Section 73.404 - [Effective until an indefinitely delayed date ] IBOC DAB operation(a) The licensee of an AM or FM station, or the permittee of a new AM or FM station which has commenced program test operation pursuant to § 73.1620 , may commence hybrid IBOC DAB operation with digital facilities which conform to the technical specifications specified for hybrid DAB operation in the (2002) First Report and Order in MM Docket No. 99-325, as revised in the Media Bureau's subsequent Order in MM Docket No. 99-325. In addition, the licensee of an AM station, or the permittee of a new AM station that has commenced program test authority pursuant to § 73.1620 , may, with reasonable notice to listeners, commence all-digital IBOC operation with digital facilities that conform to the requirements set out in the (2020) Report and Order in MB Docket No. 19-311 and MB Docket No. 13-249. An AM or FM station may transmit IBOC signals during all hours for which the station is licensed to broadcast.(b) In situations where interference to other stations is anticipated or actually occurs, hybrid AM licensees may, upon notification to the Commission, reduce the power of the primary DAB sidebands by up to 6 dB. All-digital AM licensees, may, upon notification to the Commission, reduce the power of the secondary and tertiary sidebands by up to 6 dB, even if doing so results in non-compliance with § 73.1560(a)(1) . Any greater reduction of sideband power requires prior authority from the Commission via the filing of a request for special temporary authority or an informal letter request for modification of license.(c) Hybrid IBOC AM stations must use the same licensed main or auxiliary antenna to transmit the analog and digital signals.(d) FM stations may transmit hybrid IBOC signals in combined mode; i.e., using the same antenna for the analog and digital signals; or may employ separate analog and digital antennas. Where separate antennas are used, the digital antenna: (1) Must be a licensed auxiliary antenna of the station;(2) Must be located within 3 seconds latitude and longitude from the analog antenna;(3) Must have a radiation center height above average terrain between 70 and 100 percent of the height above average terrain of the analog antenna. 72 FR 45692 , Aug. 15, 2007, as amended at 75 FR 17877 , Apr. 8, 2010; 85 FR 18151 , Apr. 1, 2020; 85 FR 78027 , Dec. 3, 2020 85 FR 18151 , 6/1/2020; 85 FR 78027 , 1/4/2021