Most FM tuner manufacturers specify S/N using an unweighted measurement, but a few use A weighting. I wondered how it might affect the S/N figure. I wrote a short computer program to find out.
FOR f = 20 TO 15000 ' Integrate noise power from 20 Hz to 15 kHz a = f ^ 2 ' Next expression is A-weight power factor a = (13681.783 * a) ^ 4 / ((a + 424.36) * (a + 148693636)) ^ 2 / (a + 11599.29) / (a + 544496.41) d = 1 / (1 + (2 * 3.141593 * 75E-6 * f) ^ 2) ' Deemphasis power factor m = d * f ^ 2 ' Mono noise power s = m + d * ((38000 - f) ^ 2 + (38000 + f) ^ 2) ' Stereo noise power um = um + m ' Unweighted mono sum us = us + s ' Unweighted stereo sum wm = wm + a * m ' Weighted mono sum ws = ws + a * s ' Weighted stereo sum NEXT f PRINT 10 * LOG(wm / um) / LOG(10) ' Result in dB PRINT 10 * LOG(ws / us) / LOG(10) ' LOG is natural log
The result is
Mono: A-weighted S/N = unweighted S/N + 1.3 dB Stereo: A-weighted S/N = unweighted S/N + 0.4 dB
The calculation assumes that composite noise rises 6 dB/octave, the composite response is flat, and the deemphasis time constant is 75 microseconds. The result is insensitive to the composite response.