47 CFR § 73.322 - FM stereophonic sound transmission standards.
(a) An FM broadcast station shall not use 19 kHz ±20 Hz, except as the stereophonic pilot frequency in a transmission system meeting the following parameters:
(1) The modulating signal for the main channel consists of the sum of the right and left signals.
(2) The pilot subcarrier at 19 kHz ±2 Hz, must frequency modulate the main carrier between the limits of 8 and 10 percent.
(3) One stereophonic subcarrier must be the second harmonic of the pilot subcarrier (i.e., 38 kHz) and must cross the time axis with a positive slope simultaneously with each crossing of the time axis by the pilot subcarrier. Additional stereophomic subcarriers are not precluded.
(4) Double sideband, suppressed-carrier, amplitude modulation of the stereophonic subcarrier at 38 kHz must be used.
(5) The stereophonic subcarrier at 38 kHz must be suppressed to a level less than 1% modulation of the main carrier.
(6) The modulating signal for the required stereophonic subcarrier must be equal to the difference of the left and right signals.
(7) The following modulation levels apply:
(i) When a signal exists in only one channel of a two channel (biphonic) sound transmission, modulation of the carrier by audio components within the baseband range of 50 Hz to 15 kHz shall not exceed 45% and modulation of the carrier by the sum of the amplitude modulated subcarrier in the baseband range of 23 kHz to 53 kHz shall not exceed 45%.
(ii) When a signal exists in only one channel of a stereophonic sound transmission having more than one stereophonic subcarrier in the baseband, the modulation of the carrier by audio components within the audio baseband range of 23 kHz to 99 kHz shall not exceed 53% with total modulation not to exceed 90%.
(b) Stations not transmitting stereo with the method described in (a), must limit the main carrier deviation caused by any modulating signals occupying the band 19 kHz ±20 Hz to 125 Hz.
(c) All stations, regardless of the stereophonic transmission system used, must not exceed the maximum modulation limits specified in § 73.1570(b)(2). Stations not using the method described in (a), must limit the modulation of the carrier by audio components within the audio baseband range of 23 kHz to 99 kHz to not exceed 53%.