to a predeter: mined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, producing blanking pulses having a fixed amplitude, passing said composite signal through an alternating current amplifier whereby the direct current component is lost, adding said composite signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black region, reinserting said lost direct current component in accordance with the height of said blanking pulses as measured from the alternating current axis of the signal, and clipping oil the upper portion of said blanking pulses at a level measured from the top of said pulses whereby said unavoidable noise is removed.

3. In a television system having a cathode ray pick-up tube of the type having a mosaic of electron emissive capacity elements and having means for producing an electron beam and directing it against said mosaic, the electrons in said beam approaching zero velocity at the point of impact on said mosaic and having substantially zero velocity in the region of said mosaic when it is dark, the method which comprises producing periodically recurring electrical pulses and so applying said pulses to said tube as to periodically block said beam whereby the output signal of said tube goes periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, producing blanking pulses having a fixed amplitude. so adding said output signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black region, and clipping on said blanking pulses at a level determined by the height of said blanking pulse with respect to the alternating current axis of the signal.

4. In a television system having a cathode ray pick-up tube of the type having a mosaic of electron emlssive capacity elements and having means for producing an electron beam and dia recting it against said mosaic, the electrons in said beam approaching zero velocity at the point of impact on said mosaic and having substantially zero velocity in the region of said mosaic when it is dark, the method which comprises producing periodically recurring electrical pulses and so applying said pulses to said tube as to pe riodically block said beam whereby the output signal of said tube goes periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, producing blanking pulses having a fixed amplitude. passing said output signal through an alternating current amplifier whereby the direct current component is lost, adding said output signal and said blanking pulses so that said noise signals are caused to extend substantially beyond said amplitude level into the "blacker than black" region, reinserting said lost direct current component in accordance with the height of said blanking pulses as measured from the alternating current axis of the signal, and clipping oil the upper portion of said blanking pulses at a level measured from the top of said pulses whereby said ,pnavoidable noise is removed.

5. In a television system, means for producing a composite signal including picture signals and periodically recurring electrical pulses which go periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, means for producing blanking pulses having a fixed amplitude, means for so adding said output signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black region, and means for clipping off said blanking pulses at a level determined by the height of said blanking pulses with respect to the alternating current axis of the signal.

6. In a television system, means including a pickup device for producing at said device a composite signal including picture signals and periodically recurring electrical pulses which go periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, means for producing blanking pulses having a fixed amplitude, passing said composite signal through an alternating current amplifier whereby the direct current component is lost, means for adding said output signal and said blanking pulses so that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black" region, means for reinserting said lost direct current component in accordance with the mean height of said blanking pulses with said noise superimposed thereon, and means for clipping off the upper portion of said blanking pulses with the direct current component restored at a uniform direct current level.

7. In a television system, a cathode ray pickup tube of the type having a mosaic of electron emissive capacity elements and having means for producing an electron beam and directing it against said mosaic, the electrons in said beam approaching zero velocity at the point of impact on said mosaic and having substantially zero velocity in the region of said mosaic when it is dark, a moving picture projector for projecting picture frames upon said mosaic whereby a pulse appears in the output signal of said tube each time a frame is projected upon said mosaic, means for producing periodically recurring electrical pulses and so applying said pulses to said tube as to periodically block said beam whereby the output signal of said tube goes periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, means for amplifying said output signal in an amplifier which removes the direct current component, means for producing blanking pulses having a fixed amplitude, means for so adding said output signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black region, means for reinserting said direct current component, means for clipping off said blanking pulses at a fixed voltage level, and means for making said reinserting means ineffective for the duration of each of said pulses caused by projection of picture frames on said mosaic.

In a television system, means for producing a composite signal including picture signals and periodically recurring electrical pulses which go periodically to a predetermined amplitude level with respect to black in the picture being transmitted, there being noise signals unavoidably superimposed on said amplitude level, means for producing blanking pulses having a fixed amplitude, means for so adding said output signal and said blanking pulses that said noise signals are caused to extend substantially beyond said amplitude level into the blacker than black region, an amplifier tube having a control grid and a cathode, a direct current inserting means comprising a unilateral conducting device having a cathode and an anode, said device being connected between said grid and the cathode of said amplifier tube with the cathode of said device connected to said grid and further comprising a condenser through which signal may be applied to said grid, means for supplying the combined output signal and blanking pulses to said grid through said condenser with the blanking pulses of negative polarity at said grid whereby they charge the condenser through said device, said direct current inserting means further comprising a discharge circuit for said condenser, said amplifier tube being adjusted to clip off the blanking pulses at a predetermined level measured from the tops of said blanking pulses.

ALDA V. BEDFORD.