US2343568A

US2343568A - Electro-optical signaling

Info

Publication number: US2343568A
Application number: US373925A
Authority: US
Inventors: Jr Laurence W Morrison
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1941-01-10
Filing date: 1941-01-10
Publication date: 1944-03-07
Anticipated expiration: 1961-03-07
Also published as: GB556430A; FR887179A

Description

` lMarch 7, 1944.

L. W. MORRISON, JR

ELECTRO-OPTICAL S IGNALING Filed Jan. 1o, 1941 3 Sheets-Sheet 1 ATTORNEY March 7, `1944. L, w. MORRISON, JR

ELECTRO-OPTICAL SIGNALING Filed Jan. 1o, 1941 3 Sheets-Sheet 5 FIG. 6

:QQQQQQzo /NTER/g-ERENCE HRE HOLD OF NT F OR SINGLE I0 PICTURE SIZE ./TO 6 VIE WING DIS- `7I-1NCE ddl-LINE RMA TELEVISION PATTERN MOV/NG l HORIZONTAL EAI? PATTERN PATTERN VIDEO FREQUENCY- CYCLES PER SECOND /N VE N TOI? L w MORE/50N JR. B V l Mam? /JV/Wff A 7` TOR/VE V Parented Mar. 7, 1944 ELECTRO-OPTICAL SIGNALING Laurence W. Morrison, Jr., Florham Park, N. J.,

assignor to Bell Telephone Laboratories. Incorporated, New York, N. Y., a corporation of New York Application January 10, 1941, Serial No. 373,925

/ A Claims.

This invention relates to signaling and more particularly to television and high speed picture transmission.

An object of the invention is to improve the transmission of signals for such types of transmission as television and high speed picture transmission over long circuits which require a large number of line amplifiers.

In recent years a new type of wide band signaling circuit has been developed utilizingl a coaxial cable. Such a system is described in considerable detail in a paper by M. E. Strieby entitled Coaxial cable system for television transmission and published in the Bell System Technical Journal for July 1933, beginning on page 438.

It was`desired 'to use such a transmission circuit for the transmission of television signals of the kind now approved by the R. M. A. (Radio Manufacturers"Association) Standards Committee. Such approved television signals include a video band of frequencies extending from about cycles per second to 2800 kc. The symbol kc. wherever used in this application signifies kilocycles per second.V Such a coaxial cable system cannot be used to transmit the videoband directly. Therefore it becomes necessary to shift this band to a position in the frequency spectrum which can be' transmitted over the coaxial cable system without undue distortion.

lApplicant was confronted with the problem of transmitting such a video band over a coaxial cable system including a large number of vacuum tube amplifiers with such a small amount of distortion that the received video band could be used to produce television images of satisfactory quality. In the solution of this problem applicant conceived the invention of the present application.

A feature of this invention is a double modulation carrier current system operating at such carrier frequencies and at such current levels in the cable amplifiers that interfering frequency components are produced in the amplifiers but at such frequencies that they do not unduly detract from the quality of the images produced in the receiver. As a result of this invention television transmission over long coaxial cable systems is made economically feasible because the amplifiers are operated at a high current level which could not be used lnsystems not embodying this invention.

.f Ina specific coaxial cable system with which applicant is" concerned the frequency band available-.for the transmission of signals extended from 64 kc. to approximately 3100 kc. This system exhibited a relatively large amount of phase distortion through the lower frequency portion of this region so that the lowest frequency suitable for television transmission purposes is approximately 150 kc. The amplifiers for this cable system had been designed primarily for telephone transmission extending only to 2064 kc. anc consequently the feedback had been allowed to fall off progressively above this point. rlherefore it is advantageous to transmit the main vside band o ver the cable in an upright position so that the high energy region of the television signal is.

' transmitted through the amplifiers at relatively 10W frequencies where less distortion occurs.

For well-knownA reasons so-called vestigial side-band transmission is employed. From previous studies it was known that too narrow a. vestigial side band caused large phase distortion in the associated vestigial shaping networks with an attendant increase in cost of equalization. It is therefore desirable to use as large a vestigial side band as possiblev providing the main side band is not reduced too much because of the finite width of the total band which can be transmitted over the coaxial cable system.

In the specific system now being considered a vestigial to main side-band width ratio offour per cent has been found to be a good compromise between these two factors.

In View of the above noted conditions where the band available for television transmission over the coaxial cable system extendsfrom kc. to 3100 kc., the width of the main side band is approximately 2800 kc. and the width of the vestigial side band is approximately four per cent of the main side band or 112 kc., the 'rst carrier frequency as it appears on the coaxial cable system must be positioned somewhere between 262 kc. and the neighborhood of 300 kc. In order to avoid the complication of using elaborately balanced modulators to suppress the wide band video signalsthe rst and 'second carrier frequencies at the transmitter were so chosen that the lower side band resulting from the rst modulation `did not overlap the video band and the lower side band resulting from the second modulation with the lowerside band and vestigial upper side band of the first modulation occupled the frequency band available for transmission over the coaxial cable system. These requirements indicate that the first and second carrier frequencies should be in the neighborhood of 8000 kc.

But these are not the only requirements. In

connection with the specific coaxial cable sys-V tem being considered pilot frequencies were employed to control the gain of the amplifiers to compensate for changes in temperature of the coaxial cable and associated apparatus. These pilot frequencies occur at 64 kc., 2064 kc.and 3096 kc. Since the intermediate and upper pilot frequencies lie Within the band of television frequencies transmitted over the coaxial cable system, they must -be suppressed before reaching the image producer or they will cause distortion in the received picture. Applicant was therefore confronted with the problem of selecting such carrier frequencies that these frequencies on the coaxial cable system' were to lie between energy' concentrations in the frequency spectrum on ,the cable system corresponding to line scanning frequencies of the video spectrum.

By taking notice that single frequency interference in the received video band, which occurs between energy concentrations at harmonics of the picture line scanning frequency, causes less noticeable distortion in the received picture than single frequency interference which coincides with such energy concentrations, applicant has been able to choose his carrier frequencies so that the amplifiers lmay be worked at a higher level than was heretofore known to be possible. In the specific case being considered where a large amount of unmodulated first carrier frequency is transmitted along with the modulated side band. applicant has so selected his carrier frequencies that the second harmonic of the frequency on the coaxial cable system corresponding to the first carrier frequency occurs nearly midway between two energy concentrations corresponding to two energy concentrations at harmonics of the picture line scanning frequency. Suchy a frequency allocation permits the requirement of the second harmonicdistortion of the cable amplifier to be reduced by a considerable amount.

In the specific television system being considered the following frequency allocation was arrived at in accordance with this invention:

Video band 30 cycles to 2800 kc. (RMA type signal) 13.23

In this system the second harmonic of the first carrier on the cable (2fo=622.546 kc.) is displaced from the position midway between the twenty-third and twenty-fourth harmonics of the line scanning frequency as they appear on the cable (2S/L+tfr+fo=23 l3.23+%z 13.23+311.273=

by only 368 cycles. Such an allocation permits of increasing the current level in the line amplifiers 23.5 decibels over what could be used if such harmonic coincided with an energy concentration frequency on the coaxial cable system. In other words the second harmonic lies near the center of the valley between energy concentrations. The

.scanning frequency.

A further advantage of this allocation resides in the ease with which the oscillators furnishing the first and second carrier frequencies may be adjusted. The pilot frequencies are all readily available at the carrier terminal stations where the carrier oscillators are located.

The proper adjustment of the second carrier oscillator is first determined by comparing its frequency with the second harmonic of the 2064 kc pilot frequency. Power from the second carrier oscillator is impressed on one set of defiecting `elements of a cathode ray oscilloscope while the second harmonic of the 2064 kc. pilot frequency, namely, 4128 kc. is impressed on the coordinate set of deiiecting elements. The frequency of the oscillator is then adjusted to give a stationary double loop pattern along the oscillator axis of the oscilloscope thus indicating that the frequency of the oscillator is four times that of the 2064 kc. pilot or 8256 kc.

The proper adjustment of the first carrier oscillator is next determined by comparing the main lower side band of the second modulator with the second harmonic of the 2064 kc. pilot. when power from the 64 kc. pilot is used as the modulating frequency of the first modulator in place of the video signals. After the frequency of the second Acarrier oscillator has been adjusted as described just above, the lower main side band of the second modulator is impressed on one set of deflecting elements of lthe abovementioned cathode ray oscilloscope while as before the second harmonic of the -2064 kc. pilot is impressed on the coordinate set of deiiecting ele-` ments. The frequency of the first oscillator is then adjusted to give a stationary eleven-loop pattern along the side-band frequency axis of the oscilloscope, thus indicating that the eleventh harmonic` of the main lower side band is equal to the second harmonic of the 2064 kc. pilot. With this adjustment the frequency of the first carrier oscillator is very nearly 7944.727 kc. The relationship between the various frequencies involved is indicated by the following equation:

wherein the symbols are the same as used hereinbefore. It follows from Equation 1 that Therefore,

fc1=s256 +64 2 3364 3) fc1 7944.727 +kc (4) From the foregoing it is seen that applicant has embodied his invention in a television system having advantages for a number of different reasons. It is not necessary always to take into account all of these reasons to gain benefits from applicants invention. For example, there may be no pilot frequencies within the region available for television transmission; still there is a real advantage in choosing the carrier frequencies so that the harmonics of the frequency `on the transmission circuit corresponding tothe first carrier frequency falls between energy concen.

trations on the transmission circuit corresponding to harmonics of the picture line scanning frequency. l

In order to assist in obtaining the advantages of applicants invention illustrative relationships will now be set forth in the form of equations using the 'symbols hereinbefore defined. The symbols m, n, n', lc and k used in these equations denote integers. The following relationships should be closely approximated in order to obtain the advantages of applicants invention with respect to (a) Second order modulation in the amplifiers. f= n fr=fc2fa- (s) (In)` Third order modulation equation,

2f.. (nf .Ln 2cv, -fql (c) /Pilot frequency elimination,

(d) Second order vmodulation and pilot frequency elimination simultaneously,

(e) Pilot elimination and third orderlmodula- This invention will now be described more in detail having reference to the accompanying drawings.

Fig. 1 illustrates schematically a-system embodying applicants invention.

Fig. 2 illustrates the circuit elements located principally at the transmitting carrier terminal of Fig. 1.

Fig. 3 illustrates the circuit elements located principally at the receiving carrier terminal of Fig. l.

Fig. 4 shows details of the circuit of the first demodulator and oscillator of Fig. 3.

Fig. 5 shows the pattern appearing on the oscilloscope when the second carrier oscillator is properly adjusted.

F'ig.v 6 shows the pattern appearing on the oscilloscope when the first carrier oscillator is also properly adjusted.

Fig.` 7 illustrates single frequency interference threshold data for the specictelevision system disclosed herein by Way of illustration.

The specii'lc embodiment of the invention described hereinbefore has been usedexperimen- 5 tally in connection with a coaxial cable system installed between New York city and Philadelphia. The geographical layout of such sysl tem is shown in Fig. 1. Asshown in that figure, the television transmitter is located in Phila- 10 delphia and the receiver in New York city. A

television camera 5 located in building A- indicated by block 6 produces video signals which are transmitted over a special circuit adapted to transmit the video band Ato transmitting carrier l5 terminal equipment 1 located in building B indicated by block 8. This circuit comprises an amplifier 9 at building A, an amplifier I0 at the Evergreen central oflice I I, an amplifier I2 at the -Spruce central oilice I3 and an ampliiier I4 at build-ing B. Cathode ray monitoring equipment I5 for producing a picture from the video signals is also provided at building B.

At the carrier terminal 1 the video band is shifted by modulation to a frequency range suitable for transmission over the coaxial cable system I B. This coaxial cable system comprises twenty ampliiiers located at five-mile intervals between building B and the Long Lines building I1. A section of coaxial cable extends from the Long Lines building I1 to the receiving carrier terminal equipment I8 in building C, indicated by the block I 3, in New York city. At building C a cathode ray monitoring equipment 20 is located for observing the pictures produced by the detected video band.

building C, an ampliiier 25 at tlieWatkins central office 26, an amplifier 21 at the Longacre central office 28, an amplier 29 at the Circle central office 30 and an amplifier 3I at building D. The so other circuit comprises a specially shielded cable an amplier 34 at building D. Either one or the other of these alternative circuits may be used by suitable connections at terminals 35 and 36.

Television camera 5 shown also in Fig. 2 comprises a cathode ray tube 40 which has a photoelectric mosaic screen 4I and suitable scanning circuits to produce R. M. A. Standard video signals. An image of the object being scanned is projected on the screen 4I by lens 42, the object being represented by the arrow 43. Video signals appear across resistance 44 andlafter amplification in4 amplifler45 are impressed on the carrier terminal equipment 1 by suitable connections at terminal 46. The video signals may be produced 05 by any other suitable type of scanner. l

Still referring to Fig. 2 this video Vsignal is impressed on a first modulator 41 to modulate a carrier frequency of 7944.727 kc. produced by oscillator 48. Modulator 41 is especially designed to transmit a controlled amount of unmodulated iirst carrier frequency along with the side bands corresponding to the video signals. The important frequencies appearing in the output circuit of modulator 41 are a band of frequencies corresponding to the video signals, upper and lower 32 provided with an amplifier 33 at building C and v side bands on each side of the 7944.724 kc. carrier and a controlled amount of this carrier, the peak to peak amplitude of which ls equal to about twice the peak to peak amplitude of the video envelope. The lower side band extends from approxmately 5144 kc. upwards to the neighborhood of the carrier. The upper side band extends upwards from the neighborhood of the car.. rier to approximately 10,744 kc.

The frequency components in the output of the first modulator 41 are impressed on a band-pass filter 49 which suppresses the video signal and most of the upper side band and transmits the lower side band and a vestigial side band comprising about four percent of the upper side band.

` The vestigial side band extends to approximately The lower side band and the upper vestigial side band are impressed on a second modulator 50 to modulate a carrier frequency of 8256 kc. pro- The important frequenduced by oscillator cies appearing in the output circuit of modulator 5| are the upper and lower side bands corresponding to the modulating frequencies and the modulating frequencies themselves. Modulator 50 is a balanced modulator which suppresses the second carrier frequency. The lower side band comprises a component at 311.273 kc. corresponding to the first carrier frequency, a main side band extending to about 3100 kc. corresponding to the lower side band from the first modulator and a vestigial side band extending down to about 200 kc. corresponding to the vestigial side band from the first modulator. The upper side band extends approximately from 13,400 kc. to 16,312 kc.

The frequency components in the output circuit of modulator 50 are impressed on a low-pass filter 52 which passes the frequencies in the lower side band only and suppresses any carrier leak from the second modulator 50, the modulating frequencies and the upper side band. These transmitted bands, as noted above, extend from about 200 kc. to 3100 kc.

The transmitted bands are passed through a predistorter network 53 and a phase and attenuation equalizer 54. After suitable amplification in amplifier 55 they are impressed on the coaxial cable system |6. It should be remembered that the amplitude of the unmodulated component of the first carrier frequency at 311.237 kc. is large compared to the amplitude of the video envelope. Therefore, as will appear later, there is an appreciable amount of second order modulation of this frequency produced in the amplifiers, particularly as the number of amplifiers is increased. By allocating the frequencies according to this invention, a larger amount of second order modulation may be tolerated than would otherwise be the case.

The three pilot frequencies are impressed on the coaxial cable system |6 from a source 56 through circuit 51. As hereinbefore noted the proper adjustment of the frequencies of oscillators 5| and 48 is indicated by comparison with certain of the pilot frequencies. The arrangement of effecting this comparison is shown in Fig. 2. A cathode ray oscilloscope 58 having a vertical defiecting circuit 60 and a horizontal deflecting circuit 59 is provided for this purpose. The second carrier frequency from oscillator 5| is first adjusted by comparing the frequency from oscillator 5| with the second harmonic of the 2064 kc. pilot produced in harmonic producer 6|. The 2064 kc. is impressed on the harmonic producer 6| by circuit $2. The 4128 kc. irequency from this harmonic producer is impressed on the horizontal deflecting circuit 59 while the carrier frequency from oscillator 5| is impressed on the vertical deflecting circuit $0 by connecting the terminal 63 to terminal 64. 'I'he presence of a stationary double loop pattern along the horizontal axis on the oscilloscope screen indicates the proper adjustment of the frequency of oscillator 5|. This frequency is four times that of the 2064 kc. pilot or 8256 kc.

With the frequency of oscillator 5| so adjusted the first carrier frequency from oscillator 48 is next adjusted by comparing the main lower sideband frequency from the second modulator 50 with the second harmonic of the 2064 kc. pilot, when the 64 kc. pilot is impressed on the input circuit of the first modulator 41. The 64 kc. pilot is supplied over circuit 65 through connection 46 to the input of modulator 41. The main lower side-band frequency from the monitoring output of amplifier 55 is impressed on vertical deflecting circuit 6D over circuit 66 which includes selective circuit 68, by connections between

terminals

63 and 61. Selective circuit 68 is selective of 375.273 kc., the eleventh harmonic of which is almost exactly equal to the second harmonic of the 2064- kc. pilot` The presence of a stationary eleven-loop pattern along the vertical axis on the oscilloscope screen indicates the proper adjustment of the frequency of oscillator 48. This frequency is very nearly 7944.727 kc. when the 64 kc. -and 2064 kc. pilots are adjusted to their normal values.

The shape of the double loop pattern |52 appearing on the screen of oscilloscope 58 when oscillator 5| is properly adjusted is shown in Fig. 5 and the shape of the eleven-loop pattern |53 when oscillator 48 is also properly adjusted is shown in Fig. 6. The boundary of the screen is represented by the circles |50 and |5|.

Referring now to Fig. 3, the transmitted band of television signals and the pilot frequencies received from the coaxial cable system I6 are impressed on a pilot suppression network 10 which suppresses all three of the pilot frequencies to a degree necessary to prevent troublesome distortions in the received picture. The remaining television signals are passed through a phase equalizer and band elimination filter 1|, an amplifier 12 and a restorer network 13 before being impressed on the modulating input circuit of a balanced demodulator 14. The carrier frequency for this demodulator is furnished by oscillator 15 which is adjusted to supply a carrier frequency of 8256 kc.

The important frequencies appearing in output circuit of demodulator 14 are an upper and lower side band and the modulating band. The demodulator 14 is arranged to suppress the carrier frequency. The lower side band extends downward from about 8056 kc. to 5144 kc, 'I'he upper side band extends from about 8456 kc. to 11,368 kc.

The frequencies in the output circuit of demodulator 14 are impressed on a band-pass filter 16. This filter 16 passes the lower side band from demodulator 14 and suppresses the rest of the frequencies in its output circuit.

This lower side band passed by bandpass filter 16 is impressed on a linear rectifier 11, the output circuit of which is connected through a low-pass lter 18 and video signal amplifier 19 to a cath- 0de `ray image producer 20. By means of a branch connection the detected video signals the may be transmitted through connection 35 to tentials for demodulator I14 are supplied by a di.

the alternative circuits extending to building D.

Since linear detection with transmitted carrier `is employed it is only necessary that the carrier frequency for demodulator 14 `have a frequency reasonably close to the second carrier frequency produced by oscillator 5| and 15 are, therefore, adjusted independently by comparison with the 2064 kc. pilots at the terminals of the coaxial cable system.` In determining the proper adjustment of oscillator 15, current of 2064 kc. .from pilot frequency source 8l is impressed on the modulating input circuit of demodulator 14 through circuit 82 and a connection between terminals'83 and 84. For this purpose the connection to the coaxial cablefsystem from terminals 84 to 85 is removed. Another way to supply the 2064 kc. pilot to the input circuit of demodulator 14 is to render ineffective the 2064 kc. pilot suppression apparatus. Of

` course no television signals should be incoming from the coaxial cable system during adjustment Telephone receiver .86 which lator 14.is a section of concentric conductor 90 and the circuit between demodulator 14 and band-pass filter 16 is also a section of concentric conductor 9|. The input network of demodulator 14 consistsfof a transformer 92, inductance coils 93 and 94, resistancesSE and 96 and condensers 91 and 98. This demodulator is of the balanced type comprising a pair of pentode vacnum-tubes 99 and |00. The television signals lare applied to the control grids of the two tubes in series and the carrier frequency from the oscillator 15 is applied to the same control grids in parallel. The 8256 kc. carrier frequency is applied through transformer |0|, the windings of which are shunted by condensers |02 and |03, respectively. The parallel network consisting of resistance |013 and condenser |05'are connected in series with the secondary winding of transformer |0|. The demodulator tubes 99' and |00 work into transformer |06 whic has an impedance ratio of 1300 to 72 ohms and a pass band of 4900 kc. to 8100 kc. This transformer 106 has an impedance of 72 ohms when measured on the low side when each half of the high side is terminated in 650 ohms shunted by a 24 mmf. (mi- This demodulator 14 is -very accurately ball anced to suppress the 8256 kc. carrier frequency which otherwise would cause an interfering com, ponent in the detected video signals produced by the linear rectifier 11 due to modulation with the transmitted first carrier of 7944.727 kc. This ac-v curate balance is obtained in the screen and the plate circuits of demodulator 1t. The screen po- Oscillators rect current source .represented by battery H5 vthrough `a combination of three potentiometersv densers, one of which subtracts capacity from one plate circuit as it 'adds it to the other. The law of variation governing these two condensers is I such that the total series capacitative impedance across both plates is at all times kept constant.

The total capacity range of each side of this condenser is about 3 to 10 mmf. A cam and lever arrangement attached to the condenser comprise a fine phase .balance control.y By proper adjustment of the phase and amplitude controls a carrier balance of the order of 75 db. down on the signal can be obtained. It has been found that although this balance may drift, it will always be better than 65 db. Since the balance requirement fore this carrier leak is about 50 db., the actual balance which can be maintained is more than sumcient for all needs.

The oscillator 15 comprises a pentode oscillator tube |20 and a pentodeampliiier tube |2| with the indicated associated circuit elements.

The procedure for adjusting the oscillator 15 to produce a frequency of 8256 kc. will now be l described The 2064 kc. pilot frequency is impressed on the modulating input circuit of demodulator 14 through` transformer 92 in vplace of the television signals received from the coaxial cable system by way of terminal 85. Telephone receiver 85 is bridged across resistance H9 through jacks and H2. Therefore, voltages developed across resistances H0 are impressed on the telephone receiver 86. Since all balanced demodulation products are present in the midbranch circuit of the demodulator, that is, in resistance ||0 which forms a series portion of the mid-branch circuit, they also are present in the telephone receiver 8B. Consequently, -when the 2064 kc. pilot frequency is impressed on the d emodulator, the fifth order modulation product will be balanced and appear in the telephone Vreis represented by the equation where fr is the frequency of the tone heard in the telephone receiver 85 and fc3 is the frequency produced by oscillator 15. The carrier frequency fc3 is then varied until a zero beat is produced in the receiver 86. When this condition obtains the frequency of the oscillator 15 is 8256 kc. providing the actual pilot frequency has its normal value of 2064-kc.

Modulation requirements The modulation requirements for the hereinbefore described specific system will now be set forth. Single frequency interference threshold data is given in Fig. 7. These curves represent a cross-plot of data obtained by experiment with cathode rray tubes of the type described in the 7c paper by M. kE. Strieby, identified hereinbefore,

ceiver 86. Such fifth order modulation product and interpreted for the R. M. A. Standard television. signal. These experiments determined the threshold of visibility for numerous horizontal and bar patterns and all results were expressed in terms of the relative brightness of the pattern and background. For a background brightness of from 0.1 to 0.5 foot lambert which is the minimum expected under television viewing conditions, the value of M y B for threshold of visibility has been found to be approximately a constant for any particular bar pattern. Curve H is for a horizontal bar pattern moving at the rate oi' one bar per second.'

Curve V is for a vertical bar pattern moving at the same rate. It is convenient to express this threshold of visibility requirement in terms of the maximum brightness of the picture. In the present system it is proposed to employ a reproducing tube which will have an effective linear range of 30 db. Then the B nl,

requirement will be given by AB AB AB BMA= +30 +2.5 +325 db. where i observed threshold of visibility.

Fig. 7 represents the single frequency interference threshold of visibility for a condition of a slowly moving pattern. This slow movement imposes a slightly more severe requirement than if the pattern were stationary. The observer is assumed to be viewing an 8" x 10" field at a distance of from 3 feet to 6 feet.

As pointed out previously, a necessary condition for low distortion, when envelope detection is employed, is the transmission of the carrier at a relatively high level with respect to the signal. This is accomplished by unbalancing the first modulator until the peak to peak amplitude of this transmitted carrier at the input terminals f the linear rectifier is 6 db. greater than the peak to peak amplitude of the modulating video signal. Since the transmitted R. M. A. signal does not contain direct current, this transmitted carrier has a constant average value.

For convenience the value of the unmodulated carrier will be used as a reference for expressing all interference requirements. Then the single frequency interference requirement at the input terminals of the linear rectier will be those given on Fig. 7 increased by 6 db.

Since both transmitting and receiving terminals contribute equally to the vestigial shaping, a receiving terminal discriminates `against the carrier by 3 db. more than at frequencies 100 kc. or more removed from the carrier. The requirement is thus increased by 3 db. at all points preceding the receiving terminal band filter where this semi-vestigial shaping is introduced.

Then, to summarize, the requirement on single tone interference introduced at the line position is 9 db. more severe than indicated by Fig. '1. For example, the requirement on a tone of 700 kc. introduced on the line, which would produce a received interference of '100-31l=389 kc. approximately, would be 83.5 db below the value of unmodulated carrier at thatv point on the line.

Second and` third order modulation of the carrier frequency Second and third order modulation requirements of the transmittedcarried, lo, may be derived on a single frequency interference basis.

As hereinbefore pointed out, the present allocation aifords a reduced' requirement of 23.5 db.

to be used for the second harmonic of the carrier frequency. This is due to the fact that the second harmonic of the carrier frequency which will occur at 622.546 kc., will fall approximately midway between the components of the line scanning frequency. Then referring to Fig. 7 the requirement on 2fn, which produces a received interference tone of 311.273 kc., will be The third harmonic of fo will produce a received interference pattern which will not have the beneilt of being an (11+ V2) multiple of the line scanning frequency and the requirement may be written as where the received interference tone will be 622 kc. approximately.

The above coaxial cable basic requirements exist for a system which employs no predistortion. The subject of predistortion and a description of the nal predistortion characteristic will be discussed hereinafter.

Pilot frequency modulation products ments in terms of fo, for a system employing no predistortion follows:

fpz ifm :$73.5 di).

In the above, where upper and lower sideband products have different requirements, the most severe case is listed. It is to be noted that the above requirements hold for each product interfering alone. The increased requirement due to the addition of a number of simultaneous interfering products will not be considered at present. When the received interference tone fromk a modulation product is a (n+1/2) multiple of the line scanning frequency, a reduced threshold requirement may be used.

In the specific television system hereinbefore described the video band and -the lower side band resulting from the first modulation do not overlap. If a balanced modulator is used for the first modulation which is carefully balanced to suppress the video band the first carrier may be chosen so that the lower side band overlaps the video band. .In such a system lower carrier frequencies may be used but they should be chosen in accordance with the teachings of this invention to effect improvement in the transmitted television signals.

What is claimed is:

1. An electro-optical image producing system f harmonic of the shifted frequency correspondn ing to said first carrier frequency lies between adjacent energy concentrations in said shifted range.

2. A television system comprising means to proa duce television signals occupying a wide band of frequencies and having energy concentrations at harmonics of the frequency corresponding to the line scanning frequency, a transmission circuit adapted to transmit said television signals, and means comprising a source of pilot current of frequency included within said wide band of frequencies but intermediate to adjacent harmonics of said frequency corresponding to the line scanning frequency and a control means to control a transmission characteristic of said circuit by pilot current from said source.

3. A television system of the kind in which the video signals are transmitted as modulations of a carrier current and a component of unmodulated carrier current is transmitted therewith to effect detection at the receiver comprising means to produce a video signal having a line scanning frequency of selected value, means to produce transmission current having frequency components corresponding to the respective frequency components of said Video signal moved upward in the frequency spectrum by a constant amount including a source of carrier frequency and means to lmodulate said carrier frequency by said video signals, the frequencies being so chosen that a selected harmonic of the component of the transmission current corresponding to said carrier frequency lies intermediate the two frequencies in said .transmission current corresponding to two adjacent harmonics of said line scanning frequency, and means to transmit said transmission current. l

4.' A system for transmitting television signals over a transmission circuit comprising a large number of amplifiers in tandem, means for pro.- ducing a video signal having energy concentraquencyv range which includes a harmonic of the shifted frequency corresponding to the first carrier current frequency and wherein said harmonic lies in the contrai portion of the frequency space between two adjacent shiftedenergy concentrations, and means for amplifying said last shifted band in said transmission circuit ampli- .fiers at a current level which produces an amount of said harmonic of the shifted frequency corresponding to the first carrier current which would cause distortion greater than is permissible under predetermined tolerances in an image produced from said amplified shifted band if said harmonic of the shifted frequency corresponding to the first carrier current frequency coincided with a shifted energy concentration.

5. A system for transmitting television signals over a transmission circuit comprising` a large number of amplifiers in tandem, means for producing a video signal having energy concentrations at integral multiples of the linescanning frequency, means for shifting the video signal band by modulating a rst carrier current of such frequency that the essential lower side band occupies a frequency range displaced with respect to the essential video band, means for selecting and transmitting said lower side band and an amount cf unmodulated rst carrie-r current at least equal to the peak to peak amplitude of the selected side band. means for shifting the selected and transmitted band, including the com ponent of unmodulated first carrier current, by additional modulation to a frequency range which includes'a harmonic of the shifted frequency corresponding to the first carrier current frequency and wherein said harmonic lies in the central portion of the frequency space between two adjacent shifted energy concentrations, and means for amplifying said last shifted band in said transmission circuit amplifiers at a current level which produces an amount of said harmonic of the shifted frequency corresponding to the first carrier current which would cause distortion greater than is permissible under predetermined tolerances in an image produced from said amplified shifted band if said harmonic of the shifted frequency corresponding to the first carrier current frequency coincided with a shifted energy concentration.

6. A television system comprising means to produce video signals of the R. M. A. type having energy concentrations at the line scanning frequency of substantially 13.23 kc. and harmonies thereof, a first modulator adapted to transmit a controlled amount of unmodulated .carrier current-along with the side bands produced therein, means to impress said Video signals on the modulating input circuit of said first modulator, a first source of carrier current of a frequency substantially equal to '7944.727 kc. adapted. to supply carrier current to said first modulator, va band-pass filter connected to the 'output circuit of said rst modulator having a lower cut-off at approximately 4900' kc. and an upper cut-off at approximately 8056 kc., a second modulator of the balanced type adapted to suppress the carrier current and transmit thc side bands having a modulating input circuit connected to the output circuit of said band-pass filter, a second source 'of carrier current of a frequency substantially equal to 8256 kc, adapted to supply carrier current to said second modulator, a low-pass lter having an uppercut-off at approximately 3300 kc. connected to the output of said second modulator, and means 'to utilize the television signals passed by said lowpass filter.

7. In a signaling system, a modulatory a source of carrier current for said modulator, a transmission circuit, a source of pilot frequency associated with saidtransmission circuit for controlling a transmission characteristic thereof, and means to adjust 'the frequency of the carrier current supplied by said source of carrier current and indicate when the frequency bears a predetermined relationship to the frequency of said pilot frequency.

8. In a television system, a modulator, a source of carrier current for said modulator the frequency of which is adjustable, a coaxial cable system having a plurality of amplifiers connected in tandem, a source of pilot frequency of 2064 kc. associated with said cable system, for regulating the gain in said amplifiers, means for indicating the proper frequency adjustment of said source of carrier frequency comprising a cathode ray oscilloscope having coordinate deflecting means,

means to impress a second harmonic of said pilot frequency on one deecting circuit, and means to impress carrier current from said source of carrier current on said other deflecting circuit, and means to vary the frequency of said source of carrier current to produce a stationary double loop pattern on the screen-of said oscilloscope along the axis corresponding to said second harmonic of said pilot frequency.

9. In a television system, a first and second modulator each having modulating input circuits and output circuits, a coaxial cable system having a plurality of amplifiers connected in tandem. means to produce two pilot frequencies of 64 kc. and 2064 kc., respectively,'associated with said cable system for regulating the gain of said amplifiers, means to impress a carrier current of 8256 kc. on said second modulator, means to impress said 64 kc. pilot frequency on the modulating input circuit of said rst modulator, a source of carrier current for said first modulator, means to select the lower side band from said first modulator and impress it upon the modulating input circuit of said second modulator, means to select a lower side-band frequency from the output of said second modulator corresponding to the lower side-band frequency from said first modulator resulting from the 64 kc. pilot frequency impressed on the modulating input circuit of said first modulator, means for indieating the proper frequency adjustment of said source of carrier frequency for said first modulator comprising a cathode ray oscilloscope having coordinate deflecting means, means to impress the second harmonic of said 2064 kc. pilot frequency on one said deflecting circuit, means to impress said selected sideband frequency from the output of said second modulator on said other vdeflecting circuit, and means to vary the frequency of said source of carrier frequency for said first modulator to produce a stationary eleven-loop pattern on the screen of said oscilloscope along the axis corresponding to said selected sideband frequency.

l0. In a television system, a balanced demodulator adapted to suppress the carrier current impressed thereon having a modulating input circuit, an output circuit for the side bands and a common circuit in which the fifth order modu lation product is present, a source of carrier current for said demodulator the frequency of which is adjustable, a coaxial cable system having a plurality of amplifiers connected in tandem. means to produce a pilot frequency of 2064 kc. associated with said cable for regulating the gain of said amplifiers, means for indicating the proper frequency adjustment of said source of carrier frequency comprising a resistance connected in series in said common circuit, a telephone rcceiver connected in shunt across said resistance. and means to impress said 2064 kc. pilot irequency on the modulating input circuit of said demodulator, and means to adjust the frequency of said carrier current to cause a zero beat in said telephone receiver.

LAURENCE W. MORRISON, Jn.