US 2268811 A
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Description (OCR text may contain errors)
Jan. 6, 1942'. p. J. FE WINGS ETAL 2,268,811
' TELEVISION RECEIVER Filed Au 5, .1938
- INVENTORS DAV/D JOHN FEW/N65 ROLANDJUHN KEMP 7+4 4 m-C/L/ ATTORNEY Patented Jan. 6, 1942 UNITED STAT ES OFFICE azsasu 'i 1 TELEVISION momma David John Rowing s. and Roland li mjxemp, Chelmsford, Essex, England, assignors to Radio Corporation of America, a corporation of Ilelaware Application August '5, 1938, Sam-N; 223,148
7 In GreatBritain August 25, 1937 2' Claims. (o1. 178-73) ,1
' if it besought to applyan ordinary A. v.0, system as well known in broadcast sound programme receiver practice, to a television receiver, there arises a serious defect which is not at first sight obvious. In such an ordinary A. V. C. system thegain control is efiected by rectifying the also to destroy desired variations corresponding to average picture brightness variations. The main object of this invention is to'avoid this defect.
According to this invention a television receiver is provided with an A. V. C. system which is responsive not to variations in general incoming carrier level but to variations in a predetermined selected signal component which, as transmitted, is of substantially constant level. A signal component available for selection and which is transmitted at a substantially constant chroniz ing signal such as developed in the final detector of a receiving...device.
I Figure 2 shows the controlling wave form and the wave forms developedby the A. V. C. device of. Figure 3.
Figure 3 is a diagram of the circuit A. V. C. ,device.
- Referring (to the drawing, synchronizing sig- I of the nals are separated out by any knownapparatus (not shown) from the total signals received and then, ifgrequired, amplified. .Thenature of the received signals received from a transmitter which transmits, signals includinga D. C. component representative of general picture brightness is" represented conventionally in Figural. Here three scan lines are .representedatl, II,
and III. I is a dull illumination picture line which has been subjected to fading; II is the separation out of the synchronizing signals will level in most present day television transmitters is provided by the synchronizing pulses (either the line synchronizing or the framing synchronizing pulses, or both) these pulses being usually maintained, as transmitted, at about 30% of the peak value of the carrier, as transmitted. In carrying out this invention the selected signal component, after separation at the receiver, is utilized to provide a gain controlling uni-directional potential which is employed in any known convenient way, e. g. as gain controlling grid bias, to control the gain of a carrier stage or stages of the receiver.
The invention is illustrated in and further explained in connection with the accompanying graphical and diagrammatic drawing which relates to one way of carrying out the invention.
Figure 1 shows a composite picture and-synbe, as shown conventionally at a in Figure 2, a
, series of unidirectional pulses of a polarity which will depend upon the method of selection employed and of an amplitude which will be (in the absence'of fading) constant. It is arranged that these pulses are in the positive direction as shown in Figure 2 at a. Since these pulses will be of relatively short duration as compared to the intervals between them, direct rectification and smoothing to produce the actual gain controlling potentialis not easy and it is therefore much to be preferred to precede rectification by a wave shape transforming operation which changes the pulse wave form into a form more convenient for rectification. One way of doing this is shown in Figure 3. Here the pulses are applied, for example through a coupling condenser I, to the grid 2 of a valve 3, e. g. a triode whose anode-cathode circuit includes a condenser 4 and whose anode receives positive potential through a resistance 5. The cathode leg contains the usual capacity shunted self-bias resistance combination 6, I, and the grid receives cut-oil bias through a grid resistance 8. The arrangement is such that during each interval between pulses the condenser 4 charges up through the resistance 5, the rate and law of charging depending upon the value of this resistance 5, the value of this potential and the size of the condenser 4. Rectilinearity of charging. is not an important desideratum and the preferred arrangement is that in which the condenser 4 becomes charged substantially to the in Figure 2, the sawtooth having flattened tops.
If fading occurs and the pulses accordingly become of less amplitude there will obviously be a less complete discharge of the condenser 4 at each pulse and the output wave. form will be, as
shown at c in Figure 2, of reduced amplitude with longer flat tops to the sawtooth. Such a modified sawtooth wave form is quite convenient for rectification. For example, and again as shown in Figure 3, this wave form may be applied through a coupling condenser 9 to a rectifier I0 and whose output, after smoothing in the usual way, is applied via lead II as gain controlling bias to the grid or grids of a variable-p. carrier amplifier valve or valves -(not shown) in the receiver. Obviously, any convenient form of rectifier may be used; for example, if a double diode is used the said double diode may be constituted by the diode section of a double-diodetriode, the triode section of which constitutes the valve 3.
Either the line or the frame synchronizing signals may be selected and utilized for the purposes of this invention. Selection of the line signals presents the advantage of involving less elaborate smoothing (since the line frequency is much higher than the frame frequency) but selection of the frame signals offers the advantage of ease of separation.
What we claim is:
1. In an automatic volume control system for radiant energy receivers wherein recurrent signals of a predeterminable amplitude are transmitted, a thermionic vacuum tube having anode, cathode and at least one control electrode, means for impressing said recurrent wave onto a control electrode of said thermionic tube, means for storing energy connected in the anodecathode path of said tube, means for storing energy in said energy storage means during intervals when said tube is in a non-conductive state, and discharging said energy when the wave form impressed onto the control electrode of said tube renders the tube conducting, said discharge being caused by said signals and proportional to the amplitude thereof, thus forming a serrated wave form, means for rectifying said wave form to develop a control potential, and means to utilize 'said control potential for automatic volume control purposes.
2.-I'n a television receiver wherein signals of predetermined amplitude are received intermittently, indicating the level of the transmitted carrier, the method of developing automatic volume-control signals which comprises the steps of storing electrical energy, discharging said energy by said signals, and controlling the amount of discharge by the amplitude of said signals, rectifying saidstored energy and utilizing the rectified energy for automatic-volume-control purposes. I DAVID JOHN FEWDIGS.
ROLAND JOHN KEMP.