Different criteria for "broad" have been applied in different contexts
and at different times. Its origin is in physics, acoustics, and radio
systems engineering, where it had been used with a meaning similar to
"wideband".[1][2] Later, with the advent of digital telecommunications,
the term was mainly used for transmission over multiple channels.
Whereas a passband signal is also modulated so that it occupies higher
frequencies (compared to a baseband signal which is bound to the lowest
end of the spectrum, see line coding), it is still occupying a single
channel. The key difference is that what is typically considered a
broadband signal in this sense is a signal that occupies multiple
(non-masking, orthogonal) passbands, thus allowing for much higher
throughput over a single medium but with additional complexity in the
transmitter/receiver circuitry.
The term became popularized through the 1990s as a marketing term for
Internet access that was faster than dialup access, the original
Internet access technology, which was limited to 56 kbit/s. This meaning
is only distantly related to its original technical meaning.
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