# FM Stereo

Discussion in 'General Electronics Discussion' started by dm99, Jan 9, 2013.

1. ### dm99

26
0
Oct 29, 2012
How does "FM Stereo" work? If you're listening to a station transmitting in stereo with a mono radio are you missing one of the channels?

8,393
1,270
Nov 28, 2011
3. ### Merlin3189

250
69
Aug 4, 2011
They thought about that when they designed it. So the two signals they transmit are the sum of the L & R channels and the difference of the L & R channels.
A mono reciever ignores the latter and just hears the sum - the two channels added together.
A stereo rcvr adds the two signals (L+R and L-R) to give just the left channel, and it subtracts the two signals (L-R subtracted from L+R) to give just the right channel.
I suppose to be algebraically correct, one ought perhaps to say,
L = 0.5 ( (L+R) + (L-R) )
R = 0.5 ( (L+R) - (L-R) )

The first signal, L+R, was modulated in the normal way, so thats what the mono receiver picks up. So no change was needed in existing receivers.
The second signal (L-R) was modulated onto a sub-carrier at a frequency outside the normal mono pass band, so that mono receivers ignored it. A stereo receiver had a wider passband and processed both signals.

Last edited: Jan 10, 2013
4. ### quantumtangles

152
3
Dec 19, 2012
Interesting. If you wanted to build a stereo FM circuit on a breadboard, what would be the most basic FM stereo circuit you could build in terms of the minimum number of basic components + an IC?

7,682
1,686
Jan 5, 2010
6. ### davennModerator

13,672
1,891
Sep 5, 2009
breadboarding anything RF particularly above 50MHz and in this case the 88-108MHz FM broadcast band is a really really bad idea
The circuit will most likely oscillate ... ie will be unstable
even building 50MHz and up on veroboard will be problematic for similar reasons

its all because of stray inductance and capacitance between adjecent strips

As I said really not a good idea

Dave