Re: [Discuss-gnuradio] Broadcast FM is crap!

[David I. Emery]

On Sun, Feb 19, 2012 at 03:19:21PM -0500, Marcus D. Leech wrote:

> So, on a related topic, how do commercial FM-MPX receivers maintain 
> proper balance between
>   L-R and L+R -- do they just have a gain control that they tweak at 
> the factory and glue in place?
> 
> If the gain balance is off, separation starts to go to heck really 
> quickly.  And obviously, you can't use
>   AGC, since that would cause a lot of noise to be injected when the 
> L-R channel was mostly zero.

        There is a well known design trick... the level of the DSBSC
difference signal in the FM baseband was chosen such that if you sample
the FM baseband out of the FM detector - including the stereo subcarrier
energy - in phase with the suppressed 38 KHz carrier (eg on peaks or
troughs of the suppressed carrier) you get either L or R depending on
whether the samples are on carrier peaks or troughs.   In effect the
difference signal sideband phasors either add  (L+R) + (L-R) producing
2L or subtract it a half cycle later producing 2R.

        This of course simply requires correct frequency response of the
FM detector and deemphasis network  and suitable flat group delay for
good stereo separation...  rather than carefully matched gain of the
baseband and subcarrier path.  


-- 
  Dave Emery N1PRE/AE, address@hidden  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."