ANALOG FM : STEREO vs MONO - MYTHS and TRUTH

Most of the people think that FM Analog STEREO has better quality than MONO .
It depends what we intend for QUALITY .

If the parameters of QUALITY are :

A) DISTORTION
B) BANDWIDTH
C) SIGNAL TO NOISE RATIO

The reality is :

A) Distortion and bandwidth are the same in MONO and STEREO
B) Signal to Noise Ratio in much better in MONO ( in the range of around 20dB or higher !)
20 dB means that , to have the same S/N ratio from a STEREO signal respect to a Mono Signal , a power 100 times higher is needed .

The obvious advantage  in STEREO is that you have a " SPATIAL BIDIMENSIONAL IMAGE " .
In other words , you can perceive if the sound of different music components come from an arc Left - Center -Right .

The big disadvantage of STEREO vs. MONO is  the big loss  of Signal to Noise , that limits the STEREO coverage respect to the MONO coverage with the same Signal to Noise Ratio .

What is written here is well shown  in the following movie , where an FM Tuner that receive a signal in a fringe area of coverage is switched from STEREO to MONO .
Listening in STEREO is severely annoying , while the same signal in MONO is quite comfortable .
STEREO is when you have red light . MONO when you have blue light .
STEREO to MONO switch was made manually three times during the movie .
Please use as results the two second switching sessiones because the first time , when switching, there was also a change of program between speech and music .
Actually most of the receivers have a " Blending system "  that decides automatically when switching from STEREO to MONO , but in fringe areas this switching is annoying , so it is obvious that to avoid this , the only way is to broadcast MONO ....

The difference of S/N Ratio MONO/STEREO is well plotted in the following pictures taken from the ITU recommendations :

https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.412-9-199812-I!!PDF-E.pdf