§ 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 § 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]

   return arrow Back to Top