Theory of AM, FM, and FM Stereo
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(A minor treatise on radio theory)
By: Steve J. Quest, BSEE
Preface
For most people associated with radio, either as users or producers, the
need to know how radio works is, for the most part, unimportant. For the
amateur radio broadcaster however, this need to know is crucial. Amateur or
`MicroPower' broadcasting is currently forbidden in many countries, tho its
popularity is growing. Since there is relatively little information
available to the MicroPower enthusiast, I have been asked to prepare a minor
treatise on the subject of radio theory. This primer will be written
specifically to educate those interested in MicroPower broadcasting, with no
assumption made to any prior electronic and/or radio knowledge. I hope this
text will prove very helpful in developing an intuitive understanding of the
principles of AM, FM, and FM Stereo radio broadcasting.
S. Quest
Jul 1, 1996
Chapter 1 - Overview of AM, FM, and FM Stereo.
The first method of transmission that was developed with the capability
of broadcasting voice and music was an Amplitude Modulated (AM) system. In
an AM system, the audio is conveyed from the transmitter to the receiver by
causing the amplitude (size proportions of the wave) of the Radio Frequency
(RF) carrier to vary in accordance with the audio waveform. Below is an
illustration of an audio waveform, an RF waveform and an AM waveform.
Vp+ .. ..
. . .
. . .
. . .
. . .
Audio 0 -------------------------------------------------
Sine wave . .
. .
. .
. .
Vp- ..
Vp+ ^ ^ ^ ^ ^
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
RF 0 -------------------------------------------------
Carrier . . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
Vp- v v v v v
Vp+ ^ ^
^ . . ^ . . ^
. . . . . . . . .
. . . . ^ . . . . .
. . . . . . . . . . .
AM 0 -------------------------------------------------
Waveform . . . . . . . . . .
. . . . v . . . .
. . .. . . . .
. . v v . .
Vp- v v
The frequency of the RF Carrier (the stations frequency) is assigned to
a given station by the Federal Communications Commission (FCC). Since it is
unlawful to operate as an unlicensed MicroPower radio station, one must
therefore choose their own frequency of operation. You must be _very_
careful, for if you were to assign yourself a frequency that interfered with
a commercial station that was outside of your reception area, but was being
received by someone inside your area (using a better receiver or antenna),
you can expect trouble with the FCC. I can not stress enough the importance
of non-interference with commercial broadcasting stations.
The AM broadcast band ranges from 540 Kilohertz (Khz) to 1.6 Megahertz
(Mhz). The stations are assigned so that the minimum separation between
carrier frequencies of adjacent stations is 10 Khz. Normally, in an average
population density, this separation is exaggerated proportionate to the
number of stations in the area, however the minimum separation between
adjacent stations must be at least 10 Khz.
A major disadvantage of an AM transmission system is its sensitivity to
electrical noise. This noise most commonly manifests itself as a rapid and
irregular variation of the amplitude of the RF carrier. The demodulator of
the AM receiver will recover such amplitude modulations and incorporate it
right along with the audio impressed upon the carrier. This disadvantage was
largely overcome however by the invention of Frequency Modulation (FM).
Under Frequency Modulation, the carrier wave deviates in frequency a small
range in accordance with the audio waveform. In an FM transmission, the
instantaneous amplitude of the audio signal determines the extent of the
deviation of the carrier from the selected center (operational) frequency.
The noise immunity of FM transmissions results primarily from the fact
that FM demodulators (the device that removes the audio information from the
modulated signal) respond to variations in frequency rather than to
variations in amplitude. By incorporating amplitude limiters or `clippers'
into the receiver, most of the amplitude variations that may have been
produced by undesired electrical noise can be removed, or clipped off. As a
result, FM transmissions are virtually free of noise as compared with AM. As
with any system however, FM is not 100% noise free. A certain amount of
noise will be generated inside the radio receiver itself, and no FM
demodulator is perfectly immune to amplitude variations.
It is also found that the high frequency portions of the audio spectrum
contribute more noise to FM reception than do the lower frequencies. The
highs therefore tend to have a lower Signal to Noise Ratio than the lows.
The noise contribution of the high frequency region can be reduced by
transmitting the highs at _increased_ relative volume levels and then
reducing the level by the same amount at the receiver. This boosting of the
highs at the transmitter is known as Preemphasis and the reduction of the
highs at the receiver is called Deemphasis. For realistic reproduction, the
amount of deemphasis at the receiver must equal the preemphasis at the
transmitter. Simple networks are utilized to achieve this as specified by
the FCC. The time constant for use within the United States is specified at
75 microseconds. In Europe and some other countries this time constant is
usually 50 microseconds.
Noise free reception by itself is not necessarily high-fidelity
reception (Hi-Fi). High-fidelity reception requires that all audio frequency
components in a musical passage be transmitted and reproduced at the
receiver. This in turn requires that the width of the transmission channel,
plus or minus from center, (bandwidth) be sufficiently wide to accommodate
the major portion of the audio spectrum.
The FM broadcast band ranges from 88 to 108 Mhz. Stations are located
in the band at 200 Khz intervals in odd tenths of a megahertz (e.g. 88.1,
88.3, 88.5 and so on.). Note that the FM broadcast band makes use of a
portion of the RF spectrum that is about 100 times as high as that used for
the AM broadcast band. The width of the transmission channel allocated for
each FM station is 200 Khz while the channel width for an AM station in the
AM band can not exceed 10 Khz. Given this greater channel width allotment by
the FCC, a much greater range of audio frequencies may be transmitted than is
possible with the narrow channels of the AM band. The FCC permits FM
transmission of audio frequencies ranging from 50 to 15,000 Hertz while AM
transmissions may not exceed 5,000 Hz. If AM audio impression was to exceed
5,000 Hz, the bandwidth would exceed the 10 Khz and thus interfere with
adjacent stations (10 Khz apart). The wider FM bandwidth is what accounts
for the higher fidelity of FM transmissions as compared with AM.
The FM-Stereo system is an enhancement of the original FM monaural
system and is capable of transmitting two independent audio channels via one
frequency modulated RF carrier. FM-Stereo receivers can receive stereo
broadcasts in stereo while monaural receivers can reproduce the same
broadcast monaurally with no detectable degradation of fidelity. In other
words, a monaural receiver is compatible with stereo signal transmission.
In regular monophonic FM transmission, the highest audio frequency
transmitted is 15,000 Hz. An examination of the system capabilities however
reveals that it is possible to transmit modulating frequencies up to 75 Khz,
well beyond the hearing range of humans. The modulating frequency spectrum
space between 15 and 75 Khz is used to transmit the second channel of audio
information required for FM stereo transmissions. The second channel of
encoded audio information is placed above the audio spectrum in the main
channel by amplitude modulating (with the carrier suppressed) a subcarrier
(carrier within a carrier) whose frequency is 38 Khz. The sidebands (a
modulated signal with suppressed carrier) of the modulated subcarrier are
then added to the modulating signal of the main audio channel to form a
composite FM signal that contains audio information that the stereo receiver
may process to form independent left and right audio channels for stereo
sound reproduction. The only difference between monaural and stereo
receivers is the fact that stereo receivers have added circuitry to derive
two independent channels (audio left and right) from the transmitted FM-
Stereo signal.
Chapter 2 - Aspects of Radio Waves.
It is a well known fact that radio waves are produced at the antennas of
radio transmitters and that these waves carry the impressed information
(acoustic audio, digital intelligence, etc.) from the transmitting antenna to
the receiving antenna. The following paragraphs will describe several
aspects of radio wave generation, propagation, and reception.
A simple radio transmitter consists of an RF oscillator, and RF
amplifier, and a transmission line that feeds the center of a simple (dipole)
antenna. Electron currents are caused to flow up and down the dipole by the
alternating voltage applied at the center of the dipole. Obviously, the
dipole antenna does not form a complete electrical circuit in the
conventional sense. The result is that electrical charges develop at the
ends of the dipole with one end being positive and the other being negative
alternately at the frequency of the carrier wave. This separation of charge
between the elements of the dipole produces an electric field in the space
surrounding the antenna. Further, a magnetic field is produced by the
electron currents flowing up and down the antenna. Thus, both electric and
magnetic fields are produced around the antenna and both exhibit sinusoidal
variation at the oscillator frequency. Since the oscillator drives the
antenna at RF frequencies, the rapid variations cause the electric and
magnetic fields to `break loose' from the antenna. When this happens, the
fields begin an outward journey from the antenna. The result is an
electromagnetic (EM) wave [a wave possessing both electrical and magnetic
properties]. The wave moves away from the antenna at the speed of light.
In most cases the orientation of the antenna determines the polarization
of the wave. Vertical antennas produce vertically polarized waves, while
horizontal antennas produce horizontally polarized waves. It is not
coincidental that waves emanating from a transmitting antenna travel at the
speed of light. In fact, light waves are EM waves of extremely high
frequency. Thus light waves and radio waves are different manifestations of
the same physical phenomenon, with only the frequencies being different.
Chapter 3 - Propagation of radio waves.
Natural atmospheric effects play an important part in the propagation of
radio waves. Solar energy produces layers of ionized (electrically charged)
air that surrounds the earth at an altitude of around 250 miles or less.
These layers are called the ionosphere. The intensity and altitude of the
layers varies with time of day, the season of the year and more importantly,
with solar activity (e.g. sunspots, solar flares, etc.). The fundamental
characteristic of the ionosphere is its variability.
The importance of the ionosphere to radio lies in the fact that charged
layers tend to refract (reflect or bend) radio waves back to earth that would
otherwise be ejected into space. The result is radio transmission over
distances much greater than that possible with the ground waves that remain
close to the surface of the earth. This phenomenon is called `skip'. The
ability of the ionosphere to return waves to earth diminishes at frequencies
above 30 Mhz. Since FM stations operate between 88 and 108 Mhz, they are not
as susceptible to ionospheric effects. FM stations are limited in range
because of the absence of skip, while AM stations are very susceptible to
ionospheric effects and can easily achieve transmissions ranging into the
thousands of miles. Since skip effects are most pronounced at night, many AM
stations (those not designated by the FCC as "clear channel" stations) are
forced to drop power or go off the air at sundown.
The strength or intensity of a radio wave is given in terms of the
strength of the electric field in units of microvolts per meter. The meaning
of this may be understood if we imagine two large metal plates separated by
a distance of 1 meter. If we placed a potential difference of 1 volt between
the two plates, the electrical field strength between the plates will be
1,000,000 microvolts per meter. If the field strength in a region is known,
the signal voltage produced in a simple straight-wire receiving antenna can
be computed by multiplying the field strength (in volts per meter) by the
length of the antenna (in meters). Inversely, measuring the voltage dropped
across a given length of antenna (one meter or multiples thereof) will give
you the field strength of a given area.
The propagation characteristics of broadcast band FM signals are
significantly different from those of AM as stated. This accounts for the
fact that FM reception distances are only slightly greater than line of
sight. Further, since higher frequencies are not reflected back to earth by
the ionosphere, long distance skip does not occur. Remember that the
principles behind this are the frequencies of the carrier waves, not the type
of modulation used.
FM broadcast waves are horizontally polarized, so horizontal dipole
antennas are normally used for external FM antennas. A horizontally
polarized beam type antenna (similar to TV antennas which are also
horizontally polarized) can also be used for greater transmission range but
only in a given direction. Omnidirectional antennas are primarily suited for
MicroPower radio broadcasts unless your intent is to focus your transmission
into a given region. Many MicroPower broadcasters utilize vertical
polarization antennas with horizontal ground planes in their designs.
Although not specified as being the "ideal" for the application, they do tend
to work well given the operation.
Chapter 4 - Principles of FM-Stereo.
The primary concern of the FCC when studying the various proposals for
FM stereo systems was that the system used should be compatible with existing
monaural FM receivers. The FM stereo system was not to affect monaural
reception to any perceptible degree. It was required that monophonic
receivers were to be able to receive the stereo transmissions and reproduce
them as monaural while the stereo receivers were able to receive the same
signal and reproduce full stereo separation. These were rather stringent
requirements, but through extensive thought and design, such a system now
exists and has been used for quite some time.
The FM `audio' spectrum as it was when FM stereo was proposed situated
the main audio portion from 50 to 15,000 Hz (as is presently used), and a
subcarrier was situated much higher, most often at 67 Khz. The subcarrier
and its sidebands occupied the spectrum between about 60 and 74 Khz. Thus it
was apparent that spectrum space was available from about 15 to 60 Khz which
could possibly be used to transmit a second channel of audio information.
The spectrum referred to here is the spectrum of the modulating signal. This
signal is fed to the FM modulator to produce a maximum deviation of the FM
carrier of 75 Khz.
A subcarrier is a part of the signal used to modulate the main carrier.
In FM transmissions it is far above the audio spectrum, so it produces no
sound at the receiver. For example, if a 67 Khz signal is added to the audio
at the transmitter the same 67 Khz signal will appear at the receiver at the
output of the FM demodulator. It can not be heard of course, due to its high
frequency. Generally, the subcarrier signal forms only a small portion of
the total amplitude of the modulating signal. It is possible to modulate the
subcarrier (AM or FM), and when this is done, sidebands are formed on both
sides of the subcarrier just as for the main carrier. Thus, the subcarrier
is capable of carrying a separate channel of audio information. All that is
required is that the circuits be added at the output of the FM detector to
pick off the higher frequency subcarrier and associated sidebands for
application to the appropriate subcarrier detector. Thus, an FM station may
transmit two programs at the same time, one on the main carrier and another
on the subcarrier, which itself rides on the main carrier. Such
transmissions are called Multiplex transmissions. In 1955 the FCC approved
subcarrier transmission of FM stations under the Subsidiary Communications
Authorization (SCA) to broadcast music to private subscribers (stores,
restaurants, etc.) provided with special SCA decoded receivers. The SCA
frequency is now standardized at 67 Khz.
The method decided upon for multiplexing FM stereo does not use the SCA
subcarrier method due to the fact that the monaural receiver would not
receive the total audio spectrum, only one channel of information. The
method used is called the `Left plus Right, Left minus Right' scheme of
multiplexing. A stereo program originates at two microphones (or comparable
devices) which generate a left (L) and a right (R) audio signal representing
the two stereo channels. If these two signals are combined in a _linear_
network, an output is produced which is the sum of the two, L + R. We can
then say that they have been added. The L + R output by itself is a monaural
reproduction of the original program, acceptable in all respects. Going
further, if the R signal is passed through an inverting amplifier operating
at unity gain, the resulting signal will be 180 degrees out of phase with the
original R. We denote the inverted signal as minus R (-R). If -R is now
added to the original L signal, the result is L - R. This signal is the
difference between the left and right channels. By itself it does not have
any entertainment value, but it is of paramount importance to FM stereo. If
the L - R signal is passed through an inverting amplifier at unity gain, the
result is -(L - R) or -L + R. This signal is also important to FM stereo.
Now suppose that we have a L + R and a L - R signal. How can the original L
and R signals be recovered from those combinations of L and R? First, add
the two combined signals (L + R) + (L - R) = L + R + L - R = 2L. We now see
that a pure left signal is attained. The amplitude is twice the original,
but this is of no consequence. Next, invert the L - R signal and then add it
to the L + R as in -(L - R) + (L + R) = -L + R + L + R = 2R. The result is
a pure right signal of twice the original amplitude. This demonstrates how
it is possible to recover the original L and R signals from the combinations
of L + R and L - R.
The system adopted for FM stereo functions as follows. The combination
of L + R signal modulates the main carrier so that monaural listeners hear a
combination of both L and R channels. A subcarrier located at 38 Khz carries
the L - R signal that, in stereo receivers, is used to effect channel
separation. Monaural receivers can not respond to the 38 Khz L - R signal as
it is way above the audio spectrum. A special type of AM modulation called
Double Sideband Suppressed Carrier (DSSC) is used for the 38 Khz subcarrier.
This is the same as ordinary amplitude modulation except that the carrier is
suppressed. Only the sidebands are transmitted. In other words, the 38 Khz
subcarrier is not transmitted, only the AM sidebands on both sides of the 38
Khz are transmitted. To demodulate a DSSC signal, the carrier must be
reinserted to restore the normal AM waveform. For Hi-Fi demodulation, the
frequency and phase of the locally generated carrier (which is reinserted)
must exactly match that of the original. To meet this requirement, a Pilot
Carrier at 19 Khz (half of 38 Khz) is transmitted along with the other
program material. This pilot carrier is used to guide the phase of the 38
Khz oscillator at the receiver. Audio frequencies from 50 to 15,000 Hz are
used to modulate the 38 Khz subcarrier. Therefore the sidebands associated
with the subcarrier range from 23 to 53 Khz.
The FM stereo transmitter consists of the following parts. First you
require a stereo audio source which may be studio microphones, tape or CD
units or similar items. The audio is then processed through an audio mixer,
preemphasis networks and a source switching arrangement. The L + R output of
the audio processing block goes to the adder input of the main FM modulator.
The FM stereo section begins with a 38 Khz oscillator which provides the
signal for the subcarrier and also applies the 38 Khz signal to a frequency
divider to obtain the 19 Khz pilot carrier which is delivered to the main
carrier modulator input. The DSSC balanced modulator receives the L - R
audio and the 38 Khz subcarrier and forms the L - R sidebands. The sidebands
are then applied to the adder input of the main FM modulator. The SCA
channel receives an SCA audio source, which due to the narrow bandwidth (7
Khz) may be of lower fidelity. The SCA signal is processed and is applied to
the main FM modulator as frequency modulated 67 Khz subcarrier and sidebands.
The main FM carrier channel begins with a carrier generator which is a
controlled oscillator running at the center frequency for your station
allocation. This carrier frequency is then frequency modulated by the
composite signal. A power amplifier array then raises the power level to the
proper level, and an antenna `flings' the multiplexed FM stereo signal out
into radioland.
--
-<squest@cris.com>---------\ ( ( | ) ) Amendment1 Congress shall make
============================> /_\ no law abridging the freedom
MicroPower FM Broadcasting-/ /\_/\ of speech, or of the press.
--- ifmail v.2.8.lwz
* Origin: AVR Technologies (1:340/13@fidonet)
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