US2227197A

US2227197A - Signaling system

Info

Publication number: US2227197A
Application number: US90657A
Authority: US
Inventors: Percival William Spencer
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1934-09-04
Filing date: 1936-07-15
Publication date: 1940-12-31
Anticipated expiration: 1957-12-31
Also published as: DE875818C; GB446663A; DE755705C; GB462228A

Description

40- w. s. PERCIVAL 2,227,197

SIGNALING SYSTEM Filed July 15,1936 2 Sheets-Sheet 1 WILL l AM g l iEaVAL ATTO RN EY 4 Dec. 31, 1940. w, a PERCNAL 2,227,197

SIGNALING SYSTEM Filed July 15, 1956 2 Shee ts-Sheet 2 INVENTO WILLIAM S. P i ClVAL BY )KZW' ATTO R N EY Patented Dec. 31, 1940 UNITED STATES PATENT or ies SIGNALING SYSTEM Britain Application July 15, 1936, Serial No. 90,657 In Great Britain July 29, 1935 9 Claims.

The present invention relates to television systems and is concerned with improvements in or modifications of the invention forming the subject of British Patent No. 446,663. Thus the present invention relates to a television or like receiver of the kind wherein there are provided means whereby, for received signals having an amplitude exceeding a predetermined value, the intensity of the light produced is maintained at a low value. In this way interference or synchronizing signals in the whiter than white sense which would produce bright spots in the reproduced picture are caused to produce grey or black spots which are far less noticeable.

According to one feature of the present invention a receiver for receiving picture signals in the form of modulations of a carrier wave is provided with two parallel channels between the input of the receiver and the picture reconstituting device each of these channels containing a signal detector, one of these detectors being arranged to pass only signals exceeding a predeterminedam-plitude and the outputs of the two channels being arranged to pass current in op- 25 posing senses to the reconstituting device.

According to a further feature of the present invention a circuit is provided for feeding back signals exceeding a predetermined amplitude from a point in the receiver to an earlier point therein in such a manner as to render the output of the receiver in response to signals exceeding this predetermined amplitude less than the output in response to signals of the predetermined amplitude.

The present invention also provides a television or like receiver wherein the input of the receiver is coupled to the reconstituting device through two parallel paths one of these paths containing an electron discharge device having 40 its control grid biased beyond anode current cut off, the arrangement being such that signals exceeding a predetermined amplitude cause current to flow in said discharge device, this current serving to reduce the amplitude of the signal applied to the reconstituting device.

According to another feature of the invention the input of a television or like receiver is coupled through two parallel paths to two different electrodes of a cathode ray tube acting as reconstituting device, one of these paths being arranged to pass only signals exceeding a predetermined amplitude which signals are arranged to decrease the brightness of the scanning spot produced by the cathode ray.

The invention will be described with reference to the drawings accompanying the specification, in which: 1

Fig. 1 shows the detector stage of a television receiver modified in accordance with this invention, a

Fig. 2 shows a modification of the arrangement of Fig. 1, r

Fig. 3 illustrates another embodiment of the invention, and

Fig. 4 and Fig. 5 illustrate the application of the invention to television receivers having a picture reconstituting device in the form of a cathode ray tube, and with reference to the accompanying drawing which is marked Fig. 6 and which illustrates a further application of the invention to a receiver having a picture reconstituting device in the form of a cathode ray tube.

Like parts are given the same references in the various figures. I

Referring to Fig. 1, the last valve l of the high frequency amplifier of a wireless receiver for the reception of television signals is coupled through a transformer 2 to a signal detector 3 in the form of a diode valve. One end of the secondary Winding of transformer 2 is connected to the anode of diode 3 through a condenser 4, and the other end thereof is earthed through condenser 5. The anode of diode 3 is earthed through choke coil 6.

The cathode of diode 3 is associated with a low pass filter comprising choke coils l, 8 and shunt condensers 9, I 0, and with a potential divider resistance II from which the modulation frequency output is obtained. The end of potential divider ll remote from the cathode of diode 3 is earthed.

' There is also provided a second signal detector constituted by a diode I 2 having its anode connected to the cathode of diode 3 and its cathode connected to the upper end of the secondary winding of transformer 2. The lower end of this winding is directly connected to the tapping point I3 in a potential divider, comprising re sistances l4 and I5, which is connected between earth and the positive pole of a source of potential (not shown) the negative terminal of which is earthed. In the absence of received signals, therefore, the potential of the cathode of diode I2 is more positive than the potential of the anode thereof to an extent dependent on the position of tapping point l3, and no current flows in diode l2.

Receivedsignals cause current to flow in diode 3, and the potential at the upper end of resistance II becomes positive relative to earth. Signals of amplitude greater than a value dependent on the position of tapping point I3, and hence on the backing-E potential applied to the oathode of diode l2, cause current to flow in diode I2, and the direction in which this current flows is such as to oppose the current flowing in resistance II due to the diode 3. The positive potential at the upper end of resistance II is thus prevented from exceeding a predetermined max-' imum value, and, in fact, because the condenser 4 attenuates signals passing through the channel including diode 3 which passes signals down to substantially the lowest values, for signal am'- I plitudes greater than that corresponding to this maximum value, the potential is less than the maximum value. The signal detectors 3 and I2 lie in parallel channels through which signals are passed from the input to the picture reconstituting device.

It will be noticed that the positive potential referred to in the preceding paragraph opposes the positive potential set up at the cathode of diode I2, and the latter potential must therefore be made at least equal to the potential set up at the upper end of the resistance II by signals corresponding to a full white picture; otherwise, current will flow in diode I2 on the peaks of the picture signals, and distortion will result.

In the modification shown in Fig. 2, the use of such a large backing-off potential for diode I2 is avoided by capacitively coupling diode I2 to the output potential divider II through a condenser Iii. In this arrangement, the diode I2 is provided with a load resistance I1, and a decoupling condenser I8 is provided. The arrangement of Fig. 2 has the advantage over that of Fig. 1 that the capacity due to the diode I2 which is efiectively in shunt with resistance I I is reduced; a further advantage is that the signal potential difference across the diode I2 at which this valve commences to pass current is fixed and independent of the value of the direct current component of the picture signals.

Theposition of the tapping point I3 in both modifications is made adjustable so that the level above which interference is reduced can be controlled.

A further arrangement according to the invention is illustrated in Fig. 3. In this arrangement, the cathode of diode I2 is connected to a tapping point I3 in a potential divider I4, I5 from which a backing-off potential is derived. Voltage pulses set up across resistance II, provided they are of sufficient amplitude to exceed the potential to which diode I2 is backed oif, cause current to flow in the diode, and the resulting positive pulse set up at the cathode of the diode is transferred through condenser I9 to the control grid of valve I, and causes grid current to flow in this valve, thus reducing its amplification. The control grid of valve I is connected through a resistance to earth, and the cathode of this valve is earthed through biasing resistance 2 I. The desired result is in this case therefore obtained by feed back from the output of the receiver to an earlier point therein.

The arrangement described with reference to Fig. 3 has the advantage that only a small capacity is placed in shunt with the output of amplifier valve I.

Referring now to Fig. 4, the output from the signal detector (not shown) of the receiver is appliedto a potential divider resistance 22, having one end earthed as shown; a tapping point in resistance 22 is connected to the control grid of a cathode ray tube picture reproduced 23 through a resistance 24, and the indirectly heated cathode of the tube 23 is earthed through a source of bias voltage 25.

The tapping point in resistance 22 is also connected to the control grid of a triode valve 26, which is biased by means of the source 21 so as to pass no anode current in the absence of signals of an amplitude greater than a value determined by the voltage of source 21. The anode of valve 26 is connected through a resistance 28 to the positive terminal of a source of anode current, not shown (the negative terminal of which is earthed) and through a condenser 29 to the control grid of tube 23.

The value of the bias rovided by source 2'! is so chosen that the electron discharge device 26 is biased beyond cut-off to such an extent that current only flows therein for received signals of an amplitude greater than that corresponding approximately to full white-that is, for interference or whiter than white synchronizing pulses. Such a pulse causes current to flow in resistance 28, and a negative pulse is thus applied to the grid of tube 23, throughcondenser 29, which opposes the interference pulse on the grid of the tube 23. By suitably choosing the values of the components of the circuit, interference pulses having an amplitude greater than that corresponding to full white may be arranged to produce grey or black spots on the screen of the tube 23. The valve 26 and its associated circuits may, if desired, be connected between the signal detector and a direct current amplifier feeding the tube 23.

In the arrangement of Fig. 5, the anode of valve 26 is connected to its associated source of current (not shown) through a resistance 30, and an electrode 46 of the tube 23 is also connected to the source through this resistance. The electrode 46 serves as an accelerator electrode, and it is arranged that interference pulses cause the potential of the accelerator to become more negative, whereby the intensity of the cathode ray beam is reduced.

In the arrangement of Fig. 6, a tapping point on the potential divider 22 is connected directly to the'control electrode of the cathode ray tube 23 and the whole voltage across the potential divider 22 is applied to the indirectly heated cathode of the tube 23 through a diode 32 and a condenser 33. The cathode of the diode 32 is connected through a resistance 34 to a point upon a potential source 35, the negative terminal of which is earthed. The indirectly heated cathode of the tube 23 is earthed through a resistance 36 and the source 25.

The tapping on the source 35 is so chosen that current only fiows through the diode 32 when the signal exceeds the maximum desired signal amplitude. For signals exceeding this maximum amplitude there is applied to the cathode of the tube 23 a positive pulse equal or approaching the voltage applied to the control electrode of the tube so that the intensity of the scanning light spot produced by the cathode ray is reduced to zero or at least to a value substantially less than that corresponding to the maximum desired signal amplitude.

Suitable values for certain of the components in the arrangement of Fig. 6 have in one example been found to be: resistance 34, 50,000 ohms; resistance 36, 20,000 ohms and condenser 33, 0.01 microfarad.

The arrangements described have been given by way of example only, and many other embodiments of the invention within the scope of the appended claims will occur to those versed in the art.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A television receiver comprising an input circuit, a reproducing device, means to couple the input through two parallel paths to the reproducing device, one of said paths comprising an electron discharge device having a cathode, an anode, and a control electrode connected in one of said paths, means to bias the control grid beyond anode current cut-off potential, means to supply signals to the input circuit, means to pass anode current through said device only duringperiods when the signals exceed a predetermined amplitude, and means to reduce the effect of the signals supplied to the reproducing device through the other of said paths in accordance with the flow of anode current.

2. A television receiver comprising an input circuit, means to supply signals to said input circuit, a cathode ray picture reproducing device having two electrodes, means to couple the input through separate paths to each of the electrodes, means to pass along one of the paths to one of said electrodes only signals exceeding a predetermined amplitude, passed signals producing an opposite efiect in said reproducing device from the efiect produced by thesignals actuating the other of said electrodes.

3. A television receiver as claimed in claim 2, wherein the two electrodes are a cathode and control electrode of the cathode ray tube, and wherein signals are fed in the same polarity sense along both paths.

4. A television receiver as claimed in claim 2, wherein the means for passing only signals exceeding a predetermined amplitude comprises a' thermionic diode having a cathode and an anode, and biasing means to maintain the cathode positive with respect to the anode.

5. A television receiver as claimed in claim 2, wherein the twoelectrodes are a control electrode and a second control electrode, and wherein signals are fed in opposite polarity sense along the two paths.

6. A television receiver as claimed in claim 2, wherein the means for passing only signals exceeding a predetermined amplitude comprises an electron discharge device having a cathode, control grid and anode, and means to bias the control grid relative to the cathode to prevent anode current from flowing.

7. A television receiver comprising input means for receiving picture signaling energy, a picture reproducing device, a first path between the input means and the picture reproducing device for passing picture signaling energy from the input means to the picture reproducing device, a second path intermediate the input means and the picture reproducing device for passing picture signaling energy from the input means to the picture reproducing device, said second path being operative only upon receipt of picture signaling energy above a predetermined amplitude level and producing an efiect opposite to the effect of the signaling energy passed over said first path on said picture reproducing device.

8. The method of reproducing pictures from electrical signaling energy, which comprises the steps of supplying picture signaling energy from which a picture is to be reproduced, passing a portion of the supplied energy through a first path to a picture reproduccr, supplying a portion of the signaling energy through a second path to the picture reproducer only when the 9 energy exceeds a predetermined value, and reproducing the picture in accordance with the difference between said two energies supplied to the picture reproducer.

9. A television receiver comprising an input circuit, a means to supply picture signals to said input circuit cathode ray picture reproducing device having two electrodes, means to couple the input circuit through separate paths to each of said electrodes, one of said paths being opera-- tive only upon the receipt of picture signaling energy above a predetermined amplitude level, said one path producing an effect opposite to the efiect of the signals passed over the other of said paths on said picture reproducing device.

WILLIAM SPENCER PERCIVAL.