جاسم محمد جاسم محمد: االسم
هندسة المعلومات واالتصاالت: القسم
الثانية \ صباحي: المرحلة
FM MODULATION
Definition
Frequency modulation (FM) conveys information over a carrier wave by varying its
frequency which is directly proportional to the instantaneous value of the input (carried) signal.
Basics
In telecommunications, frequency modulation
(FM) conveys information over a carrier wave by
varying its frequency (contrast this with amplitude
modulation, in which the amplitude of the carrier is
varied while its frequency remains constant). In analog
applications, the instantaneous frequency of the carrier is
An audio signal (top) may be
directly
carried by an AM or FM radio
proportional to the instantaneous value of the
wave.
input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete
values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of
جاسم محمد جاسم محمد: االسم
هندسة المعلومات واالتصاالت: القسم
الثانية \ صباحي: المرحلة
music and speech. Normal (analog) TV sound is also broadcast using FM. A
narrowband form is used for voice communications in commercial and amateur
radio settings. The type of FM used in broadcast is generally called wide-FM, or
W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve
bandwidth. In addition, it is used to send signals into space.
FM is also used at intermediate frequencies by most analog VCR systems,
including VHS, to record the luminance (black and white) portion of the video
signal. FM is the only feasible method of recording video to and retrieving video
from magnetic tape without extreme distortion, as video signals have a very large
range of frequency components — from a few hertz to several megahertz, too wide
for equalisers to work with due to electronic noise below -60 dB. FM also keeps
the tape at saturation level, and therefore acts as a form of audio noise reduction,
and a simple limiter can mask variations in the playback output, and the FM
capture effect removes print-through and pre-echo. A continuous pilot-tone, if
added to the signal — as was done on V2000 and many Hi-band formats — can
keep mechanical jitter under control and assist timebase correction.
FM is also used at audio frequencies to synthesize sound. This technique, known as
FM synthesis, was popularized by early digital synthesizers and became a standard
feature for several generations of personal computer sound cards.
جاسم محمد جاسم محمد: االسم
هندسة المعلومات واالتصاالت: القسم
الثانية \ صباحي: المرحلة
Applications in radio
Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in
Radio Signaling by a System of Frequency Modulation", which first described FM
radio, before the New York section of the Institute of Radio Engineers on
November 6, 1935. The paper was published in 1936.
Wideband FM (W-FM) requires a wider bandwidth than amplitude modulation by
an equivalent modulating signal, but this also makes the signal more robust against
noise and interference. Frequency modulation is also more robust against simple
signal amplitude fading phenomena. As a result, FM was chosen as the modulation
standard for high frequency, high fidelity radio transmission: hence the term "FM
radio" (although for many years the BBC called it "VHF radio", because
commercial FM broadcasting uses a well-known part of the VHF band; in certain
countries, expressions referencing the more familiar wavelength notion are still
used in place of the more abstract modulation technique name).
FM receivers employ a special detector for FM signals and exhibit a phenomenon
called capture effect, where the tuner is able to clearly receive the stronger of two
stations being broadcast on the same frequency. Problematically however,
frequency drift or lack of selectivity may cause one station or signal to be suddenly
overtaken by another on an adjacent channel.
جاسم محمد جاسم محمد: االسم
هندسة المعلومات واالتصاالت: القسم
الثانية \ صباحي: المرحلة
Frequency drift typically constituted a problem on very old or inexpensive
receivers, while inadequate selectivity may plague any tuner.
An FM signal can also be used to carry a stereo signal: see FM stereo. However,
this is done by using multiplexing and demultiplexing before and after the FM
process, and is not part of FM proper. The rest of this article ignores the stereo
multiplexing and demultiplexing process used in "stereo FM", and concentrates on
the FM modulation and demodulation process, which is identical in stereo and
mono processes.
A high-efficiency radio-frequency switching amplifier can be used to transmit FM
signals (and other constant-amplitude signals). For a given signal strength
(measured at the receiver antenna), switching amplifiers use less battery power and
typically cost less than a linear amplifier. This gives FM another advantage over
other modulation schemes that require linear amplifiers, such as AM and QAM.
In audio and music frequency modulation synthesis (or FM synthesis) is a form
of audio synthesis where the timbre of a simple waveform is changed by frequency
modulating it with a modulating frequency that is also in the audio range, resulting
in a more complex waveform and a different-sounding tone.
Refrence:
Source: http://www.juliantrubin.com/encyclopedia/electronics/fm.html
