19-01-2012, 01:01 PM
Frequency modulation
The fi rst public broadcast radio services introduced in the 1920s were in
the long-wave (LW) and then medium-wave (MW) band, using amplitude
modulation (AM) and limited by the technology of the time. Historically,
long- and medium-wave AM broadcasting has used restricted bandwidth,
approximately 4 kHz, which allows intelligible speech but is not very good
for music broadcasting, since music signals are typically 14–20 kHz in bandwidth.
Amplitude modulation also suffers from impulsive interference, such
as sparks caused by switches opening and closing, for example thermostats
and motor car ignition systems. This is because the information content of
the signal is carried by the envelope of the carrier wave, but impulses like
sparks are very wide band signals so there is energy at practically all frequencies
simultaneously, effectively adding to the envelope of the desired signal.
