CONTRACT RD-94 TASK ORDER NO. 2

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03300A001900110008-0
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 4
• Document Creation Date: December 22, 2016
• Document Release Date: September 20, 2011
• Sequence Number: 8
• Publication Date: October 30, 1957
• Content Type: LETTER

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October A. 1957
Subjects Contract RD-91.
Task Order Now 2
In accordance with Article 2 of the basic contract,
there are forwarded herewith two (2) copies of the monthly
Progress Report for September 1957 on Task Order No. 2 of Aa?91.1
This report is UNCLASSIFIED. An additional copy is being
held in y the project engineer for the use of
your personne w a-"* at this location.
In connection with this aonthly progress report
the following inforaation is subaittedi
Total expenditures to 8--31?57 ? $459431
Outstanding eosaituents as of
8031.57 ? 242
Funds resaining as of 8-jl?5' ? $140623
This da.tMnsM M, s M M sanction affaotin0 the aatlonal
dalllaa of tp. Unltad $W w within the msanln0 of the
Espisfla01 Lori, This it, U.S.C., 8aedens 793 and 7"
Ma ML AINM K tI* r.wlition of its oontent$ In any
MM M M ? sawisibm MtsM Is 009"M by SaMI.
f.24'i
Saelosures
Very truly yours,
Government Contract Adaini
CONFIDENTIAL
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UUNI-IULNIIAL
monthly Progreso Report
September 1967
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-to-noise ratio in nusic reproducing or transmission
systems.l 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 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 subharenios
are not transmitted by the system. The 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 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
1. H.F. Olson, "Electronics," Dec., 1947a
CONFIDENTIAL
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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 tbrm9nts.3 The assumption that this method of noise
reduction should be utilized for speech was based upon the belief that it
would be possible to find frequency regions in which the amplitude of the
speech formants 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 formant 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 bandwidth.5 Measurements have been made using filters which were
2. He- etcher, 'Speech and Hearing on Communication," Van Nostrand Co., Inc.,
New York, 1963 (see Figures 61 and 70).
3. Op.cito chap. 1.
4. L.L. Beranek, "The Design of Speech Communication Systems," Proo. IRS,
Vol. 36, pp. 882. Sept., 1947.
5. N.R. French and J.C. Steinberg, "Factors Governing the Intelligibility
of Speech Sounds," Jour. Acoust. Soo. Amsr., Vol. 19, Jan., 194?.
(See Figure 7)9
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! yy u e U i- I 11 1/-!L
-3-
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 sham 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. Singe
very narrow bandwidths are required to reduce the noise below the signal,
the number of bands requirod to cover the speech spectrum is quite large.
There was 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. A complete multi-channel system has been developed
in order to determine the effectiveness of this method of improving speech
intelligibility in noise. The multichannel system developed contains 110
channels covering the frequency range from 170 to 3200 cps. The bandwidth
and the threshold level of each channel is adjustable. The 110 channels are
arranged on 11 chassis with 10 channels on each chassis.
During September the design of 11 band pass filters was completed and
sent to the model shop. At present, whole speech is fed to each of the 11
chassis. The purpose of the band pass filters is to limit the frequency
spectrum of the speech applied to each chassis to only those frequencies
that the chassis can pass. In addition, a volume compressor is being designed
for each chassis. This compressor, used in conjunction with the band pass
filters, will reduce the dynamic range of the signal applied to each group of
10 channels. It is desired to compress the dynamic range of speech from 20 db
into 3 db with as simple a device as possible. As soon as these modifications
are completed, evaluation measurements will be continued.
October 4, 1957
TIA
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