Abstract:
A digital FM demodulation circuit provides improved resolution by selecting from a group of at least four of the most recent samples, a subgroup of three samples. The first and last samples of the selected subgroup are summed and the sum is divided by the intermediate sample.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to a digital FM demodulation circuit in general. More specifically, the invention pertains to a circuit for the digital FM demodulation of temporarily equidistant samples derived from an analog frequency-modulated signal by way of sampling with the aid of a sampling signal. The samples are fed to the input of a chain of two series-arranged delay stages each having a delay equal to the period of the sampling signal. The samples are coupled to the first input of an adder whose second input is coupled to the output of the last delay stage. The output of the adder is connected to the dividend input of a divider which has its divisor input coupled to the output of the first delay stage. The divider output signal is used either directly or after forming the corresponding inverse sign values to provide a demodulated digital signal. 
     One such circuit is known from the German Offenlegungsschrift No. DE 30 30 853 A1. In this publication it is stated as a general principle that the digital FM demodulation can be achieved by a corresponding interconnection of three samples. However, more specifically the digital FM demodulation is obtained from three successively following samples as follows: the first and the third sample are added and divided by the second sample. Implementing this principle in circuitry leads to the circuit described above. 
     As is well known from the sampling theorem, the sampling frequency must be at least twice as high as the highest frequency existing in the signal to be sampled. To demonstrate a disadvantage of the conventional circuit, reference is first made to the special case in which the frequency of the sampling signal is just four times as high as the frequency of the signal to be sampled, and the signal frequency is frequency-modulated in the usual way. In that case, the sign of the first sample is opposite to that of the associated third sample of three successively following (adjacent) samples. In addition the amounts of both the first and the third sample are practically identical. This implies that the sum formed from these two samples results in small a numerical value lying around the zero point which, to provide sufficient resolution, requires a correspondingly high number of digits in the associated digital signals and therefore in the corresponding digital words. This property of the conventional circuit is of importance especially in cases where the first and the third sample are obtained adjacent to the positive and the negative peak value of the signal to be sampled. 
     SUMMARY OF THE INVENTION 
     It is one object of the invention to modify the conventional circuit in such a way that the resolution, in the case of a constant, given number of digits of the samples, is independent of the temporal position of the samples with respect to the sampled signal. 
     According to the invention, this is accomplished in that four, in the most simple case, successively following or adjacent samples (sampling values) are evaluated in such a way that a particular group of three samples contained in the group of four samples is applied to the adder and to the divider to yield the best resolution. Relative thereto, the arrangement is made in such a way that the particular group of three whose first and third samples are in the vicinity of the greates slope of the signal to be sampled is selected. In the case of samples of a purely sinusoidal signal, the selected three samples would be the values in the vicinity of the zero crossover. To select the respective group of three, a comparator and bus switches are used. 
     In accordance with a further embodiment of the invention, the resolution can be still further improved by not evaluating adjacent samples (sampling values), but rather by evaluating the first one in a long sequence of samples, then two adjacent ones following after an even number of skipped samples, and the last sample (sampling value) of the sequence again following after the same number of skipped samples. 
     An advantage of the invention is that a circuit for digital FM demodulation can be realized at a given resolution with a constant number of digits in particular of the adder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be better understood from a reading of the following detailed description in conjunction with the drawings in which: 
     FIG. 1 is a schematic block diagram of an FM demodulator in accordance with the invention; and 
     FIG. 2 is a schematic block diagram of a second embodiment of an FM demodulation in accordance with the invention. 
    
    
     DETAILED DESCRIPTION 
     In the circuit of FIG. 1, the analog frequency-modulated signal FM to be demodulated is applied to the input side of the sampling stage a which is controlled by the sampling signal fa and which, at its output, provides temporarily equidistant sampling values in the form of corresponding digital signals or words. The tape or ribbon-shaped connecting lines extending between the individual partial circuits in FIG. 1 are indicative of the parallel signal processing as employed by the invention. Accordingly, each tape-shaped connecting line indicates a number of parallel lines commonly also referred to as a bus. Compared with the merely two delay stages proposed according to the aforementioned German reference, which corresponds to the first and last delay stages v1, vl in FIG. 1, there is provided, according to the invention, at least one further delay stage vm in the middle, which is of the same kind. Accordingly, the chain of delay stages contains an odd number of stages through which the sampling values are passed. 
     The input signal of the middle one of the delay stages vm is applied to the minuend input em of a comparator k. The output of delay stage vm is applied to the subtrahend input es of the comparator k. The respective inputs of the first, middle and last delay stages v1, vm, vl are applied to the respective rest contact input er of first, second and third bus switches u1, u2, u3. The respective control input of the three bus switches is connected to the minuend-greater-subtrahend-output ak of the comparator k. If, in the case of the presumed positive logic, the more positive level H of two binary levels H, L appears at output ak, i.e., when the signal at the input em of the comparator k is greater than the one at the input es, then the three bus switches u1, u2, u3 are in the positions as shown, that is, the input signals of the delay stages v1, v2, vl are through-connected by the respective bus switches u1, u2, u3. If, on the other hand, the output ak of the comparator k is a logic low or L-level, the bus switches u1, u2, u3 switch over to the respective operating contact eu, and the output signals of the three delay stages v1, vm, vl applied to the associated operating contact ea, and connected to the outputs of the respective bus switches u1, u2, u3. 
     To graphically represent the bus switches u1, u2, u3, the symbol of a mechanical switch is shown. It is evident that each bus switch consists of such a number of individual electronic switches which corresponds to the number of individual lines of the aforementioned buses. 
     The output of the first bus switch u1 and the output of the third bus switch u3 are connected to the first and the second inputs e1, e2 of an adder ad, respectively. The output of the second bus switch u2 is connected to the divisor input ds of a divider dv, and the output of the adder ad is connected to the dividend input dd of the divider dv. The output signal dm of divider dv serves directly as a demodulated digital signal, or can be fed to a stage for forming the corresponding arc sine values, with the output signal thereof then serving as the demodulated digital signal. 
     Thus, in the arrangement of FIG. 1, out of a sequence of four adjacent or, subsequently following samples, a group of three adjacent samples is selected in which the middle one of the three, has the highest value compared with the two adjacent ones. It is thereby assumed that the first and the last sample (sampling value) of the thus selected group of three, are within the area of the greatest slope of the signal to be sampled, and the resolution is optimized. 
     FIG. 2 shows a further embodiment of the arrangement in accordance with the invention in which it is possible to obtain a further improvement of the resolution. In the arrangement of FIG. 2, three samples of a long sequence of sampling values are processed each time, with an even number of adjacent sampling values between the first and the middle one, as well as between middle one and the last sampling value not being used. In the circuit diagram of FIG. 2, an equal even numbers n and r of additional delay stages vn and vr are respectively inserted before and after the middle delay stage vm. These additional delay stages are inserted such that the additional delay stages vn are arranged between the operating contact input ea of the first bus switch u1 and the rest contact input er of the second bus switch u2, and the additional delay stages vr are arranged between the operating contact input ea of the second bus switch u2 and the rest contact input er of the third bus switch u3. 
     As in the arrangement of FIG. 1, three bus switches u1, u2, u3 are utilized to select the inputs to adder ad and the divisor input ds of the divider dv. A comparator k is likewise utilized to control the bus switches u1, u2 u3. The respective inputs of the first, middle and last delay stages v1, vm and vl, respectively are applied to the respective rest contact input er of the first, second and third bus switch u1, u2 u3. The respective outputs of the first, middle and last delay stages v1, vm and vl, respectively, are applied to the respective operating contact input ea of the first, second and third bus switch u1, u2, u3. Th input sm of delay stage um is coupled to the minuend input em of comparator k. The output sm&#39; of delay stage vm is coupled to the subtrahend input es of comparator k. The comparator output ak is connected to the control inputs of the bus switches u1, u2, u3. 
     When sm is greater than sm&#39;, the output signal at ak will be an H level and the bus switches will be in the position shown. Adder ad then adds the samples so and sr&#39; providing the sum to the dd input ot divider dv. The sum is divided by the sample sm which is coupled to the divisor input ds. 
     When sm is not greater than sm&#39; adder ad adds samples sl and ss which divider dv divides by sample sm&#39;. 
     Turning back to FIG. 1, when the digital signals at the inputs or outputs of the three delay stages v1, vm, vl are indicated by s0, s1, s2, s3, then the output signal dm of the divider dv, in the shown position of the bus switches u1,u2, u3, can be written as follows: 
     
         dm=-F(s0+s2)/πs1 or 
    
     
         dm=-(2F/π) arc sin (s0+s2)/2s1, 
    
     with F indicating the carrier frequency of the analog frequency-modulated signal FM. 
     To the arrangement as shown in FIG. 2, there applies a corresponding formula: 
     
         dm=(-1).sup.(m+1)/2 F(s0+sr&#39;)πmsm. 
    
     By way of example for digital FM demodulation of SECAM television signals preferably six additional delay stages are inserted into the chain of delay stages for vn and a like number vr. 
     For this example, the following numerical values apply to the individual indices: 
     
         m=7,r&#39;=14,s=15. 
    
     In this particular case, the following will result as the output signal of the divider: 
     
         dm=F(s0+s14)/7πs7.