Download Appendix: More on Why Community Stations Must Stay on FM

Appendix: More on Why Community Stations Must Stay on FM
As stated in the main body of the NCRF submission document, the NCRF strongly opposes the
proposal to move South African community radios from FM to AM/MW frequencies. Moving
community stations away from their existing FM frequencies to AM/MW frequencies would very
seriously undermine stations’ operations and sustainability
The general technical arguments for why FM, and not
AM/MW, is the appropriate medium for South African
community radio broadcasters, have been made in
Section 1 of the main body of the NCRF submission
document.
This appendix goes into more depth on those technical
arguments.
FM and AM/MW - Introduction
Radio waves are broadcast in a number of bands
which each contain a range of frequencies and have
different ways of travelling from point to point through the Earth's atmosphere. FM and AM/MW
broadcast frequencies operate in different ways and have very different characteristics
The difference is in how the carrier wave is modulated, or altered. With AM/MW 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.
The different characteristics of AM/MW and FM broadcasting influence the appropriate use of the
two different types of frequencies.
One important characteristic of the frequencies is related to the reaction to the ionosphere. AM/MW
and FM frequencies have different ionospheric refraction. AM/MW waves refract off the ionosphere,
meaning that they stay within the Earth’s atmosphere. FM waves on the other hand, go through the
ionosphere, and disappear into space.
Appendix – More on Why Community Stations Must Stay on FM
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Comparison between the main FM and AM/MW Characteristics:
Susceptibility
to slight
changes in
amplitude
Dynamic
range,
frequency
response
Stereo
capability
Broadcast
range
Increased
night-time
ionospheric
refraction
Transmitter
power, cost
FM
With an FM broadcast,
slight changes in
amplitude don't matter,
because the audio signal
is conveyed through
changes in frequency; the
FM receiver just ignores
changes in amplitude. The
result: no static at all.
FM radio, which came
into being in the 1930's,
uses a different approach
than AM. Its virtually
immune to any type of
external interference, it
has a greater dynamic
range, and it can handle
sounds of higher and
lower frequencies.
AM/MW
With an AM/MW
broadcast, changes in
amplitude result in
static.
FM broadcasting is a
low-power, low cost
technology.This is
because FM waves can
AM/MW is a
higher-power,
higher-cost technology.
This is because
Conclusion
Audio quality is far higher with an
FM broadcast than with AM/MW.
A lower quality sound would be
highly destructive to community
stations.
FM’s better dynamic range and
frequency response makes it
much better than AM/MW for
broadcasting music, which has
much greater frequency ranging
than the human voice. Music
sounds much better on FM radio.
FM provides high audio quality
suitable for both voice and music
(community radios are typically
licensed to carry 60% talk and
40% music).
Stereo
Non-stereo
FM allows for a stereo signal,
which is also essential to quality
music broadcasting. All music
recorded in the last 40 years is
intended to be played in stereo.
FM waves pass through
AM/MW waves refract
AM/MW is good for long-range
the ionosphere. They are off the ionosphere,
broadcasting, rather than
generally line-of sight.
bending around the
short-range. FM is ideal for
Earth.
short-range, small-footprint
broadcasting. South African
community radios generally
operate in small footprints,
making FM the more appropriate
broadcast system.
FM is not noticeably
Because of increased
Dealing with night-time
affected by the
night-time ionospheric
ionospheric refraction adds
ionospheric refraction,
refraction, at sunset it is complexity and costs to AM/MW
and thus is not noticeably often necessary for
transmission, and if the refraction
affected by the rise in
AM/MW radio stations
is not properly dealt with, the
such refraction in the
to reduce power and
signal is jeopardised. Community
evenings.
directionalise their
stations will be much better able
signal in order to avoid to achieve transmission skills
interference with other
sustainability, and financial
radio broadcast signals sustainability, using FM (which is
not affected by night-time
ionospheric refraction)
AM limits the
loud-to-soft range of
sounds that can be
reproduced (called
“dynamic range”) and
the high-to-low sound
frequency range (called
“frequency response”)
Appendix – More on Why Community Stations Must Stay on FM
South African community radio
stations that do their own FM
signal transmission are able to
do so with low-cost, low-power
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travel far even when
transmitted by a
low-power transmitter.
Set-up costs
The main costs of setting
up an FM transmission
system are the
transmitter, the mast, and
simple cabling and
grounding.
AM/MW waves need
much more power than
FM waves to reach their
destination. AM/MW
transmitters are
high-power, and much
more expensive than
the FM transmitters
needed to cover the
same broadcast
footprint.
transmitters – often 250-watt
transmitters. Others pay a
monthly fee to Sentech to send
out their FM signals.
Setting up an AM/MW
transmission system
not only requires a
much more expensive
transmitter, but also a
series of connected
masts, large land
space, and expensive
cabling and grounding
systems.
Setting-up a full AM/MW
transmission system would be
prohibitively expensive for a
community-owned radio.
Community stations would not be
able to raise the money
necessary to own their own
high-power AM/MW transmitters.
If they paid Sentech to send out
their AM/MW signals, the
Sentech monthly fees would be
unsustainably high.
One of the goals of the South
African community radio
movement is for all stations to
own their own transmission
systems, in order to safeguard
their independence. Pushing
community radio stations onto
AM/MW frequencies would
seriously undermine this goal.
Technical Explanation
A Step Back
The reason FM outperforms AM/MW in audio quality
is that AM is amplitude-modulated, making it unable to
send a clear stereo signal. FM does not have this
problem, because it modulates the carrier frequency.
As a result, you get more information, i.e. full
frequency range, frequency response, and a stereo
signal. The spectrum of the audible frequencies
(20-20KHZ) with FM is covered (depending on limiting
and clipping etc.) at around 2000-16000 Hertz,
whereas AM is limited in the spectrum, with the high
and low frequencies getting "cut off". Typically, you
have around 6-8 times more effective bandwidth in FM
transmission than in AM/MW transmission. (Note: AM
means “Amplitude Modulation” a modulation
technique; MW means “Medium Wave,” which is the
band/frequency on which AM is carried. For the
purposes of this submission, we mostly refer to them
together, as “AM/MW.”
FM means “Frequency
Modulation,” carried on what are known as FM
frequencies.)
In the mid 1930s, Inventor Edwin Armstrong
saw that several problems existed with AM
frequency broadcasting:
 static interference from household
appliances and lighting
 limited audio quality (frequency
response and dynamic range)
 night-time interference between
many stations (co-channel
interference), because of ionospheric
refraction, especially in rural areas.
So he invented FM.
.
Appendix – More on Why Community Stations Must Stay on FM
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