US2273719A - Television signaling system - Google Patents

Description

Feb; 17, 1942. i.. w. MoRRlsoN. JR

TELEVISION SIGNALING' SYSTEM Filed 0G12. 2, 1940 3 Sheets-Shea?l 1 ll I.

/NVENTOR L. W MORR/SONJR @Y l Arm NEY Feb. 17,j '1942;- 'A l.. w. MoRRls'QN. JR 2,273,719Y

TELEVISION SIGNALING sYs'rEu y Filed oct. 2, 1940., sheets-sheet r2 SAME WAVE AS IN FGJA AFTER TRANEMISS/ON THROUGH 90 PHASE SHF TER 1F m= Joa, mf. PEA/r To 954k MPL/runs /s 4. 44, ,4N /Ncnsus or la MAX/MUM AMPLIFIER ur'uv SQUARE WAVE AFTER TRANSMISSION THROUGH A PRE -EHPHASIZER ALL PORT/ON` 0F THE WAVE OUTSIDE THE DASHED LlNE` ARE ASSUMED 7D BE CHOPPED OFF DY AN AMPLIFIER SAME WA VE AS F16. 5A AFTER TRANJM/JS/QN THROUGH RESTORER ATTOv EV Patented Feb. `17, 1942 Laurence W. Morrison, Jr.,v Floris-am Park. N. Jl, v assignor to Bell Telephone Laboratories, Incorporatcd, New York;A N. Y., a corporation of New York more. particularly to methods of, and apparatus for enective transmission of televisioni-sign'al's.

I iswell known that an attenuating network designed to shape a composite signal ofr ofthe purpose of emphasizing oneportion band-'of frequency components relative appimsuan october 2,. isioQseilai No. 359,401 comms. (omis-'44) This'invention relates .to electric lsignaling'and ofthe action of electrical vnetworks that this4 'i .would be the case. v

toanother and that such a network may bc used telephone plant for pre-emphasizing or pre-shaping the signal y(as, for example, in the operation of program circuits)l before transmissioni'to increasethe signal-to-noise ratio over lineshor cables. At the receiving point the sigbe restored to its original form by transmission through a complemental restoring network. Pre-emphasis has likewise been used in television transmission for this same purpose and also to reduce the effects of unwanted mod'- ulationwhen this is controlling.

Inan experimental television system employing pre-emphasis certain'defects inthe image were noted, one of thembeing displaced outlines of the'. }'edges of features within the image, which defect was especially noticeable in subjects embodying a large change of brightness at the point of demarcation such as a dress A'shirt next to a dinner Jacket. .In the attempt to find the cause of the defects'of .this system oscillographic ob- I I servationsI of the wave form at 'various points in the circuits were made and it was found that prominent peaks were present in the output current from the pre-'emphasizing network. Further investigation disolosed the fact that a portion oi' these peaks was being cutA oil by the transmitting amplifier, upon which the output oi' thepre-.emphasizing network was impressed.. Oscillographic examination of the output of the restoring network showed also the presence of valleys.. corresponding to the chopped-oil' peaks.

Froma'n analysis of the action of'pre-emphasizi ing networks it -appeared that these peaks, which it seemed were ,duc-toA amplitude and phase ,changes inthernetwork.' were so much dependent uponi-the; changes that it would perhaps be' possibleitsumciently well reduce them by an auxiliaryphase ,controlling network. Subsequent 'experimentation with correcting networks A inserted between pre-emphasizer and amplii'ler'resulted intheiinding that a material reduction-inpeaks could beA effected with the use of aphase'changing network such that there is maintained `a substantially linear phase shift of the combination of this network with the preemphasizer over the frequency range, although it was not obvious from available knowledge l These discoveries led a signal, is used in combination with the preemphasizing network to reduce the peak to peak amplitude of the signal and permit a reduction in the peak load carrying capacity of the following ampliiler or other device which cannot transmit these peaks without overload or the introduction of harmful distortions (or, conversely,

a greater signal-to-noise ratio) and there is provided a second phase controlling network which compensates (so far as is necessary to obtain the -quality of image desired) the phase shifts of the respect to the edge. effect mentioned, above.

which effect may in this way be eliminated or rendered unnoticeable,` without operating the transmitting or. power amplifier at very low emciency as `was found to be necessary to obtain this improvement in the image without the use oi' this invention.

From the above discussion of the invention it will be apparent that it has as an object the.

provision of a phase changing network such that th'e combined actionof Athe pre-emphasizer and this network results in a substantially. linear phase shift over the band of frequency components present in a signal of the kind now.

generally employed in television in such manner as to obtain, sin-.increased efficiency of operation in the handling of such signals.

Referring to the accompanying drawings,

which illustrate an embodiment of the invention herein chosen for purpose of illustration:

Fig. lis a diagrammatic showing ofone form of television system utilizing the invention;

Fig. 2 is a diagrammatic showing of one form of network for pre-emphasizing;

Fig. 3 is a diagrammatic showing of one form of phase changing network:

Figs. 4a, 4b. 45a and 5b are wave form diagrams hereinafter referred to in explanation'of the invention; v

Fig. 6a is a diagrammatic showing of'an amplifiler interstage connection;

F18. 6b is a transmission characteristic of nfto the presentinvention, in accordance with which a network having the characteristic mentioned just above over the frequency range of a signal produced by successive line scanning, or ay carrier modulated by such frequency. With switches 6 and 1 in the position shown the video signals are transmitted over line Y 5 to receiving amplier 8, which is followed by a restoring network 3, a phase equalizer I0, and the reproducingl or image forming apparatus II.

Line 5 may for example be a cable of a type which very greatly attenuates the components in the upper portion of the frequency band, so that between stages of line amplification the signal-tonoise ratio becomes excessively high. With the switches 6 and 'I placed in the horizontal position the output of amplifier 4 passes over line 20 and is impressed upon the modulator I2, the output of which passes through a second pre-emphasizer I3, a second phase equalizer I4 and an amplifier I5. The output of the latter is transmitted by l line or radio to the receiving point where it is amplied by amplifier I6, the output of which is impressed upon the restoring network' I1. A third phase equalizer IB follows the restoring network and its output terminals leadto the demodulating apparatus I 9. The output of the latter is impressed upon the line 2| which leads lto the video frequency receiving elements 8, 9, IU, II as in the alternative case in which carrier transmission is-not utilized. Lines and 2| maybe cables having thev unequal attenuating characteristic above mentioned. The carrier transmission line between amplifiers I5 and I6 may be a coaxial cable and, as is often done, the modulating apparatus may consist of two series modulators utilizing. separate carrier frequencies so chosen that intermodulation products which are not entirely eliminated by filters may fall within the upper part of the band of signal frequencies and be of such amplitude as to cause blemishes to appear in the image. components of the modulated carrier within this portion of the signal band is thenv advantageous.

Figs. 2 and 3 show known types of pre-distorty ing and phase equalizing networks respectively which may be employed to carry out the invention. These are merely given by way of illustration. Any other known suitable forms may be used for thispurpose. While two network sections are shown in each of these figures it will be understood that as many sections are used as are found necessary to accomplish the desired purpose. As is well understood by those familiar with the design of such networks the various reactive and resistive elements within the networks are designed in accordance with well known principles. These are set forth for example in the article by O. J. Zobel, Bell System Technical Journal. vblume 7, pages 438 to 534, July 1928.

For purpose of analysis, a television signal can be thought of as synthesized from a plurality of the term effective noise is used to denote the proximately as l/n, where n is the order of the term in the series. In this connection it may be noted that the spectrum of any symmetrical square wave is exhibited by Fourier series of an odd order of terms whose amplitudes are proportional to l/n. One of these series has a fundamental of 60 cycles` per second, corresponding to the frame frequency, andthe other a fundamental of 13,230 cycles per second, corresponding to the line scanning frequency. In the following discussion of Figs. 4 to 6, inclusive, there will be assumed a 13 kc. symmetrical square wave as representative of the television signal. It will not be necessary to consider the 60 cycle square wave, it being assumed that the pre-emphasizing network has a substantially fiat characteristic over the relatively narrow portion of the frequency range covered by its components, as would ordinarily be the case.

Considering the problem of pre-emphasis broadly, there are two factors to be considered; namely, the signal spectrum at the transmitting terminal, and the effective noise spectrum at the receiving terminal. By the signal spectrum at the transmitting terminal is meant the relative power as a function of frequency. In connection with the noise spectrum at the receiving terminal,

fact that the interfering effect of a given noise power may be a function of frequency. Hence the effective noise spectrum can be considered to embrace two factors, a distribution function Pre-emphasis of the square waves of various lengths, amplitudes andy phases. It has been found that in a Well known formof television signal produced by scanning the field in 441jlines, with the usual interlacle, with 30 frames per second, the high energy terms of the frequency spectrum are given by two series Aof components whose amplitudes decrease apof noise power versus frequency at the receiving terminal, and a weighting factor dependent upon the action of the human eye, which is also a function of` frequency, modifying this. To obtain the best. results from pre-emphasis we are con-l cerned with minimizing the total effective noise at the receiver while keeping the peak load capacity Iin terms of power or amplitude at the transmitter constant. This is attained by inserting a transducer ahead of the transmitting amplifier, with possibly some additional gain, and inserting at the receiver the inverse of this transducer together with such extra gain as is necessary to bring the signal level back to its original value.

When pre-emphasis is applied to audio systems we are not concerned with the phase characteristic of the pre-emphasizer since the addition of the peak powers `of the various portions 'of a spectrum gives the total peak power to a good approximation. In ordinary audio frequency operation the transmitting gain is readjusted after the insertion of the pre-emphasizer to make thepeak output power the same as before. The receiving gain is likewise readjusted after the insertion of the complementary restcrer to secure the desired transmission level.

The first thing to be noticed in video preemphasizing is that phase distortion changes the wave shape and hence the observed picture. Let us take for example our assumed 13 kc. symmetrical square wave of unit amplitude, as shown in Fig-4, Fig. 4A being a representation of this wave before transmission through a phase shifter associated with a pre-emphasizer which phase shifter puts in a phase shift of to allfrequencies. A signal. containing about 300 components has a peak to peak amplitude more than four times as great after transmission through -amplifier stages.

' described above is merely l 'the invention, as defined in the claims, may be unequal attenuation rying capacity of roughly 1I! db. in the amplifier although the average'power in the distorted 'wave is the same as the average power in the original though this phase distortionwill be taken out by the restoring network at the receiver, whic in general is the inverse of the pre-emphasizer', the introduction of the pre-emphasizer may result in a considerably peaked signal at all points of the transmission path beifore` the restorer is reached, if we assume that the load carrying capacity of some transmission element is not ex-` ceeded so that the peaks are cut oli with the resultant appearance of distortion in the image.

Fig. 5A shows a squarewav which has undergone pre-emphasis and whose peaks have been chopped off by an amplier having insuiilcient load carrying capacity. Fig. 5B shows this wave after transmission through the restorer. These drawings have been made of the wave as observed onan oscilloscope. It is seen thatthe restorer introduces valleys roughly corresponding to the peaks that have. been chopped off. These valleys result' in the added outline edge effect noted above..

Another illustration of substantial Ipeak. re-

"duction may be mentioned. This is illustrated in Fig. 6 which shows the effectwhen the wave is transmitted through a series oi 15 -ampliiier stages employed as vshown in Fig. 6A. Fig. 6B is the transmission characteristic of this seriesof Fig. 6C shows the output wave from these amplifiers while Fig. 6D shows the wave with the phase distortion equalized. The amount oi peaking abovethe square wave has been reduced by about one-half. Whilelin this case the necessary increase in load capacity may be too small to-warrant steps being taken to reduce the peaking, the example is given as an illustration ofthe general principlesv underlying the invention. l

Itis to be understood that the arrangement illustrative and that utilized in a wide variety of circuit arrangements.l The pre-emphasizing may take place in'y other apparatus, Vit being understood by those electric transmission that skilled in the art of within a band oi transmitted frequenciesv is., or may be, produced by many amano forms of networks; for example, filters and vac-I uum tube circuits such as ampliers and modulators. It is to be further understoodthatthe phase correcting networks may be placed elsewhere in the system than in juxtaposition to the pre-emphasizing networks, so long as they l are located at such points that the above nientioned advantages will result.

What is claimed is:

,WL In the art of signaling, th combination with `lelectrical conducting means of means for impressing thereon signai waves. or a carrier wave modulated in accordance with auch signal waves, of the kind produced by successive yscanning -of elemental lines ofja picture or'eld of view, said conducting means comprising a network adapted to transmit a'range of frequencieswith greater attenuation 4in one portion of the range than in another and a phase equalizer-network associated with said iirstl networkto maintain a linear phase shift of the network combination to reduce the peak amplitude or the signal wave, a third network Ahaving av transmission characteristic which is complementalto that of said iirst network, meansfor impressing the signal wave from `said conducting means upon said third network, and a fourth network associated with said third" network which corrects, at least in part, for non-linear vphase shifts in said third network. -v l 2. In the art of signaling, the combination with electrical conductingmeans of means for imfor impressing the signal wave from said ypressing thereon signal waves, or a carrier wave modulated in accordance with such signal waves,

of the kind produced bysuccessive scanning of elemental lines of a picture or eld of view,v said conducting means comprising a network adapted to transmit a range of frequencies with greater attenuation in one portion of the range than in anotherand a phase equalizer network associated with said rst network to maintain a 1inear phase shift of thefnetwork combination to reduce the peakv amplitude oi the signal wave, means `for subsequently acting upon said signal wave, which means would be overloaded without said peak amplitude reduction, a third network having a transmission characteristic which is complemental to that oi said iirst network, means amplifier upon said third network, and a fourth network associated with said third network which corrects, at least in part, for non-linear phase shifts in said third network.

. LAURENCE W. MORRISON, Jn.

Images