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 systems
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.
This exciting method of communication is reaching out beyond the frontiers of space.
By Herbert Kondo
From 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
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 carrier.
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 second.
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 (PCM).
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 a pulse).
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
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.
"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.
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 there.
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 July 1, 2019