Technical information

audio cassette undergoes this, it produces rapid variations in the playback
frequency. At the slower end of the scale, it can sound like an unwanted tremolo
or vibrato. At higher rates it can add an unpleasant harshness to the sound.
Flutter is specified in per cent and specifications of more than around 0.1% are
unacceptable. Digital sources such as CDs are immune to flutter because they
lock their playback speed to a solid-state timing device.
FM Frequency Modulation. A system of storing or transporting a signal by using it
to modulate a carrier in the frequency, rather than amplitude, domain. FM radio,
high-fidelity audio on VHS, and numerous other systems are based on FM. FM
tends to be more resistant to interference than AM.
Four-way loudspeakers See 4 way loudspeakers.
Frequency The rate at which a repetitive signal repeats, measured in cycles per
second or hertz (or its convenient multiples). Audio frequencies are generally in
the range of 20 to 20,000 hertz because those are the frequencies to which the
human ear sensibly responds. Equipment measurements often extend this range
to 0 to 100,000 hertz. Video frequencies typically deal with a range of 0 to 7
MHz because that is the bandwidth available for PAL and NTSC signals. The
human eye responds to light in the frequency range of 385,000 GHz to 789,000
GHz. The frequency of a signal can be calculated by dividing the speed with which
the signal propagates through a medium by its wavelength.
Frequency response A measure of how accurately a system reproduces different
frequencies. In the case of audio in a home theatre system, it is desirable for the
frequency response of a whole system, including speakers and subwoofer, to be
from 10 hertz to 20,000 hertz ±3dB. This performance requires a very
expensive system indeed and, in practice, very few systems will produce bass
down to anything like that bottom limit. Manufacturers who claim a frequency
response for speakers of, say, 20 to 20,000 hertz without specifying decibel
boundaries are telling you nothing. A tinny two inch transistor radio speaker can
reproduce that range, although you won't actually hear it at either extreme
because its output will be so low. Even subtle variations of less than half a decibel
across the audio band can be quite audible, especially if they're spread over a fairly
wide band of frequencies, and can thus change the character of the sound.
Indeed, with speakers the single measure most closely related to their sound is
the frequency response.
Fundamental frequency Any musical tone primarily consists of a particular simple
tone (a sine wave) and a series of higher frequency tones, where the frequencies
of those higher tones are whole multiples of the first-mentioned tone. The
frequency of that simple tone is called the fundamental frequency. When we say