EP0562896A1 - Transmission control method and system for a single-frequency and/or FM transmitter network with an overlapping area - Google Patents

Abstract

The invention relates to a method for controlling the transmission of a network of single-frequency and/or FM transmitters in which at least two transmitters can interfere with each other in an overlap region, characterised in that the said two transmitters, fed by at least one satellite, have an asymmetric transmission pattern chosen such that, in a region (R) where the transmitted fields are equal, their modulations are substantially in phase. <IMAGE>

Description

The invention relates to a method and a system for controlling a network of transmitters working, in frequency modulation and / or at a single final transmission frequency (single frequency network).

The coverage of a territory by the same program in particular in frequency modulation, for example a radio program, requires several frequencies distributed between the various transmitters to allow an overlap between their emission diagrams in order to cover all the surface to be served without Gray areas. This situation is particularly detrimental for listening to a program from a mobile receiver since it is necessary to regularly adapt the frequency of the receiver according to the frequency of the nearest transmitter. Furthermore, such a solution consumes frequencies in an already highly saturated band. It is therefore useful to design a network of transmitters, in particular in frequency modulation working at the same modulation frequency. In the interference zones between the transmitters, it is desirable for such a network of transmitters that there is phase concordance for the modulating signals and / or that the HF high frequency interference is attenuated when two transmitters are received with similar field levels.

The invention proposes to control, possibly in synchronism, terrestrial transmitters forming part of a single frequency network and / or FM, in particular broadcasting, and supplied from a transmission medium or by satellite and which use analog transmission. In this type of link, a monophonic LF program, generated from a network head station, can frequency-modulate a subcarrier according to the same characteristics as the final broadcast before being inserted for example in an existing analog video multiplex such as than the one used by "Télédiffusion de France" known as "MT4". The analog video multiplex "MT4" comprises a video channel up to 6 MHz and several subcarriers, for example four subcarriers, (7.5; 8.5; 9.2 and 10 MHz), one or two are available for transporting the BF program.

The broadcasting of programs in frequency modulation clearly distinguishes two functions: the studio-transmitter transmission which ends its mission by delivering the signals in baseband and the broadcasting proper which, in each transmitter, and starting from the basic signals in low LF frequency, composes the multiplexes, frequency module (FM) and amplifies the high frequency HF signals.

The subject of the invention is a method allowing phase concordance for the modulating signals of the message of a single frequency network.

For this purpose, a phasing of the low frequency modulating signals in overlap zones between transmitters, supplied by at least one satellite is carried out, according to the invention, with asymmetric emission diagrams so as to choose zones of interference where their modulations are naturally in phase to avoid the contribution of delay lines.

The transmitters can be available on the same terrestrial meridian.

According to a preferred embodiment, said asymmetric emission diagram is such that the phasing takes into account the difference in times of arrival at the transmitters of the signal transmitted by said satellite, and the propagation time of the signals from each transmitter. towards said area.

The phenomena of high frequency interference between two transmitters having a zone of overlap of their emissions can be notably attenuated by implementing an amplitude modulation of the signals transmitted by at least two transmitters, by an amplitude modulation signal whose frequency is high compared to the maximum frequency of the modulating signals of the message transmitted.

According to an advantageous variant, the method implements an amplitude modulation of the signals transmitted by at least one out of two transmitters having an overlap zone, by an amplitude modulation signal whose frequency is high relative to the maximum frequency modulating signals of the message sent. This reduces high frequency interference (HF). Amplitude modulation has a modulation rate advantageously less than 30% and preferably between 10 and 20%.

The invention also relates to a method for controlling a network of transmitters working in particular in frequency modulation with a single final transmission frequency and comprising a head-end transmitter for transmitting an analog multiplex including at least a first sub- carrier modulated by a low frequency LF program and transmitters at the final transmission frequency for the implementation of the above method characterized in that each transmitter is arranged to receive an unmodulated transposition frequency, of frequency lower than the final frequency of transmission and to transpose without demodulating it, the first subcarrier using the transposition frequency, to the final transmission frequency.

According to this advantageous variant of the invention, in order to produce a network of possibly synchronous single frequency transmitters, the subcarrier present in the analog video multiplex is not demodulated but it is directly transposed to the final transmission frequency of the transmitter corresponding (in the case of a plurality of frequencies) or to the final transmission frequency of the network (in the case of a single frequency network) by using (possibly after multiplication) a transposition frequency which, in the second case, is common to all transmitters and which can accompany a modulated subcarrier in a multiplex including video or which can be transmitted to transmitters through any transmission medium, for example a subcarrier taken on a video channel of a satellite or which can possibly be produced by a local oscillator of good stability, in particular for some of the transmitters, of end of rese at.

To control a synchronous monofrequency network, the same transposition frequency must be available. Each transmitter of the network must for this purpose have a signal of the same frequency F₂ which we call "beacon".

The "beacon" can thus be transmitted to "senders on a channel parallel to the program channel. In the case of satellite or terrestrial hertzian links, it can take the form in analogical multiplexes of an additional subcarrier or a free subcarrier. It can also use a different route, the most advantageous case being as mentioned above, the distribution of this beacon by satellite. Note that this beacon can have other profitable uses such as stabilization of in the television and radio transmitters and rebroadcasters.

More specifically, according to the invention, a method for controlling a network of transmitters working, in frequency modulation, at a single final transmission frequency is such that the transmitters are supplied by an analog head-end signal, in particular a multiplex analog, for example video, including at least a first subcarrier modulated by an FB program consists in using a reference signal, in particular a second unmodulated subcarrier, and of frequency lower than the transposition frequency, multiplied in each transmitter, to transpose the subcarrier modulated by the LF program to the final transmission frequency.

• 1 - Une atténuation des interférences HF lorsque deux émetteurs sont reçus avec des niveaux de champ semblables.
• 2 - Une concordance des phases pour les signaux modulants en zones de recoupement.
• 3 - Une fréquence d'émission rigoureusement identique pour les les émetteurs du réseau.

According to its preferred embodiment, the invention makes it possible to simultaneously satisfy the three conditions which have hitherto excluded the implementation of a FM single frequency network.

• 1 - Attenuation of HF interference when two transmitters are received with similar field levels.
• 2 - A phase concordance for the modulating signals in overlapping zones.
• 3 - A strictly identical transmission frequency for the transmitters on the network.

The preferred embodiment of the invention makes it possible to simultaneously fulfill these three conditions in networks where the transmitters are supplied by HF links with analog transmission (terrestrial radio-relay systems, satellite or any other HF link modulated in FM).

• la figure 1 représente l'entrée du multiplex de tête de réseau et un émetteur d'un réseau monofréquence selon un mode de réalisation préféré de l'invention.
• la figure 2 illustre le phénomène d'interférence HF entre trois émetteurs monofréquence diposés en triangle.
• les figures 3a et 3b représentent un mode préféré de réalisation de mise en phase en basse fréquence sans ligne à retard.

Other characteristics and advantages of the invention will appear better on reading the following description in conjunction with the drawings below:

• FIG. 1 represents the input of the headend multiplex and a transmitter of a single frequency network according to a preferred embodiment of the invention.
• FIG. 2 illustrates the phenomenon of HF interference between three single frequency transmitters arranged in a triangle.
• Figures 3a and 3b show a preferred embodiment of phasing in low frequency without delay line.

Referring to FIG. 1, a composite signal of the analog video multiplex type (for example transmission multiplex MT4.E) including, in addition to a video signal VI (video channel VVI), modulated subcarriers (BF1 ... BF4) in frequency (channels V₁ to V₄) by a LF program (or by a stereo composite signal) and an unmodulated subcarrier EF₂ (channel BO) of frequency F₂ constitutes a headend 10 by a terrestrial network (radio beam) or by satellite to a plurality of transmitters.

• 1) Le signal BF2 devient le programme complet et module F₀ à 8,5 Mhz dans les mêmes conditions que l'émission finale (excursion + ou - 75 KHz).
• 2) La fréquence F₂ dite de "balise" (voir plus loin), produite dans un générateur extérieur, est introduite sur le BUS de sortie du multiplex émission.
• 3) La démodulation sur la voie de réception à 8,5 Mhz est supprimée de manière à sortir à la fréquence F₀ un signal SF₀ encore modulé.
• 4) Une voie de réception supplémentaire, calée sur la fréquence F₂ de la balise est ajoutée et elle n'est pas non plus démodulée.

By way of example, FIG. 1 describes a mode of transmission of the two frequencies (F₀, and F₂) necessary for the system by an image / sound multiplex known under the name "MT 4" of Télédiffusion de France. This type of multiplex is transmitted in a radio channel from 0 to 10 Mhz using the 0 to 6 Mhz band for the image and is completed by four subcarriers at (7.5 - 8.5 - 9.2 and 10 Mhz) frequency modulated by four low frequency LF sound channels. Since the bandwidth is not suddenly cut, an additional frequency around 11 MHz can be introduced at a low level so as not to disturb the balance of the multiplex (transmission of the frequency F₂). With the existing multiplex, the invention proposes four modifications of the standard multiplex "MT 4" to adapt it to the needs of the system.

• 1) The signal BF2 becomes the complete program and module F₀ at 8.5 Mhz under the same conditions as the final emission (excursion + or - 75 KHz).
• 2) The frequency F₂ called "beacon" (see below), produced in an external generator, is introduced on the output BUS of the transmission multiplex.
• 3) The demodulation on the 8.5 MHz reception channel is suppressed so as to output at frequency F₀ a signal SF₀ still modulated.
• 4) An additional reception channel, set on the frequency F₂ of the beacon is added and it is not demodulated either.

In FIG. 1, a power transmitter (11 ... 50) of the transmission network receives from the network head 10 a composite signal SC constituted by an analog video multiplex (for example reception multiplex MT 4.R) comprising in addition the video signal SVI (“VID” channel), five subcarriers SBF₁, SF₀, SBF₃, SBF₄ and SF₂ corresponding respectively to the transmission channels V₁ to V₄ and B₀. Two of these subcarriers (F₀ and F₂) are used for network transmitters. The composite signal SC is therefore demultiplexed by input circuits 11 of the receiver 12 to selectively obtain, thanks to a circuit comprising a filter 2 and an intermediate frequency circuit FIV₂, the subcarrier SF₀ (of frequency F₀ for example 8.5 MHz ) frequency modulated by the program BF and thanks to a circuit comprising a filter 5 and an intermediate frequency circuit FIF₂, a signal SF₂ of frequency F₂, for example an unmodulated subcarrier. For example, the frequency F₁ is 8.5 MHz and the frequency F₂ is 10.77 MHz. Other circuits (1, FIV₁, CBF₁), (3, FIV₃, CBF₃), (4, FIV₄, CBF₄) and VID allow to obtain low frequency signals respectively SBF₁, SBF₃, SBF₄, as well as a video signal Please. Circuits 1, 3 and 4 are filters at the respective frequencies of the subcarriers, the circuits FIV₁, FIV₃ and FIV₄ are intermediate frequency circuits, and the circuits CBF₁, CBF₃ and CBF₄ are circuits for generating LF signals. The two signals F₁ and F₂ feed a transposer TRA comprising a frequency multiplier 20 to produce a transposition signal FT, a signal mixer 30 and a filter 40 allowing signals to pass through at the transmission frequency (for example 105.5 MHz) which can be the final frequency sought in frequency modulation of the network. The signal SF₀ is sent with the transposition signal SFT whose frequency is approximately 97 MHz (in the aforementioned example after multiplication by 9 in the multiplier 20) to the inputs of the mixer 30 to be transposed to the final emission frequency FX in 105.5 MHz frequency modulation (F₀ + FT = Fx). A bandpass filter 40 calibrated on the transmission frequency Fx is implemented to attenuate the undesirable products created in the mixer 30. The signal SFx thus obtained can enter directly into the amplifier 50 of the transmitter, or pass through a amplitude modulator MOD in the case where this function is implemented to attenuate interference dips (see below). The signal supplied at the output of the mixer 30 is filtered by the filter 40 before being applied to the high frequency amplifier HFA referenced 50 which amplifies it to the desired transmission power. In this way, the final frequency of the transmitters is stabilized from two frequencies, coming exclusively from the head of the network. Note, however, that the frequency instability of the broadcast signal is not very harmful, since it affects all transmitters in the same way.

Of course, it is possible to implement in the multiplex a frequency different from the standard frequencies.

The transposition signal SFT can also be generated by a local oscillator 10 of good stability referenced 25 in particular at the end of the network if the frequency of the beacon is not available.

In particular in the case of overlap zones between the transmitters, the phasing of the low frequency signals in the overlapping zones of the transmitters, phasing making it possible to avoid distortions of the message, is carried out by an optional delay line DL referenced 15 acting on the analog program signal SF₀, the delay being adjusted as a function of the relative geographic position of the transmitters.

For example, two transmitters 50 km apart, powered by a satellite serving as the head of the network and located on the same longitude. The satellite-transmitter route is approximately 40 km shorter for the one located the most southerly, hence a travel time difference equal to 130 µs to be compensated for by delay line.

The signal SF₀ of frequency F₀ equal to 8.5 MHz in the example, if necessary crosses an analog delay line DL referenced 15, calculated as a function of the differences in the path of the signal SF₀ on the transmission channels upstream of two neighboring transmitters . To perform this operation, the signal SF₀ to be delayed can for the circumstance be converted to digital and then returned to analog after processing. To use conventional digital / analog converters, the frequency F₀ can be lowered to 1 MHz and then reset to F₀ provided that the same transposition generator (known per se) is used at the input and at the output of the operation.

The signal SF₂ of frequency F₂ (10.77 MHz in the aforementioned example) passes through a multiplier X referenced 20 to obtain the signal SFT at the transposition frequency FT, here of value F₂ X 9 = 97 MHz.

The delayed signal SF₀ and the signal SFT combined in the mixer 30 give at output a signal SFx of frequency Fx, here 105.5 MHz. Note that current technology allows a multiplier 20 to multiply by any value.

The phenomena of high frequency interference between two neighboring transmitters of any network of single frequency transmitters can be attenuated by amplitude modulation, at a low rate, the signal carrier, which, in the example described, is supplied at the output of the filter. 40, by any signal of high frequency relative to the modulating message signals, for example 100 KHz. A frequency equal to at least twice the maximum frequency of the modulating message signals is then chosen (38 kHz for a stereo broadcast).

Are, in fact, two transmitters A and B synchronized on the frequency 100 MHz and distant of 50 km. At a point C distant 25 km from A and B, the fields received are theoretically equal and of the same phase. In this case, every 3 meters (wavelength) of 100 MHz corresponding to band 2 transmissions), the fields add up to double the field received from a single transmitter ("belly"), and all three meters interspersed, they retreat ("knots") to cancel the received field. By getting closer to one transmitter, the effect is attenuated because the emission of this transmitter gradually wins out over that of the other.

For a fixed station reception, the choice of the location of the receiver can resolve the anomaly, but it is not the same for a mobile. For example, for a vehicle traveling from A to B, or vice versa, the program is hatched at a rate proportional to its speed (for example 10 Hz for a speed of 120 km / h).

This phenomenon, very annoying for mobile receivers, is in practice attenuated by "ground waves" and multiple reflections of the received signals. However, if superabundant transmission powers are not used, the dips, even attenuated, remain and may be lower than the sensitivity of the receivers.

By modulating in amplitude (AM modulator referenced 35) at a low adjustable rate (10 to 30%) by any high frequency FC signal (for example inaudible modulating signal of the order of 100-120 kHz) compared to the modulating signals of the message, cancellations or excessively large reception differences between two transmitters are avoided. The signal FC is produced by an oscillator 0SC referenced 36. Two lateral bands are thus induced at the limit of the high frequency spectrum.

The compensation rate can be determined in the field according to the terrain data and the requirements of the broadcaster. The compensation must be all the stronger as the troughs are pronounced.

In the example above, and 'in the vicinity of point C (where the effect is maximum), the theoretical difference between the maximum field and the minimum field is 21 dB for a modulation rate of 10% and 16 dB for a modulation rate of 20%. These differences are likely to be at least partially made up by the automatic gain control circuits of the receivers.

According to an advantageous variant, it is also possible to use the modulation of the weakly amplitude-modulated signal to broadcast narrowband messages.

The amplitude modulator comprises (see fig. 1) an OSC oscillator referenced 36, for delivering a frequency of the order of 100 kHz. This frequency makes it possible to modulate the signal SFx in the amplitude modulator AM referenced 35 with an adjustable modulation rate between 0 and 20%. The output signal from the modulator 35 passes through a filter 37 also centered on the transmission frequency Fx to supply the amplifier 50 with a signal S'Fx conforming to FM broadcasting standards.

The amplifier which receives the SFx signal with a power of the order of 1 Watt brings this power to the value necessary for the transmitting antenna A.

The development of the entire network requires a detailed study to determine the locations of the transmitters, their powers, the value of the delay lines or their suppression, and the AM modulation rate when necessary.

The distance between transmitters thus depends on the bandwidth of the program to be transmitted. The power of the transmitters depends on the distances thus established. An example is given in Figure 2. A group of three transmitters A, B and C placed at the vertices of an equilateral triangle (elementary set to cover a surface), 50 km from each other broadcasts a monophonic program (frequency signal maximum 15 kHz). Close to any of the transmitters A, B or C, reception is ensured by the near dominant transmitter. In the center O of the triangle and on the perpendicular bisectors there is HF interference and BF phase matching. On the other hand, on a line x - x 'distant in X by 20 Km from transmitter A and therefore by 30 Km from transmitter B, the difference in path BX - AX = 10 Km. The wavelength of a signal of 15 kHz is 20 km and for a signal of 7.5 kHz it is 40 km. This results in phase opposition for the frequency of 15 kHz therefore a significant attenuation for this frequency and a loss of 3 decibels for the frequency of 7.5 kHz due to the quadrature for this signal. Consequently, depending on the quality to be obtained on the amplitude / frequency response, the maximum distances between transmitters can be defined. Note that the program undergoes in the most unfavorable sites a loss of high frequencies with conservation of the essential of the message. The intervention of the third emitter of the basic triangle has the effect of attenuating the HF interference at the center O of the triangle. Similarly, on line x - x 'the 3rd transmitter improves the level differences as soon as we leave point x located on the axis connecting transmitters A and B.

The calculation of a section using delay lines, appears as an example in Figure 3a, which represents a distribution by satellite in the most unfavorable case where the two transmitters A 'and B' of the network are 50 km apart due exclusively to a deviation in latitude. In this case, the distance traveled to reach a point located 50 km north of the transmitter A 'is approximately 40 km greater. Therefore, to put these signals back in phase at the input of A 'and B', the components SF₀ at the input of the transmitter A 'must be delayed by 130 microseconds. Note that for two transmitters located at the same latitude the delay becomes unnecessary, this is the most favorable case.

According to the invention, the use of delay lines, which constitute an expensive element of the assembly, is avoided, thanks to the location of the transmitters and to the shape of the radiation diagrams. This solution will now be described in connection with Figures 3a and 3b.

Are transmitters A '- B' - C ', located every 50 km on the same meridian and supplied by a geostationary satellite SAT facing south. The invention proposes to establish for each transmitter radiation patterns similar to that of FIG. 3b. This diagram is practically that of a panel antenna for which the front / rear difference is approximately 20 dBs. The axis of fire of the antenna being directed to the North, that is to say opposite to the satellite, it follows that the critical area of overlap where the fields are equal (point R) is 45 km in front of the transmitter A 'and 5 km behind the transmitter B'. At point R, the signal BF starting from the transmitter A 'covers 45 km. Starting from a point H in phase with A', it travels HB '+ B'R = 45 km also. The delay line is then unnecessary. As soon as one departs from this critical zone, one of the transmitters A 'or B' becomes preponderant.

The production of transmitters with desired emission diagrams can be carried out by referring to the work by S. Lacharnay "Antennes pour la Radiodiffusion" Document 1.83 published by "Editions Radiodiffusion Télévision".

The construction of a single frequency network can use a mixture of the two LF phasing proposals (with or without delay line for a given transmitter). This solution makes it possible to establish breaks in the totalization of delays on cascaded transmitters.

Claims (6)

Method for controlling the transmission of a network of single frequency and / or frequency modulation transmitters in which at least two transmitters are capable of interfering with one another in an overlap zone, characterized in that said two transmitters are supplied by at least one satellite and have an asymmetric emission diagram chosen so that, in an area (R) where the fields emitted are equal, their modulations are substantially in phase. Method according to claim 1, characterized in that said transmitters are arranged on the same meridian. Method according to either of Claims 1 and 2, characterized in that the said asymmetrical emission diagram is such that the phasing takes into account the difference in arrival times at the transmitters of the signal transmitted by the said satellite, and the time of propagation of signals from each transmitter to said zone (R). Method according to one of claims 1 or 3, characterized in that it implements an amplitude modulation of the signals transmitted by at least one transmitter out of two (12) having an overlap zone, by an amplitude modulation signal whose frequency (FC) is high compared to the maximum frequency of the modulating signals of the message transmitted. Method according to claim 4, characterized in that the amplitude modulation has a modulation rate of less than 30% and preferably between 10 and 20%. Method according to one of the preceding claims, characterized in that each transmitter is supplied with a head-end signal including at least a first subcarrier modulated by a low frequency program (LF) and in that each transmitter (12) uses a reference signal (SF₂) of frequency lower than the single final emission frequency (FX) to transpose, without demodulating, the first subcarrier (SF₀) modulated by the program (BF) to said single final frequency d (FX).

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