US2210523A

US2210523A - Television system

Info

Publication number: US2210523A
Application number: US193257A
Authority: US
Inventors: Blumlein Alan Dower
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1935-01-15
Filing date: 1938-03-01
Publication date: 1940-08-06
Anticipated expiration: 1957-08-06
Also published as: FR800602A; BE413315A; US2212933A; GB489231A; US2227066A; FR49228E; FR832232A; GB455375A; DE754557C; GB490150A; DE893806C

Description

1940- A. b. BLUMLEIN 2.210.523

TELEVISION SYSTEM Filed March 1, 1958 Early! INVENTOR A. D. BL UM E/N avpfg W ATTORNEY Patented Aug. 6, 1940 UNITED s'rs rr'r Fries assignor to Electric & Musicai industries Limitcd, Hayes, Middlesex, England, a company of Great Britain Application March 1, 1938, Serial No. 193,25? In Great Britain March 4, 193? 3 Claims.

This invention relates to television systems, and is particularly concerned with a modification of the system described in the specification of copending patent application No. 59,022 filed January 14, 1936, which describes circuits for obtaining synchronism between the operation of scanning at a receiver with that at a transmitter.

In the specification of co-pending patent application No. 59,022, the problems involved in obtaining synchronism are discussed, and alternative synchronising pulse separating circuits are described, such circuits being for use in television systems employing frame pulses of relatively long duration and line pulses of relatively short duration, but of the same amplitude. In the arrangements described in the specification, a condenser is charged through a resistance and shunt ed by a valve which is positively biased, and is, therefore, conducting in the intervals between pulses. The condenser charges during pulses and a saw-tooth voltage waveform is produced, the pulses of which are of duration equal to that of the pulses producing them but of greater amplitude in the case of the longer frame pulses. By applying both derived saw-teeth pulses to an amplitude selecting device the pulses of larger amplitude operate to produce pulses which are applied to the frame deflecting apparatus.

, The specification referred to only discloses circuit arrangements which depend upon the components having shorter time constants between pulses than during pulses. Actually the time constant between the pulses is much shorter than the gaps between them.

The object of the present invention is to provide a pulse separating circuit which will give correct separation for interlacing frame pulses while employingtime constants which are longer during the intervals between pulses than during pulses.

According to the present invention in a modification of a television receiver described in the specification of co-pending patent application No. 59,022, the converting means for producing derived frame and line pulses from frame and line synchronising pulses includes a condenser charged from a source of potential, a resistance connected across said condenser and a device for controlling the charge in said condenser the time constant of said resistance and condenser being greater than the intervals between the synchronising pulses but rendered effectively short by means of a unilaterally conducting device such as a diode valve which serves to maintain a predetermined voltage across the condenser in the intervals between pulses. The device for controlling the charge in the condenser may be a thermionic valve to the input circuit of which the mixture of pulses to be separated is applied.

In a particular circuit arrangement embodying the invention the resistance and condenser are connected in the anode circuit of the controlling valve and a diode valve or the equivalent is connected between a source of potential and the anode of the controlling valve in such a manner that the diode is only rendered conducting when the charge in the condenser reaches a predetermined value. The derived frame pulses are applied to a saw-tooth waveform generating circuit through a diode valve the cathode of which connected to. the anode of a controlling valve and the anode of the diode is connected to a source of positive potential through a coil coupled with the coils of a blocking oscillator circuit. I

In order that the invention may be more clearly understood and readily carried into effect, an impulse separating circuit operating in accordance therewith, will now be described by way of example with reference to the accompanying drawing in which:

Fig. 1 is a circuit diagram, and

Figs. 2 and 3 show explanatory waveforms.

Referring to Fig. 1 of the drawing, the valve i which although represented as a triode, may be a tetrode or a pentode, is arranged to be selfbiassing, and a resistance 2 is connected between the rid and cathode of the valve for this purpose. Picture signals and synchronising signals are together feed to the grid of the valve I through a condenser 3, the synchronising signals being in the positive sense and of such amplitude that while they will cause anode current to flow in the valve l the anode current will be cut off at all picture signal amplitudes. A resistance t is connected between the anode of the valve and the source of high tension voltage and a condenser 5 is connected across the resistance i. The circuit connections and method of operation up to a point marked A in the diagram are typical of known arrangements for separating synchronising pulses.

Diode valves 6 and iare shown having the anode of one and the cathode of the other connected to the anode of the valve I and derive their operating potentials from the high tension source supplying the charging current for the condenser 5.

The operation of the circuit will be described with reference to Figs. 2 and 3 of the drawing,

the former showing a typical synchronising signal waveform comprising three line synchronising pulses I followed by two frame pulses f. Fig. 3 shows the two curves A and B drawn with reference to a line CD representing maximum high tension voltage. The curve A represents the voltage obtained at the point A in the circuit shown in Fig. 1 independently of any apparatus following that point in the circuit.

quired to fire the blocking oscillator generating On the application of a synchronising pulse to the grid of valve l, the valve is rendered conductive and the condenser 5 tends to charge negatively, as shown by the straight falling portions of curve A, and in the intervals between the synchronising pulses the potential of the anode of valve 8 tends to return towards the H. T. voltage as shown by the rising curved portions of the curve A. The time constant in this instance is such that the discharge of the condenser 5 occupies a time somewhat greater than that of a line period.

On the application of the frame pulses to the grid of valve l, the anode potential tends to fall further than in the case of the line pulses, and to pass a limiting value a at which point the frame scanning apparatus is operated. It will be seen that the time taken for the potential of point a to be reached, depends upon the amount of charge in the condenser 5 left over from the preceding line synchronising pulse.

If the synchronising pulses are arranged to effect interlacing, some frame pulses will occur mid-way between line pulses and others will occur at the same time as line pulses. Due to the 3 different positions of alternate frame pulses with respect to the line pulses, there are different charges in the condenser 5 at the occurrence of alternate frame pulses so that the times after the beginning of the frame pulses at which the po-. tential of the condenser rises to the value rethe deflecting waveform are different, resulting in one frame scan being a little late with respect to the other and inaccurate interlacing.

The curve B represents the voltage across the condenser 5 resulting from the connection of the diode valves 6, l as shown. The condenser 5 is prevented by the diode 6 from discharging to the H. T. potential represented by the line CD, the cathode of the diode being connected to the positive terminal of the high tension source through a tapping point 8 in a potential divider including resistances 9, ill and Ii, the point 8 being decoupled by the condenser IZ. The diode 5 is thus biassed so that it becomes conductive only when the voltage at the anode reaches a certain value as the condenser 5 tends to discharge to the high tension potential.

During the interval between the line synchronising pulses, the potential of the anode of valve I therefore rests at the value indicated by the straight horizontal portions of the curve B. On the occurrence of a synchronising pulse the potential falls as in curve A, and at the cessation of a synchronising pulse the potential rises. If the diode 6 were not connected as shown, the curve would proceed along the dotted line shown. As soon as the potential of the anode reaches the critical value of the cathode of the diode G, the rise of the anode potential ceases, so that its potential is maintained fixed as shown in the full line of curve B. It will thus be seen that in eifect an artificial short time constant is given to the potential rise by the diode 6. On the occurrence of a frame synchronising signal the valve remains conducting for a longer period, and causes the potential of the point A to drop below the limiting voltage applied to the diode l, the anode of which is held through the coil l3 at the potential at the tapping point l4 which is decoupled through a condenser 15. As soon as the potential across the condenser 5 drops below the level b, current flows through the coil l3 which may be coupled for example, with the coils of a blocking oscillator controlling the frame scanning. It will be seen from examination of the curve B that the time at which the potential b is reached is unaffected by any remnant of the preceding frame synchronising pulses. If desired, the diode i may be biassed from a lower potential such as that represented at b, so that it will not conduct until the occurrence of the second, third or even the fourth frame synchronising pulse.

-While the valve I has been shown to be of the triode type, it will be understood that a valve of the screen grid or pentode type may be employed, and in such a case the line frequency pulses de; rived from the line frequency synchronising pulses and fed to a line frequency saw-tooth waveform generating circuit may be taken from the screening grid of the valve.

I claim:

1. In a television receiver wherein line and frame synchronizing impulses of different duration are received, comprising a discharge tube having a cathode, an anode and a control electrode, means for applying the synchronizing impulses to said control electrode, circuit means including a source of potential for maintaining said anode positive with respect to said cathode, a resistance and a condenser connected in parallel and included in the anode circuit, a second discharge tube having a cathode and an anode, a second resistance, means for connecting the anodes of the discharge tubes together, means including said second resistance for connecting the cathode of said second discharge tube to the positive terminal of the source of potential, and resistance means for connecting the cathodes of said discharge tubes together whereby the condenser may be charged on the application of synchronizing impulses in accordance with the length of the impulses, and whereby the condenser may be discharged to a predetermined value during a time period materially less than the frequency of occurrence of the impulses.

2. A television receiving circuit including a discharge tube having a cathode, a control electrode and an anode, means for applying line and frame synchronizing impulses to said control electrode, the frame impulses being of a greater duration than the line impulses, a source of potential, means for connecting the negative terminal of the source of potential to said cathode, means including a resistance and a condenser connected in parallel for connecting the positive terminal of the source of potential to said anode, a second discharge tube having a cathodeand an anode, means for connecting the anode of said second discharge tube to the anode of said first mentioned discharge tube, a resistanceconnected across said source of potential, and means for connecting the cathode of said second discharge tube to a point along said resistance whereby said condenser may be charged on the application of synchronizing impulses in accordance with the length of the impulses, and whereby the condenser may be discharged toa predetermined value during a time period materially less than the frequency of occurrence of the impulses. I

3. In a television receiver wherein line and frame synchronizing impulses of difierent duration are received, comprising a discharge tube having a cathode, a control electrode and an anode, means for applying the synchronizing impulses to said control electrode, a source of potential, means for connecting the negative terminal of the source of potential to said cathode, means including a resistance for connecting the positive terminal of said source of potential to the anode of said discharge tube, a condenser connected in parallel with said resistance, a second resistance connected across said source of potential, a pair of diodes each including a cathode and an anode, means for connecting the cathode of one of said diodes to a point along said second resistance relatively adjacent the positive end thereof, means for connecting the anode of the other diode to a point along said second resistance relatively adjacent the negative end thereof, and means for connecting the other electrode of each of the diodes to the anode of said first mentioned discharge tube, whereby the condenser may be charged on the application of synchronizing impulses in accordance with the length of the impulses, and whereby one of said diodes may be rendered conductive a predetermined and fixed length of time after the initiation of a frame synchronizing impulse.

ALAN DOWER BLUMLEIN.