Sec.  73.1570   Modulation levels: AM, FM, TV and Class A TV aural.

   (a) The percentage of modulation is to be maintained at as high a level as
   is consistent with good quality of transmission and good broadcast service,
   with maximum levels not to exceed the values specified in paragraph (b).
   Generally, the modulation should not be less than 85% on peaks of frequent
   recurrence, but where lower modulation levels may be required to avoid
   objectionable loudness or to maintain the dynamic range of the program
   material, the degree of modulation may be reduced to whatever level is
   necessary for this purpose, even though under such circumstances, the level
   may  be  substantially less than that which produces peaks of frequent
   recurrence at a level of 85%.

   (b) Maximum modulation levels must meet the following limitations:

   (1) AM stations. In no case shall the amplitude modulation of the carrier
   wave  exceed 100% on negative peaks of frequent recurrence, or 125% on
   positive peaks at any time.

   (i) AM stations transmitting stereophonic programs not exceed the AM maximum
   stereophonic transmission signal modulation specifications of stereophonic
   system in use.

   (ii) For AM stations transmitting telemetry signals for remote control or
   automatic transmission system operation, the amplitude of modulation of the
   carrier by the use of subaudible tones must not be higher than necessary to
   effect reliable and accurate data transmission and may not, in any case,
   exceed 6%.

   (2) FM stations. The total modulation must not exceed 100 percent on peaks
   of frequent reoccurrence referenced to 75 kHz deviation. However, stations
   providing  subsidiary  communications services using subcarriers under
   provisions of  Sec. 73.319 concurrently with the broadcasting of stereophonic or
   monophonic programs may increase the peak modulation deviation as follows:

   (i) The total peak modulation may be increased 0.5 percent for each 1.0
   percent subcarrier injection modulation.

   (ii) In no event may the modulation of the carrier exceed 110 percent (82.5
   kHz peak deviation).

   (3) TV and Class A TV stations. In no case shall the total modulation of the
   aural carrier exceed 100% on peaks of frequent recurrence, unless some other
   peak modulation level is specified in an instrument of authorization. For
   monophonic transmissions, 100% modulation is defined as +/−25 kHz.

   (c)  If  a  limiting  or compression amplifier is employed to maintain
   modulation levels, precaution must be taken so as not to substantially alter
   the dynamic characteristics of programs.

   [ 44 FR 58735 , Oct. 11, 1979, as amended at  47 FR 13165 , Mar. 29, 1982;  49 FR 14508 , Apr. 12, 1984;  49 FR 15081 , Apr. 17, 1984;  49 FR 27147 , July 2, 1984;
    49 FR 47610 , Dec. 6, 1984;  49 FR 48312 , Dec. 12, 1984;  51 FR 26251 , July 22,
   1986;  56 FR 64872 , Dec. 12, 1991;  65 FR 30004 , May 10, 2000]