Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Radio waves...





The way I understand the question is that, for example, when
people talk, we fluctuate the wavelengths and amplitude in such a way
that it forms the words we hear. If a DJ is talking (or music is
playing), what would the radio waves look like so that they transmit the
sounds.

In amplitude modulation, a carrier frequency is mixed additively with the
audio frequency wave. The resultant wave therefore has a spectrum of
frequencies above and below the carrier frequency. (I'm leaving out the
complication of the 455 kHz frequency used in superhet radios.) Thus the
entire signal occupies approximately 15 kHz on either side of the carrier
(the AM system doesn't transmit hi-fi up to 20 kHz audio, it only goes to
15 kHz if memory serves). Thus the two sidebands contain identical
information, the carrier carries none. In some commercial and short-wave
radio band the carrier is suppressed before transmission. In some the
carrier and one sideband is suppressed. These conserve bandwidth and
transmitted power. In either case, a carrier is supplied at the receiving
end for the demodulation process.

Am radio in the US retains both bands and the carrier. I've heard some
proposals of AM stereo by putting one channel of audio on each sideband.
Ordinary radios would get the mixed (mono) signal, but radios could be
designed to separate the sidebands and get stereo. I haven't kept up with
this, it may already be used.

In FM radio they wisely allocated enough bandwidth to each station to
include adio up to 20kHz, and a subcarrier for the stereo difference
signal, and still another subcarrier for other services (background music
for stores and businesses, second language services, etc.) I suspect this
second subcarrier isn't much used anymore. TV bandwidth also accomodates
subcarriers for color signal, stereo sound, and still another for
closed-captioning, second language, etc.)

Subcarriers on an FM signal are often amplitude modulated. Sometimes phase
modulated. Too bad these textbooks which have a "technology" orientation
seldom cover this very interesting technolgy which touches all of us, for
better or worse.

Find a ham radio operator in your area to bring you up to date on the
latest technology. This post is off the top of my head, and someone surely
can improve on it. The Radio Amateur's Handbook has lots of good info on
this, but my copy is at the office, and I am at home.

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
......................................................................