Doppler effect In 1968, Schnitzler noted certain bats lower the
animal echolocation emission frequency by 13 to 16 kHz, compensating for
Doppler shifts caused by the bat’s own movement.
Doppler shift compensation, dynamic frequency modulation, ensures that the returning echo frequency is optimally adjusted for the bat's auditory fovea.
Magnetic tape storage FM is also used at
intermediate frequencies by analog
VCR systems (including
VHS) to record the
luminance (black and white) portions of the video signal. Commonly, the
chrominance component is recorded as a conventional AM signal, using the higher-frequency FM signal as
bias. FM is the only feasible method of recording the luminance ("black-and-white") component of video to (and retrieving video from)
magnetic tape without distortion; video signals have a large range of frequency components – from a few
hertz to several
megahertz, too wide for
equalizers to work with due to electronic noise below −60
dB. FM also keeps the tape at saturation level, acting as a form of
noise reduction; a
limiter can mask variations in 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. These FM systems are unusual, in that they have a ratio of carrier to maximum modulation frequency of less than two; contrast this with FM audio broadcasting, where the ratio is around 10,000. Consider, for example, a 6-MHz carrier modulated at a 3.5-MHz rate; by
Bessel analysis, the first sidebands are on 9.5 and 2.5 MHz and the second sidebands are on 13 MHz and −1 MHz. The result is a reversed-phase sideband on +1 MHz; on demodulation, this results in unwanted output at 6 – 1 = 5 MHz. The system must be designed so that this unwanted output is reduced to an acceptable level.
Sound 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 in several generations of
personal computer sound cards.
Radio in Buffalo, New York
Edwin Howard Armstrong (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio. He patented the regenerative circuit in 1914, the superheterodyne receiver in 1918 and the super-regenerative circuit in 1922. 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. The first experimental station,
W2XMN, went on the air in 1937. As the name implies, wideband FM (WFM) requires a wider
signal bandwidth than
amplitude modulation by an equivalent modulating signal; this also makes the signal more robust against
noise and
interference. Frequency modulation is also more robust against 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 part of the
VHF bandthe
FM broadcast band). FM
receivers employ a special
detector for FM signals and exhibit a phenomenon known as the
capture effect, in which the
tuner "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously).
Frequency drift or a lack of
selectivity may cause one station to be overtaken by another on an
adjacent channel. Frequency
drift was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner. A wideband FM signal can also be used to carry a
stereo signal; this is done with
multiplexing and demultiplexing before and after the FM process. The FM modulation and demodulation process is identical in stereo and monaural processes. FM is commonly used at
VHF radio frequencies for
high-fidelity broadcasts of music and
speech. In broadcast services, where audio fidelity is important, wideband FM is generally used. Analog TV sound is also broadcast using FM. Narrowband FM is used for voice communications in commercial and
amateur radio settings. In
two-way radio, narrowband FM (NBFM) is used to conserve bandwidth for land mobile, marine mobile and other radio services. 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 methods requiring linear amplifiers, such as AM and
QAM. There are reports that on October 5, 1924, Professor
Mikhail A. Bonch-Bruevich, during a scientific and technical conversation in the
Nizhny Novgorod Radio Laboratory, reported about his new method of telephony, based on a change in the period of oscillations. Demonstration of frequency modulation was carried out on the laboratory model.
Hearing assistive technology Frequency modulated systems are a widespread and commercially available
assistive technology that make speech more understandable by improving the signal-to-noise ratio in the user's ear. They are also called
auditory trainers, a term which refers to any sound amplification system not classified as a
hearing aid. They intensify signal levels from the source by 15 to 20 decibels. FM systems are used by hearing-impaired people as well as children whose listening is affected by disorders such as
auditory processing disorder or
ADHD. For people with
sensorineural hearing loss, FM systems result in better speech perception than hearing aids. They can be coupled with behind-the-ear hearing aids to allow the user to alternate the setting. FM systems are more convenient and cost-effective than alternatives such as
cochlear implants, but many users use FM systems infrequently due to their conspicuousness and need for recharging. ==See also==