US2485611A - Broadcasting system - Google Patents

Description

Oct. 25, 1949. LABFN ETAL 2,485,611

BROADCASTING SYSTEM Filed April 7, 1944 5 Sheds-Sheet 1.

ATTORNEY Patented Oct. 25, 1949 UNITED STATES PATENT OFFICE BROADCASTING SYSTEM Emile Labin, New York, and Donald D. Grieg,

Forest Hills, N. Y., assignors to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application April 7, 1944, Serial No. 529,932

3 Claims.

This invention relates to radio broadcasting systems and more particularly to radio multichannel broadcasting in the ultra-high frequencies and the selective reception of such transmitted channels.

'The ultra-high frequency broadcasting system known as FM (frequency modulation) is being developed along the same lines as the standard low frequency AM (amplitude modulation) broadcasting systems. The method used is the following: In the larger centers of population, New York city for example, the studios of the several broadcasting stations are situated conveniently to the program originating sources. From these studios the programs are transmitted by line to the respective transmitting stations which are located primarily for maximum populated area coverage and which transmit on individual authorized frequencies. At the receiving end the listener, of course, separates the various stations by frequency tuning.

While this system of broadcasting has proved entirely satisfactory for the lower frequencies, the same advantages do not necessarily follow at the higher frequencies. In fact, at these higher frequencies several major disadvantages arise. To give a few examples:

There can only be a limited number of optimum transmitting locations about the area to be served by the broadcaster, and since height of the transmitting antenna in such area is all important at the higher frequencies, those stations not possessing these important locations are at a serious disadvantage with respect to their competitors. InNew York city for example, there is only one Empire State building dominating in height the entire city and centrally located. The broadcasting company controlling this location enjoys, therefore, a monopoly which even a considerably larger amount of power at other locations cannot overcome.

The multiplicity of transmitting equipment required is an additional disadvantage. At the higher frequencies the broadcast coverage area is limited to horizontal distance which practically limits the range to about 20 to 30 miles. In order to link areas at distances greater than this, and to extend the networks on nationwide scale, it has been envisaged to use relay stations linking the important service areas. However, with the existing multiplicity of transmitting stations a consequent multiplicity of relay stations will be required which would entail considerable economic waste aside from the technical problem of duplication of the optimum of relay location.

The listeners receiver in an area served by the several broadcasting stations is also at a distinct disadvantage with the existing system. With the various transmitting stations located at different compass points with respect to the receiving location, it is not possible for a single simple receiving antenna to be operated at maximum efficiency due to the problem of reflections from nearby buildings. This is especially true with facsimile and television, although not much less so with music and voice broadcasting. The practical elimination of these reflective effects entails the use of a directive antenna. Hence, in order to receive all the stations, in addition to the normal receiving tuning, a definitely impractical rotation of the antenna or a switching anv tenna system is necessary. This problem alone makes it virtually imperative that all stations for a given area be located geographically as close together as possible.

In addition to the above-mentioned disadvantages there are still others such as the duplication of transmitting apparatus and transmitting antenna systems, and the reduced efficiency of duplicate operation.

It is an object of this invention to provide a multi-channel broadcasting system which overcomes the above-mentioned disadvantages of existing and other heretofore proposed broadcasting systems.

Another object is to provide a multi-channel broadcasting system incorporating the unique advantages of ultra-high frequency pulse transmission.

.Another object is to provide a method and means for multi-channel broadcasting over a common carrier frequency and for selectively receiving at points in the vicinity of the broadcasting station any desired one of a plurality of channels.

A further object of the invention is to provide a method and means for synchronizing receivers to the transmission sequence of the signals of a multi-channel transmission system.

Still another object of the invention is to provide a relatively simple and inexpensive ultrahigh frequency receiver for selective reception of the channels transmitted on a common carrier frequency with the signals thereof interleaved with the signals of other channels.

A feature of the invention is the character of channel transmission. Each channel is represented by a train of pulses timed to interleave in sequence with pulses of other channels to form a single train of pulses. The pulses of one of 3 the channels are modulated with an identifying characteristic distinct from the pulse characteristics of other channels for synchronizing receivers to the transmission sequence of the channels. The pulses of the other channels, and the pulses of the identified channel if desired, are modulated in some manner, such as the time relation of the pulses of the respective channels,

- according to intelligence to be conveyed thereby.

The identifying characteristic may involve the shape of the pulses or it may comprise a frequency signal selected outside of the frequency band of the intelligence with which the channels are modulated.

It is acknowledged that a system has been proposed heretofore for multiplex communication between two points by interleaving the signals of a plurality of channels, the signal of each channel varying in amplitude according to intelligence-for such channel. The receiver for that multiplex system was synchronized to the transmitting sequence by including a further channel, the amplitude of which was made greater than the average maximum amplitude of the other channels. That type of multi-channel transmission, however, has a number of disadvantages among which are the following: The transmitter-rereiver circuits, for example, must have accurate linear characteristics to avoid intermodulation and distortion. Because of the amplitude modulation feature of such system, limiters cannot be used for minimizing interference, and the maximum signalto-nois ratio obtainable for the expenditure in band width cannot be realized, and further, the average power required varies. In the case of fading present in AM systems, the effects thereof are amplified through all repeaters which may be used to extend the range of the system thereby necessitating the use of A. V. C. circuits. The receiving circuits must also have accurate linear characteristics.

According to the pulse modulating principles of our invention, we overcome the disadvantages present in amplitude modulation systems since modulation of intelligence by our invention involves the time relation of the pulses as distinct from amplitude, while the synchronizing signals utilize pulse shape or time modulation outside the frequency band of the intelligence with which the channels are modulated.

The interleaved channel pulses are broadcast according to our invention over an antenna located at a vantage point such as at the top of the highest building of a city or on a tower located at some other high elevation within the area to be served. The antenna preferably is of the omni-directional type although it may have certain directional characteristics if desired. Certain channels may be assigned to local studios located within the area of the broadcasting station and others may be assigned to national broadcasting chains. All of the channels are broadcast in the form of pulses interleaved in a single train at a common carrier frequency. This clearly permits the broadcasting of a plurality of different programs from one high point so that receivers located within the range of the station may be equipped with directional antennas arranged in alignment with the antenna of the transmitting station. The directional antenna of the receivers may be fixed in position and the tuning for different programs effected :by time selection of the channels, the time selection being based upon the timing of the synchronizing channel. This selection of stations by time relation is, of course, independent of the carrier frequency used. Thus, a simple and universal tuning mechanism may be used which remains unchanged regardless of the band of frequencies used for the broadcast system. The receiving antenna, of course, need not be directional but for best reception with a minimum of reflection interference from buildings and the like, the directional type will, in most cases, be preferred. Furthermore, the directional receiving antenna need not be fixed on one broadcasting antenna but may be adjustable for alignment with other broadcasting sources.

A further feature is that intermodulation due to lack of suflicient'frequency selectivity is avoided by the use of the time sequence of channels.

The above and other objects and features of the invention will become more apparent upon consideration of the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic panoramic view of New York city and vicinity illustrating the centralized multi-channel broadcasting principle .of our invention;

Fig. 2 is a block diagram of a multi-channel broadcastng system utilizing pulse width as the distinguishing characteristic for the synchronizing channel;

Fig. 2A is a block diagram illustrating the synchronizing channel portion of a modified multichannel broadcasting system;

Fig. 3 is a schematic wiring diagram of a phase shifter, time modulator and pulse width shaper for one of the channels of the system of Fig. 2;

Fig. 4 is a graphical illustration used in explaining the operation of the circuit of Fig. 3;

Fig. 5 is a block diagram of a radio receiver capable of selectively receiving channels transmitted by the system of Fig. 2;

Fig. 5A is a modified form of radio receiver for use with the modified broadcasting system illustrated by Fig. 2A;

Fig. 6 is a graphical illustration used in explaining the receiving operations of Figs. 5 and 5A;

Fig. '7 is a block diagram of another modified form of multi-channel broadcasting system employing pulse frequency modulation; and

Fig. 8 is a graphical illustration used in explaining the multi-channel transmission of Fig. 7.

Referring to the panoramic view of New York city and vicinity shown in Fig. 1, the multi-channel broadcasting station is shown located, for illustrative purposes, in the Empire State building in which affords the highest centralized location for the broadcasting antenna I2. The antenna may be of any known omni-directional type. The'building may house several of the studios while other studios and sources of signals may be located in still other buildings as indicated at 2, 3 and N, each studio being connected by line to the control room of the broadcasting station. By this arrangement a number of programs is supplied for modulation of different trains of pulses which, when properly timed, interleave in sequence to form a single train of pulses to modulate a single transmitter for simultaneous transmission at a common carrier frequency F1 over antenna I2. Thus each studio, rather than being identified by an individual transmitting frequency, is represented by a train of pulses interleaved in time position with the pulses carrying programs from other studios. Listeners at receiving points such as 2|, 22, 23, 24 and 25 tune their respective receivers by time channels rather than frequency to choose a desired one of the programs transmitted.

In addition to the selective reception, the multichannel transmission may be relayed by repeated broadcasting stations where the receiving antenna thereof is in visual alignment with the transmitter antenna l2 as indicated at 26. If the repeater broadcasting station is beyond range as indicated at 27, relay repeaters such as indicated at 28 and29 may be located at appropriate points in-between. The repeater broadcasting systems rebroadcast the channel pulses after amplification either at the same or at different carrier frequency, as the case may require.

in Fig. 2, the separate studios are shown con'-' nected by lines to one or more pulse modulating channels of the system. Studio I, for example, is shown connected through switch 30 to channel I which includes a modulator 32 indicated, for example, as P. T. M. (pulse time modulator) coupled to a pulse width shaper 34. The modulator 32 is controlled by a base Wave produced by generator 35. The additional pulse modulating channels are also controlled by the base wave from generator 35 but the timing thereof is controlled by individual phase shifters as indicated at 36 for channel 2, the difference in timing with respect to channel I being indicated by the symbols ta, ta, etc. The modulation of the pulses of each channel may be controlled by an appropriate switching arrangement as indicated at 31 and 38. The synchronizing pulses of channel I may or may not be modulated with program intelligence as may be inferred from the use of switcn 30. For conservation of space in the train of channel pulses, it is best to maintain the synchronizing pulses free of programs for with a program the time modulation thereof would also vary the deblocking pulses in a like manner. This would, therefore, require much greater interval between the pulses of adjacent channels, otherwise crosstalk might result.

Since one of the channels is to be used fOr synchronizing the receivers, as one method it is necessary to characterize the pulses thereof with a width distinct from the width of the pulses of other channels. In Fig. 2, this is accomplished by differently shaping the width of the pulses for channel I. This is done by adjusting the pulse width shaper 34 to a width different from the pulse widths determined by the pulse width shapers 42, 43, etc. This method of identifying the synchronizing pulses is clearly shown by curve 6a of Fig. 6. The pulses of channel l are considerably wider than the pulses of the other channels which, in this case, comprise a given narrow width. The interleaved train of channel pulses produced at the outputs of the shapers 34, 42, 43, etc., is applied to a carrier translator fill for translation to a common carrier frequency to be transmitted over omni-directional antenna l2. The antenna shown is of the type disclosed in U. S. patent to A. Alford, 2,283,897.

Studio 3 is shown in Fig. 2 to be connected to two different channels. This is to indicate that a given studio may have more than one channel for broadcasting purposes. It will also be understood, in this connection, that while in existing broadcasting systems programs are generally transmitted for voice and musical programs, other types of intelligence may also be transmitted. For example, pulses may be transmitted in this manner as facsimile signals. the pulses being used directly as the facsimile build-up characters. The transmitted pulses may also constitute a sound track for television as well as synchronizing pulses required for controlling the line and frame scanning in facsimile and television transmission. In addition, the two channels may serve for binaural sound transmission for a single program.

The multi-channel broadcasting system may be provided with synchronizing signals by means other than characterizing the shape of the pulses. In Fig. 2A, a synchronizing channel is shown which may be substituted for the synchronizing channel I in Fig. 2. The modulator 32a is supplied with signals of frequency f1 from source 52. n

This frequency is selected outside the frequency band of the intelligence with which channel I and the other channels are modulated. If desired, the frequency of the signal may be selected within the lower and upper band limits of the intelligence frequency but, in that case, each studio channel would require a filter for removing the selected frequency from the intelligence. It is to be understood, however, that where we specify that the synchronizing signal is selected outside of the intelligence, we intend to include a selection arrangement of this character.

Fig. 3 shows a circuit of one form of P. T. M. modulator 32a such as may be employed in the channels of the systems of Figs. 2 and 2A, together with a phase shifter 35a and a pulse width shaper 34a. The phase shifter 36a comprises a condenser-resistor network CR1Rz, the relative values of which determine the phase shift of the base wave applied thereto from base wave generator 35, Fig. 2. Assuming that there is no phase shift of the base wave required as in the case of channel I, then the wave may be regarded as of the phase position indicated by wave 10 in curve 4a, Fig. 4, when applied to primary coil 12 of the coupling transformer 73.

The modulator circuit includes two secondary coils 14 and T5 coupled to the control grids of two vacuum tubes 16 and H in push-pull arrangement similar to a full-wave rectifier. The modulator amplifies and, in effect, full-wave rectifies the wave 7!), curve 4a, Fig. 4, to obtain a cusper wave 80, curve 4b. Curve 4c shows the cusper wave for channel 2 determined in time position by the phase position 10a, curve 4a, of the base wave.

Time modulation of the cusper wave is produced by applying the signal intelligence over input connection 82, with respect to ground, to primary coils 83 and 84 on the transformer 13. While the rectification of these waves may be symmetrical relative to zero axis 61, it is shown for purposes of illustration as being offset by different biasing potentials 65 and 66. This gives the effect of an offset axis 61a about which modulation takes place. The signal intelligence operates, in effect, to vary the wave 10 relative to its offset axis 61a as regards the full-wave rectification. This relative variation between the wave and the zero axis thereof is illustrated in curve 4a by the upper and lower modulation limits 86 and 81. When the input signal varies the relative relation between the offset axis 61a and the wave 10 as indicated by limit 86, the cusper wave 90, for example, is displaced as shown by the broken line 90a, and when varied to the opposite limit 87 it is displaced as shown by broken line 90b. It will be observed that the signal wave thus varies the time positions of the cusps 9|, 92, 93 and 94 in push-pull manner toward and away from each other thereby decreasing or increasing the time interval between successive cusps.

For transmission purposes, the cusps are clipped from the wave and, if desired, may be shaped to form substantially rectangular pulses. A suitable shaper circuit is shown at 34a, Fig. 3, it being understood, however, that other known shapers may be used instead. By providing each channel with a shaper circuit of this character, the shape of the pulses of each channel may be controlled.

The shaper 3 a may be of any known character capable of clipping between two desired levels, but preferably is of the character disclosed in our copending application, Serial No. 437,530, filed April 3, 1942. clipping circuit wherein the plates of the two diodes NH and IE2 are connected together and the cathode of tube IfII is connected through coupling condenser M3 across the load resistor I04 of the modulator 32a The cathode of diode IflI is connected through a resistor M6 to a potentiometer M27. The plates of the two tubes are connected through a resistor I88 to a second potentiometer I69. The cathode of the tube 522 is connected through a resistor Hi! to ground.

The two diodes HM and W2 operate together as a gate clipper for clipping the cusper wave 8E8, for example, between limit levels i I l and I I2, curve 41), the width of the gate being controlled by the adjustment of potentiometer I8? and the position of the gate relative to the wave is controlled by adjustment of the potentiometer I139. The gate position of the clipping levels Ill and H2 on wave 86, produces a pulse of the width W1, curve 4d. The pulse id of curve Ad is shown to be amplified, it being understood that the pulse width shapers may include one or more amplifying stages as indicated at H5.

For channel 72, the cusper wave 90 is clipped at gate levels H3 and lit as indicated in curve 40. These different gate levels are effected by adjusting potentiometer me. This gate clipping results in the pulse width W2 which is less than the width W1. It will thus be clear that by proper adjustment of theshaper circuits, the pulses of any channel such as channel I may be made distinctive from the pulses of the other channels. This distinction in Width is shown in curve to, Fig. 6, the wider pulses of channel I being used as synchronizing pulses for the receivers.

Referring to Fig. 5, we have shown in block diagram 9. receiver of the character synchronizable to the transmission sequence of the channel pulses by means of the distinction in pulse width between the synchronizing channel and the other channels of the system. The receiver includes a carrier frequency amplifier and detector l2il whereby the carrier pulses of curve 6a are translated into the video pulse form of curve 51). The video pulses are applied to a synchronizing channel selector I22 which, in this case, may be a pulse width discriminator of any known type.

If desired, a pulse width discriminator of the character disclosed in our copending application, Serial No. 487,072, filed May 15, 1943, now Patent No. 2,440,278, may be employed. This width discriminator includes a resonant circuit which is shock-excitable in response to the leading and trailing edges of the pulses and a dampling circuit having a vacuum tube connected across the resonant circuit for damping out the oscillatory energy following one or two undulations, as thecase may be. The output from the It comprises a double diode circuit is provided with a threshold clipperstage which is adjustable to pass energy in response to undulations produced from pulses of a width corresponding to the tuning of the resonant circuit. Should the pulses be of a width different from one-half the period of the frequency to which the circuit is tuned, the undulations thereof will be less than that required to produce conduction in the threshold clipper stage. Thus, pulses varying in width from the desired width are blocked, pulse energy being passed by the clipper stage only in response to undulations produced from pulses of the desired width.

The output pulse energy from the synchronizer channel selector I22 is applied to an adjustable delay device I25 which, preferably is of a known form of pulse translation type of delay circuit, whereby the pulse energy may be adjusted in time with reference to the timing of the,synchronizing pulses. The retarded pulse energy of the device I25 is applied to a mixer I25 to which is also applied the video pulse output of the detector I20. It will be understood, of course, that the pulse output of the selector I22 may be shaped to provide a pulse of a width corresponding'to the total time space occupied by the pulses of each channel throughout the maximum limits of modulation thereof. This output pulse energy It which we call deblocking pulses is shown by curve 60. Retardation of the pulse energy is shown to be an amount it; thereby aligning the deblocking pulses lb, as shown in broken line, with the pulses of channel 4. Curve 6d represents the mixer action whereby the deblocking pulse Ib lifts the pulses of channel 4 above the threshold level I21 of the mixer 526. Thus, the mixer passes only pulses of channel 4 to the demodulator I30. The demodulator 530 may be, in the case of P. T. M. modulation, of the character disclosed at I30a.in Fig. 5A, whereby the pulse displacement is translated into amplitude modulated pulses and applied to filter I32 where the envelope energy of the pulses is obtained for application to the speaker 34.

The receiver circuit of Fig. 5A is a modification of the receiver of Fig. 5, adapted for synchronization by a synchronizing signal of the character employed by the transmitter circuit of Fig. 2A. The video output of thedetector 520 is fed over line I40 to a mixer tube I42. At the same time, deblockingpulses from a pulse generator I44 are fed over line I46 to a second grid of the tube I42. The pulse generator I44 is controlled by the control wave produced by generator I43 whose period of operation is slightly greater than the period T of base Wave II! employed at the transmitter (see wave 4a, Fig. 4, and wave 6b, Fig. 6). These deblocking pulses serve to produce in conjunction with the incomiIlg pulses timed to add thereto an ouput series of pulses from tube I42. Assume, for example, that pulses of channel I of curve 6a are applied over line I40, the deblocking oscillator willoperate to produce a plurality of rectangular pulses such as lb of curve 60 timed in spaced relation according to the pulses of channel i. Tube I42 is also biased in the proper manner so that only the boosted pulses of channel I appear in the output thereof, A. D. C. restorer rectifier tube I50 may beprovided across the input from deblocking oscillator I44 to eliminate the effect of variation of the peak value of the incoming deblocking pulses and thus provide a proper level to work with the mixer circuit of tube I42 to leave only the desired channel pulses in the output of the tube I42.

The output pulses from tube I42 have the same cadence frequency T as the original modulated pulses produced at the transmitter, Fig. 2. The pulses of the plate circuit of tube I42 are applied through a phase reverser II to the control grid of the demodulator tube I54 and cause tube circuit I56 connected to the screen grid of the tube to oscillate at a desired frequency producing in the output circuit I51 a combined wave in the form of a combination of the wave generated in the circuit I56 and the incoming pulses from tube I42. The circuit I56 is preferably tuned to some harmonic of the cadence frequency as determined by the period T, curve 6b, so that as the pulses are displaced due to modulation signals the output pulses of tube I54 will be raised to different levels depending upon their time displacement. Accordingly, in the output of tube I 54 will appear a modulation envelope of pulses carrying signal modulations according to the amplitude measurements thereof. For a further understanding of the principles of this type of demodulator, reference may be had to the copending application of D. D. Grieg, Serial No. 459,959, filed September 28, 1942, now Patent No. 2,416,306.

A low-pass filter I60 is provided to by-pass the high frequency pulse components and pass on the modulation frequencies that define the signal envelope. The channel synchronizing signal is passed over filter I6I tuned to pass only the synchronizing frequency such as f1, Fig. 2A, which in turn serves to synchronize the operation of the control wave generator I43. Since the pulses of channel I are modulated with the synchronizing signal frequency ii, the generator I43 will be controlled thereby and the deblocking pulse output of generator I44, which may be of the cusper type the same as modulator 32a, Fig. 3, with suitable shaper, is timed to continuously deblock the pulses of channel I. In the absence of a synchronizing frequency signal on the pulses passed by the tube I42, no synchronizing pulse energy will be passed by the filter I6I and the control wave generator I43 will continue to operate at a frequency slightly different from the cadence frequency T. The wave output of generator I43 will thus continue to cause the channel pulses to be deblocked in rotation until the pulses of the channel carrying the synchronizing frequency signal ii are deblocked, whereupon the generator I43 is brought into synchronism.

For channel selecting, the deblocking pulses of generator I44 are applied to adjustable delay device I 45, whereby the pulses are retarded a selected amount such as t4, for example, for deblocking pulses of channel 4. The retarded pulse energy is applied to mixer I'I0 together with channel pulses over connection I12. The channel pulses passed by the mixer may then be applied to a reproducing circuit such as indicated at I30 (which may be of the type shown at I 30a, Fig. 5A), I32 and I34 of Fig. 5. Since the control generator I43 is synchronized to the transmitter, alternatively for demodulation, the output of generator I43 shown in Fig. 5A may be applied to a harmonic generator I80 the output of which is suitably phased at I82 and applied to mixer I84 together with channel pulses from mixer Ill]. The channel pulses are thus translated into amplitude modulated pulses according to their occurrence on the harmonic wave as more particularly described in the aforementioned copending application, Serial No. 459,959.

In Fig. 7, a broadcasting system according to the principles of our invention is shown wherein the pulses are modulated according to frequency or recurrence rate, that is, the modulating signal causes a variation in the time interval between successive pulses according to the instantaneous value of the signal. This type of modulation we call P. F. M. (pulse frequency modulation). The system of Fig. 7 is along the same lines of the system shown in Fig 2. A base wave generator 35 is employed to control the timing of the pulses for the different channels. The timing between channels is controlled by a series of phase shifters such as phase shifter 36 for channel 2. Channel I, in this embodiment, is not provided with a modulator since the pulses of this channel are used as synchronizing pulses only. The base wave from generator 35 is translated directly into pulses of the desired width by a suitable wave-topulse translator 200. The base wave after having been shifted the desired amount by shifter 36 is applied to the FM modulator 202 for channel 2. The output of the modulator is applied to a translator 204 which may be similar to the translator 200 except that it is arranged to shape the pulses to a width different from the width of the synchronizing pulses. The additional channels of the system follow the arrangement of the channel 2. Thus, an output of channel pulses similar to those shown in curve 6a of Fig. 6 will be produced by the system of Fig. '7. Such a curve is shown for three channels, for example, by curve 8b, of Fig. 8.

For selective reception, a receiver of the character shown in Fig. 5 is employed, the demodulator in such case being an integrator circuit measuring the variation in average value of the pulses with modulation.

To illustrate further the fact that P. F. M. modulation may be applied to pulses in a multichannel system, the pulses of channel 2 in curve 8b are shown modulated according to the principles of P. F. M. modulation by a signal wave 220 shown by curve 8a, Fig. 8. The limits of pulse displacement according to the principles of P. F. M. modulation are selected so that the modulation will not interfere with the pulses of adjacent channels. The frequency modulation of the pulses caused by the signal 220 is indicated by the broken line positions of the pulses of channel 2. Assuming that a normal interval of if exists between the pulses of channel 2 in the absence of modulation, then as a positive signal is applied as shown at point 22I on wave 220, the pulse corresponding to such instantaneous signal value will 'be spaced from the preceding pulse by an interval t+At. Should the signal value increase a corresponding amount from point 22I to 222, the interval between the channel pulses occurring at these two points is t+2At. At the next point 223 of pulse occurrence on the curve the signal value is shown to be the same as at 222, therefore the interval between points 222 and 223 will also be t+2At. A decrease in the signal value to pulse recurrence point 224 will decrease the interval proportionately as indicated by the interval t-l-At. Since the next pulse occurrence point 225 is substantially at zero signal value, the interval between points 224 and 225 will be t representing zero potential for the signal. A swing of the signal in the negative direction causes the intervals between pulses to decrease as shown for the negative portion 220a of the signal wave. During this oscillation of the signal wave, it will be noted that the variation in pulse spacing causes the pulses to be displaced from normal positions throu hout the cycle until the wave returns to the zero center line 240. By controlling the limits of signal swings or by limiting the degree of FM modulation of P. the base wave at the transmitter, the pulses can be maintained within the allotted limits between pulses of adjacent channels in the train of pulses.

From the foregoing description, it is readily apparent that our invention provides maximum usage of the optimum transmitting location of each service area. Rather than a single station at each location, many stations or studios may now have equal advantage of such a location thereby resulting in improved and more uniform service to the public.

Furthermore, greater efficiency and lower cost operation results due to the elimination of the duplication of transmitter andantenna installation as required by the present existing system. The relative simplicity of the pulse method of modulation, aside from the question of multiplexing, allows further economy in operation. This is especially true at the very high frequencies where transmitting equipment such as magnetrons and Klystrons must be used.

An objection. might be raised relative to the excessive band width required for the pulse method of transmission. However, in actuality the band width necessary for equivalent transmission, is equal to, or even somewhat less than that required for existing FM broadcasting. As.- sume that a top audio frequency of. 14 or 15 kc./sec. is required for high fidelity transmission. Since the ratio of pulse frequency to the highest modulation frequency for the pulse system may be assumed of the order of four, the fundamental pulse frequency required is approximately 50 kc./sec. This allows a timing of 20 microseconds between the pulses of each channel. If it is further assumed, as a hypothetical example, that a total of ten channels are required, which is probably an adequate number for servicing a given broadcast area, a maximum timing of two microseconds is allowed between pulses of adjacent channels. This time interval must include the pulse width as well as the modulation displacement. In order to achieve a satisfactory signal-to-noise ratio, a pulse buildup time in the neighborhood of 0.25 microsecond would be required, which would necessitate a modulation band width of approximately 1T4 megacycles. For double sideband R. F. transmission the total required band width for the ten channels would thus be 2.8 megacycles.

In the case of frequency modulation, for the same top audio frequency mentioned previously, current practice is to utilize a modulation index of five with a modulation swing of :75 kc., requiring an R. band width of 0.25 megacycle per channel. For the total of ten channels plus the frequency guard channels, a total of approximately 3.0 megacycles band width is required. Thus it. is seen that the bandwidth necessary for both the pulse and FM systems of modulation for ten channels is of substantially the same order of magnitude.

There are further advantages to be mentioned for the pulse broadcast system. At the receiving location no R. F. tuning is required of the receiver since a common carrier frequency is used for all stations. A fixed carrier frequency tuned receiver may thus be used, and since station selection takes place after detection, a simpler and more economical receiving system may be utilized as hereinbefore described. Furthermore, there is no antenna problem arising from the reflection effects from nearby buildings because of the sin-' gle transmission point. Thus a simple fixed die. rective antenna can be used for receiving all stations.

An important advantage of the multiplex broadcast method is the simplicity of relaying. A na tionwide broadcast relay system linking the im: portant service areas may be built-up in which one series of stations operating at a common carrier frequency provides the relay requirements for the several different broadcasting stations or studios. In the case of PM, as many separate and duplicating relay networks would be required as While we have described above the principles of our invention in connection with specific apparatus and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on our invention and the scope of the accompanying claims.

We claim:

1. A broadcasting system comprising a transmitting antenna mounted at a vantage point to transmit energy over a designated area, means for producing a plurality of series of signal pulses, each series constituting a separate signal channel, means for interleaving all of said series of pulses, means for modulating the signal pulses of each channel according to the intelligence to be transmitted during their respective channels, means for modulating a carrier wave of a given frequency with the resultant pulse train, means for feeding the modulated carrier wave to the transmitting antenna for radiation of energy therefrom, a plurality of separate points for producing signals to provide the aforesaid intelligence modulation of the signal pulses; and a receiver comprising means for receiving signals of said given carrier freouency over a directive receiving antenna aligned to receive from said transmitting antenna, means for removing the carrier frequency from said received signals to reproduce the signal pulses, adjustable selecting means for separating the signal pulses of a desired channel, demodulating means for translating the modulation of the se-.

lected separated signal pulses into another form of modulation, and a utilization device responsive to said other form of modulation.

2. A broadcasting system according to claim 1 wherein characteristic synchronizing pulses separate the signal pulses into cyclically recurring groups, and means in the adjustable selecting, means of the receiver responsive to the synchro-- nizing pulses.

3. The method of broadcasting from a plurality of studios a plurality of signal channels over a single carrier frequency comprising the following steps: producing a plurality of groups of cyclically recurring signal pulses, each recurring pulse con REFERENCES CITED 7 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 913,521 Latour Feb. 23, 1909 1,877,561 Davis Sept. 13, 1932 1,887,237 Finch Nov. 8, 1932 Number 14 Name Date Coyle Sept. 26, 1933 Montani Apr. 7, 1936 Riggs July 21, 1936 Finch Oct. 20, 1936 Tidd Dec. 22, 1936 Luck July 13, 1937 Luck Apr. 5, 1938 Finch Mar. 8, 1938 Mertz Jan. 2, 1940 Bumstead July 16, 1940 Beatty Sept. 16, 1941 Gray Oct. 7, 1941 Deloraine et a1 Nov. 18, 1941 Heising Feb. 17, 1942 Henroteau Mar. 3, 1942 Kell Apr. 21, 1942 Goldsmith May 5, 1942 Blumlein Feb. 16, 1943 Grieg Apr. 1, 1947 Chatterjea et a1. Oct. 28, 1947 Deloraine et a1. Oct. 28, 1947 Deloraine et a1 Apr. 6, 1948

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