Stereo S/N Penalty

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.

October 8, 202088–108 MHz