An electrical filter for removing noise from a voice communications signal is reported; seven sample values of the signal are obtained continuously, updated and subjected to filtering. Filtering is accomplished by adding balanced, with respect to a mid-point sample, spaced pairs of the sampled values, and then multiplying each pair by a selected filter constant. The signal products thus obtained are summed to provide a filtered version of the original signal

Edwards, T. R.

Zeanah, H. W.

English

NASA Technical Reports Server

United States Patent [191 
Edwards et al. 
D I G I T A L  
T O  C N V E R T E R  ANALOG
1 11 3,949,206 
[451 Apr. 6, 1976 
AV'6vn- C 
[ 54 1 FILTERING DEVICE 
[75] Inventors: Thomas R. Edwards; Hugh W. 
Zeanah, both of Huntsville, Ala. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[22] Filed: Dec. 17, 1974 
[21] AppI. No.: 533,608 
3,665,171 511972 Morrow .............................. 2351152 
3,798,560 311974 Taylor ................................. 3281167 
3,821,527 611974 Kang ................................... 2351152 
Primary Examiner-Joseph F. Ruggiero 
Attorney, Agent, or Firm-George J. Porter; L. D. 
Wofford, Jr.; John R. Manning 
1571 ABSTRACT 
An electrical filter for removing noise from a voice 
communications signal in which, for example, seven 
sample values of the signal are obtained continuously 
updated and subjected to filtering. Filtering is accom- 
plished by adding balanced, with respect to a mid- 
point sample, spaced pairs of the sampled values and 
then multiplying each pair by a selected filter con- 
stant. The signal products thus obtained are summed 
to provide a filtered version of the original signal. 
1 Claim, 1 Drawing Figure 
[52] U.S. CI. ..................... 235/156; 325142; 333118 
[ 5 1 ] Int. CL2 .......................................... G06F 15/34 
[ S S ]  Field of Search .................... 2351152, 156, 181; 
3281167; 333118, 28, 70 T; 325142, 65 
[561 References Cited 
UNITED STATES PATENTS 
3,370,292 211968 Deerfield ................................ 34315 
D E L A Y  C H A I N  OF REGISTERS 
https://ntrs.nasa.gov/search.jsp?R=19760014278 2020-03-20T02:14:28+00:00Z
April 6, 1976 
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t 
3,949;206 
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It will be assumed that the spectrum of audio signal 12 
FILTERING DEVICE is in the range of 300 to 3,000 Hertz and that the sam- 
pling rate of analog to digital converter 10 is at least 
ORIGIN OF THE'INVENTION 10,000 samples per second. The digitally encoded sam- 
The invention described herein was made by empldy- 5 ple values from A-D converter 10 are fed and shifted, 
ees of the United States Government and may be man- sequentially, through a delay chain of shift registers 
ufactured and used by or for the Government of the 14-20, which will rhus contain, at,any instance, a series 
rposes of digital representatioiiq of the sampled data points, as 
there- parenthetically labeled. Shift registers 14-20 form a 
10 delay chain of registers which enable simultaneous 
presentation of what are in fact time-spaced samples: In 
accordance,with the invention, an odd number of regis- 
ters are employed. This is done for the purpose of un- 
ambiguously establishing the center point at mathemat- 
Weighted averaging of the thus sampled data is ac- 
, complished as follows. The first and last, in time, sam- 
ple bits of registers 20,and 14 are added in digital adder 
hasJong been 22 to provide a first input to digital multiplier 24. The 
a problem. Many types of filters have been devised to Zo next to first and next to last, in time, sample outputs 
remove the noise. Usually they have bee f the reac- from registers 19 and 15 are added in digital adder 26 
tance type which function to block frequencies above and provide a first input to digital multiplier,28. The 
and below the signal spectrum, approximately 300 to second from first and second from last, in time, sample 
3,000 Hertz in the case of voice signals. W,ith the devel- outputs from registers 18 and 16 are added in digital 
opment of the digital computer, efforts have been made 25 adder 30 and the sum provided as a first input to digital 
to solve the noise problem digitally by converting Sam- multiplier 32. Finally, the center sample bit, the output 
pled values ofa  signal to digitally ewoded bits and then of digital registel: 17, is applied as a first input to digital 
performing, at very high speeds, varioqs modifications multiplier 34. 
to the encoded bits. The difficulty has been that the To each of digital multipliers 24,28,32, and 34 is fed 
computer programs employed to accomplish this task 30 a multiplying filter constant, Co, C,, C,, and Cs, respec- 
have been too complex, too demanding of computer tively. The selection of constants is based upon the 
hardware, and too expensive. particular characteristic of filtering desired, and as a 
It is the object of this invention to provide a new and typical set of constants to achieve a low-pass filter 
improved mCanS Of filtering COmmUniCatiOnS Signals which has the effect of removing high frequency noise, 
which substantially reduces the hardware requirement 35 noise higher than 3,000 Hertz the maximum frequency 
and yet enables extremely effective filtering. of the communications signal shown, reference is made 
to Table 1 of the Savitzky and Golay reference referred 
to above. This table indicates that where seven points SUMMARY OF THE INVENTION 
are sampled that filter factors progressively applied 
* 
partic- 
15 ical position zero, a replacemdnt value. 
The removal of 
In accordance with the invention, a communications 
signal to be filtered is continuously sampled at equally 40 would have the following weighted values: 
time-spaced sample points to provide a continuously 
updated plurality of time-spaced electrical samples of Sample Point Factor 
the signal. Sample pairs, balanced with respect to a 
-03 -2 
-02 3 
-0 I 6 
00 I 
6 01 
mid-point sample, are added to thus provide a plurality 
lected filter constant and the products obtained are 02 3 
of signal sums. Each of the signal sums and the mid- 45 
point sample are then separately multiplied by a se- 
summed to provide a filter output. Specific filter fac- 03 -2 
tors for low-pass and high-pass filters is described . 
herein. Other' filter facto; may be determined from 50. 
multiparameter optimization theory and regression 
calculations, examples of which have been published 
by A. Savitzky and M. J. E. Golay, Anal. Chem., 36, 
1627 ( 1964). and J. Steiner, Y. Termonia, and J. Del- 
tour, Anal. Chem., 44, 1906 (l972), and T. R. Ed- 55 CR. 
wards, Double Convolution Coefficients (to be pub- 
lished ). 
The present invention makes use of only the last four 
constants, which are 7, 6, 3, and -2; and they are ap- 
plied, in that order, as second inputs of multipliers 24, 
28, 32, and 34 for the constant inputs Co, C,, C2, and 
AS an example of a set of constants for a high-pass 
filter, reference is made to Table 3 of the Golay and 
Savitzky reference and wherein equated filter constants 
would be 0, 1, 2. and 3, respectively. Other filter char- BRIEF DESCRIPTION OF THE DRAWING 
The drawing is an electrical schematic circuit dia- 60 acteristics for use with the present invention may be 
determined in accordance with the same theoretical gram of an embodiment of the invention. 
I 
consideration described in the Savitzky and Golay ref- 
erence. While seven sample points are utilized in the 
example, additional ones may be employed by addition 
DESCRIPT1oN OF THE PREFERRED 
EM BO DIM ENT 
Analog to digital (A-D) converter 10 converts sam- 65 of shift registers, adders, and multipliers. The effect of 
additional samplings is that of raising the upper limit of 
thc bandpass frequencies. However, it is a significant 
finding by thc applicant that an extremely cffectivc 
pled data points A-G of input audio signal 12 to a like 
designated series of digitally encoded values, each 
valuc ruprcsentativc of one of the sampled data points. 
3,949,206 
3 4 
filtcring job can be accomplished by using only seven 
sampling points, with double convolution coefficients, 
and with the additional consideration of hardware. 
The compensated outputs of multipliers 24, 28, 32, 
and 34 arc digitally added by adder 36 and the output 5 
of adder 36 applied to digital to analog converter 38, 
which converts the signal information back to analog 
form as dcpicted by wavcforrn 40, a waveform which is 
a replica of a portion of input waveform 12 but with a 
substantial quantity of signal noise eliminated. 
said second summing means, and the outputs of thc 
third and fifth of thc chain of said shift registers 
being connected to said first and Second inputs of 
said third summing means; 
first, second, third, and fourth product means, each 
comprising means for multiplying first and second 
applied inputs, an output of said first summing 
mcans being connectcd as a first irtpbt to first prod- 
uct means, the output of said second summing 
means being connected as a first input to said sec- 
ond uroduct means. the outDut of said third sum- What is claimcd is: 
I. A filtering system for removing noise above 3,000 
cycles from a continuous audio frequency signal of 
frequencies in thc rangc of 300 to 3,000 Hertz compris- 
ing: 
analog-to-digital convcrsion means rcsponsive to 
such a said signal for providing as an output a con- 
tinuous chain of digital signals representative of 
sampled points on said signal at a ratc of at least 
10,000 samples per second; 
a chain of scvcn serially-connected shift registers, the 
input of the first of said shift registers being con- 
nected to the output of said analog-to-digital con- 
version means; 
first, second, and third summing means, each having 
first and sccond inputs and an  output, outputs of 
the first and Past of said chain of said shift registers 
being connectcd as said first and second input sig- 
nals to said first summing means, the outputs of the 
second and sixth of said chain of shift registers 
being connected as said first and second inputs of 
ming means being connected as first input to said 
third product means, and the output of the fourth 
of said chain of shift registers being connected to 
the first input of said fourth product means; 
filter constant means for applying weighted values to 
said product means, a value of -2 being coupled to 
said second input of said first product means, a 
value of 3 being connected to the second input of 
said second product means, a value of 6 being 
connected to the second input of said third product 
means, and a value of 7 being connected to the 
second input of said fourth product means; 
fourth summing means responsive to the outputs of 
said product mcans for providing as an output the 
sum of the outputs of said product means; and 
digital-to-analog conversion means responsive to the 
output of said fourth summing means for reproduc- 
ing said signal as modified by removal of signal 
noise content above 3,000 cycles. 
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Filtering device

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