In the United States, the FCC (Federal
Communications Commission) decides who is able to
use which frequencies for which purposes, and it
issues licenses to stations for specific
frequencies. Other countries has their own
regulating organizations. It is the operator's
responsibility to run the transmitter in
accordance with the frequency regulating authority
of their country.
General broadcasting links
The most commonly used radio broadcasts are
the FM radio broadcasts operating at around
88-108 MHz frequency band. FM broadcasting
offers around 50 Hz to 15 kHz bandwidth with
stereo sound. The deviation used in FM broadcast
is 75 kHz (peak deviation). Typical transmitter
power for FM station can be from 10W up to 100
kW.Those broadcasts were originally mono
broadcasts, but were later converted to stereo.
The stereo broacasts use special technique to be
compatible with mono receivers. The reason for
this is that in order to avoid the wrath of all
the ownersof monaural FM sets, the FCC in its
wisdom decreed that a "compatible"system
would be necessary before they would approve FM
stereo. Theengineers quickly noted that the A+B
signal from two microphones gives apassable
monaural signal (especially if
"one-point" miking is used).Now the
problem was how to get A and B out of A+B. To do
this the (A-B) signal is also needed. The way to
do the stereo to send (A-B) in some clever
wayand the receiver could reconstruct A and B by
"matrixing". The methodadopted was to
"multiplex" this (A-B) signal onto the
main carrier byusing it to modulate a SUBcarrier
located at 38 KHz. Double-SidebandSuppressed
Carrier (DSBSC) modulation was chosen. This gave
a "lowersideband" extending downward
from 38KHz (less 20Hz or so) to 23KHz(because
the highs were cut off at 15 KHz.) A "pilot
carrier" was putat 19 KHz which allows the
receiver to recover the precise frequencyphase
of the 38 KHz carrier so that recovery of the
(A-B) signal couldproceed. The compatiblity was
good, because pilot at 19KHz and A-B from 23 to
38 kHz are so high in frequency that most people
wouldn't hear them, andmost older monaural FM
sets and loudspeakers won't reproduce
themaudibly anyway. In addition to the stereo
broadcasts, some radio stations also transmit
extra SCA audio (for example to transmit MUZAK)
with their broadcasts. This audio is transmitted
usign FM broascast at 67 kHz carrier (uses 53 to
81 kHz frequency range). The ratings of FM
transmitter, antenna, radio-frequency cable, and
tower depend upon the desired coverage and its
geography. Ordinary FM receivers can catch
signals on limited coverage ares. Sophisticated
ones can also receive the broadcasts from longer
distance.Several test instruments are essential
in an FM station. Most critical are spectrum
analyser, audio analyser, FM demodulator, field
strength meter, RF power meter, oscilloscope,
multimeter and programme amplifier. A backup
power system (usually generator) is necessary to
keep the station working when normal supply
fails. Modern FM radio stations usually nowadays
heavily process the audio before they transmit
it. Typical processing is the FM broadcasting
following: Audio Amplifier adjusts level of
input signals from left and right channels
torequired intensity. Usually this stage
includes nowadays some some form of automated
signal level control, compression and/or
limiting. Stereo Coder converts the left and the
right channel signals into .L+R. and .L-R.
elements. It multiplexes them with a
synchronising pilot-signal of 19 KHz. It can
also combine signals of traffic radio, radio
data system, or Subsidiary Communications
Authorisation channels. Modulator superimposes
the signal on Carrier Frequency. Synthesiser can
set transmitter's Rated Frequency in steps
(usually 10 KHz steps over the entire range
88-108 MHz). It synchronises the signal with
Reference Frequency (from stabilized cystal
source). Audio Amplifier, Stereo Coder,
Modulator and Synthesiser together make up the
Exciter portion of an FM transmitter. Power
Amplifier intakes a weak signal from exciter and
amplify it to the high power that is sent to the
transmitting antenna. A normal FM station
transmits at power of hundreds or thousands of
watts. There are strong incentives for music to
be processed in such a waythat it's
"louder" and more attention-getting,
when played over FMradio. There are a bunch of
reasons for this, having to do withcompetition
between stations (the station with the loudest-soundingsignal
is believed to have a better chance of
"grabbing" a listenerthan one with a
quieter-sounding signal) and the conditions
underwhich popular music is often played back
(in cars, boomboxes, Walkman-and MP3-players
with cheap headphones, etc. in conditions with
highambient noise levels). Most popular-music FM
station directors feelthat it's to their
advantage to squash the &*^%$ out of the
music thatthey broadcast.Usually the music
dynamics are squashed downinto a couple of dB of
actual dynamics when they are broadcasted to FM
radio stations. Automatic dynamics controlling
and compressing equipment also help the people
working at the radio studio: you don't have to
be very careful how loud you play back
something, everythign comes out at around same
volume (in earily history there was a separate
people for controlling the audio level and
quality that gets transmitted out).FM adio
broadcasting have differences between USA and
Europe. North American FM broadcast channels are
on the odd 200KHz frequencies: 99.5MHz,
100.1MHz, etc. In Europe, channels can be on any
multiple of 100KHz, even or odd. This means that
some digital tuner from the USA will not tune
European stations properly. FM stations in
Europe use a different pre-emphasis than those
in North America, 75 and 50 microseconds
respectively. Using wrong pre-emphasis has
effect on the frequency response in reception.
Am broadcasting is the first voice
broadcasting system in use and it is still with
us. In amny thigns AM is worse than FM on
technical perspective. From a technical
perspective, there should be no contest between
AM and FM. The 9 kHz RF channels in the LF and
MF bands set the maximum audio bandwidth for AM
at 4.5 kHz. In practice, the frequency response
of most AM radios is typically -3 dB at 3.5 kHz,
whereas FM offers 15 kHz bandwidth as well as
stereo. AM reception is relatively stable even
on moving car, this is why some people prefer it
on car and on voice programs. At the end of the
1990s, AM listening is showing signs of decline.
AM remains viable for news and sports services,
but is less likely to be successful for music
formats.
Local broadcast stations in USA use an
amplitude-modulated (AM) transmitter connected
to a vertical antenna with a minimum of 120
buried radials for a ground plane. The radiated
field has a strong ground wave extending as far
as several miles from the antenna. This ground
wave can cause noise problems to other systems.
Communications towers at a height nearly equal
to the quarter wavelength of the AM station
frequency not only influence the radiation
pattern of the broadcast station, but also
provide considerable energy in the ground return
of the tower. If your tower is within the
influence of a broadcast tower or electrical
substation, you should expect some of this
energy to appear in the ground system of your
station. A tower grounded at its base without
other connections will not bother anyone other
than to produce pattern disturbance. However, a
closed current loop is provided when conductive
appendages such as microwave dishes, VHF, UHF
and 800MHz/900MHz antennas are attached. Cables
leaving the tower at some elevation usually are
attached to the electronic equipment, thus
providing another current path separate from the
tower ground. Often the shunt current is of
sufficient magnitude that it interferes with the
station ground.
Sky-wave propagation is generally regarded as
a disadvantage in the MF bands used for AM
broadcasting. However, it also offers the
possibility of covering large areas with a
single transmitter, especially in the MF and HF
bands. Where there are low levels of both
co-channel interference and man-made noise,
sky-wave coverage is very attractive for
international broadcasting. A major problem for
sky-wave services is that multipath propagation
through the ionosphere causes time-varying
selective fading.
AM broadcasting bands are nowadays adapting
also to new digital technologies. The
combination of advanced digital modulation
schemes with new algorithms for the digital
compression of audio signals offers tremendous
potential - even within 9 kHz or 10 kHz RF
channels. Digital systems can offer enhanced
performance - probably giving performance
equivalent to monophonic FM services - whilst
being much less fragile than AM in terms of
immunity to interference and selective fading.
Digital Radio Mondiale (DRM) is investigating
such systems, with the objective of agreeing a
singlestandard for digital radio in the AM
bands. This could be used as the long-term
replacement of AMbroadcasting in the HF bands,
as well as in the LF and MF bands. Ideally, the
DRM solution will be applicable to existing AM
transmitters with only minor modifications.
Unfortunately, the real cost of switching from
AM to digital services is in thepurchase of
millions of new radios that listeners need to
buy to get those new services. AM broadcast
stations un USA have frequencies is 10 kHz
steps. In Europe AM station frequencies have
generally 9 kHz steps.
- AM
broadcasting - is the end in sight? -
The introduction in the 1950s and 1960s of
FM broadcasting sounded the death-knell for
AM broadcasting. As FM was so much better
than AM, it seemed obvious that AM would
disappear within 10 years - or 20 years at
the most. In fact, AM is still with us. Why
has it refused to lay down and die?
- AM
Broadcasting: The New Cellular Design Factor
- As cellular systems have sprouted
towers across the landscape, a seemingly
unlikely conflict has arisen between
cellular operators and old-fashioned AM
broadcast stations. Awareness of this
important, but obscure, situation has just
been heightened by the Federal
Communications Commission (FCC). Typical
cellular towers are just the right height to
reradiate signals transmitted from AM
broadcast stations.
- A
Chronology of AM Radio Broadcasting
- Using
Synchronized Transmitters for Extended
Coverage in AM Broadcasting - This paper
provides an overview of "simulcasting"
the technique of extending radio broadcast
coverage areas by using multiple
transmitters operating on the same
frequency. The paper discusses how
simulcasting works and new solutions that
address specific technical obstacles that
have historically limited use of this
technology. This paper is primarily directed
at chief engineers, directors of
engineering, professional engineers and
consulting engineers who desire a technical
introduction to simulcasting and transmitter
synchronization.
- DRM
(Digital Radio Mondiale) - DRM is a full
stereo fully digital broadcast system. The
quality of the broadcasts are close to that
of FM radio but transmitted on normal AM
bands.
- Technical
Standards and Requirements for AM
Broadcasting Transmitters (BETS-5) -
This document contains the technical
standards and requirements for the issuance
of a Technical Acceptance Certificate (TAC)
for AM broadcasting transmitters. The
information in this document is specific to
Canada.
- Low
Power Broadcasting FAQ (Frequently Asked
Questions)
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