"APPARATUS FOR THE TRANSMISSION OF RADIOWAVES IN

ISOFREQUENCY"

The present invention relates to an apparatus for the transmission of radio waves, particularly by FM equipments, serving the purpose of allowing a wide coverage and to broadcast a signal correctly receivable using only one broadcast frequency.

It is common knowledge that to obtain an adequate radio broadcasting coverage, particularly in FM, one often recurs to multiple broadcasting frequencies shared among various transmission equipments on the territory, in such a way not to originate noise in transmissions and to avoid blind areas where no signal reaches in. However this involves a series of both technical and economical complexities, due to the necessity to use multiple frequencies in order to give continuity to the signal.

Therefore transmission systems in isofrequency have been developed, that allow the transmission of radio waves in frequency modulation (FM) by means of equipments geographically far away from each other but that share the same frequency of transmission. These systems require, however, a special attention in order to avoid the destructive consequences of the broadcasted waves. It is herein referred particularly to those radio broadcasting stations that, having decided to serve with the best possible coverage a particular geographical area using a plurality of transmitters at the same frequency, run into the so called "overlapping" phenomenon, that is the accumulation of the waves in a determined overlapping area, said phenomenon tending to destroy the result of the "addition" of transmission and yielding an inaudible programme. It is common knowledge, in fact, that the various transmitters broadcast the same signal with time delays different from one another, therefore on the overlapping areas, distortion phenomena of the received signal are perceived.

In order to solve this problem, many isofrequency synchronization systems and equipments have been proposed, that, starting from empirical evaluations stating that delays exceeding a set value of 5 μs are unacceptable, make use of delay managing devices for signals coming from contiguous transmitting aerials, in such a way to make the signal coherent in the overlapping areas. Systems of this kind are described in EP0291676B1 , EP0370915B1 and EP0575246B1.

Another system to make isofrequency broadcasts audible in particularly difficult areas consists of using particular devices called "gap fillers". It deals with low power signal repeaters able to receive and transmit on the same channel, thus not occupying different

or additional frequencies with respect to the main transmission channel. The disadvantage of gap fillers is that of heavily affecting operating costs, since they are as a matter of fact additional broadcasting installations, that have to be made compliant to radiation, installation, and building standards, as much as a bigger installation.

Further systems of transmission of radio frequencies in isofrequency in overlapping areas are described in application EP0562896A1 , describing a system for a satellite signal broadcasting of isofrequencial units, and in US6011977, relative to a system for channel delay riequalization for isofrequency working repeaters making use of a GPS, Global Positioning System.

The main object of this invention is therefore to allow not only generation of perfectly isofrequential transmission waves, but also to perform the calculation process carried out on the modulation in such a way to make the overlapping areas perfectly isomodulated, making this possible by means of using synchronization apparata disciplined, for example, by Global Positioning Systems.

The experimental stage with two transmitting units, suitably positioned in such a way to create an overlapping area where analysis of transmission quality is performed, has yielded excellent results both in terms of listening and in terms of measurements. The isochrony of the carrier used and perfect synchrony in the time domain of modulation has granted to the overlapping area a level of audio quality absolutely comparable to that obtained with a single transmitting unit, even with delays above 5 μs. The empiric results confirm the analytical evaluation and numerically, via mathematical simulations, proving that the 5 μs limit can be extensively overcome.

The present invention will now be described in deeper detail with reference to a particular exemplary and non limiting embodiment, and annexed drawings, in which:

Fig. 1 is a block diagram of the installation with the apparatus, called after its project name "ISOCAST", carrying out the isofrequency radio waves transmission according to the present invention; and

Fig. 2 and 3 represent two graphic curves indicating distortion, expressed in dB, as the function of the protection ratio.

The basic principle involved in this invention is that two equipments which are isochronous and isomodular working in the same time-frequency domain are suitable to supply an optimal listening of the broadcasting on overlapping areas.

The theoretical condition is thus respected if one ore more units equipped with this device object of the invention broadcast a signal in FM on geographically adjacent areas.

The perfect synchronization both in terms of carrier and modulation broadcast allows to ease-off protection parameters so that the equipments will not disturb each other or will disturb each other in a limited way.

Components, or blocks, that constitute the device according to the invention are illustrated in the block diagram of Figure 1 herein attached. In this figure, A is the device serving to generate in an isochronous way frequencies required to the operation of the entire apparatus, device A being fit to reconstruct a time-frequency signal that can be spread on the territory. In the illustrated embodiment, the synchronization device A includes a GPS receiver, with relative aerial, that manages the frequency generators, checking the output frequency beside distributing PPS (pulse per second) signal for the synchronization of the modulation processes. The different generators are carried out using voltage controlled oscillators, like quartz oscillators (OCXO and/or VCXO), suitable to grant optimum stability to temperature variations. The frequency values generated vary according to the kind of device serving the apparatus. For example, in Figure 1 some of the possible frequencies have been pointed out, given the possibility to choose according to efficiency and/or cost.

For example, for the prototype used in the experimentation, the synchronization device A broadcasts a signal, coming from an analogical bridge at 381 ,8 MHz, suitably amplified, filtered and then converted to intermediate frequency (IF) of 10,7 MHz. The conversion is carried out via a mixer E, described herein, using a voltage controlled oscillator (VCO) at 371 ,1 MHz linked to the reference at 10 MHz via a numerical PLL.

Letter B indicates a device used to receive the audio feed signal. It can be supplied to the device object of the invention by means of a radio bridge, by satellite or by a numerical or analogical telephonic net section. The choice, made during the installation phase, is based on purely systemistic grounds, that is, suggested by how the user has configured the net. The main aspect of receiver B lies in the coherent demodulation of the signal received, fit to avoid frequency offset phenomena, rather than phase rotation. Its output can be an intermediate frequency or base band depending on the RDS (radio data system) application situated at the downstream end of the apparatus, named F and subsequently described. The configuration at intermediate frequency recovers RDS information from the input signal and outputs them with no modification. The base band configuration allows, instead, the insertion of a local RDS packet by means of subsequent block F, hereinafter described.

The synchronous digital demodulator C serves to the digital demodulation of the intermediate frequency, allowing digital buffering and the creation of digital delay. This is the heart of the device according to the invention, since it allows to synchronize the modulation on the territory by means of an elastic memory. This memory, which in this embodiment uses the PPS synchronization signal, allows to assign a delay on the modulation necessary to limit interference phenomena on the overlapping areas. The transmitting units, indeed, will have to be set in such a way to have, on the overlapping areas, a relative delay in the modulations that is the least possible on the lowest protection geographical areas or with a power rate among the units nearest to 1. This condition is even more necessary as the powers of the two units tend to be similar.

In the prototype made, the synchronous digital demodulator C receives the signal at the intermediate frequency of 10,7 MHz at a level of 0 dBm, converting it to a digital stream by undersampling. The operation of numerical sampling is performed in a synchronous way as for both the starting moment, using the PPS signal, and as frequency, using the reference at 32,768 MHz.

The numerical stream obtained is transferred to the subsequent digital synchronous modulator D by means of an external bus. The modulator D, in fact, serves the coherent modulation of the signal after the demodulation carried out by the preceding device C. The process, inverse in respect to the preceding one, allows to get back the signal modulated before the traslation to the final frequency. The modulator D is able to obtain the modulated signal using either the intermediate frequency coming from the block C or from the base band.

This digital synchronous modulator serves to introduce the net section delay. This experimentation has used a delay that can be set from 0 μs to a maximum of 50 ms, with a minimum step of 2 μs. The setting of this value is obtained simply keystroking the front panel of modulator D, which processes the stream in a synchronous way using the reference at 32,769 MHz and PPS. Particularly for the regeneration of the analogical signal at intermediate frequency 10,7 MHz ₁ a direct synthesis modulator is used.

Using a direct synthesis modulator warrants the numerical precision both of the carrier signal (10,7 MHz) and of the modulator signal in FM.

Downstream of modulator D a mixer E is present, deputed to the coherent transposition of the signal modulated at the channel frequency to broadcast. Even in mixer E one specifically uses frequencies generated in an isochronous way, therefore extremely stable and precise. In this embodiment, the mixer E uses a reference frequency at 84,4

MHz, coming from apparatus A and necessary to grant the precision and stability of the signal in FM obtained.

The apparatus of the invention, besides, is equipped with an RDS synchronous synthesizer named F, deputed to the generation of the RDS data packet. This synthesizer F is timed by the PPS signal for the start frame, besides having a pilot frequency at 57 KHz and the symbol frequency at 1187,5 Hz, them too managed by and referred to the GPS signal.

The apparatus of the invention is completed by the device G, deputed to the numerical conversion of the audio signal in base band. This sampling uses traditional converters at an elevated daterate. Both the sampling frequency and the sampling moment are managed by the GPS signal or other time-frequency synchronization systems.

As can be viewed by the description of the various components of the device "ISOCAST", the whole process of data acquisition is completely synchronous. This condition, theoretically demonstrated, allows the exact replica of an FM signal even if generated at different sites. During lab test phase, "ISOCAST" devices have yielded an output signal in radiofrequency identical (it matches exactly), such to allow the perfect isomodulation and isosynchrony which lie at the base of physical-mathematical principle previously stated.

The previous description shows that the device for the transmission of the radio waves in isofrequency fulfils the intended objects of the present invention and embodies a considerable technical progress, thanks also to the widespread diffusion of digital treatment of signals capable of granting considerable precision. Of relevant importance is the possibility to regenerate extremely time stable frequencies, besides having convenient and economical absolute time reference, by means of the GPS.

A considerable characteristic of the apparatus of the invention is also the possibility to function with a delay in the broadcast signals that can be set between 0 μs to a maximum of 50 ms, a value considered excessive, up to now, for a correct audibility of a radio broadcast transmitted by an isochronous and isomodulated installation. Diagrams at Figures 2 and 3, in fact, widely show how the 5 μs parameter is too restrictive for listening to an acceptable radio signal.

The integration of the various functional blocks in a unique apparatus allows the introduction of several advantages, too, both technical and economical, for various kinds of broadcasting.

It has to be pointed out, however, that an exhaustive description has been given of an exemplary and non limitative embodiment of the apparatus of this invention, and many modifications, variations, additions and/or substitutions of elements may be carried out, without falling out of the scope of the present invention, as defined by the annexed claims.