October 1960 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Pulse modulation comes in many forms, including pulse position modulation
(PPM), pulse width modulation (PWM), pulse frequency modulation
(PFM), pulse amplitude modulation (PAM), and pulse code modulation
(PCM). In addition to providing a nice introduction to the concept
of pulse modulation, author Herbert Kondo covers the basics of each
type and then discusses their application in various communications
systems. The first time I recall encountering pulse modulation
was in the mid-1970s with radio control system for model airplanes.
Pulse position modulation was the scheme used in both AM and FM
sets. Modern R/C systems use frequency hopping spread spectrum (FHSS),
direct sequence spread spectrum (DSSS), or a combination thereof.
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This exciting method of communication is reaching out beyond the
frontiers of space
By Herbert Kondo
Satellite 1959-delta the message came loud and clear: a huge belt
of electrons circles the planet earth thousands of miles out in
space. Our 1959-delta had further jolting news: the outer Van Allen
radiation belt, once thought to expand after a solar eruption, actually
shrinks. Even more striking was the news that there is a huge interplanetary
"atom smasher" centered about the sun.
Satellite 1959-delta, commonly known as "Explorer VI," had a
lot more to say. But how it said it is just as interesting as what
it said. A great deal of Explorer VI's information was sent by a
five-watt transmitter that used pulse modulation, the most sophisticated
modulation system known today. So important is this new communications
system that it is already used for telegraphy, radar, multi-channel
microwave transmission, and telemetry, as well as space communications.
Basic Theory. The idea of pulse modulation has
been around a long time. In telegraphy, the familiar "dots and dashes"
of the Morse code are pulses produced with a switch or key. Ham
operators have long been using a form of pulse modulation when they
key their high-frequency transmitters to send out pulses of electromagnetic
energy in code. Television servicemen come across a form of pulse
modulation in the gated-beam tube.
The principle behind the pulse modulation system is actually
ridiculously simple: information is impressed on a train of pulses
instead of directly on a continuous-wave carrier. But if it's as
simple as that, why all the excitement about it? What does pulse
modulation have that more familiar forms of modulation - AM and
FM - don't have?
For one thing, pulse modulation offers practically noise-free
transmission and reception - even more so than FM. To visualize
this concept, let's consider a train of ideal pulses - pulses with
vertical sides, as shown in (A) of Fig. 1. Noise is picked up during
transmission, resulting in the waveshape shown in (B). With suitable
clipping and limiting circuits, we can reproduce only that part
of the pulse signal between the dotted lines, as shown in (C). Having
done this, we can then re-transmit this new signal free of noise.
Fig. 1. - Original signal amplitude of pulses (A) is
affected by noise in transmission (B). Electronic dipping
restores original signal (C).
Fig. 2. - Information contained in the modulating signal
in (A) is shown as it would appear using the various pulse
transmission methods (B through F). Binary numbers corresponding
to signal amplitudes can be transmitted in the PCM system.
Pulse modulation has another outstanding advantage. It uses transmitter
energy more efficiently than either AM or FM because of the simple
"on-off" nature of the pulses. This means that a pulse transmitter
will have a longer range than an AM transmitter of the same power.
All pulse modulation systems boil down to two basic principles:
(1) A message signal modulates a train of pulses which are applied
to a subcarrier. (2) The subcarrier then modulates a high-frequency
The relation of a subcarrier to a carrier can be made clear by
an analogy. Let's suppose that there are five messenger boys on
the same subway train in New York City. Each boy is carrying a message
to a different destination (receiver). If we think of the subway
as the carrier, then each messenger boy is a subcarrier. The message
each boy carries is the modulated signal.
Sampling. The most important idea in pulse modulation
is sampling, a concept which we come across almost every day. For
example, if you've never heard a stereophonic recording, you can
listen to a "stereo sample" record and get a good idea of what stereo
is like. Another widely known use of sampling is the public-opinion
poll which bases its findings on selective sampling techniques.
If we want to transmit a conversation by pulse modulation, we
take samples of the conversation - thousands of samples each second
- and then transmit them in the same order in which they were spoken.
Each pulse is actually a single sample; its height, width, or position
indicates the instantaneous value of the sound sent.
For good reproduction, it has been shown that the number of samples
per second must be greater than twice the highest frequency of the
signal we wish to send. Thus, if the highest frequency in a telephone
conversation is 4000 cps, we must take at least 8000 samples each
Types of Modulation. Another basic concept in
pulse modulation is the modulation itself. When we modulate a carrier
wave, we ordinarily alter its amplitude (AM), its frequency (FM),
or its phase (PM). The nice thing about a pulse is that there's
another characteristic we can use for modulation, namely, time.
If we alter the timing of the pulses, we are effectively changing
their position relative to one another - this is actually done in
pulse position modulation (PPM). In pulse width modulation (PWM),
we alter the width of the pulses; in pulse frequency modulation
(PFM), the frequency of the pulse changes. We can also alter the
amplitude of the pulses to produce pulse amplitude modulation (PAM).
And we can even code the pulses, as is done in pulse code modulation
Let's take a closer look at all of these pulse modulation techniques
and find out how a sine wave - see Fig. 2(A) - is transmitted in
each system. Later, we'll see how pulse width modulation and pulse
code modulation are used in transmissions from satellites and in
multi-channel telephone communications.
PPM. Pulse position modulation, widely used
in radar and in microwave relays, depends on a modulating signal
varying the position of the pulses. A separate generator produces
a series of marker pulses which act as reference points. With PPM,
the relative position of the signal pulse and the marker pulse are
important, as shown in Fig. 2(B).
PWM. In pulse width modulation, the width or
duration of the pulses varies directly in accordance with the modulating
signal, as shown in Fig. 2(C). Also known as pulse duration modulation
(PDM), PWM varies either the leading or the trailing edges, or perhaps
even both edges, of the pulses. For example, if the leading edges
of the pulses were spaced at equal time intervals, the trailing
edges could then be varied (displaced in time) in accordance with
the amplitude of the modulating signal. Since pulse width modulation
requires relatively simple circuitry, it is the ideal type of pulse
modulation for use in outer space vehicles.
PFM. Pulse frequency modulation is somewhat
similar to ordinary FM, except that the basic carrier consists of
equally spaced pulses rather than a sine wave. The occurrence of
the pulses varies with the amplitude of the modulating signal, as
in Fig. 2(D).
PAM. In pulse amplitude modulation, the height
of the pulses varies directly in accordance with the modulating
signal, much like the amplitude modulation of a continuous-wave
(c.w.) carrier. In Fig. 2(E), the positive-going portion of a sine
wave increases the height of the pulse train, while the negative-going
portion of the signal decreases the height.
Fig. 3. - Satellites can send a number of messages
over a single transmitter by sampling each signal with a rotating
commutator, then converting the sampled information to PWM signals
for transmission to earth.
PCM. Pulse code modulation uses the presence
or absence of a pulse to convey information. In the sample shown
in Fig. 2 (F), the code makes use of a group of four positions,
which may be "filled" with either a pulse or a space (absence of
PWM in Outer Space. If we were to make a block
diagram of the telemetry system used in the Vanguard rocket, it
would break down into the five simple blocks shown in Fig. 3. (See
"Telemetering - Vital Link to the Stars," in the November 1959 issue
of Popular Electronics for a complete discussion of telemetry.)
In Fig. 3, a rotating sampling switch - called a commutator -
samples a number of contacts which are connected to devices that
measure outer space data (cosmic and ultraviolet rays, X-rays, etc.).
Information from the contacts is then sent to the keyer which triggers
a one-shot multivibrator (itself a special type of PWM generator).
With this arrangement, the multivibrator produces pulse signals
whose width varies in accordance with the information (voltage)
supplied to it by the commutator and keyer. The PWM signals are
fed to the oscillator, which modulates the transmitter that sends
satellite performance information to earthbound receiving stations.
"Explorer I," which discovered the Van Allen radiation belt, also
used pulse width modulation. The initial output of the cosmic ray
channel, which carried the Van Allen radiation information, was
a pulse width signal which then frequency-modulated a subcarrier
oscillator. The subcarrier, in turn, phase-modulated the carrier
of the satellite's transmitter. This rather complex sequence of
modulation techniques also occurred on the cosmic dust transmissions
from Explorer 1.
In the PCM system, amplitude of actual signal (A) is
sampled at regular intervals. The samples are rounded off
to whole-number pulse amplitudes - a quantized signal (B)
- and then converted to binary numbers. Binary code chart
(C) gives decimal value of binary numbers.
PCM in Communications. Of all forms of pulse
modulation, the most exciting is pulse code modulation. Says a one-time
Bell Telephone Laboratories scientist: "It's the most sophisticated
communication technique around. It has the advantage of an extremely
high signal-to-noise ratio, plus the added element of secrecy. PCM
is statistical in nature, and it's hard to jam any statistical communication
system - the less predictable the system, the harder it is to design
electronic countermeasures against it."
Suppose you bought a VTVM kit for $29.17, tax included. If a
friend asked you how much you paid for it, you might tell him that
it cost $30.00. Would you be lying? Not at all - you are perfectly
justified in rounding off the numbers to the nearest easily remembered
figure. People are doing this sort of thing all the time. The same
technique is used in pulse code modulation.
For example, if the amplitude of the signal we wish to send is
4.7 volts, PCM would send it through as 5 volts; if the signal amplitude
is 2.37 volts, PCM would transmit it as 2 volts. This simplification
is necessary because the signal has to be coded, and the code uses
only whole numbers.
Let's suppose we want to send the signal shown in Fig. 4 (A).
Sampling pulses sense the amplitude of the signal to be transmitted.
Pulse A, which has a value of 3.2 volts, is changed to an amplitude
of 3 volts as shown in Fig. 4(B). Pulse B, which has a value of
3.8 volts, is changed to an amplitude of 4 volts. This process of
simplifying the original signal in terms of whole numbers is called
quantizing the signal; the result is known as a quantized signal
- see Fig. 4(B).
Once the signal is quantized, it must be coded for transmission
(hence the name, pulse code modulation). For this, the binary code
is used (see "The Language of Digital Computers," Popular Electronics,
January 1958, p. 68).
Each quantized pulse representing the amplitude of the signal
at a given point must be changed into a group of pulses in the PCM
binary code. Always keep in mind this distinction between the quantized
pulse and the pulse group: the quantized pulse is a sampling pulse,
whose value will be determined by its amplitude; the pulse group
represents the original signal in binary language.
In a binary pulse group, only the presence or absence of a pulse
has meaning. If the code is a three-pulse group, as shown in Fig.
4(C), then the far-right position has a value of 1 if a pulse is
present, or 0 if the pulse is absent. The middle position would
have double the first position's value, or 2, if a pulse were present,
but would again have a value of 0 if there were no pulse. The far
left position would have double the value of the middle position,
or 4, if a pulse were present, but a value of 0 if no pulse were
Suppose our quantized pulse has a value of 3. Then, in a three-pulse
binary code, there would be a pulse in the far right (1) and middle
(2) positions only (1 + 2 = 3). If the quantized pulse has a value
of 7, then all three pulses in the group would be needed (1 + 2
+ 4 = 7).
With a three-pulse binary group, we can send out the waveshape
shown in Fig. 4(B) using any of seven values. For greater "fidelity"
in reproducing the waveshape, we would need a large number of samples,
and larger binary pulse groups would be required. A five-pulse
group, for example, gives 32 different amplitudes; a seven-pulse
group gives 128 different amplitudes.
The binary-coded signal is ultimately fed to an r.f. transmitter,
which is turned alternately on and off by the binary pulses.
Multiplexing and PCM. Bell Telephone Laboratories has many plans
for pulse code modulation. For example, they envision a 24-voice-channel
PCM telephone system which would allow 24 people to talk at the
same time over a single line.
If you've had any experience with present-day "party lines,"
you know it's impossible for two people to talk over the
same line at the same time. How, then, can 24 people do it? The
answer is multiplexing, a kind of sampling technique. The type
used in telephony is time-division multiplexing.
Let's consider a case where six people are sharing a single telephone
line. Three of them are talking in city A and three are listening
in city B. By means of a rotating commutator in city A, each speaker
is rapidly hooked up to the line in succession. At the same time
a second commutator in city B, synchronized with the commutator
in city A, samples the line and distributes each speaker's voice
to the intended listener in city B. It's possible to have as many
as 176 simultaneous conversations over a single line using PCM.
Multiplexing, incidentally, is the method used by earth satellites
to transmit different types of information back to earth. Instead
of hooking up 24 talkers in sequence, we can hook up 24 transducers
which give information about temperature, cosmic ray density, magnetic
field strength, etc. Each transducer modulates a subcarrier oscillator,
which in turn modulates the regular high-frequency carrier. Both
time-multiplexing and PCM were used in the Explorer VI.
PCM offers great possibilities as a television transmission
system, and Bell Labs is actively at work on this idea also. In
microwave radio, PCM promises practically interference-free transmission.
And since a PCM signal is easily applied to magnetic tape, it is
ideal for missile and satellite telemetering as well.
Compared to other forms of pulse modulation, PCM has the sole
disadvantage of a wider bandwidth requirement. But as telemetry
systems move from the lower megacycle bands to the 2200-mc. region,
this disadvantage becomes less and less important.
An Exciting Future. Pulse modulation is no longer
just theory-it is a reality. Young as it is, pulse modulation is
the giant behind the front-page news of space exploration.
As we explore the frontiers of outer space, and as we search
for ways to improve and increase the information-handling capacity
of our existing communications systems, it becomes increasingly
evident that pulse modulation is one of the most exciting developments
of modern electronics.
Posted May 14, 2014