CONTRACT RD-94 TASK ORDER NO. 2

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03300A001900110019-8
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 11
• Document Creation Date: December 22, 2016
• Document Release Date: May 3, 2011
• Sequence Number: 19
• Publication Date: February 12, 1957
• Content Type: LETTER

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February 12, 1957
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Contract RD-9%j,
Task Order No* 2
In accordaw with Article 2 of the basic eantract,
them are forwarded herewith two (2) copies of the Monthly
Progress Report for January, 1957 on Task Order No. 2 of
RD-91. The "part is dated February 11, 1957. ThIs "no", ~ s
TJNCLASSIPIED. An additional copy is being held in
by the pro jest engineer for the use of your personnel while
at this location.
In connection with this monthly progress report,
the following information is submitted:
Total expenditures to 12-3l$6 $15,068
Outstanding commitments as of
12? -5b
Funds remaining as of 12 1.56
,n anv
.'.&j by law.
Now
Assistant Manager
Goverment Contract Administration
TRR: nah
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Faelosures
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CONFIDENTIAL
Monthly Progress Report
January 19 7
Task Order No. 2
Contract No. RD-9+
Audio Poise Reduction to
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.involvos,a principle
which has been used successfully to improve the sigai-lrtolnoiser'-
ratio in music reproducing or transmission systems77.. 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 of the
non-linear elements contain the fundamental, and also harmonics
and subbarssonics of the-fundamental. However, . since the pass band
oft he input and output -bsndpass filters is no greater than an
octave, the harmonics and subbarmonios are not trs emitted by the
system. The function of the doerlinear 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 in-
operative, 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 overall noise level is reduced. When the
music signal in a given channel is greater than the noise, the
channel conducts and allows the composite signal to pass. Thus a
channel conducts only when the desired signal is greater than the
1. H. F. Olson, "Electronics", Dec. 1947.
CONFIDENTIAL
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2.
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
is necessary to find frequency regions in which there are times
when 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 formants3. The assumption that this method of noise reduc-
tion could be utilized for speech was based upon the belief that
it would be possible to find frequency regions in which the ampli-
tude of the speech formants would be greater than the noise a sub-
stantial part of the time.
A study has been made to determine what bandwidths are required
in order to obtain speech Iormant 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. 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 bandwidths. The critical bandwidth is a function of
frequency. It is different when listening with one or two ears.
The critical bandwidth for two ears as a function of frequency is
shown by the upper curve of Figure 1. Measurements have been made
2. H. Fletcher, "S eech and Hearing on Communication", Van Nostrand
Co. , Inc. NYC 1953 (see figures 61 and 70)
3. Op. cit. chap. 1
4. L. L. Beranek, "The Design of Speech Communication Systems",
Proc. IRE, vol. 35, pp.6S2, Sept. 19#7.
5. N. R. French and J. C. Steinberg, "Factors Governing the In-
telligibility of Speech Sounds" Jour. Acoust. Soc. Amer.
Vol.19, Jan. 1947 (see figure 75
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3?
using filters which were both narrower and wider than the critical
bandwidth. Both pure tones and speech mixed with continuous speo-
trua type noises have been studied. The res*Ults 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 bonds,
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 is no satisfactory way of evaluating the effect
upon speech intelligence of small contributions from many narrow
bands without building a many channeled circuit and evaluating it
by making articulation measurements. From the information available
from studying a few channels throughout the speech spectrum it seems
possible that some improvement in intelligibility can be effected,
but this improvement may prove to be small.
In view of the fact that there is no convenient way to eval-
uate the contributions of a few narrow band channels to speech intelli-
gibility, a complete multi-channel system will be developed in order
to determine the effectiveness of this method of improving speech
intelligibility in noise. The system under development will contain
approximately 80 frequency channels in the frequency range from 700
to 3200 cps. The bandwidths of these channels will be 3 db narrower
than the critical bands. The bandwidths of these channels as a
function of frequency are shown by the lower curve of Figure 1.
During January, the design of a ten channel prototype circuit
has been completed. Bight of these circuits will be required to
form an 80 channel noise reducer. The circuit schematic and chassis
layout drawings have been completed, and the circuit is now under
construction in the model shop. It is planned to complete and test
this prototype before the other seven are built. A block diagram of
this circuit is shown in Figure 2. The prototype chassis will require
a seven inch panel. The total 80 channel noise reducer circuit will
be housed in a seven foot relay rack. A view of the planned relay
rack is shown in Figure 3. A block diagram of the 80 channel circuit
is shown in Figure 4. A schedule of the frequency of each channel
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if.
and the chassis position is shown in Table I. The approximations
which these channels have to the desired curve are shown in Figure 1.
Efforts to purchase special low pass filters for use in its
output of each 10 channel circuit, as shown in Figure 2, and obtain
delivery in less than three months have been unsuccessful. There-
fore, it was deemed necessary to design and construct these low
pass filters. The filters for all eight groups have been designed.
The one for the prototype chassis is now under construction.
ESR tnan
Feb. 11, 195`!
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Table I
Band Center, Bandwidth and Chassis
Position of each Channel
Chassis
Channel
Band
Band-
Low Pass
No.
No.
Center
CPS
width
CPS
Out off
CPS
l
700
20
2
20
7
20
70
4
20
760
20
780
20.
1000
6
20
7
820
20
8
8!}0
20
20
10
880
20
r11
900
22
112
2
2
9
22
l
22
ia
94
%6
22
2
15
988
22
1200
16
1010
22
17
1032
22
18
32
1
22
1
9
076
22
20
1098
22
22
2
2.120
I-
1168
24
24
2}
1
1192
1216
24
00
~
3
2S
0
~
28
1i94
29
2h
26
30
131
26
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2.
Chassis Channel
No. No.
Table I (Coat).
Band
Center
CPs
31 32
13394
33
1420
1
34
1448
4
3
1476
37
1532
38
9
L r
1620
rtt2
1680
1710
742
1
5
IitZ
1806
47
1870
49
1938
52
1972
52
5
54
22078
6
55 -
91.34.
56
2150
58
2190
2
59
2270
60
2310
Band-
width
CPS
Low Pass
Out off
CPS
1700
2000
2400
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Table I (font).
3?
Chassis Channel Band Band? Low Pass
No. No. Center width Out off
CPS CPS CPS
0
62 2390 0 40
6 2.130 40
61 2470 40
~ 2800
7 66 1 2 .58
6
02
67 2
44
68
69 2264263&6-
6gp6 - ~
4
l
70 2734 44
2 2880 4 8 8
288 48
F, 2926 48
2M 8
8 71 3026
[78 3078 52
52
3234 52
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N
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CONFIDENTIAL
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CONFIDENTIAL
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CONFIDENTIAL
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CONFIDENTIAL
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