August 1962 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
are hereby acknowledged. See all articles from
(Random Access and Correlation for Extended Performance), was an
early form of frequency hopping spread spectrum (HFSS) devised by
the Martin Company (now Lockheed-Martin). It was used for secure
voice communications and worked by sampling speech in small 'slices'
and then transmitting each slice modulated onto a carrier whose
frequency was determined by a predetermined sequence of center frequencies.
A receiver with a matching sequence key would then decode the speech
and, with appropriate filtering, reassemble it into its original
content. President John F. Kennedy had such systems at his disposal
whether on the road or in the Oval Office. General Electric (GE)
had a different idea it dubbed 'Phantom' that spread the signal
over a very wide bandwidth. Today, we refer to it as Direct Sequence
Spread Spectrum (DSSS). Not to be outdone, Hughes Aircraft devised
the 'Vocoder,' which is akin to synthesized speech where a series
of numbers, each representing a single syllable, is played back
sequentially to emulate a human voice. When this article was published
in 1962, the required circuitry took up a lot of volume, required
a lot of power, and was very costly. Today, an IC for a couple bucks
does the job a million times better and consumes a few milliwatts.
See all articles from
The Secret Keepers
By Ken Gilmore
latest methods of radio communications defy detection by any listener
- friend or foe.
Most radio communications systems are like
"party lines" - anyone can listen in. But electronics scientists
have been working overtime to come up with the equivalents, radio-wise,
for the more desirable (and costly) "private lines." Their objective:
to allow our military and government officials to transmit secret
information on the air with the full assurance that it can be "received"
only by those listeners it is intended for.
Perhaps the best
known gadget of this kind is President Kennedy's "scrambler." Thanks
to this device, the transmitters in his private automobiles and
airplanes take his words and turn them into a kind of electronic
"hash." Then a special receiver which is set for the right "code"
unscrambles the hash and turns it into intelligent speech again.
The result is that no unauthorized listener can eavesdrop on the
Electronics engineers are coming
up with a number of devices to allow "private radio communications.
And some of them - already being tested by the armed services -
do the job by performing a series of ingenious electronic tricks.
The Orlando Division of the Martin
Company has come up with a system called RACEP (short for Random
Access and Correlation for Extended Performance) . One of the more
promising schemes to insure secrecy on the airwaves, RACEP is based
on a principle that is really quite simple - electronic circuits
are capable of switching millions of times a second, but our ears,
by comparison, are very slow.
Therefore, suppose an electronic
circuit were designed to snip tiny samples out of words being spoken.
Let's say this circuit takes 8000 such samples every second, and
that each sample is one microsecond long.
The equivalent of a private telephone system, the Martin Co's
RACEP needs no wires or central switching facilities.
Another new communications device, Hughes' vocoder, "condenses"
speech into basic sounds, reproduces it artificially.
The RACEP system transmits during only 1 microsecond out of
125, but its "chopped-up" waveforms contain enough data for
Pulse-modulated RACEP equipment similar to that shown above
handles as many as 70 separate conversations at the same time
and on the same frequency.
Vocoder transmits up to ten conversations in a bandwidth normally
If General Electric's "Phantom" were used on the standard broadcast
band, its carrier would occupy about the same space as 20 conventional
stations. But only a special wide-band receiver specifically
"cued" to a particular Phantom transmitter would respond to
Now suppose you're talking by radio and speak a 1-syllable word
which has a fundamental frequency of 200 cycles - about average
for a man's voice. During one cycle of your voice signal, the sampling
circuit will take 40 1-microsecond samples.
The pulses generated
by this sampling technique will trace out the shape of your voice
waveform quite accurately. Using just these pulses, decoding equipment
at the receiving end can reconstruct the original 200-cycle voice
signal so well that the human ear can't tell it from the original
"unsliced" signal. Your voice, in other words, has been transmitted
faithfully by a series of pulses.
Now, to take it one step
further, suppose the transmitter keeps shifting its frequency, so
that each pulse is sent out on a different wavelength. A receiver,
in order to pick up this tricky signal, must be set to synchronize
with the pulses at the proper repetition rate. And, at the same
time, the receiver must keep changing frequency exactly in step
with the transmitter, so that it's tuned in to each pulse at the
right time and at the right frequency.
Your words will be
heard clearly on this special receiver, of course, but they'd be
lost on any radio not set up to receive them properly. Military
planners are excited about RACEP because it would be almost impossible
for enemy electronics experts - even if they knew the principles
involved - to analyze the waveforms and build equipment capable
of intercepting and untangling the scrambled RACEP signals.
Another big advantage: a RACEP user can call any receiver whose
code he knows, simply by setting up his transmitter to broadcast
its pulses in that code. Battlefield units could call each other
as easily as dialing a telephone.
Let's say you want to
call receiver 35. Just as you can call a friend on the telephone
if you know his number, you could call receiver 35 by dialing its
number on your transmitter. The code you dial sets up your transmitter
to broadcast a series of coded pulses at a specific repetition rate.
Furthermore, each of the pulses is sent out on a slightly different
frequency. Each receiver, on the other hand, is set up to receive
signals which are broadcast at a predetermined pulse rate and which
change frequency in a pre-determined pattern.
If you transmit
the pulse pattern which receiver 35 is set up to receive, its operator
will hear your words as clearly as though you were speaking over
a regular radio. Other receivers, not set to detect this particular
combination of pulse rate and frequency changes, very likely won't
hear a thing.
RACEP brings with it another advantage,
too. Your voice is sampled only one microsecond out of every 125.
The system, then, is working for one microsecond, and idle for 124.
Your transmitter is on the air only 1/125 of the time you are speaking,
so many other transmitters can be operating in the same frequency
band at the same time without interfering with you or with each
other. Even if an occasional pulse does happen to synchronize with
another in both time and frequency, this slight interference would
be so brief as to be unnoticeable.
at the Martin Company have found that scores of conversations can
be going on simultaneously in a band about 4 mc, wide without seriously
interfering with each other. Even in such busy systems as air-to-ground
radio, each individual is using his radio only a small percentage
of the time. Therefore, systems planners estimate that up to 700
receivers could be operating in one area with the RACEP system.
RACEP isn't the only new
communications system. General Electric researchers have come up
with an entirely different approach which they call "Phantom."
The principle, again, is rather simple. A radio transmitter
- one used by a regular commercial radio station, for example -
may broadcast on a carrier frequency of 1000 kc. If it broadcasts
a 5000-cycle note - about the highest frequency transmitted by most
AM stations - this signal modulates the carrier so that the final
output signal contains frequencies between 995 and 1005 kc. Engineers
call this a bandwidth of 10 kc. (1005 - 995 = 10 kc.).
Your receiver has a bandpass of about 10 kc., too. As you tune across
the dial, you shift the position of this bandpass. When you tune
to 1000 kc., the bandpass is centered around this frequency so that
you receive all frequencies between 995 and 1005 kc. and thus hear
the program the station is transmitting.
system, however, would stretch the audio signal over an extremely
wide band of frequencies - perhaps 200 kc. or more. The transmitted
signal, then, would cover a band of frequencies from 900 to 1100
kc. Since it is spread over such a wide area, only a tiny fraction
of the signal would fall within the bandpass of an ordinary receiver.
It wouldn't be possible to tune in on the wideband
Phantom signal simply by having an extra-wideband receiver, either.
If you had this kind of setup, a jumble of stations broadcasting
on frequencies within the band You were covering would come tumbling
in. To get around this problem, Phantom designers "tag" the transmitted
signal with a special waveform. The Phantom receiver lets in only
signals which are identified by this waveform and rejects all others.
You may have heard Phantom broadcasts without knowing
it. General Electric has transmitted Phantom signals more than 2000
miles across the country to test the system. Because this special
waveform is spread over such a wide frequency band, its amplitude
in the bandpass of any normal receiver is very low - so low that
you wouldn't notice it even if you happened to be tuned in somewhere
on the broad band of frequencies across which the Phantom signals
go skittering. And if your receiver were sensitive enough to hear
the Phantom signal, you would probably think it was just ordinary
Incidentally, GE engineers who didn't know
the exact waveform tried to intercept the messages during the test
transmissions, just to see whether an enemy could break the "code."
The results: they couldn't. Said one, "It's like a combination lock.
Even if you know the principle on which it works, that doesn't mean
you can open it without knowing the combination of the particular
lock you want to open."
Phantom systems can use literally
thousands of "combinations" or special identifying waveforms, and
they can also change from one to another rapidly. Thus, even if
someone happened to stumble on the code accidentally - about as
likely as opening a combination lock by chance-it wouldn't do him
much good. Next time he tried, the combination would have been changed.
Engineers at Hughes Aircraft have come up with still another way
to transmit messages secretly, although the gadget they use to do
it wasn't originally developed for that purpose. Their basic approach,
as a matter of fact, isn't even new.
Back during the
1930's, Bell Laboratories scientists built a gadget they called
a "vocoder." It consisted of a cabinet full of sound generators,
filters, and other circuitry, and it was designed to create a reasonable
facsimile of the human voice. If you turned on the right combination
of circuits and did it fast enough, the vocoder produced a series
of speech-like sounds.
These electronically generated
words were quite intelligible. In fact, Bell's vocoder created a
sensation at the New York World's Fair in 1939, where an operator
played it from a keyboard much like that on a piano. By pressing
the right combination of keys in the right sequence, he could make
the vocoder "speak" whole sentences.
in the secrecy sweepstakes makes use of the old vocoder principle.
Essentially, the spoken words to be transmitted are fed into an
analyzing circuit which determines several important characteristics
of the various sounds which go to make up each word - pitch, intensity,
and so on. This information, electrically coded, is sent on to a
receiver, which, much like the earlier Bell Labs unit, turns these
signals into intelligible speech.
The voice signal
to be transmitted is applied to the inputs of a series of 12 bandpass
filters. The output of each filter is determined by how much sound
energy the word or syllable being spoken contains in that particular
Since the outputs from these circuits
are rectified, the sound energy going through a particular filter
shows up as a d.c. voltage. The louder the sound applied to the
input of any specific filter falling within that filter's frequency
range, the higher the voltage at the output of that filter.
is actually based on an idea that dates back to the 1930s. Its reconstructed
naturally sounds somewhat different than the original
voice, but it is still intelligible.
A final circuit - called the pitch extractor - finds out
two things. First, it determines the presence or absence of pitch.
And second, if sounds with a definite pitch are present, it determines
By way of explanation, a vowel -
an "a," for example - is produced when our vocal cords generate
a sound of a certain frequency. A consonant, on the other hand -
such as an "s" - is a less specific sound (a hiss, in this case),
requires no movement of the vocal cords, and is at no particular
The pitch extractor transmits an encoded
electrical signal which determines whether pitch is present, and,
if so, what its frequency is. The signals from the pitch extractor
and the 12 filters go to a time multiplexer which forms them into
a single composite signal for transmission by radio.
At the receiving end, a time de-multiplexer splits up all of the
signals again and sends each one to its proper circuit. The signal
from the pitch extractor is applied to a relay, which turns on one
of two circuits. If there is no pitch present at the transmitter,
the relay turns on a "hiss generator" which produces white noise.
If pitch is present, the relay activates a "buzz generator" which
puts out a sound rich in harmonics and similar to that produced
by the human larynx. The buzz generator operates at the same fundamental
frequency that the pitch extractor detected in the speech at the
Now, either the hiss or the buzz
(depending on which one happens to be present at any given moment)
is applied to the inputs of all the bandpass filters in the receiver.
Suppose. at one particular moment, that the person back at the transmitter
is saying "a." The fundamental frequency of his "a" might be 300
His particular voice quality - the characteristics
of his voice which allow his friends to distinguish his speech from
someone else's - is determined, among other things, by the relative
strengths of the various harmonics of this basic 300-cycle tone.
Let's say. for example, that the second harmonic-600 cycles - is
twice as strong as the fundamental, and that the third harmonic
- 900 cycles - is half as strong as the fundamental.
Again, for the sake of illustration, let's say that bandpass filter
No. 1 at the transmitter has put out a signal of 4 volts, corresponding
to the intensity of the 300-cycle fundamental. Bandpass filter No.3,
carrying the second harmonic, would have put out a signal twice
as large - 8 volts. Filter No.5, transmitting the third harmonic,
would have produced only 2 volts.
At the receiving
end, these signals of varying strengths are applied to corresponding
filters. Number 3, then, amplifies the output of the buzz filter
- which, you'll remember, is operating at the same 300-cycle fundamental
- twice as much as number 1 and four times as much as number 5.
The result is a sound very close to the original "a" spoken into
The vocoder was originally designed
to squeeze voice signals into a narrower bandwidth and make space
for more messages in the crowded radio spectrum. And it does this
very efficiently. The encoding vocoder generates 13 signals: one
from each of the 12 filters and one from the pitch extractor. Each
of these 13 signals can be squeezed into a channel just 25 cycles
wide, and all 13 taken together require a total bandwidth of only
Normally, communications channels such
as those used by the military, commercial airlines, and so on, are
some 3000 cycles wide - about the same as a telephone channel. With
the vocoder, about nine conversations can be squeezed into the band
space usually taken up by only one.
A vocoder operating
as described above is said to be an analog device, that is, the
voltage output of the separate circuits varies continuously as the
input signals change, and these constantly changing values are transmitted
continuously. But the vocoder can also be operated as a digital
encoder and decoder.
When operated digitally, a sampling
circuit checks each of the individual circuit outputs some 50 times
a second. The series of pulses obtained by this method is transmitted
to a receiver where an unscrambler separates the various pulses.
Then, it sends each to the circuit in the receiver corresponding
to its counterpart in the transmitter.
As you may
have guessed, digital operation gives the vocoder several outstanding
advantages. First, it can operate reliably in the presence of tremendous
amounts of interference - amounts which would paralyze an analog
system; consequently, a digital system is far harder to jam. Second,
signals from a digital vocoder can easily be encoded - by turning
them into a kind of electronic "hash" something like that used with
President Kennedy's scrambler. Then, a special unscrambler at the
receiving end turns the scrambled signals back into words. To anyone
listening without an unscrambler set specifically for the message
being transmitted, the signal sounds like pure gibberish.
Thus, with such tricky electronic devices as these, our military
forces and government officials can have all the advantages of radio's
instant communications. And they can also have another advantage
that radio has seldom offered - the assurance that their messages
have traveled through the ether in such a manner that only the persons
they are intended for will ever know what they were all about.