PROGRESS REPORT NO. 9 CONTRACT NO. RD-53-SA RESEARCH ORDER NO. IRD-4

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03153A001600010008-8
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 28
• Document Creation Date: December 23, 2016
• Document Release Date: March 5, 2014
• Sequence Number: 8
• Publication Date: October 15, 1954
• Content Type: REPORT

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PROGRESS REPORT NO. 9
Contract No. RD-53-SA
Research Order No. 1RD-4
Prepared by:
Approved by:
Project Engineer
Chief Engineer
Submitted: October 15, 1954
Covering period from September 1, 1954 to Septemher 30, 1954
COPY NO. 2
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INTRODUCTION
Thia is the ninth progre.,}, report on Res..ar _h and Dev dopment
Task 4. This month's report covers work done on a pulse
duration modulated system that functions in a manner to provide
security against detection by conventional receivers.
Previous progress reports have discussed the various ad-
vantages of pulse communications over conventional A. M. -
F. M. systems. However, it was noted that kgonventioThal
receiver could detect the pulsed signal, on condition that a
strong signal was present. In order to avoid this possibility,
additional circuits to provide security were designed into the
CYPL
equipment. The term security will be defined for this purpose,
as the prevention of the reproduction of a transmitted intel-
ligence by conventional A..M. or F. M. receivers.
DISCUSSION
Lxperiments conducted with a pulse time modulated system in-
dicated the following results. For proper reception, the signal
to noise ratio of the input signal as measured at the detector
output, was as follows: The PTM receiver, 2:1; The F. M.
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receiver, 8:1; And the A.M. receiver, 18:1.
Theoretically, various methods may be employed to provide
security under these conditions of detection. The method
that was chosen, and described in this report, is a means
of masking inte1iigence of noise. Mask-
ing may be defined as the number of decibels by which a
listeners threshold of audibility for a given tone is raised
by the presence of another sound.
All communications systems normally include some noise,
and invariably it is an undesireable feature. In general
both the masked sound and the masking sound have highly
complex wave forms and frequency structures.
Two books used as reference for this phase of the work are
"Fundamentals of Acoustics" by L. E. Kinsler and A.R. Frey, and
"Acoustic Measurements" by L. L. Beranek. They both describe
masking experiments using pure sinusoidal tones. In performing
experiments of this type, the masking tone is operated steadily
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at some particular intensity level, and the intensity of the
signal tone is raised from a level below audibility to one
that is just distinguishable.
The masking of one pure tone by another is most apparent
when the two tones are of approximately the same frequency.
?
In general a tone masks signals of higher frequency more
effectively than it does those of lower frequency. As an
example of relative masking effects, a signal of 1000 cps.
and an intensity level of 40 db is completely masked by a
400 cycle tone whose intensity level is 80 db, but it is well
above the threshold of audibility in the presence of a 2000
cycle tone of the same intensity.
This is readily explained by a consideration of the aural
harmonieg generated by the masking tones. For the 400 cycle,
80 db tone these harmonics have frequencies of 800, 1200. 1600,
etc. cps. and have loudness approaching that of the fundamental,
so that, since one or another of these harmonics
will be approxi-
mately the same frequency as any signal in the upper audible range,
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it will provide effective masking. On the other hand, all the
aural harmonics of the 2000 cycle tone have frequenci4s of
4000 cps. or more and do not mask the 1000 cycle signal.
The band of speech frequencies most important for
bility is that extending from about 1000 to 2500 cps. However,
in the interests of making speech sound more natural, such as
distinguishing male from female voices, it is usual to extend
the band from 250 to 2750 cps.
From the above considerations it should be expected that a pure
tone would be most effective in masking speech when its
frequency was about 500 cps., for its more intense harmonics
would be spread across the frequency band required for good
articulation. Similarly it may be concluded that distorted waves
such as sqruare waves or series of pulses would be more effective
in masking speech than are pure tones, for such masking signals
are themselves rich in harmonic friequencies. The technical
literature quoted previously bear this out.
When a pure tone is sounded in the presence of a random
noise only the noise in a. narrow frequency band on either side
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serves to mask it. A quantity known as the "recognition
differential" (RD) is sometimes used as a measure of the
conditions under which a signal is recognizable in the presence
of a noise background. The RD is defined as the difference,
expressed in decilAls, between the signal and background intensity
levels, when the signal can be recognized just 50 percent of the
time.
Under certain conditions of signal bandwidth, noise background
bandwidth, spectrum level of noise at signal frequency, and shape
and frequency of the noise spectrum, the RD may be a negative
value, meaning the signal may be smaller than noise. In general,
for the same intensity of noise level, if the transmitted frequency
range is narrowed, the RD is higher than if the entire speech and
noise spectrum is transmitted.
1 The equipment as designed for the masking application, utilizes
unlimited frequency range for both signal and noise at the modulator.
However, the demodulation cuts off at 3000 cps. The design
permits a signal to masking noise ratio of 1 to 2.
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DESIGN OF EQUIPMENT
There were two general circuits considered for this noise
masking application. One would provide three distinct
pulses; two of which would contain noise modulation and the
third the signal. Because of the complexity of the trans-
mitter circuitry, this design was temporarily shelved in
favor of the second circuit.
In general, the system used has some distinct advantages
over the three pulse system. It is essentially a two pulse
system, which basically simplifies the circuitry. Furthermore
It lends itself to a more efficient system in so far as duty cycle is
concerned.
Possibly the greatest advantage pertains to this unique distinc-
tion. The three pulse system would have the lagging edge of each
pulse as the deviating edge. It would be apparent to a monitor
that the pulses contain some intelligence. By sampling each
pulse in turn, the pulse carrying the signal could be demodu-
lated and the intelligence derived. The two pulse unit, on the other
hand, has no deviation on the first pulse, and causes both edges of
the second pulse to deviate. The leading edge deviates with noise, and
the lagging edge carries the signal. This condition would be more
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confusing to an observer. Sampling either pulse would not
result in signal. The only means of deriving intelligence
is to sample the lagging edge exclusively.
The modulator is indicated in Figure 1. The design incorporates
a gas tube as a noise generator. The usual sporadic current
flow of this type of tube is put through a plate resistor. The
resulting noise voltages are. coupled to additional noise ampli-
fiers prior to performing noise modulation.
The basic controlling units are the saw tooth generators and
rnultivibrators. A blocking oscillator acts as the basic frequency
source, which provides a frequency of 6000 cycles per second.
This pulse is fed to two parallel sections which comprise the
signal and noise sections.
The noise section of the circuit eliminates the positive portion of
the trigger pulse. By using the negative portion, a delay of 5
microseconds is obtained. This negative pulse is amplified and
made positive, before triggering a saw tooth generator. The
output of the saw tooth generator and the output of the noise
amplifier are both coupled to a multivibrator. The output of
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the multi-vibrator is taken as a negative pulse. It is delayed
fifteen microseconds and has its lagging edge deviating. The
maximum deviation attained is ten microsecords.
The signal section is essentially unchanged from the original one
channel circuit as described in the previous report. The blocking
oscillator triggers a saw tooth generator. This is coupled to a
multivibrator along with the modulating audio signal. The output of the
multivibrator is taken off as a positive pulse that is 28 microseconds
wide. The lagging edge deviates to correspond with the audio, and
the maximum deviation is 5 microseconds. This results in a signal
to noise which at the peak audio signals is no better than 1 to 2.
The outputs of the two multivibrators are combined and the noise
pulse cancels the center section of the signal pulse. This results
In two distinct pulses. The first pulse has neither edge deviating.
The second pulse has both edges deviating; the leading edge is
the noise factor and the lagging edge contains the signal.
The demodulator, for purposes of this test, was designed to accept
the input as an ordinary audio amplifier, or to provide the special
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demodulation necessary to separate the signal from the noise.
This was accomplished by switching the input either directly
to the audio amplifiers or to the noise demodulator.
The circuit is represented by Figure 2. The double pulse
P.D.M. input is fed to two parallel sections. The noise de-
modulator section consists of two pulse amplifiers, the output
of which is differentiated. This replaces the original two pulses
with a series of sharp pulses that trigger a multivibrator. The
differentiator was necessary because the multivibrator had a
tendency to follow the shape of the relatively wide pulses.
The output of the multivibrator is taken out as a positive pulse.
The stage is designed to provide a pulse that is wide enough to
cover the first pulse and part of the second pulse. At this point
the positive multivibrator pulse is mixed with the original pulses
which have a negative polarity due to one stage of amplification.
The multivibrator pulse cancels out the first pulse and the leading
or noise edge of the second pulse. This leaves the second pulse with
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a new and fixed leading edge, but with the lagging or signal
edge unaffected. The new pulse is further amplified and then
switched to a cathode follower. This in turn feeds the signal
to a series of low pass filters and audio amplifiers, termin-
ating in a loud speaker.
The parallel path consists of a pulse amplifier that handles
both pulses, and then immediately switches them to the cathode
follower.
TESTING
The system was tested on a closed wire basis. One test
involved the useCof the noise demodulator. The output was
a noise free signal, with reasonable fidelity.
_
The second test was made with both the noise and signal
coupled to the audio section. The result was complete
masking of the signal by the noise. In addition to the absence
of intelligibility, and of even greater importance, there was
no apparent indication of the presence of speech. It sounded
as though it was industrial noise.
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The waveforms at various positions were recorded and the
test points are indicated on Figure 3 and 4. The oscillograms
are shown as follows:
Fig. 5 The plate of the blocking oscillator shows a
negative pulse.
Fig. 6 The output of the blocking oscillator used as
the trigger pulse. It has a cycle duration of
160 microseconds or a frequency of 6000 cps.
Fig. 7 The input to the noise modulated channel, the
positive half of the trigger pulse has been
eliminated, This results in a delay of 5 micro-
seconds.
Fig. 8 The pulse used to trigger the noise channel saw tooth
generator, one stage of amplification has converted
it to a positive pulse required for triggering.
Fig, 9 The output of saw tooth generator.
Fig. 10 The input to the multivibrator, the11-51aw tooth
generator output is reshaped by rectification to
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Fig.
11
Fig.
12
Fig.
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provide the conventional saw tooth wave
form. The pulse now has a total d*lay with
respect to the blocking oscillator of 15
microseconds.
The plate of the gas tube noise generator; it
is evidently white noise.
The output of the noise amplification Judi-
cates an amplifier gain of 18 db.
The output of the noise channel rnultivibrator;
this is taken off as a negative pulse. The lag-
ging edge is noise modulated. The leading
edge has been delayed 15 microseconds.
Fig. 14 The output of the signal etiAAnel saw tooth generator.
Fig. 15 The input to the signal channel raultivibrator, the
w awe has been reshaped to the conventional saw
tooth shape and has the proper positive polarity.
The pulse is delayed only 3 microseconds.
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The input to the audio amplifier; it is a 1000 cps. tone.
The output of the last audio amplifier; the saw tooth
Fig.
16
Fig.
17
Fig.
18
trigger pulse is superimposed.
The output of the signal channel multivibrator; it
is taken out as a positive pulse with the lagging
edge signal modulated. The pulse is 28 micro-
seconds wide.
Fig. 19 The two rnultivibrator outputs have been combined.
The noise pulse has cancelled out the center section
of the signal pulse. The first pulse of the two has
no moving edges since they both represent the
stationary leading edges of the original pulses.
The second pulse has both edges deviating. The
leading edge is noise modulated, and the lagging
edge is signal modulated.
The following oscillograms were taken at the demodulated section.
The test points are indicated on Fig. 4.
Fig. 20 The combined pulses after two stages of amplification.
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Fig. 21 The input to the multivibrator; this has been
differentiated to insure more positive trigger-
ing action. It was found that the original
pulses tended to cause double triggering, which
effect was eliminated by using the sharp
differentiated pulses.
Fig. 22 The output of the multivibrator, taken out as a
positive pulse. It is about 20 microseconds
wide, and wide enough to cover the first pulse
and the leading noise edge of the second pulse.
Fig. 23 The original pulses after one stage of ampli-
fication. They are now of negative polarity
which is required for cancellation.
Fig. 24 The input to the mixer, the multivibrator pulse
has cancelled out the first pulse and part of the
second. The leading edge is now fixed and the
lagging edge is signal modulated.
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Fig. 25 The output of the mixer, indicates further amplification
to bring the level up to the required value before
switching to the audio and filtering sections.
Fig. 26 The input to the cathode follower indicates an
additional gain of 16 db.
Fig. 27 The output of the first low pass filter, the 6000 cps.
ripple id about 20 per cent of the signal.
Fig. 28 The output of the second low pass filter indicates a
ripple of about 8%.
Fig. 29 The secondary of the output transformer indicates
a reasonably smooth sine curve and shows the
overall fidelity of the system.
CONCLUSION AND FUTURE PLANS
A pulse duration modulated system was designed to incorporate
security against conventional detectors, irrespective of the
signal strength at the receiver.
This was designed on a basis of masking the signal in a back-
ground of white noise.
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The modulator and demodulator were tested and it was
found that with the use of a signal to masking noise ratio
of 1 to 2 there was no intelligibility. In addition, there
was no indication of the presence of signal in the noise.
The demodulator gave every indication of atmospheric
and/or industrial noise, with no audible sign of signal.
It is planned to test the transmitter and receiver for
operating range and system efficiency.
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FiG, FIG
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SENS1TIV IT Y- V/CM 35
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SENSITIVITY- V/CM ?
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SENSITIVITY- VC 18
5NEEP -,ct, SEC/CM 4,2
SIGNAL A-I
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SWEEP--u.. SEC/CN
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SIGNAL H-1 ?
Declassified in Part - Sanitized Copy Approved for Release 2014/03/05: CIA-RDP78-03153A001600010008-8
Declassified in Part - Sanitized Copy Approved for Release 2014/03/05: CIA-RDP78-03153A001600010008-8
SENSITIVITY- V/Cm 0-13 -
5VVEEP 5EL/cm to
SIGNAL I-1
SENSITIVITY- V/CM
SWEEP -Ax. SEC/CIVI
SIGNAL
FIG, FIG,
28 29
SENSITIVITY - V/CM 0.04-
SVNEEP-Ak SEC/CM I 30
siGNAL J-1
SE.NSITIVITY- V/CM
SWEEP-AA. SEC/CM
SIGNAL
Declassified in Part - Sanitized Copy Approved for Release 2014/03/05: CIA-RDP78-03153A001600010008-8