Frequency modulation

Frequency modulation (FM) is the encoding of information in either analog or
digital form into a carrier wave by variation of its instantaneous frequency
in accordance with an input signal. This is typically accomplished using
radio waves. The most typical use is radio broadcasting.

Frequency modulation requires a wider bandwidth than amplitude modulation by
an equivalent modulating signal, but this also makes the signal more robust
against 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 insisted on
calling it "VHF radio").

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 processed used in "stereo
FM", and concentrates on the FM modulation and demodulation process, which
is identical in stereo and mono processes.

Technology

The harmonic distribution of a simple sine wave signal modulated by another
sine wave signal can be represented with Bessel functions - this provides a
basis for a mathematical understanding of frequency modulation in the
frequency domain.

A rule of thumb, Carson's rule states that nearly all the power of a
frequency modulated signal lies within a bandwidth of

     2(Δf + fm)

where Δf is the peak instantaneous deviation of the carrier from the
centre frequency and fm is the highest modulating frequency.

Note that frequency modulation can be regarded as a special case of phase
modulation where the carrier phase modulation is the time integral of the FM
modulating signal.

Frequency shift keying (FSK) refers to the simple case of frequency
modulation by a simple signal with only two states, such as in Morse code or
radio-teletype applications.

Manchester coding may be regarded as a simple version of frequency shift
keying, where the high and low frequencies are respectively double and the
same as the bit rate, and the bit transitions are synchrounous with carrier
transitions.

When used in supervisory signaling in telephony, the term frequency-change
signaling has been used to describe frequency modulation.

The phrase frequency-modulated, an adjective, should have a hyphen.

How to - Physics - History - Companies - Internet - Video Games - List of Phobias - September 11, 2001
Radio - Timelines - Chemistry - Genealogy - Family - Film - SARS - Cancer - Medicine - DVD - Calendar
Countries - Disease - Health Science - Dentistry - Economics - AIDS - Law - Autism - Statistics - Bible
Recipes - Architecture - Computers - History of the Internet - Personal computer - Apple Macintosh
War - Presidents of the United States - United States Constitution - Universe - Philosophy - Animals
Biology - United States Constitution - Marketing Topics - Sports - Television - History of Computing

Mortgages - Mobile Phones - Car Credit - Wedding rings - Art Prints