US2200009A - Television and like receiver - Google Patents

Description

y 1940. T. c. NUTTALL 2,200.009

TELEVISION AND LIKE RECEIVER Filed April 17, 1959 K W 1 Jung m 0 i J 11 T T Ipmw rol Patented May 7, 1940 UNITED STATES PATENT OFFICE TELEVISION AND LIKE RECEIVER Application April 17, 1939, Serial No. 268,397

- In Great Britain April 29, 1938 8 Claims.

This invention relates to improvements in television and like receivers and is especially concerned with circuits for separating line frequency and frame frequency synchronising impulses which are transmitted as excursions of a carrier in the same sense from a datum level. Synchronising impulses employed in television systems commonly comprise two series of substantially rectangular impulses of different periodicity which are transmitted as variations of carrier amplitude in a range outside that occupied by picture signals, the two sets of impulses being of the same amplitude but having different time lengths. It is advantageous to separate the impulses of lower periodicity from those of higher periodicity before applying them to control an oscillation generator which is required to be operative at the lower periodicity in order to prevent incorrect operation of the said oscillation generator, especially in systems in which intercalated scanning is employed.

The present invention seeks to provide an improved method of effecting this separation.

According to the present invention there is provided a method of separating synchronising impulses of a greater duration from synchronising impulses of a lesser duration which comprises applying original impulses to a network such as to produce a time delay at least equal to the time length of the shorter impulses, mixing the delayed impulses with the original impulses and applying the combined impulses to a device adapted to pass only signals exceeding the individual amplitude of the delayed and original impulses when mixed.

According to a feature of the present invention synchronising impulses are applied to the input terminals of a network of which the output is unterminated so that reflection occurs with the result that signals applied to the input terminals of the network reappear at these terminals after a time equal to twice the delay time of the network.

In order that the present invention may be more particularly described reference is now made to the accompanying diagrammatic drawing, of which:

Figure 1 is a diagram illustrating the method of operation of the present invention;

' Figures 2 and 3 are block schematic diagrams illustrating different methods of carrying out the present invention, and

Figure 4 shows the circuit of a delay network suitable for carrying out the present invention in one mode.

In Figure 1, A represents the waveform of synchronising impulses received from a transmitter, the higher frequency impulses being represented by l and a lower frequency impulse by 2. B represents the same signals delayed by a time greater 5 than the time length of the higher frequency impulses but less than the time length of the lower frequency impulses, while C represents the combination of original and delayed impulses. It will be seen that in C the lower frequency im- 10 pulses have been increased in magnitude so that by applying the impulses to a device passing only signals exceeding a value indicated by the line 3, effective separation of low frequency from high frequency impulses is obtained.

In Figure 2 mixed high frequency and low frequency synchronising impulses are applied to the terminals 4 and pass on by two different channels to a mixing device I and thence to a limiting device 5 passing only signals above a chosen magnitude. One of the two channels comprises a delay network 6 while the other introduces no delay. If the delay introduced by the network 6 is greater than the duration of the higher frequency impulses the effect described in relation to Figure 1 will occur and, by suitable adjustment of the cut-off point of the device 5, low frequency synchronising impulses only will appear at the terminals 8.

In Figure 3 use is made of the fact that reflection of signals will occur from the open end of an unterminated network. Signals are applied at the terminals 4 through a resistance 9 to a delay network 6, the output terminals III of which are unterminated. Thus at the terminals ll signals will appear comprising both applied and reflected impulses and if, as before, the delay period of the network 6 and the cut-oil? point of the limiting device 5 are suitably chosen, low frequency impulses only will appear at the terminals 8.

As described, the invention calls for the use of a non-dissipative network designed to simulate a length of line having input and output terminals. However, the output terminals of this network are open circuited and are not required for connection to any other part of the circuit, only the two input terminals being utilised; I may therefore carry out the invention by the use of any two-terminal network which produces appropriate reactance variations. This is an advantage in so far as it allows greater freedom in the design of the network.

' The nature of the required reactance variations with frequency may be determined by comparison with the case of an ideal open-circuited line. "As is well known, the reactance in this case follows a cotangent law, points of infinite and zero reactance occurring at equidistant-frequency intervals. It is not possible to reproduce this law exactly with a finite number of circuit elements, and the greater the accuracy required the greater the number of elements which must be used. I have found, however, that for many purposes the simplest form of circuit shown in Figure 4 is suflic-iently accurate. This circuit, which is fed from terminals 4 through resistance 9 as before, comprises a series inductance l2 shunted at either end by condensers l3 and M respectively. For separating low frequency impulses of approximately 40 micro-seconds duration from high frequency impulses of approximately 10 microsec-- onds duration the following values were found satisfactory in practice:

Resistance 9 ohms 7850 Inductance l2 millihenries 20 Condenser I3 microfarad 0.0001

Condenser i l do 0.0005

I claim:

1. A method of separating synchronising impulses of a greater duration from synchronising impulses of a lesser duration which comprises applying original impulses to a network such as to produce a time delay at least equal to the time length of the shorter impulses, mixing the delayed impulses with the original impulses and applying the combined impulses to a device adapted to pass only signals exceeding the individual amplitude of the delayed and original impulses when mixed.

2. A method of separating relatively long and short electric impulses which comprises producing a time delay of the impulses for an interval at least equal to the length of the short impulses, but less than the length of the long impulses, mixing the delayed impulses with the original impulses, whereby a difference in amplitude is obtained for the long and short impulses, and separating the long and short impulses by selection according to the difference in amplitude.

3. A method of separating relatively long and short electric impulses which comprises producing a time delay of the impulses for an interval at least equal to the length of the short impulses but less than the length of the long impulses, adding the delayed impulses to the originally applied impulses, whereby only the long impulses acquire a larger amplitude than the originally applied impulses, and selecting only the impulses exceeding the individual amplitude of the delayed and originally applied impulses.

4. A method of separating relatively long and short electric impulses which comprises applying the impulses to an open circuited artificial transmission line to produce reflected impulses delayed by at least the length of the short impulses but less than the length of the long impulses, whereby Y the reflected impulses are added to the originally applied impulses and selecting from the combined originally applied and reflected impulses producing a delay at least equal to the duration,

of the short impulses but less than the duration of the long impulses, a mixing device for the originally applied impulses and the delayed im pulses, whereby an increase in amplitude of the long impulses is obtained, and an amplitude selecting device for the combined impulses from the mixing device.

6. Apparatus for separating relatively long and short electric impulses comprising a two-terminal time delay network producing a delay which is at least one-half the duration of the short impulses, but less than one-half the duration of the long impulses, a mixing device, means applying the impulses to said mixing device and to said network, whereby the originally applied impulses are combined with reflected impulses from said network with a resulting increase in the amplitude of the long impulses and an amplitude selecting device for the combined impulses from the mixing device.

7. Apparatus for separating relatively long and short electric impulses as claimed in claim 6, wherein the two-terminal time delay network has a reactance-frequency characteristic approximating to that of an ideal open circuited line.

8. Apparatus for separating relatively long and short electric impulses as claimed in claim 6, wherein the two-terminal time delay network comprises at least one inductance and two condensers producing a zero reactance at one frequency and infinite reactance at another frequency.

THOMAS CAYTON NUTTALL.

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