Multifrequency digital signal receiver

• US 4,223,185 A
• Filed: 11/14/1978
• Issued: 09/16/1980
• Est. Priority Date: 11/18/1977
• Status: Expired due to Term

• Current Assignee: Le Materiel Telephonique
• Original Assignee: Le Materiel Telephonique
• Inventors: Picou, Claude H.
• Primary Examiner(s): Moffitt, James W.
• Assistant Examiner(s): Popek, Joseph A.
• Application Number:
  US05/960,723
• Time in Patent Office: 672 Days
• Field of Search: 179/84 VF, 328/138, 324/78 D, 364/484, 364/726, 364/728, 343/100 CL
• US Class Current: 708/404
• CPC Class Codes: H04Q 1/4575 which are transmitted in digital form

Abstract

A multifrequency code receiver for signals consisting of the sampled and encoded sum of q sinusoids of amplitudes A_f, whose frequencies f are regularly spaced apart by ΔF_i =pΔF_O (ΔF_O being a divisor of the sampling frequency F_E) the receiver possessing a heterodyne device transforming each value f into F_L -f, a multiple of ΔF_O, a counter counting up to r, where r=F_E /F_T, F_T being the divisor of F_E greater than (q-1)ΔF_i, filtering circuit, a fast Fourier transform (FFT) computer for N_T =F_T /ΔF_O samples producing the amplitudes A_f.

Claims

1. A digital receiver for multifrequency signals, each multifrequency signal comprising the sum of several sinusoids which are sampled at a frequency F_E, then quantified and linearly encoded, each sinusoid having the form A_f sin2π

ft, whose frequencies f define a code and are selected out of q predetermined frequencies constituting an arithmetical series based on Δ

F_i ≐

pΔ

F_O (p being an integer, Δ

F_O being a divisor of F_E, said receiver including means for calculating the fast Fourier transform (FFT) of an input signal, characterized in that said receiver includes in series connection;

(a) digital frequency shifting means for generating a frequency F_L at or close to the center of the band of said q frequencies, and for changing each of said q frequencies to a frequency F_L -f, whose absolute value is a multiple of Δ

F_O ;

(b) low-pass digital filtering means with a cut-off frequency f_p equal to the highest of the frequencies |F_L -f|, for attenuating the components of frequencies F_L +f due to frequency shifting to a level beneath a predetermined level A_min ;

(c) means, connected to the output of said filtering means, for counting up to r and for retaining one sample of the multifrequency signal out of every r samples taken, r being chosen equal to F_E /F_T, where F_T is a divisor of F_E greater than 2f_p and a multiple of Δ

F_O,

said fast Fourier transform calculation means being connected to the output of said counting means and including N_T =F_T /Δ

F_O inputs fed, each second, with F_T samples obtained from said filtering and said counting means, and v outputs, each corresponding to one, and only one, frequency f of an input digital signal.

2. A digital receiver in accordance with claim 1, in which Δ

F_i is a divisor of F_E, characterized by the fact that F_L is chosen equal to one of the two center frequencies of the spectrum of the q frequencies if q is even, or to the center frequency if q is odd, f_p assuming the values (q/2)Δ

F_i (q even) or (q-1/2)Δ

F_i (q odd), and N_T being equal to F_T /Δ

F_i.

3. A digital receiver, in accordance with claim 1, in which Δ

F_O =2Δ

F_O (Δ

F_O and Δ

F_O being divisors of F_E) and in which q is even, characterized by the fact that F_L is equal to the center frequency of the band of the q frequencies, f_p assuming the value (q-2)Δ

F_O and N_T being equal to F_T /Δ

F_O.

4. A digital receiver in accordance with claim 1, in which Δ

F_i =pΔ

F_O (p>

2), Δ

F_O being in this case the highest common divisor of F_E and Δ

F_i, characterized by the fact that;

if p and q are even, F_L is equal to the center frequency of the band of the q frequencies, f_p assuming the value [p(q-1)/2]Δ

F_O,

if p is odd and q is even, F_L is ±

Δ

F/2 from the center frequency of the band of the q frequencies, f_p assuming the value [p(q-1)+1/2]Δ

F_O,

if q is odd, F_L is chosen equal to the center frequency of the spectrum of the q frequencies, f_p assuming the value [p(q-1)/2]Δ

F_O.

5. A digital receiver in accordance with claim 1, in which the digital frequency shifting means is divided into first and second similar computing elements, said first and second computing elements being characterized by:

a read-only memory comprising a plurality of memory cells functionally arranged in rows and columns containing the address of a quantified value of the phase φ

=2π

k×

F_L /F_E with respect to that of the local frequency F_L, the number of addresses being reduced to h, the latter value representing the denominator of the irreducible fraction q/h=F_L /F_E, and the addresses having the h values 0, 2π

q/h, 2(2π

q/h) . . . , (h-1)(2π

q/h, and of which each row contains at the intersection points with the columns the digital value of cos φ

or sin φ

;

means for receiving the multifrequency signal samples at the frequency F_E, in successive groups of h; and

a multiplier which generates the product of the digital value of each sample and either cos φ

or sin φ

, where φ

is the address corresponding to the order of the sample within each group of h.

6. A digital receiver in accordance with claim 5, wherein said low-pass digital filtering means comprises first and second identical digital filters operating at the frequency F_E, said first and second filters being connected to the outputs of said first and second computing elements, respectively, each of said filters being a recursive filter having an attenuation less than a predetermined value, A_max, in the pass-band from nF_E -f_p to nF_E +f_p (n=0, 1, 2, 3 . . . ∞

), said attenuation being greater than said predetermined value, A_min, in the part of the spectrum beyond the frequency F_L +f_min, f_min being equal to the lowest frequency in the spectrum of the q frequencies.

7. A digital receiver in accordance with claim 6, wherein the attenuation A_max of said digital filters is negligible, each of said filters is connected to the input of said means counting up to r, each of said counting means is connected to the input of said fast fourier transform calculation means, said calculation means including N_T input pairs and v output pairs of known orders, each pair corresponding to one and only one of the frequencies f, said calculation means producing, in digital form, the complex components A_f exp(jφ

_f) of the spectrum, each output pair being followed by a digital multiplier generating A²_f cos² φ

_f for one output and A²_f sin² φ

_f for the other, and said receiver further comprises q digital adders each receiving the digital values of the same order and each producing on its output the digital value A²_f corresponding to this order.

8. A digital receiver in accordance with claim 6, wherein the attenuation A_max of said digital filters is not negligible, each of said filters being connected to the input of said means counting up to r, each of said counting means being connected to the input of said fast fourier transform calculation means, said calculation means including N_T input pairs and V output pairs of known orders, each pair corresponding to one and only one of the frequencies f, said calculation means producing, in digital form the complex components A_f exp(jφ

_f) of the spectrum, each output pair being followed by a digital multiplier generating A²_f cos² φ

_f for one output and A²_f sin² φ

_f for the other, and said receiver further comprises q digital adders each receiving the digital values of the same order and each producing on its output the digital value A_f² corresponding to this order, the effect of the attenuations in the pass-band affecting each value of A²_f associated with a frequency f being corrected by a plurality, but not more than q, multipliers connected to the outputs of said q digital adders, for multiplying the outputs by the coefficients by a₁, a₂ . . . a_q, respectively.

9. A digital receiver in accordance with claim 8, wherein to compensate for receipt of a spurious signal of frequency f_c, whose transform (F_L -f_c) by frequency shifting (|F_L -f_c |<

F_T /2) lies in the transition zone of the digital filters, the receiver further comprises auxiliary means providing attenuation at this frequency F_L -f_c, said auxiliary means comprising third and fourth digital filters of the same nature as said first and second digital filters and having the same passband from nF_E -f_p to nF_E +f_p, (n=0, 1, 2 . . . ∞

) but operating at the frequency F_T, said coefficients a₁, a₂ . . . a_q being modified to allow for the additional attenuations due to said third and fourth digital filters within the pass-band.

10. A digital receiver in accordance with claim 8, in which 2F_T is a divisor of F_E and wherein to compensate for receipt of a spurious signal of frequency f_c, whose transform (F_L -f_c) by frequency shifting (F_T >

|F_L -f_c |>

F_T /2) lies in the transition zone of said first and second filters, the receiver further comprises auxiliary means for providing attenuation at this frequency F_L -f_c, said auxiliary attenuation means comprising first and second counters each counting up to r/2, first and second auxiliary digital filters each having the same pass-band from nF_E -f_p to nF_E, (n=0, 1, 2 . . . ∞

) but operating at the frequency 2F_T, and first and second counters counting up to 2, said coefficients a₁, a₂ . . . a_q being modified to allow for the additional attenuations due to the auxiliary digital filters within the pass-band.