FM radio works the same way that AM radio works.
The difference is in how the carrier wave is
modulated, or altered. With AM radio, the amplitude, or overall strength, of the signal is varied to
incorporate the sound information. With FM, the frequency (the number of times each second that the
current changes direction) of the carrier signal is varied.
FM signals have a great advantage over AM signals. Both signals are susceptible to slight changes in
amplitude. With an AM broadcast, these changes result in static. With an FM broadcast, slight
changes in amplitude don't matter -- since the audio signal is conveyed through changes in
frequency, the FM receiver can just ignore changes in amplitude. The result: no static at all.
AM (or Amplitude Modulation) and FM (or Frequency Modulation) are ways of broadcasting radio
signals. Both transmit the information in the form of electromagnetic waves. AM works by modulating
(varying) the amplitude of the signal or carrier transmitted according to the information being sent,
while the frequency remains constant. This differs from FM technology in which information (sound) is
encoded by varying the frequency of the wave and the amplitude is kept constant.
AM FM
Stands for AM stands for Amplitude Modulation FM stands for
Frequency
Modulation
Origin AM method of audio transmission was first FM radio was
successfully carried out in the mid 1870s. developed in
the United
states in the
1930s, mainly
by Edwin
Armstrong.
Modulating In AM, a radio wave known as the "carrier" or In FM, a radio
differences "carrier wave" is modulated in amplitude by the wave known as
signal that is to be transmitted. The frequency and the "carrier" or
phase remain the same. "carrier wave"
is modulated in
frequency by
the signal that
is to be
transmitted.
The amplitude
and phase
remain the
same.
Pros and cons AM has poorer sound quality compared with FM, FM is less
but is cheaper and can be transmitted over long prone to
distances. It has a lower bandwidth so it can have interference
more stations available in any frequency range. than AM.
However, FM
signals are
impacted by
AM FM
physical
barriers. FM
has better
sound quality
due to higher
bandwidth.
Frequency Range AM radio ranges from 535 to 1705 KHz (OR) Up to FM radio
1200 bits per second. ranges in a
higher
spectrum from
88 to 108 MHz.
(OR) 1200 to
2400 bits per
second.
Bandwidth Twice the highest modulating frequency. In AM Twice the sum
Requirements radio broadcasting, the modulating signal has of the
bandwidth of 15kHz, and hence the bandwidth of modulating
an amplitude-modulated signal is 30kHz. signal
frequency and
the frequency
deviation. If the
frequency
deviation is
75kHz and the
modulating
signal
frequency is
15kHz, the
bandwidth
required is
180kHz.
Zero crossing in Equidistant Not equidistant
modulated signal
Complexity Transmitter and receiver are simple but Tranmitter and
syncronization is needed in case of SSBSC AM reciver are
carrier. more complex
as variation of
modulating
signal has to
beconverted
and detected
from
corresponding
variation in
frequencies.(i.e.
AM FM
voltage to
frequency and
frequency to
voltage
conversion has
to be done).
Noise AM is more susceptible to noise because noise FM is less
affects amplitude, which is where information is susceptible to
"stored" in an AM signal. noise because
information in
an FM signal is
transmitted
through varying
the frequency,
and not the
amplitude.
History
AM method of audio transmission was first successfully carried out in the mid 1870s to produce
quality radio over telephone lines and the original method used for audio radio transmissions. FM
radio was developed in the United states mainly by Edwin Armstrong in the 1930s.
Differences in Spectrum Range
AM radio ranges from 535 to 1705 kilohertz, whereas FM radio ranges in a higher spectrum from
88 to 108 megahertz. For AM radio, stations are possible every 10 kHz and FM stations are
possible every 200 kHz.
Pros and Cons of AM vs. FM
The advantages of AM radio are that it is relatively easy to detect with simple equipment, even if
the signal is not very strong. The other advantage is that it has a narrower bandwidth than FM,
and wider coverage compared with FM radio. The major disadvantage of AM is that the signal is
affected by electrical storms and other radio frequency interference. Also, although the radio
transmitters can transmit sound waves of frequency up to 15 kHz, most receivers are able to
reproduce frequencies only up to 5kHz or less. Wideband FM was invented to specifically
overcome the interference disadvantage of AM radio.
A distinct advantage that FM has over AM is that FM radio has better sound quality than AM
radio. The disadvantage of FM signal is that it is more local and cannot be transmitted over long
distance. Thus, it may take more FM radio stations to cover a large area. Moreover, the presence
of tall buildings or land masses may limit the coverage and quality of FM. Thirdly, FM requires a
fairly more complicated receiver and transmitter than an AM signal does.
Popularity
FM radio became popular in the 1970s and early 80s. By the 1990s most music stations switched
from AM and adopted FM due to better sound quality. This trend was seen in America and most
of the countries in Europe, and slowly FM channels exceeded AM channels. Today, speech
broadcasting (such as talk and news channels) still prefers to use AM, while music channels are
solely FM.
Technical Details
AM was initially developed for telephone communication. For radio communication, a continuous
wave radio signal called double sideband amplitude modulation (DSB-AM) was produced. A
sideband is a band of frequencies higher (called upper sideband) or lower (called lower
sideband) than the carrier frequencies which is a result of modulation. All forms of modulations
produce sidebands. In DSB-AM the carrier and both USB and LSB are present. The power usage
in this system proved inefficient and led to the double-sideband suppressed-carrier (DSBSC)
signal in which the carrier is removed. For greater efficiency, single-sideband modulation was
developed and used in which only a single sideband remained. For digital communication, a
simple form of AM called continuous wave (CW) operation is used in which the presence or
absence of carrier wave represents binary data. The International Telecommunication Union
(ITU) designated different types of amplitude modulation in 1982 which include A3E, double
sideband full–carrier; R3E, single-sideband reduced-carrier; H3E, single-sideband full-carrier;
J3E, single-sideband suppressed-carrier; B8E, independent-sideband emission; C3F, vestigial-
sideband and Lincompex, linked compressor and expander.
FM radio characteristics and services include pre-emphasis and de-emphasis, stereophonic FM
sound, Quadraphonic sound, Dolby FM and other subcarrier services. Pre-emphasis and de-
emphasis are processes that require boosting and reducing certain frequencies. This is done to
reduce noise at high frequencies. Stereophonic FM radio was developed and formally approved
in 1961 in the USA. This uses two or more audio channels independently to produce sound heard
from various directions. Quadraphonic is four-channel FM broadcasting. Dolby FM is a noise
reduction system used with FM radio, which has not been very successful, commercially.
Below is an old training video from the U.S. Army that talks about the technical workings of AM
and FM radio.
In radio communication, the message signal wave (low frequency) is combined with a carrier signal
(high frequency). In this combination, one or more characteristics of the carrier wave are varied with
respect to message signal. This variation is termed as modulation and it is needed so that message
can be transmitted over long distances and no undesired signal mixing takes place. Depending on
several factors such as range, application and budget, modulation can be casted into three
types: Amplitude Modulation, Frequency Modulationand Phase Modulation. Out of these three
types, the former two are widely known as they form a major commercially applicative part of radio
communication. In this article, we will discuss common difference between AM and FM which will
enhance our learning in terms of these two technologies.
1. Evolution: Formulated in the 1870s, AM is a relatively older modulation process compared to FM
which was found in the 1930s by Edwin Armstrong.
{ 2. Technology: AM stands for amplitude modulation where the amplitude of the carrier is
modulated as per the message signal. The other aspects of the carrier wave such as frequency
phase etc. remain constant. On the other hand, FM means frequency modulation and in it only
frequency of the carrier wave changes while amplitude, phase etc. remain constant.
3. Frequency range of working: Amplitude modulation works between 540-1650 KHz while FM works
at 88-108MHz.
4. Power Consumption: FM based signal transmission consumes a higher amount of power than an
equivalent AM based signal transmission system.
5. AM vs FM: Signal Quality: Signal quality is a lot superior in FM than AM as amplitude based
signals are more susceptible to noise than those which use frequency. Moreover, noise signals are
difficult to filter out in AM reception whereas FM receivers easily filter out noise using the capture
effect and pre-emphasis, de-emphasis effects. In capture effect, the receiver locks itself to catch
stronger signal so that signals received are more synced with that at the transmitting end.
In pre-emphasis, de-emphasis process, the signal is further amplified to a higher frequency at
sending end (pre-emphasis) and vice versa at receiver end (de-emphasis). These two processes
reduce down the chances of a signal to get mixed with other signals and make FM more immune to
noise than AM.
6. Fading: Fading refers to power variation during signal transmission. Due to fading, the power with
the signal received can vary significantly and reception wouldn’t be of a good quality. Fading is more
prominent in amplitude modulation as compared to frequency modulation. That is why, AM radio
channels often face the problem where sound intensity varies while FM radio channels have constant
good reception.
7. Wavelength Difference between AM and FM: AM waves work in the range of KHz while in FM
waves work in MHz range. As a result, AM waves have a higher wavelength than the FM ones. A
higher wavelength increases the range of AM signals as compared to FM which have a limited area
of coverage.
8. Bandwidth consumption: AM signals consume 30KHz of bandwidth for each while in FM 80KHz
is the bandwidth consumed by each signal. Hence, over a limited range of bandwidth, more number
of signals can be sent in AM than FM.
9. Circuit Complexity: Aforesaid, Amplitude Modulation is an older process and has a very simple
circuitry. On the other hand, frequency modulation requires a complicated circuitry for transmission
and reception of signal. The signals sent in FM are more modulated and emphasized at the
transmitter and they are thoroughly checked and corrected at the receiving end. This is why circuitry
for FM signals is very complicated.
10. Commercial Aspects: Setting up an AM based radio communication system is very economic
as there is no complicated circuitry and processes are easy to understand.
AM and FM are two very popular and very different methods of sending information over the
airwaves. AM is amplitude modulation while FM is frequency modulation. But what is modulation? It is
the act of modifying a certain aspect of the carrier frequency in accordance to the information being
sent. It is then clear that AM modifies the amplitude of the carrier frequency while FM modifies its
frequency.
AM is the older an easier of the two technologies to implement. The receiver detects the changes in
the carrier frequency’s amplitude and amplifies it to drive a speaker. The simplicity of the technology
made it easy to build radio receivers in great quantities. The main problem associated with AM
broadcasting is the fact that it is very susceptible to various weather conditions that deteriorate and
distort the signal. The simplicity of the design also limits the broadcast to a single audio channel,
making it inadequate for stereo sound.
FM is a development over AM broadcasting and it provides a lot of substantial benefits thus it is much
more complex compared to AM. The first and most substantial benefit is its ability to send out two
channels of information at the same time with the use of advanced algorithms. This allows the station
to broadcast left and right audio channels for full stereo sound. Since most environmental factors that
distort radio waves only affect its amplitude and not the frequency where FM stores the actual voice
signal, the data in the FM signal doesn’t degrade as easily as AM. This also means that FM signal
quality doesn’t degrade linearly as you get further from the transmitting station.
Range wise, AM takes the cake with its much greater distance travelled. FM signals usually drop of at
around 50 miles from the station, but AM waves can be refracted in the atmosphere resulting in
greater range. The range is also one of the reasons why talk radio prefers AM even if the sound
quality is not very high.
The advancement in technology also meant that the degree in complexity and price between AM and
FM receivers have become moot. Most manufacturers can even manage to place the whole circuitry
for both AM and FM inside a single chip, turning them into a package instead of competing against
each other.
Summary:
1. AM broadcasting is simpler than FM but the difference in complexity and price are very marginal at
present.
2. AM is more prone to signal distortion and degradation compared to FM.
3. FM doesn’t degrade linearly with distance.
4. AM usually broadcasts in mono which makes it sufficient for talk radio.
5. FM can transmit in stereo making it ideal for music.
6. AM has a longer range than FM.
