CONTRACT RD-94 TASK ORDER NO. 2

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03300A001900110013-4
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 12
• Document Creation Date: December 22, 2016
• Document Release Date: May 3, 2011
• Sequence Number: 13
• Publication Date: June 20, 1957
• Content Type: LETTER

Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
June 20, 1957
Subject: Contract RD-9L
Task Order No. 2
In accordance with Article 2 of the basic contract,
there are forwarded herewith two (2) copies of the Monthly
Prof;ress Report for May, 1957 on Task Oruer No. 2 of ftD-94.
This renn-f- IQ . LASSIFIED. An additional copy is being
held in by the project engineer for the use of 25X1
your personnel while at this location.
In connection with this monthly progress report,
the following information is submitted:
Total expenditures to 4-30-57 $30,605
Outstanding commitments as of
4-3a-57 135
Funds remaining as of 4-30-57 *29,576
Very truly yours,
Assistant Manager
Government Contract Administration
TRR:mr
f -14608
Enclosures
cc:
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-03300A001900110013-4
CONFIDENTIAL
Monthly Progress Report
may 1957
Task Order No. 2
Contract No. RD-94
Audio Noise Reduction Circuits
The object of this project is to develop a noise reduction circuit
suitable for use in separating speech intelligence from a signal containing
speech and noise when the speech intelligence is masked by the noise. The
proposed method involves a principle which has been used successfully to
improve the signal-ft.-noise ratio in music reproducing or transmission
systems*1 The system used for music contains bandpass filters which pass
frequencies over a range of an octave or less. These filters are used
at the input and output of a non-linear element. The output or the non-
linear elements contain the fundamental, and also harmonics and subharmonios
of the fundamental. However, since the pass band of the input and output
bandpass filters is no greater than an octave, the harmonics and subharmonios
are not transmitted by the system. 1e function of the non-linear element
is to reject all noise signals below a given amplitude or threshold level.
The threshold levels of the non-linear devices in each channel can be
adjusted so that, in the absence of desired signal, the noise is rejected.
When the desired signal is greater than the threshold level, the non-linear
elements allow the composite signal to pass. 'thus, for passages of recorded
music, when the music signal is below the noise level in a given frequency
channel, the channel is inoperative, and its output is eliminated from the
total output, Since the contribution of this channel to the total output
would have been only noise, the over-all noise level ie reduced. when the
1. H.F. Olson, "Electronics," Dec. 1947.
7- J
EN I
LLAL
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-03300A001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
musio signal in a given channel is greater than the noise, the ahannsl
conducts and allows the composite signal to pass. Thus, a channel conducts
only when the desired signal is greater than the noise, and rejects when
noise alone is present.
In order to apply this method of noise reduction to speech, when
the wide band speech signal-to-noise ratio is very low, it was believed
necessary to find frequency regions in which the speech amplitude is
greater than the noise. Although the long time average spectrum of speech
is continuous, and similar in shape to the spectrum of room noise,2 the
short time spectrum of various speech sounds contains regions of maximum
energy called speech formants.3 The assumption that this method of noise
reduction should be utilised for speech was based upon the belief that it
would be possible to find frequency regions in which the amplitude of the
speech formats would be greater than the noise a substantial part of the
A study has been made to determine what bandwidths are required in
order to obtain speech torment amplitudes above the noise when a wide band
speech sample is just intelligible in noise. It is known that for noises with
a continuous spectrum it is the noise components in the immediate frequency
region of the masked tone which contribute to the masking.4 When a very
narrow band of noise is used to mask a pure tone, the masking increases as
the bandwidth is increased until a certain bandwidth is reached. After this,
as the bandwidth is increased, the amount of masking remains constant. This
bandwidth at which the masking reaches a fixed value is termed the critical
2. H. Fletcher, "Speech and Hearing on Commaunication&" Van 1dostrand Co., Ina.,
New York, 1953 (see Figures 61 and 70).
3. Op.oit. chap. 1.
4. L.L. Beranek, "The Design of Speech Coinmunioation Systems." Pron. IRE,
Vol. 35, pp. 882, Sept. 1947.
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
bandwidth: Measurements have been made using filters which were both
narrower and wider than the critical bandwidth. Both pure tones and speech
mixed with continuous spectrum type noises have been studied. The results
of this study show that, for the narrowest permissible bands which can be
used to pass speech formants, the number of times the speech formant amplitude
in a given band exceeds the noise is small. Also, in these bands, the speech
amplitude is never considerably greater than the noise. Since very narrow
bandwidths are required to reduce the noise below the signal, the number of
bands required to cover the speech spectrum is quite large. There was no
satisfactory way of evaluating the effect upon speech intelligence of small
contributions from umny narrow bands without building a many channeled
circuit and evaluating it.
In view of this fact a complete multi-channel system has been
developed in order to determine the effectiveness of this method of improving
speech intelligibility in noise. The multi-channel a tem developed contains
80 channels covering the frequency range from 700 to 3200 cps. The bandwidth
of each channel is adjustable and each has been set so that it is one halt that
of the critical band when the signal is 3 db above the threshold level. A
schedule of the band centers and their bandwidths is contained in the progress
report for January 1957.
During May final adjustment and assembly of the 80 channel noise
reducer was completed. A by-pass circuit was built which has the sans pass
band as the noise reduction circuit. The response-frequency characteristic
of the by-pass circuit and the noise reduction circuit is shown in Figures
1 and E. The input-output characteristic.of the noise reducer at 1000 cps.
is shown in Figure 3, Curve A. Some preliminary measurements have been
5. N.R. French and J.C. Steinberg, "Factors Governing the Intelligibility of
Speech Sounds," Jour.Aooust.Soo.Amer., vol. 19. Jan. 1947 (see Figure 7).
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
made using this noise reducer on speech mixed with both flat noise and
simulated room noise. In these measurements a recording of continuous
discourse was played back through a mixing amplifier. The noise level mixed
with the speech was adjusted until the threshold of intelligibility waa
reached when the composite signal was passed through the bypass circuit.
The same composite signal was then passed through the noise reduction circuit
at various levels above the threshold. A block diagram of the test circuit is
shown in Figure 4. It has been found that when the maximum of the composite
signal is about 5 db above the threshold wane speech components are passed
free of noise. However, the information contained in these components is
not sufficient to provide speech intelligibility. It was found that the
composite signal mist be at least 15 db above the threshold in order to
provide some intelligence, and maximum intelligence occurs when the maxfrxua
of the composite signal is from 20 to 25 db above the threshold of the noise
reduction circuit. At this level both speech and noise are passed by the
noise reduction circuit.
In order to maintain a more constant level fbr the maxi.mxm amplitude
of the composite signal, a compression anplifier was added between the nixing
amplifier and the noise reduction circuit. The response curve and compression
characteristic are shown in Figures 5 and 6. The effect of the compression
amplifier on the input-output characteristic of the noise reducer is shown
in Figure 3, Curve B. The evaluation measurements were repeated using speech
mixed with noise.
The results of these preliminary tests indicate that passing only the
speech information which is greater than the noise does not provide sufficient
6. J.E. Hawkins, Jr. and S.S. Stevens, 'The Masking of Pure Thnes and of
Speech by Whits Noise," JASA, Vol. 22, p. 11, January, 1950.
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
information to give speech intelligence. However, when the composite signal
is 15 to 20 db above the threshold of the noise reducer, and considerable
noise is passed, it is believed that the intelligibility of continuous
discourse is improved.
In view of these results, it is planned to make further evaluation
measurements and modifications of the circuit in an effort to improve its
performance. Tests will be performed for various conditions of the noise
reducer and noise samples. For example, the thresholds of the noise reducer
channels can be adjusted as a function of frequency; the bandwidths of the
channels can be adjusted; the characteristic of the compression amplifier
can be varied. The results of these further measurements will give a
better evaluation of this method of noise reduction.
PER: pad
June 10, 1957
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-03300A001900110013-4
t
I
- -
--- - ----------
I
- _J- -.. -{
r
-- 1
_:
I
_
---
-- ---
-
4---
~-
F
-
i
-
-
clm
I
is
T
I
L
-
II
1
7-, 71
-
1 71 -
W
{
L
t
1
'
i
L.
-77
tt
.
2_.
7 7-;
r
a -
11 -1
-J-
--
L
~-T
Na
2
O
k-RDP78-03300A001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
-- -r
--
- -
--~ - - -
1
_7
{
-
2.1
W
r
; 1
I t
-
I 1
_
~
(888
_~{t
T
T 7 !7.
7--
t
i
r_
1
1 `
~
4
9
I I I
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-03300A001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
,_ _i _ 1
1 -
1
- 1
-
,.
-t r"i
.
...~1
.i
r
..~_I
.. fl't
T + 14
~t~~I+ !
1
. :.~
'
1r
rlj~?
1
t
Y
1
T
T-+h
r
t
+' t
.
.
r l
~
1 Y
11}F
<
1
t
f'
~
t
,
TL
'
t t.,
f
tr
t--1
t;
,-t-
-
t
r
+^
i
I -
{
1
j
-7
14-
,-LI
"f
' IS
f
f
~. .
.
..:
I
It
. r
r
- I
t
fi
r
1: i
I
I
} L ?I i
--
_j
k
i i
I
1
1 1
~1-
~
1
I
,
::.
----
'
.-.
41
7''
1
7
}
77
+i
r
1
;
I
f
L
f
~
1f
zL
r
l~~
'
tr
I
~
.
Ii
1
t
,
I.
r
'
1,
l~
r
t
r
r
;
+
~
~
r
V
-
ti-
Ti
T
+
trr
~L
t
li
~
}_,
+ 4;
t-~~
C
r
t
`r
i._
I_,
rI-I
,
1( +
r! '
l~
i
F+`
-
1 4
I
T: 71
t
a
hr.
i
1
1 '-
r l
.
+.
-
.
j
t
J-j
r
+.
.:
777
7=
- 1 `
Li~.yr~..
?
f 1
J
1
-t-
71
t
r
1
,
t
. ~'.
U-j
C 'r
qtr
Tl'
r~
.
1.4r'
I 1r
. 1
t' 7
~_,^ 1
i
~L-
,
:+
? .
j
,
ttt r
t-L
T-t
1" 7
1++
T
:14
_
?+-' r
I'I+)
- IL Y
-f
, I
i _i *
.~. - 1
h +-I
1 rI
-
rt,T
_I ': I
A
'-j i
+
, t
I' :.
i
I
`
-
*
I1 t
r
1.
j
~ ~:
y
I
.. 1 '.
+
}
r
r I.
+
+ {
7
I
,
t 1'
~rrL I
I+t
,
I
..
j .
.
` I 't
rF
+ tl .
->t
.t t+
1:n
t +t{
t~
a
i
I ,
r
1 t
rt
.,;
a,
I
F
x:,
'
LS
t
?- Y
r
*
}
f
+ F
~
I,-Ji
71.
Tt
f
t' t
17f
,~
r
;
'
'
T
i .
.
1
J
'J
'_
~..
:J
+
r
t L~.
+
t
--i{
Ij
I , r
+ i:)v
, i
.
;
+
r
t'
7
+I
if
r
r1r
t
f
,
i
T
*
}
r
t.i
ij
F
1
I
i * I
I.;:1
rl
rt
"
4i
t,
kJ ~{
k
' r
I' ~r
f r,
#-
y?}~
r
r
i, I
y
}
-11-
s1
,
1
f
,ft
h_
1
f
~,
_
L
:
,-
,
'yT
t:..t
ttt_
t
Vt:i
,
r ?
1~.
~ 1
;
(-~_
rr''
~i '
ri7
7?_
i -
~
,
+ }
{
-
L
'
t
r
~..,
J.7i
I
lC
t
+T'
i
IJ
r
i
1 I
r
~
-~~I
{:
`~
~
1'
ILL
I _
1_
.
+F {
r
1
t~'
}
,
C
+ i
.
1
.
~r
.
r
_
I
l
:
.
{
L
_
+
'
~,
1
I
ti ,*r
-
h
i r
-t_
L~
jj
f 1
.1!
~i
ll
I I+I
F~
111 t
-
i+. }t L
ll.'~
yL
}
.1 F1.
1`
77-
14 4
i
.
rr l
1
?
TIC}TL.'
I
'~
4
1
T
.
~,
{t
tI `'
T
t r
1
1
r
tl
1i
L1
?
t
:.
+
} fti
!
+
a J
ti i
r.
r' I r
1 .n
~:
I
{
1
E
j
;
4--i
+
i
r
.
~.
.
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
C
I
0
0
LZ
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
 Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-0330OA001900110013-4
r
1
0
_, --. ~
~1
b4 IL
_~-F
-
-~
?~-~
Imo-
I
--
. .
- ~
._
~ '
-
I I
~
~ I I
I I
I
_
r
-f_
I
tI
I
T-FT T-
_-
-
r1=
_
_
_
_
-7j -J
I T
I
i 4-
11
YY
V
i -1
?
C?
RFIU
EIV I I
A. L
Sanitized Copy Approved for Release 2011/05/03: CIA-RDP78-03300A001900110013-4