US2212558A - Signaling system - Google Patents

Description

Aug. 27, 1940. i A, B Em Q 2,212,558

SIGNALING SYSTEM Filed April 24, 1936 KZM,

A Hornet Patented Aug. 27, 1940 UNITED STATES SIGNALING SYSTEM Alan DowerBlumlein, Ealing, London, England,

assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application April 24, 1936, Serial No. 76,108

In Great Britain April 24, 1935 10 Claims.

The present invention relates to the transmission of impulses of different kinds over a common channel, and is particularly but not ex-'- clusively concerned with television and like systems.

The invention is applicable, for example, to television systems of the kind in which synchronising impulses of different kinds are transmitted in the line and frame intervals respectively, that is to say, in the intervals between the scanning of successive lines and frames of an object of which an image is to be transmitted. A known system of this kind, in which the present invention may be employed with advantage, ,is that described in British specification No. 425,220, in which the synchronising signals transmitted are of unidirectional, impulsive character, leading edges at the line scanning frequency being arranged to persist throughout the framing intervals, in order to ensure that, at the receiver, the scanning oscillation generator which is held in step by the line impulses shall not fall out of step during the frame intervals.

The present invention accordingly provides 26 apparatus for the transmission of spaced electrical impulses of different kinds, from either of two separate sources respectively, into a common output circuit, in which there are provided switching means arranged, when in one condi- 80. tion, to permit the transmission of impulses from one of said sources to said output circuit, and, when in another condition, to permit the transmission to said output circuit of impulses from the other of said sources, characterised in that 85 means are provided for automatically preventing the change-over of I said switching means from one of said conditions to the other from taking place except during the spacing periods between the impulses from at least one of said 40 sources.

The invention further provides apparatus according to the preceding paragraph, particularly for television and like systems, wherein said two sources are arranged to generate respectively line 4!; frequency synchronising impulses of relatively short duration, and auxiliary synchronising impulses of a longer duration, said auxiliary impulsesproviding leading edges which recur in phase with and at the same frequency as said 50 line frequency impulses, and wherein means are provided for generating switching impulses at frame frequency and forutilising-said switching pulses to control the change-over of-saidswitching means, the arrangement beingsuch that ll auxiliary impulses are transmitted to said output circuit during the framing intervals and I that line frequency impulses are transmitted at other times.

In accordance with the invention, therefore, there is provided an oscillator whose frequency 5 is twice the desired line frequency and from this energy there is derived square pulses of three different frequencies. The first of these derived impulses has a frequency equal to the desired line frequency and the pulse duration is of the 10 order of 10% of the duration of ,one complete cycle for the line frequency. The second set of derived pulses has a frequency equal .to that of the master oscillator, i. e., twice the line frequency, but the pulses have a duration equal to 15 approximately 40% of the line duration, while the third set of impulses has a frequency equal to that of the desired'frame frequency and has a duration of the order of two or more cycles of the line frequency. The line and frame fre- 20 quencies are fed to a switching device which, in one operative position, passes only the line frequency impulses, while in the other operative position passes the frame frequency impulses.

Since the arrangement is to be used for interlaced television systems, a non-integer ratio of line frequency to frame frequency is provided, and in order to insure that alternate framing impulses occur between the lineimpulses, the impulses whose frequency is twice that of the line impulses are combined in a triggering device which serves to actuate the switching device.

. The effects of combining the frame impulses and I following frame impulse, the leading edge of' the frame impulse occurs halfway between two successive line pulses. By choosing the non-integer ratio to be an odd number of halves, it will be apparent that the line scanning starts at the edge of the picture atone frame and in the middle of the picture for the succeeding frames, and this action is cyclically repeated. One of the very important advantages arising from the provisions of the invention is the fact that the pulses of twice the line frequency insure that the frame impulses alternately coincide in their initiation with the leading edge of the line impulse while the intermediate frame impulses are initiated halfway between two successive line impulses.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawing, which illustrates the application of the invention to a television trans- .mission system. In the drawing,

Fig. 1 shows a part of a television transmitter, and

Fig. 2 is an explanatory diagram to which reference will be made in the course of the description of Fig. 1. r

The television transmitter to be described by way of example is one employing interlaced scanning in which the object of which an image is to be transmitted is completely scanned in two traversals thereof, the path followed by the scanning in one traversal interlacing with that followed in the next traversal. The pictures scanned in each traversal may be either the same picture or successive frames of a motion picture film. For convenience, the frequency at which traversals of the object take place will be referred to as the frame frequency.

Referring to Fig. 1, master oscillations of a issued April 28, 1931 to James N. Whitaker for Dot multiplex. Master oscillations from generator 3 are fed to frequency-dividers I and 5, the outputs of which comprise respectively oscillations at line frequency and impulses of approximately rectangular form recurring at the frame frequency. The frequency-dividers 4 and 5 conveniently take the form of multi-vibrators of any known or suitable kind, such as described in the paper entitled A Convenient Method for Referring Secondary Frequency Standards to a Standard Time Interva by Hull and Clapp, in the Proceedings of the Institute of Radio Engineers for February 1929, beginning at page 252 et seq., and in particular, at page 258 et seq., or in the paper entitled Universal Frequency of Standardization From a Single Frequency Standard by J. K. Clapp, which appeared in the Journal of the Optical Society of America, volume 15, 1927, at page 25, andthe impulses produced by frequency-divider 5 are preferably arranged to havea duration of the order of a line period.

Line frequency oscillations from frequencydivider 4 are fed to a multivibrator 6, which is preferably of the kind described in; I British Patent No. 425,220, and is arranged to generate line synchronising impulses of substantially rectangular wave form, and of a duration equal to about of a line period; the impulses generated by multivibrator '6 are illustrated in Fig. 2b.

Oscillationsv from master generator 3 are fed directly to a further multivibrator I, also preferably ofthe kind described in the above-identifled British Patent No. 425,220, which is arranged.

to generate synchronising impulses at twice the line frequency, as illustrated in Fig. 2c, each of these impulses having a duration substantially equal to 40% of a line period.

Frame frequency impulses are fed from frequency-divider 5 to another multivibrator 8 which may also be of the type described in British Patent No. 425,220, through a heptode valve I9, the function of which will be described later and which will at this stage be assumed, for purposes of explanation, to be replaced by a direct connection; multivibrator 8 is arranged to generate a set of frame impulses, of the form shown in Fig. 2d, the frame impulses recurring at the rate of 50 per second, in the example under consideration, and each having a duration between 2. and 3 line periods. The frame pulses serve, in a manner which will be more fully described hereinafter, as switching pulses.

Line frequency synchronising impulses from multivibrator 6 are fed to the fourth grid (proceeding from the cathode) of a heptode valve 9 through a condenser I0 associated with a leak resistance II, and. broader pulses from multivibrator I are fed to the corresponding grid of a second similar heptode I2 through a condenser I3 having associated therewith a leak resistance I4. The anodes of heptodes 9 and I2 are connected together, and through a resistance I5 to the positive terminal of a' source (not shown) of anode current, the negative terminal of which is earthed. The output of the apparatus is taken from the anodes of the heptodes through a condenser I6. The second, third and fifth grids of each of the two heptodes are connected together and to a point at a suitable positive potential in the anode current source, and the two valves are suitably biased by means of a resistance II, associated with a by-pass condenser I8, connected in their common cathode circuit.

It will be appreciated that by feeding switching impulses of frame frequency (such as those generated-by multivibrator 8) to the first grids of heptodes 9 and I2 in opposite polarity and at suitable amplitudes, it can be arranged that the mutilated broader pulses which have less than their normal duration. As will be clear to those versed in the art, if such a mutilation is permitted to-take place, the scanning operation taking place at the receiver may be upset, and..

correct interlacing may be lost.

The frame-frequency impulses from frequency-divider 5 are accordingly not fed directly to the multivibrator 8, but are fed in the positive sense, through condenser 22, to the first grid of the heptode I 9, the anode of which isconnected through resistance 20 to the positive terminal of the anode current source and through condenser 2| to the input of multivibrator 8. The broader pulses from multivibrator I are fed in the negative sense to the fourth grid of heptode I9 through'conde'nser 23, and heptode I9 is so biased, by means of biasing resistance 24, that no anode current flows therein except in the intervals between the pulses applied to'its fourth grid. That is to say, the broader pulses from multivibrator I bias heptode I9 to cut off,

and if a frame-frequency impulse from frequency-divider 5 arrives at the first grid of heptode I9 while a broader pulse from multivibrator I is operative upon the fourth grid :regularly at the rate of 50 per second-successive frame synchronising signals, constituted by groups of the broader pulses of Fig. 2c, will comm'ence'in different phases with respect to the line I synchronising signals. Thus the first broad pulse of the frame signal following that shown in Fig. 2e'wil1 commence half a line periodafter the leading edge of the preceding line pulse.

The provision of broad pulses at twice line frequency during' the frame intervals not'only enables the' line scanning generator at the receiver to be maintained in synchronism during those intervals, but also provides frame signals which arecapable of producing'very accurate interlacing.

The invention is notlimited to television systems, and many modifications thereof within the scope of the appended claims will occur to those versed in, the art.

1. In a signaling system, .the combination of a first pulse generator, 2. second pulse generator, a third pulse generator, all of said generators bearing a predetermined frequency relation among themselves, selective transmission means for alternately transmitting pulses from second and said third generators, and coupling means between said further switching means to said first named switching means for'restricting the selection making to time periods. between pulses from said second generator.

2. In a signaling system, the combination of a first pulse generator, a second pulse generator,

' a third pulse generator, a source of control frequency, means to control the frequency of all of said generators from said source, selective transmission. means for alternately transmitting pulses fromsaid first and said second generators, switching means for making said alternate selections, further switching means controlled j by pulses from said second and said third generators, and coupling means between said further switching means to said first named switching means for restricting the selection making: to time periods between pulses from said second generator.

3. In a signaling system, the combination of a first pulse generator, a second pulse generator, a third pulse generator, 9. source of oscillations for controlling the frequency of all of said generators, a frequency divider between said source.

and said first generator, selective transmission means foralternately transmitting pulses from said first and said second generators, switching I periods between pulses said first and said second generators, switching means for making said alternate selections, furtherswitching means controlled by pulses from said,

a 3 means for making said alternate selections, further switching means controlled by pulses from said second and said third generators, and coupling means between said further switching means to said first named switching means for restricting the selection making to time periods a between pulses from said second generator.

4. In a signaling system, the combination of a first pulse generator, a second pulse generator, a third pulse generator, frequency control means for maintaining predeterminedfrequency relationship among all of said generators, selective transmission means for alternately transmitting pulses from said first and said second generators,

switching means for making said alternate selections, further switching means controlled by pulses from said second and said third generators, and coupling means between said further switching means to said first named switching means for restricting the selection making to time periods betweenpulses from said second generator.

5. In a signaling system, the combination of a first pulse generator, a second pulse generator. a third pulse generator, all of said generators bearing a predetermined frequency relation among themselves, selective tr ansmission means for alternately transmitting pulses from said first and said second. generators, thermionic switching means for making said alternate selections, further switching means controlledby pulses from saidsecond andsaid third generators, and coupling means betweensaid further switching means to said first named switching means for. restricting the selection making to time from;said second generator.

6. In a' signaling system,vthe combination of a first pulse generator, a second pulse generator, a third pulsegenerator, all of said generators bearing a predetermined frequency relation among themselves, selective transmission means for alternately transmitting pulses from said first and said second generators, multi-vibrator switching means for making said alternate selections, further switching means controlled by pulses from said second and said third generators, and coupling means between said further switching means to said first named switching means for restricting the selection making to time periods between pulses from said second generator. i e 1 7. In a signaling system, the combination of a first wave energy generator, a second energy wave generator, a third wave energy generator, means for alternately transmitting waves from said first and said second generators, switching means actuated in accordance with energy from said third generator for determining the time period of said alternate transmission, and means 'under the control of energy from said second generator for actuating said switching means only during intervals between waves from said second generator.

8. In a signaling system, the combination of a first wave energy generator,'a second energy wave generator, 9. third wave energy generator, means for alternately transmitting waves from said first and said second generators, switching means actuated in accordance with energy from said third generator for determining the time coupling means between said second generator and said control means for restricting the actuation of said Switching means to time intervals 1 occurring only between waves from said second generator.

9. In a signaling system, the combination of a first wave energy generator, a second energy wave generator, a third wave energy generator, frequency control means for maintaining predetermined frequency relationship among all of said generators, means for alternately transmitting waves from said first and said second generators, thermionic switching means actuated in accordance with energy from said third generator for determining the time period of said alternate transmission, and means under the control of energy from said second generator for actuating said switching means only during intervals between waves from' said second generator.

ALAN DOWER BLUMLEIN.

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