US2209436A - Television system - Google Patents

Description

E. L. c. WHITE 2,209,436

TELEVISION SYSTEM I Filed Oct 5, 1938 2 Sheets-Sheet l fro LIA/F SCANNING DEV/CE r0 FkAM; Scam/ma DEV/CE 0R com/mm AT 14) INVENTOR A'l 'ToRNEY July 30, 1940.

Jul 30,1940. L. c. WHITE TELEVISION SYSTEM Filed 001;. 5, l938 I 2 Sheets-Sheet 2 12 6 10 FRAME/FREQUENCY 7 I J. 5:/ 5: snwroom ENERGY kL/NEFREOUENCK SAW room 9 ENERGY INVENTOR 5 LC. NH! 5 ATTORNEY 25 pulse because the positionof that pulse varies with the inductance so that the potential reached 25 3 being transmitted in the blacker than black sense. ent in a r uit f n at a awt th wav as 'Patented July 30,1940 V I I 2.20

UNITED srArss rArsNr orricr.

Industries Limited, Hayes, Middlescx', England, a company-of Great liritain Application October 5, 1938, Serial No. 233,342

- In Great Britain October 15, 1937 4 Claims. .(Cl. 178-695) I This invention relates to television systems and of the tube which is) so negatively. biased as to 'is particularly concerned. with circuits for obbe conducting only when the synchronizing pulses y taining synchronism between the operation of are applied in the positive sense between the grid scanning at. a receiver with that at' a transmitter. and cathode of the tube.

5 In certain television systems known at the The object of the present invention-is to provide 5 present time synchronizing is'eifected by utilizing an improved frame and line pulse separating synchronizing pulses which are transmitted incircuit in which irregular operation of the circuit the blacker than black sense, these pulses being due to interference or to the effect of adjacent superposed upon the picture signals so that they pulses upon each other is avoided or reduced.

10 can be separated from the picture signals by a According to the present invention a circuit 10 device which discriminates between the ampliarrangement for developing signals which are tude of pulses applied to it. Insuch systems, it used to effect selection from a, mixtureof pulses is usual to make the frame synchronizing pulses such as synchronizing pulses which are of difof greater duration than the line frequency synfering duration, those pulses which exceed a cerchronizing pulses, and various proposals have tain duration, includes a switching device such 1 been madefor effecting discrimination between as a thermionic valve which is conductive and these two typesof pulses. applies a substantially constant potential to an In a television system employing interlaced inductance for the duration of each synchronscanning in which the and numbered lines in one izing'p n i ren e d nonon uc ive the 50 frame are interlaced with the even numbered end of the pulse, the energy stored in the ingfl lines in another frame, it'is necessary that the ductance being diverted into a capacity which discriminating device should not be affected by may be constituted by the strayfcapacity of the the position of the line synchronizing .pulse iminductance or by a condenser provided. for the mediately preceding the frame synchronizing purpose, said capacity being effectively-in shunt in'alternate frames b a-half a line period. bythe capa y s'proport nal t th duration Bythe disclosure of British Patent 180455375, of the pulse. The developed signals may be the problems involved in obtaining synchroutilized in any desired manner to effect selection nism are discussed, and alternative synchronizing of -the pulses of longer duration and known pulse separating circuits are described, such cirfo ms of amplit de Sele ting dev es may be used 30 cults beingfor use in television systems employfor this purpos In a pa t u a ap l ation of ing frame pulses of relatively long duration and the present-invention the potential resulting from line pulses of relatively short duration, but of e charging f id capacityis applied to the the same amp itude, both line and frame pulses grid of a di ch g device constituting an In-certain arrangements described in the named f rm f r. deflecting purposes. The inductance British patent; a condenser is charged through and capacity may be shunted by a. resistance in reslstance and shuntedflby a t which is order to prevent the oscillatory condition set up P sitively biased, and is, therefore,'conductin'g in by each p e from re t s- 40 the intervals between pulses. The condenser In order that the invention m y be more clear? 40 charges during pulses and a sawtooth voltage. 31 u and readily ca into ect, a waveform is produced, the pulses of which are of ternative forms of circuit arrangement embodyduration equal to that of the pulses producing ing the invention will now be described more fully them but of greater amplitude in the case of the by way of example with reference to the accom- I longer frame pulses. By applying both derived panyingdrawings wherein: g 45 sawtooth pulses to an amplitude selecting device Fig. 1 shows'one suchtform-of circuit; the pulses of larger amplitude operate to produce Fig. 2 is an explanatory diagram illustrating pulses which are applied to the frame deflecting .waveformsoi grid voltage and anodecurrentj apparatus. I Y Fig. 3 illustratesa possible modification of Fig.

In one arrangement described in the patent 1; and I m referred to, an inductance is inserted in series Fig.4 shows a still further modification where'- with a resistance in the anode lead of a tube and in the so-called blocking oscillator tubes are used the time constant of the coil and resistance are in thescanningsystem. h

made longer than the duration of a frame pulse. Referring to the drawings, the synchronizing The synchronizing Pulses are applied to the grid signals are applied from a suitable input circuit in the positive direction to the control electrode or grid of a tube I through a condenser 2, a grid leak resistance 3 being provided so that D. C. establishment occurs at the grid of the tube I.

The synchronizing signals are of sufl'icient amplitude to cause anode current in the tube l to be cut-oil in the intervals between the pulses, the potential waveform applied to the grid being as sh'ownat 4 in Fig. 2a of the drawings. It will be seen-that operation in this manner permits picture signals also to be present. at the input of the tube 1. This tube is preferably of the pentode type or it may be a tetrode having a characteristic curve without the usual kink. The tube employed is operated at the portion of its characteristic curve at which the tube impedance is low.

When the tube I is rendered conductive by a synchronizing pulse, the anode potential drops to a low value as indicated in the curve 5 of Fig. 2b and current starts to increasein an inductance 6 inthe anode lead of the tube I. At anode current cut-ofl which occurs at the end of the pulse the current in the coil is diverted into the capacity 1, represented by a condenser in'the drawage at the anodeof the tube i increases'positively. The capacity is charged in a quarter of a free oscillation period of the natural frequency of the inductance 6 and capacity 1 which should be about 10 micro-seconds or less in the case of high definition television systems employing line frequencies of the order of 10,000 per second or higher. 1

The amount to which-the anode potential ofthe tube l rises is substantially proportional to the duration of the pulse as iilimtrated in Fig. 2b.

it maybe arranged that the oscillation following a frame pulse is of suiiicient, amplitude to trigger a hot cathode grid controlled discharge tube (such, for example, as a tube of the type known 'as thyratron or "885 type) but that the oscillation following a line pulse is of mmcient amplitude to do so. As shown in the drawings, the grid of a grid controlled tube is connected or coupled to the anode of'tube I, through a condenser 9.

A resistance I0 is connected across the circuit including inductance 6 and capacity I, the value of the resistance. being chosen to .give critical damping or almost critical damping. The pursynchronizing pulses are of greater timeduration oscillator type, as shown more particularLv by than the line synchronizing pulses.

The modification of the circuit of Fig. 1 which has beenshoyvn in Fig. 3 consists in the provision of an extension ll of the inductance coil 6 the effective coupling of the portion l l to 6 enabling the opsitive line frequency pulses to be passed to a line scanning device connected to the terminal II. The frame scanning device is connected to the terminal Ill, or preferably to a tap H in order to low the eflec'tive impedance of the coil 6. This facilitates the connection of the-,in-

ductance to other apparatus.

When the,sc'anning devices are of the blocking Fig. 4, the pulses from the tapping points I! and II may be fed to the oscillators l6 and II through condensers i and 20 in the manner shown. Referring to this drawing, it will be seen that the circuit arrangement of the tube i is similar to 5 that shown in Fig. 3 and the connection from the end of the inductance B is taken through a condenser Hi to the control grid of a blocking 1 oscillator tube It, the screening grid of which is shown coupled by a transformer H to the control grid. The value of the condenser I 5 is chosen small compared with the value of the blocking oscillator grid condenser l8. The blocking oscillator valve operates in the manner set forth and described by Tolson and Duncan in their U. S. Patent #2,101,520 granted December 7, 1937 (see also British Patent #402,629). A sawtooth potential; at theline frequency is taken from the terminal I 9. a

A connection is taken from the tapping point i4 through a condenser 20 to the control grid of a further blocking oscillator tube II, the connectiohs of which are similar to those of the tube It. Again, in this case the value of the condenser 20 is chosen smaller than the blocking oscillator grid condenser 22. Sawtooth potentials at the frame frequency can be taken from the terminal 23. By suitable choice of the values of the coupling condenser l5 and 20, the strength of the synchronization can be adjusted to give 80 least trouble from interference.

Having now described the invention, what is claimed is the following:

1. An apparatus for segregating pulses having substantially the same amplitude and of diifer- 86 ing duration comprising a thermionic valve having anode, cathode and at least one control electrode, means for rendering said valve normally non-conducting, an inductive member connected in the anode-cathode circuit of said valveha means for impressing said impulses of differing duration onto a control electrode of said valve whereby the valve is rendered conducting during receipt of said impulses, a capacity connected substantially in parallel with at least a portion of said inductance, and a load circuit connected to the inductance and condenser. parallel combination.

2. Apparatus in accordance with claim 1 wherein there is provided in addition 'a damping resistance connected substantially in parallel with said condenser.

' 3. Apparatus for separating signals having substantially the same amplitude and differing durations comprising a thermionic vacuum tube,

'having anode, cathode and at least one control electrode, means for rendering said tube normally non-conducting, atapped inductive member connected in the anode-cathode circuit .of said tube, a condenser conncctedsubstantially,

"in parallel with a tapped portion of said inductive member, a pair of load circuits, means for connecting said load circuits to differing sections of said inductive member, and' means for impressing said, signals of differing duration, onto a control electrode" of said tube whereby said tube is rendered conducting during the receipt of said signals. I 4 Apparatus in accordance with claim 3 wherein there is provided in addition a resistance connected substantially in parallel with said condenser.

ERIC LAWRENCE cAsnmG wm'rs.

Images