FAZAMY wrote:
Please explain what is meant by a station transmitting a sub-signal.

I presume you mean a "subcarrier"? (also known as "SCA" in the States
and "SCMO" in Canada)

A very high-pitched signal (usually either 67 or 92KHz) is added to the
audio before transmission. This high-pitched signal is modulated with
additional information. In the early days of subcarriers, this
"additional information" was a second audio program, usually background
music. ("Muzak") More recently, it's been used for "talking books" for
the visually impaired, and for various types of data transmission.

These subcarrier transmissions are not intended for reception by the
general public - indeed, I believe the Electronic Communications Privacy
Act makes it *illegal* for non-subscribers to monitor subcarriers.

Another data subcarrier at 57KHz is used for "RDS", a data transmission
system that *is* intended for the general public. It transmits a
continuous station ID, and can provide other information like format,
frequencies of other transmitters carrying the same program, and a text
string commonly used to show the name of the song being played.

Actually, FM stereo depends on yet another subcarrier, at 38KHz. This
subcarrier contains a mix of the right and left channels, with the right
channel 180 degrees out of phase. Mix the 38KHz subcarrier with the
monophonic signal transmitted the normal way, and the right channel
cancels out and you get only the left. Flip the subcarrier 180 degrees
out of phase, mix it with the monophonic signal, and the *left* channel
cancels out and you get only the *right*. Feed each to a separate
speaker and voila: stereo.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com