US2278788A - Television transmission system - Google Patents

Description

April 7, 1942. u. KNICK 2,278,788

TELEVISION TRANSMISSION SYSTEM Filed June 14, 1939 2 Sheets-Sheet l INVENTOR BY (/LE/CHK/V CK 3,.

TTORNEY Patented Apr. 7, 1942 UNITED STATES PATENT OFFICE TELEVISION TRANSMISSION SYSTEM Ulrich Knick, Berlin-Steglitz, Germany, assignor to Fernsch Aktiengesellschaft, Berlin-Zehlendorf, Germany Application June 14, 1939, Serial No. 279,082

Germany June 14, 1938 5 Claims.

ners are located at considerable and different distances from the transmitter, it will be appreciated that the returning synchronizing signals will no longer be in phase with the master impulses or with respect to each other, due to the various degrees of phase delay incurred by transmission over difl'erent lengths of wire lines.

This is particularly inconvenient in case it is desired to switch from the transmission of picture signals supplied from one scanner to that of signals from another scanner. It hasbeen proposed to employ phase-delay networks in lines arriving from the scanners in order to be able to make the phase of the synchronizing impulses returning from the different scanners the same. This method, however, has the great disadvantage that such delay networks, through which the picture signals are also fed, cause appreciable distortion of the latter. Furthermore, such networks must be carefully and accurately balanced. The danger also exists that the constants of such networks are changed by exterior influences such as temperature, humidity and the like.

It is the object of this invention to overcome the above disadvantages; to provide a method by means of which the synchronizing impulses returning from all scanners are made to have the same phase; to provide a method by means of which compensation of the phase delay of the synchronizing impulses due to the transmission over wire linestakes place before transmission; and to provide a method by means of which the phase correctness can be automatically regulated.

Referring to the drawings, Fig. 1 shows a schematic layout while Fig. 2 shows circuit means by way of example for carrying out the invention.

Broadly considered, this invention provides for having available at the transmitter a master signal, to generate at the transmitter synchronizing signals which are transmitted by wire to the inthat of the master signal and to regulate the phase of the synchronizing signal generator so that the phases of the master signal and of the returning synchronizing signal are in a predetermined relation, which isthe same for the syn chronizing impulses returning from all scanners. This regulation may be donebyobservatidn and manual readjustment, or automatically by deriving from the phase comparison a regulating voltage and using this voltage to regulate the phase of the synchronizing signal generator.

Thus in this manner the phase delay due to wire transmission is compensated for before transmission of the synchronizing signals.

The invention shall now be explained more in detail in connection with the drawings Fig. 1 schematically shows a layout embodying my invention. Central station I may be at the transmitter. This, however, is not necessary inasmuch as block I indicates the junction point of the communication channels to the individual scanners with the common transmission channel to the wireless transmitter. Thus, block I may indicate a mixing desk, a switching arrangement or the like. Picture scanners 2, 3 and 4 are located at various distances of considerable difference from junction point I. Synchronizing impulse generators 8, 9 and iii are preferably of the type having a master impulse generator at double line-scanning frequency and a number of frequency-dividing stages by means of which line-synchronizing and framing impulses are derived from this master frequency. Blocks i2, Ho and 12b are means of phase comparison. The master signal generator H in a preferred embodiment of the invention is merely a terminal of the 60-cycle alternating current power mains. Leads l3, Ba and l3b connect the respective phase comparators with their associated synchronizing impulse generators, while wires 5, 5a, 6, 6a, 1 and 1a connect television scanners 2, 3 and 4 with their respectiveassociated synchronizing impulse generators and phase comparators.

Thus, Fig. 1 shows three separate scanner channels, each comprising a synchronizing impulse generator, a lead through which synchronizing impulses are fed to the scanner, a lead through which'the returning synchronizing impulse ls fed to the phase comparator, a lead feeding the master signal to the phase comparator,

and a lead feeding the regulating voltage produced by the phase comparator to the synchronizing impulse generator. Common to all three channels is the master signal source I l. Prefa is fed to the synchronizing impulse generator,

thus adjusting the phase of the latter in such a manner that the returning framing impulses, and thereby also the line-synchronizing impulses, will be in a different predetermined phase relation with the master signal. It will be understood that in this manner, by comparing the returning framing impulse with a master signal of the same frequency, the phase of the outgoing framing signal produced by the synchronizing impulse generator is so controlled and advanced to such a degree that the phase delay incurred in wire transmission is compensated for at least in part so that the framing impulses returning from all scanners will arrive in the same phase. Inasmuch asthe line-synchronizing signals are in a rigid phase relation with the framing impulses if produced by synchronizing generators, as briefly described above, the phase of the line-synchronizing impulses will be shifted in the same manner as that of the framing impulses.

While I have shown in Fig. 1 separate synchronizing impulse generators and phase comparators for each television scanner channel, this is not absolutely necessary. If all synchronizing impulse generators and phase comparators are identical, it is also possible to operate a system with a multitude of scanners with only two synchronizing impulse generators and phase comparators, inasmuch as only one set of apparatus is required for the signals being transmitted by wireless, while the other set is required for preparing anotherscanner channel to which it is desired to switch over. I

Fig. 2 shows, by way of example, in more detail a means for carrying out the method in accordance with my invention. Block l indicates a sine-wave oscillator comprising an oscillator tube It and a frequency-regulating tube I| to which a regulating voltage is applied through lead 3|. It will be understood that any oscillator circuit allowing frequency control by means of a regulating voltage can be used as well. Oscillator I5 is made to oscillate with double line frequency which, according to present standards, is 26,460 cycles. The generated sine-wave is applied to a device l8, indicated by block diagram only, which converts the sine wave into a square-top wave of the same frequency. This square-topped wave is fed through a chain of frequency dividers I8, 20, 2| and 22, the output of which is a 60-cycle square-topped framing impulse. Such frequencydividing chains are well known in the television art and may comprise a plurality of multivibrator circuits tuned to different frequencies and each synchronized by the preceding impulse generator operating at a higher frequency. Inasmuch as various devices of this type are well known in the art, it is not deemed necessary to explain these in further detail as any well-operating chain can be used for this purpose. The line-scanning frequency is preferably derived from the square topped impulses of 26,460 cycles produced by device l8 by means of a frequency divisiorrof 1 to 2. This is not shown because it is not deemed essential to the invention. lt will, however, be understood that such a chain will produce linesynchronizing impulses atfa frequency of 13,230 cycles and framing impulses at 60 cycles which 2,27s,7ss

are in a rigid phase relation with respect to each other.

The output of divider 22, which is a 60-cycle preferably square-topped framing impulse, is fed through lead 23 to a television scanner 24 placed scanner. Line-synchronizing impulses may also be superimposed upon the picture signals and framing signals. The specific way in which the framing impulses are returned is not material so long as return lead 25 carries returning 60-cycle framing impulses. In case a composite television signal is returned, a synchronizing separator 26 may be provided, the output of which must be a positive -cycle square-topped impulse. In case an auxiliary carrier is made use of, it is, of course, necessary toprovide a rectifier. Such means, being well known in the art, are not believed to require any detailed explanation. The returning positive 60-cycle framing impulse is then fed to the control grid of tube 28 in phase comparator 21. In the plate circuit of tube 28 there is connected a transformer primary winding 28 in series with a shunt-connected resistor 32 and smoothing condenser 33. The master signal, mentioned earlier in this description with a frequency of 60 cycles is fed to the secondary winding 30 ofthe same transformer. This master signal is preferably sinusoidal and can be derived directly from the 60-cycle power mains as it is common to interlock the television scaning pattern with the alternating-current power mains.

Phase comparator 21 operates as follows: The cathode of tube 28 is so biased that no plate current can flow in the absence of a positive SO-cycle square-topped impulse upon the grid. Upon arrival of such an impulse, the tube draws current and a voltage dropis produced across resistor 32. The time constant of the shunt combination of 32-33 is so chosen that the voltage across resistor 32 is substantially direct. It is, of course, also possible to replace condenser 33 by more elaborate filtering means shouldthis be found necessary. It is evident that the plate current in tube 28 is a function of the instantaneous grid voltage as well as the instantaneous plate voltage, which latter is cyclically varied by means of the induced voltage from the transformer secondary winding. Thus as the duration of the 60- cycle impulses is only a fraction of V second, it may readily be seen that the plate current in tube 28 will vary in accordance with the phase relation between the 60-cycle sine wave applied to transformer secondary winding 38 and the 60- cycle impulses applied .to the control grid of tube 28. Thus a direct current regulating voltage is fgenerated; across resistor 32, the magnitude of which varies in accordance with the phase shift between the 60-cycle sine wave and the squaretopped impulses. This voltage is fed through lead 3| to the control grid of regulator tube ll of the master sine-wave oscillator l5, thus regulating its frequency and phase in such a manner that the phase relation between the 60-cycle master sine Wave and the returning framing impulses automatically adjusts itself to the same relation regardless of the length of leads 23 and 25, that is, the distance of the scanner from the impulse generating equipment or, in other words, the phase delay incurred by the wire transmission.

While I have explained my method of television transmission in connection withv some specific means, I am well aware that this method can be carried out by other means within the scope of the appended claims and I do not wish to be limited to the apparatus above described.

What I claim is:

1. In a televisionsystem, apparatus including a source of a master signal, a source of synchronizing signals and a phase comparator, a television scanner remote from said apparatus, means for transmitting said synchronizing signals to said scanner, means for returning said synchronizing signal from said scanner to said apparatus and specifically to said phase comparator, means -i'or applying said master signal to said comparator, means for deriving from said comparator a regulating voltage in accordance with the phase difference between said returning synchronizing signal and said master signal, and means for controlling said source of synchronizing signals with said regulating voltage.

2. In a television system, apparatus including a source of a master signal, a source of synchronizing signals, said master signal and said synchronizing signals having the same frequency, and a phase comparator, a television scanner remote from said apparatus, means for transmitting said synchronizing signals to said scanner, means for returning said synchronizing signals from said scanner to said apparatus and specifically to said phase comparator, means for applying said master signal to said comparator, means for deriving from said comparator a regulating voltage in accordance with the phase difierence between said returning synchronizing signals and said master signal, and means for controlling said source of synchronizing signals with said regulating voltage. I

3. In a television system including apparatus for generating synchronizing signals located at a first point and a television scanner at a second point remote from said apparatus, the method of operation comprising the steps of generating synchronizing signals at said first point, trans- .mitting said signals to said second point, returning said signals to said first point, and controlling the phase of said signals at the point of generation in such a manner as to compensate for at least a portion of the phase delay incurred by said signals in transmission between said points. 4. In a television system including apparatus for generating synchronizing signals located at a first point and a, television scanner at a second point remote from said apparatus, the method of operation comprising the steps of generating synchronizing signals at said first point, transmitting said signals to said second point, retuming said signals to said first point, and advancing the phase of said signals at the point of generation in such a manner as to compensate for at least a portion of the phase delay incurred by said signals in transmission between said points. 5. In a television system including separate apparatus for generating a master signal and synchronizing signals, said apparatus being located at a, first point, and a television scanner located at a second point remote from said first point, the method of operation comprising the steps of generating a master signal and synchronizing signals at said first point, transmitting said synchronizing signals to said second point, returning said synchronizing signals to said first point, comparing the phases of said master signal and said returning synchronizing signals, adjusting the value of a regulating voltage in accordance with the phase shift between said master signal and said returning synchronizing signals, and controlling the phase of said synchronizing signals at the point of generation in accordance with said regulating voltage, so as to compensate for at least a portion of the phase delay incurred by said signals in transmission between said points.

ULRICH KNICK.

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