From: Dave.Medin@mailbox.bison.mb.ca (Dave Medin)
Newsgroups: sci.electronics
Subject: Re: How does stereo FM radio work?
Date: 24 Sep 92 22:41:48 CST

Organization: Intergraph Corporation, Huntsville AL
Reply-To: medin@catbyte.b30.ingr.com

In article <sehari.717354096@vincent1.iastate.edu>, sehari@iastate.edu (Babak Sehari) writes:
|> In <1992Sep24.125938.3630@research.nj.nec.com> markl@research.nj.nec.com (Mark Lee) writes:
|> 
|> >I don't know if this is a FAQ, but I have never seen this question asked
|> >here and I want to know.  How do they broadcast and receive the two
|> >channels of information needed for stereo on FM radio?  More
|> >specifically, how is the modulation and demodulation of the two sets of
|> >audio done on a single carrier frequency?  Do they use asymmetric side
|> >bands or what? And if so, how is this done?  And why was FM stereo in
|> >common use long before anyone broadcast AM stereo?
|> 
|> >I have asked several people who know high-frequency electronics about
|> >this and never gotten an intelligible answer.  I'd appreciate it if
|> >someone could point out to me a decent text or article, appropriate for
|> >someone like me who already understands basic mono AM and FM.
|> 
|> >I'm just curious.
|> 
|> >--Mark
|> >markl@research.nj.nec.com
|> 
|> 1. To transmit stereo FM signals first they add the left channel (L) to the
|> Right channel (R) Then they subtract L-R then they transmit the addition of
|> two signal (L+R) after passing through 18KHz low pass filter using regular
|> FM transmite. This way every body can receive the "L+R signal", then add
|> 18KHz to the L-R signal and send it through the same channel. Now, all the
|> receivers that have stereo capability isolate this signal first, subtract
|> 18Khz from it then add and subtract it from L+R signal:
|> 
|> (L+R)+(L-R)=2L
|> (L+R)-(L-R)=2R

The above is not correct. First, the L+R and L-R signals are obtained
as the poster intimated. The L-R signal is then used to modulate
a 38KHz subcarrier in a DSBSC (double sideband supressed carrier--look
at it as standard AM with the carrier removed) manner with a balanced
modulator. Then, the L+R channel, the modulated 38 KHz subcarrier,
and a 19 KHz tone (the "pilot" tone, which is fixed phase-related
to the 38 KHz subcarrier) are summed together in a signal that FM
modulates the RF carrier in the 88 - 108 MHz band.

There are really three signals contained in the FM modulation your radio
receives--the L+R or "mono" information, the encoded L-R, and the
pilot tone used to indicate the presence of the L-R (this tone is what
lights the "stereo" light on your radio and enables the stereo
recovery process). For a mono radio, the radio signal is FM demodulated
and the pilot and subcarrier are filtered out. For a stereo radio, the
signal is first FM demodulated as a baseband (L+R) plus subcarrier
signal. Then the pilot is used to recreate the carrier of the
38 KHz DSBSC signal and it is demodulated with an ordinary AM detector
yielding the L-R signal. Mathematically, the L-R and L+R signals
yield L and R. The spectrum going into or coming out of the FM modulator/
demodulator looks like this (not quite to scale):



0 Hz				 19 KHz              38 KHz
________________________________   |   ________________|________________
|  				\  |  /					\
| 	L+R			 \ | /		DSBSC-modulated L-R	 \
|				  \|/					  \


Stereo TV functions in the same manner, but the pilot tone is at 15 KHz and
the L-R channel is compressed/expanded before/after modulating/demodulating
the subcarrier.

Note that there are a few details like preemphasis (the boosting of high
frequencies) I left out for simplicity. Multiple subcarriers at higher
frequencies can also be applied (known as SCA in the FM world and) to
carry ancillary information like background music, pager signals, etc.

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