WO2012131735A1

WO2012131735A1 - Digital microphone radio transmission system

Info

Publication number: WO2012131735A1
Application number: PCT/IT2011/000098
Authority: WO
Inventors: Michelangelo CARROZZO
Original assignee: Aeb Industriale S.R.L.
Priority date: 2011-03-31
Filing date: 2011-03-31
Publication date: 2012-10-04

Abstract

A radio transmission system (1) of the type comprising a receiver (2) provided with a receiving antenna ( 3) and with at least one output port for connection with suitable apparatuses for broadcasting the respective output signal and a transmitter (4) associated with a specific microphone capsule (5) and provided with a respective transmitting antenna. The transmitter (4) comprises a digital modulator (6) according to the mode known as "4FSK", constituted by -means (7) for converting the serial digital signal to a multi- level signal, the serial digital signal, containing the aiidio and telemetry information, coming from a line pertaining to the microphone capsule (5), with the interposition of an amplifier (5a), an analog to digital converter and a data compression unit, -a constant group delay Gaussian baseband filter (8) for the multi -level signal, placed downstream of the conversion group (7), -an electronic potentiometer (9) for modulating and controlling the level of the output signal from the Gaussian filter group (8).

Description

RADIO TRANSMITTER APPARATUS

Technical Field

The present invention relates to a radio transmitter apparatus.

Background Art

Every radio transmitter apparatus is generally constituted by a transmitter and by a receiver which are combined with some accessory components. This complex apparatus can be used for occasions such as conferences, live music shows, theater performances, radio and television broadcasts and other applications where the use of wireless microphones with high-level audio performance is necessary.

At present, internationally, the frequencies assigned for the use of UHF radio microphones are generally comprised between 470 MHz and 870 MHz.

The band assigned for these purposes thus presents an overall coverage of around 400 MHz: the end user is required to use only the frequencies allowed by the legislative prescriptions in force in the specific country where the radio transmitter apparatus is used.

Radio transmitter apparatuses of the known type are specifically made by the respective producers according to the different frequency bands (or according to the country, seeing that for each country the dedicated bands for this service are known) for which they are intended.

Purely for the purposes of example, a model "w" could be made which covers frequencies comprised between 470 MHz and 570 MHz, a model "x" that covers frequencies comprised between 570 MHz and 670 MHz, a model "y" that covers frequencies comprised between 670 MHz and 770 MHz and, lastly, a model "z" that covers frequencies comprised between 770 MHz and 870 MHz.

A user who wishes to use radio microphones to take advantage of all of the band comprised between 470 MHz and 870 MHz (as is the case for example in Italy) has to order four different models from the producer, according to the band that applies to that country.

The necessity of covering the frequency band with a plurality of different products dedicated to it implies problems of a logistics nature (each type of product must necessarily be properly stored in an individual area of the storeroom with consequent problems of organization and encumbrance).

This implementation choice furthermore determines the need to make estimates of the applicability of each type of product as a function of the distribution and consumption thereof (some frequencies may be used in many countries while others may be less widespread).

In countries where there are no restrictive obligations on the selection of a specific selected range of frequencies within the general band indicated above, the user will find himself forced to purchase all the products for each specific frequency sub-interval in order to select the one, on a case by case basis, that is least subject to the radio interference present (for example the transmissions of digital television stations that occupy the same band).

Moreover, when the simultaneous presence of multiple radio transmitter apparatuses in a determined area is necessary, the user will be forced to resort to products that are dedicated to specific frequency sub- intervals.

Radio transmitter apparatuses of the known type (each of which therefore operates in a specific frequency sub-interval) are particularly subject to a form of interference known as "phase noise" which is produced by the oscillators necessary for tuning. Phase noise consists of undesired and unpredictable fluctuations which, inevitably, over time disturb the linearity of the trend of the phase of a sinusoidal signal generated by a real source. Disclosure of the Invention

The aim of the present invention is to solve the aforementioned problems, by providing a radio transmitter apparatus that is particularly versatile and suitable for operating, selectively or inclusively, on all the possible frequency sub-intervals of the frequency band assigned for the use of UHF radio microphones.

Within this aim, an object of the invention is to provide a radio transmitter apparatus that has low sensitivity to the type of disturbance known as "phase noise".

Another object of the invention is to provide a radio transmitter apparatus that has low sensitivity to interference from digital television transmissions.

A further object of the present invention is to provide a radio transmitter apparatus at low cost, that is easily and practically implemented and safely applied.

This aim and these objects are achieved by a radio transmitter apparatus of the type comprising a receiver provided with a receiving antenna and with at least one output port for connection with apparatuses for broadcasting the respective output signal and a transmitter associated with a specific microphone capsule and provided with a respective transmitting antenna, characterized in that said transmitter comprises a digital modulator according to the mode known as "4FSK", constituted by

- a group for converting the serial digital signal to a multi-level signal, said serial digital signal, containing the audio and telemetry information, coming from a line pertaining to said microphone capsule, with the interposition of an amplifier, an analog to digital converter and a data compression unit,

- a constant group delay Gaussian baseband filter for the multi-level signal, placed downstream of the conversion group,

- an electronic potentiometer for modulating and controlling the level of the output signal from said Gaussian filter group.

Brief description of the drawings

Further characteristics and advantages of the invention will become better apparent from the description of a preferred, but not exclusive, embodiment of the radio transmitter apparatus according to the invention, illustrated by way of non-limiting example in the accompanying drawings wherein:

Figure 1 is a schematic view of the basic components of the radio transmitter apparatus according to the invention;

Figure 2 is a functional block diagram of a transmitter of a radio transmitter apparatus according to the invention;

Figure 3 is a first part of the circuit diagram of a transmitter of a radio transmitter apparatus according to the invention;

Figure 4 is a second part of the circuit diagram of a transmitter of a radio transmitter apparatus according to the invention;

Figure 5 is the initial part of the functional block diagram of a receiver of a radio transmitter apparatus according to the invention;

Figure 6 is the final part of the functional block diagram of a receiver of a radio transmitter apparatus according to the invention;

Figure 7 is a first part of the circuit diagram of a receiver of a radio transmitter apparatus according to the invention;

Figure 8 is a second part of the circuit diagram of a receiver of a radio transmitter apparatus according to the invention;

Figure 9 shows the trend of the signals emitted by various transmitters in a radio transmitter apparatus according to the invention.

Ways of carrying out the invention

With reference to the figures, the reference numeral 1 generally designates a radio transmitter apparatus.

The radio transmitter apparatus 1 comprises a receiver 2 provided with a receiving antenna 3 and with at least one output port for connection with suitable apparatuses for broadcasting the respective output signal and a transmitter 4 associated with a specific microphone capsule 5 and provided with a respective transmitting antenna. _*

According to the invention, the transmitter 4 comprises a digital modulator 6 according to the mode known as "4FSK". This digital modulator 6 is constituted by a group 7 for converting the serial digital signal to a multi-level signal. Essentially, the conversion group 7 receives the serial digital signal as an input, containing the audio and telemetry information, coming from a line pertaining to the microphone capsule 5, with the interposition of an amplifier, an analog to digital converter and a data compression unit, and converts it to a suitable multilevel signal.

Specifically, the serial digital signal containing all the audio and telemetry information can undergo the conversion to a multi-level signal by adopting a conversion group 7 of the type of a block known as "R-2R 4FSK shaper", shown graphically for the purposes of example in the accompanying figures (see Figure 3). This conversion step makes it possible to transmit two bits per symbol, i.e. double the data, while maintaining the same base band space as specified by the radioelectric emission standards applied in Europe (ETSI EN 300 422-1_V1.3.2).

The digital modulator 6 also comprises a constant group delay Gaussian baseband filter 8 for the multi-level signal; this Gaussian filter group 8 is placed downstream of the conversion group 7.

Essentially, the multi-level serial data output by the conversion group 7 will then be filtered through a Gaussian filter group 8, which, for example, can be implemented according to the circuit diagram shown in Figure 3 wherein it is designated: "Constant group delay Gaussian baseband filter". This filter will give the composite signal its final form with regard to the transmission band, in compliance with the standards.

This embodiment makes it possible to transmit a large quantity of data compared to the more restrictive European ETSI EN 300 422-l_V1.3 standard.

The digital modulator 6 lastly comprises an electronic potentiometer 9 for modulating and controlling the level of the output signal from the Gaussian filter group 8. The electronic potentiometer 9 can be implemented according to the circuit diagram shown in Figure 3 wherein it is designated: "base band level modulation control electronic potentiometer".

A digital modulator 6 having the characteristics described above has the following advantages which are indispensable for the implementation of the transmitter 4 according to the invention:

-the possibility of implementation and production adopting a low number of commonly-found components;

-filtering of the base band in compliance with the ETSI bandwidth standards EN 300 422-l_V1.3. and subsequent revisions;

-maintenance of the filtering and modulation characteristics over all of the working band of the radio transmitter apparatus 1 , i.e. from 470 MHz to 870 MHz;

-low energy consumption, essential for a radio transmitter apparatus 1 in that it determines its autonomy, which must be at least 6 - 8 hours if powered by normal batteries;

-the possibility of not using modulators with digital to analog converters of the signal, which are normally contained within integrated circuits of the "Digital Signal Processor" type. Such digital to analog signal converters are expensive, consume a large amount of energy and exhibit considerable difficulty in filtering the base band in compliance with standards. The presence of these components would therefore appear to wholly compromise the performance levels and the functional characteristics of the transmitter 4 according to the invention.

According to a possible embodiment of undoubted practical and applicative interest, the transmitter 4 comprises an oscillator device 10 for generating the radio frequency signal.

This oscillator 10 is adapted to generate the carrier frequency of the transmitter 4, in an interval between 470 MHz and 870 MHz, modulated according to the mode known as "4FSK".

The oscillator 10 constitutes the radio frequency signal generator of the transmitted signal and has some basic characteristics and advantages that make it specifically adapted to application in a transmitter 4 according to the invention:

-coverage of all of the 470 MHz - 870 MHz frequency band with a single PLL synthesising oscillator block. It is important to note that normally (i.e. in transmitters of the known type) a plurality of them is needed, with corresponding circuital complications, increase in encumbrances and increase in energy consumption with a decrease in battery life;

-low phase noise compared to other PLL synthesising oscillators of such large bandwidth (400 MHz);

-high linearity of signal amplitude over all of the band; this characteristic makes it possible to avoid the use of feedback loops to compensate for the amplitude of the signal over all the band. These feedback loops would require numerous other circuit portions, thus compromising the encumbrance of the transmitting circuit and the energy consumption (which would be too high for the application).

The oscillator 10 can be implemented according to the circuit diagram shown in Figure 4 wherein it is designated: "470— 870 MHz ultralinear very low power PLL VCO".

Within this specific embodiment, attention is drawn to the fact that the transmitter 4 can positively comprise a filter of the type known as "π" 1 1 , of variable frequency, for eliminating the spurious harmonics present in the 4KFS-modulated radio frequency carrier signal.

This filtering is essential to remain in compliance with the European ETSI radio emission standards EN 300 422-l_V1.3 and subsequent revisions.

Indeed, the radio frequency carrier signal is normally affected by spurious harmonics which must necessarily be contained within precise limits according to the ETSI EN 300 422-l_V1.3 standard.

For this reason the carrier signal coming from the oscillator 10, possibly with the interposition of other components, must be provided to the filter 11 which will "clean" the radio frequency carrier signal of the higher- order spurious harmonics.

The filter 1 1 can be implemented according to the circuit diagram shown in Figure 4 wherein it is designated: "470— 870 MHz variable cut pigrec filter".

The transmitter 4 furthermore comprises a broadband antenna 12, integrated in the printed circuit, comprising the electronic components and their interconnection wiring.

The antenna 12 is adapted to the emission of frequency signals comprised between 470 MHz and 870 MHz: in particular it can transmit in all of the frequencies in this interval with the same intensity and without particular impedance matching losses.

The fundamental characteristics of the antenna 12 according to the invention are the reduced encumbrance and the constant standing wave ratio (SWR) and emission over all of the 470 MHz-870 MHz band.

The receiver 2 of the radio transmitter apparatus 1 according to the invention comprises at least one radio frequency tuner 13, constituting the terminal part of the receiver 2 itself, with low intermodulation and high dynamics.

The possibility of selecting the working frequency band produces a strong attenuation of the other frequencies present in the radio signal received, thus minimising the intermodulation disturbances.

The high dynamics ensures that the amplifying stages of the tuner 13 do not enter a state of saturation and/or intermodulation even when a plurality of transmitters 4 are arranged at close mutual distance.

The tuner 13, which is constituted by various parts or "sub-blocks", constitutes the radio frequency front-end of the receiver 2 and has the basic characteristic of being capable of being tuned automatically as a function of the preset frequency to be received; the principle of this function is described by the charts shown in Figure 9. The reception within the working band is tuned: thanks to this contrivance all the other frequencies outside of the received frequency will be attenuated with the result that no intermodulation disturbances will be introduced by other transmitters 4 present in the signal range, or external disturbances. This characteristic makes it possible to simultaneously use a high number of transmitters 4 (or transmitters of other types) without the one disturbing the other due to any products of intermodulation.

The high dynamics of the tuner 13 ensures that it is able to work in the best conditions without causing saturation or intermodulation of the amplification stages when there are transmitters 4 very close to the receiver 2; the main characteristic that distinguishes this peculiarity is normally referred to as the "blocking point" i.e. the high immunity of the receiver 2 to being "blinded" by signals that are too strong.

The tuner 13 is therefore capable of receiving very weak signals coming from transmitters 4 very distant from the receiver 2 even if there are other transmitters 4 very close to it. This characteristic is fundamental for the correct functioning of a radio microphone and particularly so for the radio transmitter apparatus 1 due to the fact that it has broadband reception.

The tuner 13 can be implemented according to the circuit diagram shown in Figure 7 wherein it is designated: "Radio frequency tuned frontend with low intermodulation and high dynamic".

It is very important to note that the receiver 2 can effectively comprise a first local oscillator 14 for first, wide-span frequency conversion, operating in a frequency interval substantially comprised between 713.950 MHz and 11 13.95 MHz.

The first oscillator 14 has the following characteristics:

-the possibility of implementation and production adopting a low number of commonly-found components with minimisation of encumbrances; -high linearity on all of the frequency band generated (the linearity of this component is such as to not require linearization feedback circuits), with considerable savings both in materials and in energy consumption (although in the receiver 2 the containment of energy consumption is more marginal than in the transmitter 4 because it is powered directly by mains electricity). In any case, the reduction of energy consumption is important with regard to the dissipation and heating of the receiving circuit;

-low phase noise.

The oscillator 14 can be implemented according to the circuit diagram shown in Figure 7 wherein it is designated: "First PLL synthesized Wide span Local oscillator 713.950—1 1 13.95 MHz".

According to a possible embodiment of undoubted practical and applicative interest, the receiver 2 comprises an automatic analog/digital converter 15 with an automatically set threshold value for converting the signal from multi-level to three-bit digital.

The automatic analog/digital converter 15 is one of the most important components of the radio transmitter apparatus 1 : it makes it possible to convert the multi-level 4FSK band signal to a 3 -bit digital signal formatted for the FPGA {Field Programmable Gate Array) block.

The characteristics and advantages of the automatic analog/digital converter 15 are:

-eliminating the need for further integrated components, of the Digital Signal Processing type with integrated A/D converter, which are generally much more expensive;

-simplifying the management software for processing the signal with a reduction of the associated costs;

-the possibility of implementation and production adopting a low number of commonly-found components with minimisation of encumbrances; -the absence of adjustments and/or calibrations, since it automatically adjusts itself to the amplitude of the base band signal.

The automatic analog/digital converter 15 can be implemented according to the circuit diagram shown in Figure 8 wherein it is designated: "Automatic threshold three bit A/D converter".

It should be noted that, in order to increase the quality of the audio signal, there are two radio frequency tuners 13 present in each receiver 2, each of which is connected to a respective broadband antenna 3 and each of which comprises a bandpass passive filter 16, directly associated with a respective antenna 3.

A low-noise amplifier 17, of the type with gallium arsenide technology, a 7 dB attenuator 18 and a subsequent bandpass filter 19 with tunable voltage, for the frequency band from 470 MHz to 870 MHz, are connected in cascade to the bandpass filter 16.

The voltage tunable bandpass filter 19 is designed to feed a low noise post-amplifier 20, of the type with gallium arsenide technology, the outputs of which are connected to the inputs of a further 7 dB attenuator 21.

In practice the radio frequency signal picked up by an antenna 3 passes through the bandpass passive filter 16 (in the accompanying graphic examples, this is designated "470— 870 MHz bandpass passive filter") which filters the signal received, thus attenuating any disturbing frequencies below and above the functional field of the system.

Subsequently, the signal passes through the low noise amplifier 17 (in the accompanying graphic examples, it is designated "Low noise GaS frontend amplifier") constituted by an amplifier integrated circuit using gallium arsenide technology which has high dynamics and low noise characteristics. The signal thus amplified then passes through the 7 dB attenuator 18 (in the accompanying graphic examples, it is designated "7 dB attenuator") which is intended to reduce the intermodulation distortion in the terminal and linearize the impedance of the terminal so as to be able to equally amplify all the signals comprised between 470 MHz and 870 MHz.

At the output of the attenuator 18, the signal enters the voltage tunable bandpass filter 19 (in the accompanying graphic examples, it is designated "voltage tunable bandpass filter 470— 870 MHz") which tunes the receiver 2 to the set frequency; basically, if we tune the receiver to 500 MHz, the filter 19 will automatically program itself to center the band on 500 MHz. This is very important because in this way any products of intermodulation are impeded, when there are other radio microphones (or other transmitters 4) or undesired disturbances within the working frequencies of the radio transmitter apparatus 1.

The "prefiltered" signal will then be applied to the second amplifier 20 (in the accompanying graphic examples, it is designated "Low noise GaS post amplifier") which has the same characteristics as the amplifier 17 and whose function it is to further amplify the signal.

Downstream of the amplifier 20 there is the second 7 dB attenuator 21

(in the accompanying graphic examples, it is designated "7 dB attenuator") which linearizes the impedance in conformance with the needs of the components controlled by it and thus limits the distortion with strong signals, such as for example when one or more transmitters 4 are very close to the receiver 2.

It is important to note, in order to highlight the compactness and structural simplicity of the receiver 2 according to the invention, that the two voltage tunable bandpass filters 19 (present in the two radio frequency tuners 13) are controlled by a single microcontroller 22 which is designed for automatic tuning i.e. for centering the band on the tuned frequency (in the accompanying graphic examples, it is designated "Arm Cortex M3 System microcontroller and firmware update management").

Downstream of each radio frequency tuner 13 there is a respective first integrated radio frequency mixer circuit 23 with high dynamic characteristics. To a respective second input 24 of the first integrated mixer circuit 23, a signal is connected which comes from the first local oscillator 14 for first, wide-span frequency conversion.

The first integrated mixer circuit 23 (in the accompanying graphic examples, it is designated "Hi dynamic RF mixer downconverter") is designed to mix the signal coming from the respective radio frequency tuner 13 with the signal coming from the first local oscillator 14, producing as an output a radio frequency signal at a first intermediate frequency, having a possible exemplary value of the order of 243.950 MHz.

Downstream of each first integrated mixer circuit 23, a respective bandpass filter 25 is connected (in the accompanying graphic examples, it is designated "blocco 243.950 MHz first IF narrow band saw filter"); this filter 25 has a narrow band centered on the first intermediate frequency, having a possible exemplary value of the order of 243.950 MHz.

Downstream of this filter 25 a second integrated mixer circuit 26 is connected (in the accompanying graphic examples, it is designated "Second IF Mixer and 4FSK base band demodulator") which is controlled by a second local oscillator 27 (in the accompanying graphic examples, it is designated "Second PLL synthesized fixed frequency local oscillator 254.650 MHz"), operating on a preset, constant frequency value.

With specific reference to the exemplary value of the first intermediate frequency cited above (243.950 MHz), the second oscillator 27 will operate with a frequency of 254.650 MHz, feeding a signal as an output at a second standard intermediate frequency equal to 10.7 MHz.

This signal in turn will be subsequently filtered by a narrowband bandpass ceramic filter 28 (in the accompanying graphic examples, it is designated " 10.7 MHz second IF conversion narrow band double ceramic filters"), which is basically a narrowband filter centered on the second intermediate frequency.

The output signals from the filter 28, at the second intermediate frequency, suitably demodulated through the respective second integrated mixer circuit 26 controlled by the second local oscillator 27, are subsequently fed into the inputs of an electronic selector 29 provided with a specific sensor for comparing the quality of the signals (coming from the two different radio frequency tuners 13).

The output of the selector 29 thus at any given instant sends only the signal having the best quality downstream.

The selector 29 is controlled by a digital signal processor 30 (in the accompanying graphic examples, it is designated " l°st DIGITAL SIGNAL PROCESSING for audio expander and telemetry data") for expanding the audio signals and extracting the data for the telemetry present in the signals at its inputs.

The automatic analog/digital converter 15 with automatically set threshold value, for converting the signal from multi-level to three-bit digital, is connected downstream of the electronic selector 29, with the optional interposition of a digital lowpass filter 31 (in the accompanying graphic examples, it is designated " 10th order digital lowpass filter").

The three-bit signal, output by the automatic analog/digital converter 15, is fed into the inputs of a programmable digital integrated circuit 32 (in the accompanying graphic examples, it is designated "FIELD PROGRAMMABLE GATE ARRAY <FPGA> for clock recovery system and received data formatting") for extracting the synchronism of the signal with the clock pulse of the receiver 2 and for formatting the signal at its inputs as a function of this clock pulse. Part of the operations performed by the programmable integrated circuit 32 are known as "clock recovery".

Downstream of the programmable digital integrated circuit 32 the digital signal processor 30 is connected, which is designed to expand the audio signals and extract the data for the telemetry present in the signals at its inputs, to decompress the data and to extract the data for the telemetry from the signal to its inputs. The audio signal output by the signal processor 30, conveniently processed by a digital to analog converter 33 (in the accompanying graphic examples, it is designated "D/A audio converter"), can be fed into the acoustic broadcasting system.

Alternatively, this signal can also be converted by means of a suitable interface 34 of the optical type (in the accompanying graphic examples, it is designated "optical audio output interface") into an optical output signal.

The signal processor 30 can optionally be controlled by specific components 35 for interfacing with the end user.

The transmitter 4, according to a specific embodiment of particular implementation interest, comprises, connected downstream of the microphone capsule 5, an audio preamplification stage 36 (in the accompanying graphic examples, it is designated "Low noise Programmable audio preamp"), preferably of the programmable type, to which there is connected, in cascade, a suitable analog to digital signal converter 5a (in the accompanying graphic examples, it is designated "A/D AUDIO converter").

In practice, in order to set the transmission frequency or the gain of the audio preamplification stage 36, it is necessary to first set them on the receiver 2 by means of a suitable knob, buttons and dedicated menu, and subsequently the receiver 4 will send the set data to the transmitter 2 by infrared (or other type of connection) simply by bringing the transmitter 4 close to the receiver 2 at the appropriate window.

The transmitter 4, moreover, downstream of the analog to digital signal converter 5 a, comprises a digital signal processor 37 (in the accompanying graphic examples, it is designated "digital signal processing") constituted by the sequence of the following components:

-a station 38 for compressing the digital audio signals (in the accompanying graphic examples, it is designated "digital audio compression"),

-a converter 39 (in the accompanying graphic examples, it is designated "audio data and telemetry serialization with FEC") of the signal coming from the compression station 37 according to a protocol for the serial transmission of the audio data and of the other information, of the telemetry type,

-a control and management unit 40 (in the accompanying graphic examples, it is designated "general purpose management machine controller") designed to control all the interface components of the transmitter 4, including screens 41 (in the accompanying graphic examples, it is designated "OLED display"), displays, command button panels 42 (in the accompanying graphic examples, this is designated "user button interface"), communications ports 43 e.g. infrared (in the accompanying graphic examples, ^' this is designated "IRDA interface"), clock pulse generator 44 (in the accompanying graphic examples, it is designated "system clock generator"), inverter 45 (in the accompanying graphic examples, it is designated "A/D voltage converter"), all associated with the electrical power supply 46 (in the accompanying graphic examples, it is designated "2 batteries AA size") and the like.

Downstream of the digital signal processor 37, the digital modulator 6 according to the mode known as "4FSK" is connected, which is in turn connected in cascade to the oscillator device 10 for generating the radio frequency signal, adapted to generate the carrier frequency of the transmitter 4, in an interval between 470 MFIz and 870 MHz, modulated according to the mode known as "4FSK".

The signal output by the oscillator device 10 is fed into the input terminals of an ultralinear power amplifier 47, active for the amplification of the signal in the power range comprised between 10 mW and 50 mW, selectively pre-settable by the operator and operating in an interval of frequencies comprised between 470 MHz and 870 MHz.

Downstream of the ultralinear power amplifier 47, the filter of the type known as "π", 1 1, of variable frequency is connected for eliminating the spurious harmonics present in the 4KPS-modulated radio frequency carrier signal, the output of which arrives at the input of the broadband antenna 12 integrated in the printed circuit comprising the electronic components and their interconnection wiring.

The invention, thus conceived, is capable of undergoing numerous modifications and variations, all within the scope of the appended claims; in addition, all the details may be replaced by other, technically equivalent elements.

In the embodiments illustrated, individual characteristics shown in relation to specific examples may in reality be interchanged with other, different characteristics, existing in other embodiments.

In addition, it should be noted that anything found to be already known during the patenting process is understood not to be claimed and to be the subject of a disclaimer.

In practice the materials employed, as well as the dimensions, may be any according to requirements and to the state of the art.

Where the technical features mentioned in any claim are followed by reference numerals and/or signs, those reference numerals and/or signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference numerals and/or signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference numerals and/or signs.

Claims

1. A radio transmitter apparatus of the type comprising a receiver (2) provided with a receiving antenna (3) and with at least one output port for connection with apparatuses for broadcasting the respective output signal and a transmitter (4) associated with a specific microphone capsule (5) and provided with a respective transmitting antenna, characterized in that said transmitter (4) comprises a digital modulator (6) according to the mode known as "4FSK", constituted by

-a group (7) for converting the serial digital signal to a multi-level signal, the serial digital signal, containing the audio and telemetry information, coming from a line pertaining to said microphone capsule (5), with the interposition of an amplifier (5a), an analog to digital converter and a data compression unit,

-a constant group delay Gaussian baseband filter (8) for the multi-level signal, placed downstream of the conversion group (7),

-an electronic potentiometer (9) for modulating and controlling the level of the output signal from said Gaussian filter group (8).

2. The radio transmitter apparatus according to claim 1 , characterized in that said transmitter (4) comprises an oscillator device (10) for generating a radio frequency signal, said oscillator (10) being suitable for generating the carrier frequency of the transmitter (4), in an interval between 470 MHz and 870 MHz, modulated according to the mode known as "4FSK".

3. The radio transmitter apparatus according to claim 2, characterized in that said transmitter (4) comprises a filter of the type known as "π" (11) of variable frequency for eliminating the spurious harmonics present in the 4KFS-modulated radio frequency carrier signal.

4. The radio transmitter apparatus according to claim 3, characterized in that said transmitter (4) comprises a broadband antenna (12) integrated in the printed circuit comprising the electronic components and their interconnection wiring, said antenna (12) being adapted for the emission of frequency signals comprised between 470 MHz and 870 MHz.

5. The radio transmitter apparatus according to claim 1, characterized in that said receiver (2) comprises at least one radio frequency tuner (13), constituting the terminal part of the receiver (2), with low intermodulation and high dynamic, the selection of the working frequency band determining a strong attenuation of the other frequencies present in the radio signal received, thus minimising the intermodulation disturbances, the high dynamics ensuring that the amplification stages of the tuner do not enter a state of saturation and/or intermodulation even when a plurality of transmitters (4) are arranged at close mutual distance.

6. The radio transmitter apparatus according to claim 1, characterized in that said receiver (2) comprises a first local oscillator (14) for first, wide- span frequency conversion, operating in a frequency interval substantially comprised between 713.950 MHz and 1 1 13.95 MHz.

7. The radio transmitter apparatus according to claim 1, characterized in that said receiver (2) comprises an automatic analog/digital converter (15) with automatically set threshold value for converting the signal from multilevel to three-bit digital.

8. The radio transmitter apparatus according to one or more of the preceding claims, characterized in that there are two of said radio frequency tuners (13) of said receiver (2), each of which is connected to a respective broadband antenna (3) and each of which comprises a bandpass passive filter (16), directly associated with a respective antenna (3), to which the following are connected, in cascade: a low noise amplifier (17), of the type with gallium arsenide technology, a 7 dB attenuator (18), a subsequent bandpass filter (19) with tunable voltage, for the frequency band from 470 MHz to 870 MHz, designed to feed a low noise post-amplifier (20), of the type with gallium arsenide technology, the outputs of which are connected to the inputs of a further 7 dB attenuator (21).

9. The radio transmitter apparatus according to the preceding claim, characterized in that said two voltage tunable bandpass filters (19) are controlled by a microcontroller (22) which is designed for automatic tuning i.e. for centering the band on the tuned frequency.

10. The radio transmitter apparatus according to claims 6 and 8, characterized in that downstream of each radio frequency tuner (13) there is a respective first integrated radio frequency mixer circuit (23) with high dynamic characteristics, at a respective second input (24) of said first integrated mixer circuit (23) a signal being input which comes from said first local oscillator (14) for first, wide-span frequency conversion, said first integrated mixer circuit (23) being designed to mix the signal coming from the respective radio frequency tuner with the signal coming from said first local oscillator (14) thus producing as an output a radio frequency signal at a first intermediate frequency, having a possible exemplary value of the order of 243.950 MHz.

11. The radio transmitter apparatus according to the preceding claim, characterized in that a respective bandpass filter (25) is connected downstream of each of said first integrated mixer circuits (23), a filter (25) with a narrow band centered on the first intermediate frequency, having a possible exemplary value of the order of 243.950 MHz, a second integrated mixer circuit (26) controlled by a second local oscillator (27) being connected downstream of said filter (25), operating on a preset, constant frequency value, for example, with reference to the exemplary value of the first intermediate frequency, said second oscillator (27), operating with a frequency of 254.650 MHz, thus providing, as an output, a signal at a second standard intermediate frequency equal to 10.7 MHz, said signal in turn being subsequently filtered by a narrowband bandpass ceramic filter (28), a narrowband filter centered on the second intermediate frequency.

12. The radio transmitter apparatus according to the preceding claim, characterized in that the signals at said second intermediate frequency, conveniently demodulated by means of the respective second integrated mixer circuit (26) controlled by a second local oscillator (27), are fed as input to an electronic selector (29) provided with a sensor for comparing the quality of said signals, the output terminals of said selector (29) conveying only the signal having at any given instant the best level of quality, said selector (29) being controlled by a digital signal processor (30) for expanding the audio signals and extracting the data for the telemetry present in the signals at its inputs, said automatic analog/digital converter (15), with automatically set threshold value, for converting the signal from multi-level to three-bit digital, being connected downstream of said electronic selector (29), with the optional interposition of a lowpass filter (31) of the digital type.

13. The radio transmitter apparatus according to claims 7 and 12, characterized in that the three-bit signal, output by said automatic analog/digital converter (15), is fed into the inputs of a programmable digital integrated circuit (32) for extracting the synchronism of the signal with the clock pulse of said receiver (2) and for formatting the signal at its inputs as a function of said clock pulse, said digital signal processor (30) being connected downstream of said programmable digital integrated circuit (32), for expanding the audio signals and extracting the data for the telemetry present in the signals at its inputs and for decompressing the data, the audio signal output by said signal processor (30), conveniently processed by an analog to digital converter (33), being fed into the acoustic broadcasting system.

14. The radio transmitter apparatus according to claim 1, characterized in that said transmitter (4) comprises, connected downstream of said microphone capsule (5), an audio preamplification stage (36), preferably of the programmable type, to which there is connected, in cascade, a suitable analog to digital signal converter (5 a).

15. The radio transmitter apparatus according to the preceding claim, characterized in that, downstream of said analog to digital signal converter (5a), it comprises a digital signal processor (37) constituted by the sequence of the following components:

-a station (38) for compressing the digital audio signals (38),

-a converter (39) of the signal coming from the compression station according to a protocol for the serial transmission of the audio data and of the other information, of the telemetry type,

-a control and management unit (40) designed to control all the interface components of said transmitter (4), including screens, displays (41), command button panel (42), communications ports (43), e.g. infrared, clock pulse generator (44), inverter (45), all associated with the electrical power supply (46) and the like.

16. The radio transmitter apparatus according to claim 15, characterized in that said digital modulator (6) is connected downstream of said digital signal processor (37), according to the mode known as "4FSK", which modulator is in turn connected in cascade to said oscillator device (10) for generating the radio frequency signal, adapted to generate the carrier frequency of the transmitter (4), in an interval between 470 MHz and 870 MHz, modulated according to the mode known as "4FSK", the signal output by said oscillator device (10) being fed into the input terminals of an ultralinear power amplifier (47), active for the amplification of the signal in the power range comprised between 10 mW and 50 mW, which can be selectively preset by the operator and operates in an interval of frequencies comprised between 470 MHz and 870 MHz.

17. The radio transmitter apparatus according to claim 16, characterized in that connected downstream of said ultralinear power amplifier (47) there is said filter of the type known as "π" (11) of variable frequency, for eliminating the spurious harmonics present in the 4KFS- modulated radio frequency carrier signal, the output of which arrives at the input of said broadband antenna (12), integrated in the printed circuit comprising the electronic components and their interconnection wiring, said antenna (12) being adapted for the emission of frequency signals comprised between 470 MHz and 870 MHz.