§  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%.

   [ 51 FR 17029 , May 8, 1986]