47 CFR § 73.756 - System specifications for double-sideband (DBS) modulated emissions in the HF broadcasting service.
(a) Channel spacing. The nominal spacing for DSB shall be 10 kHz. However, the interleaved channels with a separation of 5 kHz may be used in accordance with the relative protection criteria, provided that the interleaved emission is not to the same geographical area as either of the emissions between which it is interleaved.
(b) Emission characteristics -
(1) Nominal carrier frequencies. Nominal carrier frequencies shall be integral multiples of 5 kHz.
(2) Audio-frequency band. The upper limit of the audio-frequency band (at - 3 dB) of the transmitter shall not exceed 4.5 kHz and the lower limit shall be 150 Hz, with lower frequencies attenuated at a slope of 6 dB per octave.
(3) Modulation processing. If audio-frequency signal processing is used, the dynamic range of the modulating signal shall be not less than 20 dB.
(4) Necessary bandwidth. The necessary bandwidth shall not exceed 9 kHz.