I was doing some experiments with this block this evening, and what I
discovered was that it requires a much better-quality signal to produce
even half-way decent audio output than my “first principles” FM
stereo decoder in my “simple_fm_rcv” application. The PLL block uses a
PLL to derive the 38KHz injection tone to re-constitute the carrier
for the DSB-SC “L-R” signal. I simply use the pilot tone itself, and
double it with a multiplier/filter.
On stronger signals, the PLL block works just fine. But on weaker ones,
my approach appears to be superior. I wonder why? Not being a real
DSP guru, I just play one on the Internet 
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium