AM or FM?

Abstract

In this paper AM and FM radio stations have been explored. I have reviewed the history

of each of the broadcasting schemes. The research question to be explored is: Why is

FM considered to be better than AM? My research shows that the AM radio is not

popular because of problems with interference; also the sound quality is not very good

either as compared to FM. At the same time, AM is a cheaper broadcast mechanism than

FM as it is easier to set up and maintain an AM radio station. FM might have a better

sound quality, however it is more expensive. Also the FM waves do not bend around the

earth like AM stations do. Thus makes the FM waves disappear into space quicker even

though they have a better coverage than AM signals. space than AM signals.

Problem Statement

I will be looking at the history of radio, from AM to FM. I want to understand how radio

came to be and why there is an AM and FM. I think it would help me understand some

of the reasoning behind the history of technology.

Research Background – History of Radio

The term radio means several different things to different people. To some, the

term radio is limited to the golden age of broadcasting between 1920s to the 1940s

where a sophisticated broadcasting service provided a wide array of programs for people

of varied listening tasters. People belonging to this category barely separate the technical

aspect of broadcasting from the programs and radio stars. To other, radio broadcasting

takes on a business aspect where huge and growing chains of radio stations operated

with syndicated programs and heavy advertising. And for still others the term radio does

not mean broadcasting at all, but instead refers to the transmission of voice and data,

amateur or ‘ham’ station operators or even broadcast reception on the internet. (Sterling,

Encyclopedia of Radio, Volume 1, 2003) None of these meanings are mutually

exclusive in defining the term radio; they just serve to illustrate the pervasive role of

radio in human society.

Wireless communication started with simple long and short burst of signals, representing

dots and dashes, in the work of Marconi and Morse and quickly developed in to radio

telegraphy – an indispensable part of erstwhile commerce and emergency services,

especially rescue of life and property at sea. Soon however, the idea of using speech,

rather than code took hold of the imagination of inventors, researches, and even

amateurs. The earliest experiments involved spark transmitters, which failed as they did

not generate continuous waves and hence could not transmit speech. Some

experimenters like Charles Herrold tried using telegraphic instruments themselves, but

this method too failed as the speech was found to be distorted to the point to

unsuitability. (Greb & Adams, 2003)

The success of voice transmission using radio is entirely attributed to the development

of wave-generating systems in the first decades of the 20th century. Following this

development, several inventors like Reginald Fessenden from Canada, Lee de Forest,

Charles Herrold and Frank Conrad from America successfully carried out experiments

with radio to transmit voice and music. Their successes led to the production of wireless

phone or radio-telephone that allowed people to communicate over distances without

wires, person to person. Most experiments during this period concentrated on the best

means of achieving two-way communication with radio. The system they were trying to

perfect is now referred to as narrowcasting. (Greb & Adams, 2003)

These early radios were made by hand, were hard to adjust and maintain, and delivered

uneven and sometimes unpredictable performance. This meant that there was a lot of

interference which was a primary concern for initial radio researchers. Some radical

thinkers however began to explore the premise of how deliberate broadcasting i..e one

person sending messages to many, would actually be a beneficial things – and radio

broadcasting was born at this point. (Greb & Adams, 2003; Sterling, Encyclopedia of

Radio, Volume 1, 2003)

AM broadcasting history

AM radio broadcasting history can be directly traced to the work of early radio

researchers like Frank Conrad, Lee De Forest, Reginald Fessenden, and Charles Herrold,

as mentioned above. Indeed the first technology to be used for radio broadcasting was

based on the amplitude modulation (AM) of the radio signals. Two underlying electronic

discoveries were instrumental in the development of AM radio – the invention of the

vacuum tube in 1904 by Fleming that laid the foundation of radio communication and

the invention of the vacuum triode amplifier in 1906 by De Forest that made radio

broadcast possible. AM broadcast was initiated for the first time in 1920 when radio

station KDKA, Pittsburgh went on the air. (Rastogi, 2010)

By 1941 all US commercial stations – about 600-700 at any one time – operated only

with AM transmission and competed only with other AM outlets. The radio industry at

this time was small and friendly. By 1941, the largest cities had a dozen stations, while

smaller towns had only one or two stations, and large parts of the country had no local

radio service at night. (Sterling & O'Dell, 2009)

From the day it started, AM radio broadcasting grew rapidly across the country and

around the world. The technology used amplitude modulation to modulate radio signals

in the long and medium wave bands. This technology was employed in the first 50 years

and had sufficient good quality to develop an expanding and enthusiastic radio audience

in virtually every country of the world. National coverage can be easily provided by one

or more high-power AM transmitters operating in the long and medium wage bands.

Also if the quality was not a limitation, the spectrum requirements were relatively

undemanding. (Woods & Woods, 2005) AM radio frequencies range from 550 to 1600

kilohertz. AM signals bounce off ionosphere, so that they can be received a long

distance from where they are transmitted. However, when the antenna is not of a

sufficient quality of when the station’s signal is weak, AM can develop considerable

interference, which is one of its main limitations. (Huff, 2001)

FM broadcasting history

The frequency modulation (FM) history was first envisioned by an American named

John Carson in 1922. Another man, Howard Armstrong patented the technology in

1933. However, few radio companies were initially interested in the idea mainly because

AM was already well entrenched in the broadcasting field. In fact, up until the World

War II, AM was the only available mode of military broadcast, even though Armstrong

had given free patent rights to the military during the World War II. It was not until the

end of the World War II that FM broadcast started gaining in popularity and developed

commercially. (Rastogi, 2010; Sterling, 2003)

Armstrong found that using a channel 20 times wider than AM i.e. about 200kHz

created an analog signal that had a much better frequency response and could avoid both

artificial and environmental interference. An FM signal need only be twice as strong as

more distant competing transmitter to suppress the interfering signal. However, FM

signals are propagated by direct line-of-sight means day or night, which limits the

transmitter coverage to a radius of less than 60-70 miles depending on the local terrain.

(Sterling & O'Dell, 2009)

The FM development opened the door to technological development in radio with the

discovery that FM signals operated on the same wavelength and did not interfere with

each other as in case of AM signals. The FM receivers simply picked up the stronger

signals. This meant that FM stations could be placed closer together as compared to the

AM stations. This in turn allowed more radio stations to be established. It was now

possible to pick FM radio signals in places where AM signals could not reach. This

remains true even today. It is impossible to tune in to any AM station while travelling

through a tunnel or a bridge and also it is difficult to find the signal in any place below

certain levels. FM broadcasting first started in the 1950s in an effort to overcome the

vagaries of propagation at the lower radio frequencies and also to very significantly

reduce problems associated with amplitude noise and propagation disturbances. Thus

FM transmissions could offer high fidelity radio making it more popular than the AM

for long, medium and short wave bands. FM broadcasting was essentially a local service

requiring multiple transmitters operating on different frequencies in order to provide

national coverage. As can be it, it was and still is very inefficient in terms of its use of

radio frequency spectrum. (Woods & Woods, 2005; Russo, 2010)

The growth of FM broadcasting was initially very chaotic. At the very start Armstrong

lost the fight to regain the 40MHz band for FM that had been a part of its broadcasting

license prior to the 2nd World War. The new band allotted was 88 to 103 Mhz which was

initially problematic owing to the difficulty in obtaining transmitters in the range. In fact

by the end of 1948 over 400,000 prewar FM receivers became useless because of FCC’s

decision to reallocate the frequency bands to FM. FM also faced additional problems for

its expansion as it had to compete not only with the AM radio but also the recently

introduced service of television. The AM operators jointly owned FM stations as well

and the higher cost of implementation made FM the 2nd choice while setting up newer

stations as compared to AM stations. Duplicated programming on many AM-FM

stations also made the newer radio service pointless to most potential listeners and as

few FM networks offered special programming, there was little reason to switch to a

newer service. (Sterling & O'Dell, 2009)

However, as mentioned the FM transmission was superior than AM transmission and the

differences in quality became increasingly obvious with the rise of rock music and its

many branches in 1960s such as psychedelic, folk rock, metal, and reggae. Further, the

shift from the 45 record to MP meant that the 3-minute song limit of AM increasingly

couldn’t handle the interests of sixties audiences and the demands of their music. By late

1960s FM had gained popularity among music lovers shift started the shift of popular

allegiance from AM to FM, which has never shifted back since then. (Hilmes, 2011)

Comparing AM & FM

Today the audience share enjoyed by FM radio is about 7 percent. Currently the AM

radio is primarily limited to news and talk radio.

The table below compared the Amplitude Modulation and Frequency Modulation

techniques:

Amplitude Modulation Frequency Modulation

Amplitude of carrier is varied according to

the amplitude of modulating signal

Frequency of carrier is varied according to

the amplitude of modulating signal

AM has poor fidelity due to narrow FM has better fidelity due to larger

bandwidth bandwidth

AM is inefficient as most of the power is in

the carrier and hence is wasted

In FM all the transmitted power is useful

There is significant noise interference There is minimal noise interference

AM broadcasts operates in MF and HF

range

FM broadcasts operates in VHF and UHF

range

In AM only the carrier and two sidebands

are present

In FM theoretically infinite sidebands are

present

AM transmission equipment is simple FM transmission equipment is complex

Transmitted power varies according to

modulation index

Transmitted power is constant and does not

vary according to modulation index

Modulation depth is limited and cannot be

increased beyond 1

Modulation depth has theoretically no limit

and can be increased by increasing

frequency deviation

(Chitode, 2009; Singal, 2010)

The table below presents a comparison of AM and FM broadcasting technology

AM Broadcasting FM Broadcasting

Medium wave band (550-1705 kHz). Each

station operates on 10 kHz wide channel,

107 AM channels are assigned to many

stations in different areas to reduce

interference

Since 1945 band shirt, FM allocation is in

the 88-105 MHz band. Each station is

assigned a channel of 200 kHz with several

channels assigned in different areas

Medium wave signals propagate by both

sky and ground waves and coverage varies

by time of day

VHF signal utilize direct line-of-sight

waves which results in limited coverage,

though the coverage pattern does not

change with time of day

Static cannot be electronically separated or

eliminated, unless it is 20 times more

powerful than an interfering transmitter

Static is separated and eliminated at the

FM receiver, though multipath fading is

possible due to line-of-sight propagation

AM radio channel can pass an audio signal

10 kHz wide, though most receivers are

limited to 5 kHz or less cutting out much

of the base and some of the treble tones

audible to human ear

FM stations can transmit audio signals

with bandwidth up to 15 kHz allowing

broadcast of most tones audible to human

ear

(Keith & Sterling, 2009; Frost, 2010)

Conclusion

The radio industry has changed dramatically from its beginnings. The start of the

radio transmission was accompanied by a push towards AM broadcasting scheme. As it

was an easier and cheaper mode of transmission and was a tried and tested technology, it

had no competition from other technologies. From its inception, FM had remained in the

background. While leading radio executives were aware that the static-free sound of FM

was far superior to the sound produced by AM, it was seen as a luxury addition and not

a broadcasting necessity. However, in the later 1960s and 1970s, FM started to come on

its own. This was because of the emergence of new music and younger audience who

were ready to experiment and who had more discerning ear for music. With a large

section of audience preferring the quality provided by FM, it interested advertisers. This

gave the required push to FM broadcasting. Even now FM happens to be the broadcast

station of choice for popular music and hardly anyone is known to tune specifically to

AM for this purpose.

The discussion above may have given the impression that FM broadcasting has

shut down the AM broadcasting for good. However this is not the case. AM

broadcasting is still extremely popular in many parts of the world. Countries such as

Russia, India, China, Brazil, and many African countries use the technique as they need

to cover very large areas. AM transmissions are very well suited due to the far reaching

wave propagation. With the advent of digital radio, the quality of services in AM band –

the chief cause for concern – has improved drastically and is as good as FM, depending

on environmental conditions. However, in European countries AM is not considered to

be a suitable option as the operational costs are high and the far reaching properties of

the system means that the regional broadcasters have to add additional measures for

combating interference. In this situation FM is the more attractive option. (Beutler,

2011) The present research has shown that the two broadcasting technologies are here to

stay in their niche areas in the foreseeable future.

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