Over the years I've seen various figures quoted for the S/N difference between monophonic and stereophonic FM reception. I decided to write a short computer program to calculate the number.
FOR f = 20 TO 15000 ' Integrate noise power from 20 Hz to 15 kHz
d = 1 / (1 + (2 * 3.141593 * 75E-6 * f) ^ 2) ' Deemphasis filter power factor
b = b + d * f ^ 2 ' L+R power
s = s + d * ((38000 - f) ^ 2 + (38000 + f) ^ 2) ' L-R power
NEXT f
PRINT 10 * LOG((b + s) / b / .91 ^ 2) / LOG(10) ' .91: 9% pilot, LOG is natural log
The result is 23.0 dB for a frequency range of 20 to 15000 Hz and 9% stereo pilot. The calculation assumes that the deemphasis time constant is 75 microseconds, the composite response is flat, and composite noise rises 6 dB per octave. The result is insensitive to composite droop.
I measured the S/N difference for six tuners using their wide IF filters and a 200–15000 Hz audio bandpass filter. The values ranged from 21.3 to 22.9 dB with an average of 22.2 dB. The calculated result for 200–15000 Hz is 22.7 dB.
88–108 MHz