Sec. 73.1570  Modulation levels: AM, FM, and 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 station. 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 
&177;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]