So to set you in the correct time period. Its early 2010, an evening and in my laboratory (or better said my bedroom). I was tinkering with a FM bug transmitter . Specifically I had trouble making my transmitter sounding as loud as the other commercial stations and actually i was trying to make my transmitter sound bright and not dull. So one day I was googling for "fm transmitter" and one of the results was a manufactures' website stating "our new pre-emphasis filter ...". Have you spotted the keyword? Pre-emphasis was the word that got me to this page. So this was the key for a nice , bright sound in FM. That was fine for quite some time but then another question stuck to my mind. How they put to channels of audio in the same frequency. So stereo transmition became my next goal. So finally I found how. For an fm transmition to be in stereo the two channels have to be combined to one mono signal (in other words added together so you end up with a L+R mono signal). Then you have to extract the difference between the two channels (again that means subtracting the L channel from R channel ) and again you end up with a L-R signal which contains the stereo information but there is a problem. The L+R and L-R use the same frequencies and so if you were to put the back together they will interfere with each other. So the trick is to move the L-R higher. The easiest way of moving the L-R is to amplitude modulate the L-R and then throw the carrier away. Up until this moment we have the L+R and L-R to coexist but there is a another problem . We have to some how know if the program is transmitted in mono or stereo. By inserting a pilot into the signal we can check if the pilot is there to determine whether the transmition is in mono or stereo. This was just a brief introduction to the World Of FM Stereo. Another post will come soon with a more technical explanation and with some rules that have to be obeyed for this system to work. Thanks for reading. Comments are welcomed.
