Abstract:
A method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone, comprising the steps of: 
     extracting the pilot signal from the modulated signal; 
     obtaining the suppressed carrier by multiplying the pilot signal by the pilot signal in quadrature; 
     adjusting the level of the suppressed carrier; and 
     reconstructing the modulating signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone, particularly for frequency modulation, and to an apparatus for implementing it. 
     In conventional signal transmission methods a signal is broadcast by means of suppressed-carrier amplitude-modulated carriers in the presence of a reference tone, known as pilot tone, whose frequency is half that of the signal to be broadcast and whose amplitude is variable. 
     Demodulation of the signal upon reception occurs by reconstructing the carrier by means of a phase-locking oscillation circuit (PLL) which uses the pilot tone as a reference for locking. 
     This method is widely used in frequency-modulation (FM) radio broadcasts for stereo signals. 
     A drawback of the above-described method resides in the fact that although it is suitable for analog implementations, it is poorly suited to digital circuits, since it is impossible to produce digital PLLs. 
     Moreover, the phase precision of the reconstructed carrier is always affected by an unpredictable error, which can be reduced to very small values in high-level and high-cost equipment. 
     Finally, the analog implementation requires a large number of components which require calibrations, with the consequent problems in terms of cost, stability and quality in general. 
     SUMMARY OF THE INVENTION 
     A principal aim of the present invention is to provide a method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone which overcomes the drawbacks of the prior art, allowing fully-digital decoding. 
     Within the scope of this aim, an object of the present invention is to provide a method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone which allows to work over a very wide frequency range. 
     Another object of the present invention is to provide a method for decoding a suppressed-carrier signal in the presence of a pilot tone which is insensitive to variations in the level of the pilot tone. 
     Another object of the present invention is to provide a method for decoding a suppressed-carrier signal in the presence of a pilot tone which allows to produce circuits having the chosen precision by increasing the precision of the computing circuits. 
     Another object of the present invention is to provide a method which is highly reliable and relatively easy to provide at competitive costs. 
     This aim, these objects and others which will become apparent hereinafter are achieved by a method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone, comprising the steps of: 
     extracting the pilot signal from the modulated signal; 
     obtaining the suppressed carrier by multiplying the pilot signal by said pilot signal in quadrature; 
     adjusting the level of the suppressed carrier; and 
     reconstructing the modulating signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the present invention will become apparent from the following detailed description of a preferred but not exclusive embodiment of the method according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of the apparatus that implements the method for decoding a suppressed-carrier modulated signal in the presence of a pilot tone, according to the prior art; and 
     FIG. 2 is a block diagram of the apparatus that implements the decoding method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, it is shown a conventional apparatus that implements a conventional decoding method. 
     According to this conventional method, the modulated input signal, broadcast by broadcasting means, is sent to a filter 1 to separate the carrier and to a synchronous demodulator 2. 
     The signal in output from the filter 1 is sent to a PLL oscillator 3, whose output is sent to the synchronous demodulator 2. The signal in output from the synchronous demodulator is then sent to a low-pass filter 4, whose output is the final decoded signal. 
     With reference now to FIG. 2, the method according to the invention is described. 
     The suppressed-carrier modulated signal in the presence of a pilot tone is input to a filter 5 for separating the pilot tone and into a synchronous demodulator 6. 
     The signal in output from the filter 5 is sent to a 90° phase shifter, designated by the reference numeral 9, to means 7 for computing the level of the carrier, and to means 8 for computing the carrier, which also receive in input the signal in output from the phase shifter 9. 
     The signal in output from the means 7 for computing the level of the carrier enters a low-pass filter 10, whose output is sent to carrier level adjustment means 11 whose output is sent to the synchronous demodulator 6. 
     The signal in output from the synchronous demodulator 6 is sent through a low-pass filter 12, whose output constitutes the decoded signal. 
     With reference to the above-described decoding apparatus, the decoding method according to the invention is as follows. 
     First of all, the pilot signal is extracted from the modulated signal by means of a conventional phase filtering performed by the filter 5. 
     Assume that the expression: 
     
         cos (d) sin (2 wt) 
    
     represents the suppressed-carrier modulated signal, and that the expression 
     
         pilot=p sin (wt) 
    
     represents the pilot signal. 
     The total signal, i.e., the suppressed-carrier modulated signal in the presence of a pilot tone, is therefore: 
     
         S.sub.tot =cos (d) sin (2 wt)+p sin (wt). 
    
     The pilot signal p sin (wt) is thus obtained in output from the filter 5. 
     Then a quadrature pilot tone is computed by using a 90° phase shifter 9 or another equivalent procedure which produces a similar phase shift with a known gain. 
     The quadrature pilot tone is therefore: 
     
         pilot(90°)=p cos (wt). 
    
     At this point, the carrier is computed by multiplying the pilot tone in output from the pilot tone separator 5 by the quadrature pilot tone (i.e., the pilot tone phase-shifted by 90°) output by the 90° phase shifter 9. This multiplication is performed by the carrier computing means 8. 
     One thus obtains: 
     
         B=p.sup.2 sin (2 wt)/2 
    
     The carrier level computing means 7 square the pilot tone and the filtering means 10 filter the high-frequency components. 
     One thus obtains: 
     
         A=(p.sup.2 -p.sup.2 cos (2 wt)), and by filtering one obtains 
    
     
         A.sub.filtered =p.sup.2 /2 
    
     The carrier level adjustment means 11 normalize the carrier output by the carrier computing means 8. This normalization is performed by dividing the carrier by the square of the filtered pilot tone. 
     Therefore, one obtains B normalized  =B/A filtered , i.e.: 
     
         B.sub.normalized =sin (2 wt). 
    
     At this point the modulating signal is reconstructed by multiplying, by means of the synchronous demodulator 6, the modulated signal in input by the normalized carrier. 
     Therefore, the demodulated signal is equal to: 
     
         S.sub.demodulated =cos (d)/2-cos (-d+4 wt)/4-cos (d+4 wt)/4+p cos (wt)/2-p cos (3 wt)/2 
    
     The signal in output from the synchronous demodulator 6 is appropriately filtered by means of the low-pass filter 12. 
     One thus obtains the demodulated filtered signal: 
     
         S.sub.demodulated-filtered =cos (d)/2 
    
     In practice, it has been observed that the method according to the invention fully achieves the intended aim, since it allows to provide fully digital decoders which operate over a wide frequency range. 
     Moreover, decoders provided according to the invention do not require calibrations and are insensitive to variations in the level of the pilot tone. 
     The method thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. 
     Thus, for example, the carrier normalization stage can be replaced with a gain control loop which has the purpose of adjusting the amplitude of the pilot tone so that the signal obtained by multiplying the pilot tone by itself and by the gain of the phase shifter 9 has a unit value. 
     It is also possible to replace the phase shifter 9 with a delay line and to choose the sampling frequency so that the delay line is equivalent to a delay (phase shift) of 90°. 
     Finally, all the details may be replaced with other technically equivalent elements. 
     In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art.