E. LPFM Emissions and Bandwidth
51. We believe that the extent to which LPFM stations would degrade FM radio service on the 2nd-adjacent channel would be considerably limited by their lower ERP and HAAT levels. In addition, we seek other technical means for further reducing this interference potential. In this regard, we could restrict out-of-channel emissions by establishing a strict spectral emission mask and/or by reducing the transmission bandwidth for LPFM stations. We are also proposing to require FCC certification of transmitters used at all LPFM stations, which we believe would be necessary to ensure compliance with out-of-channel emission requirements, particularly if the LPFM service does not include 2nd- and 3rd-adjacent channel protection requirements. We are vitally concerned that such stations meet transmitter out-of-channel emission limits and other standards related to interference protection of stations on adjacent channels. We note that uncertified equipment has on numerous occasions caused dangerous interference to aviation frequencies. 72 We do not believe that a certification requirement would overly burden small operators, given the recent streamlining of our certification procedures. 73 We seek comments on this proposal, including not only on burdens of compliance, but specific harms that could result from not requiring transmitter certification. We also ask whether a modulation monitor should be required or, alternatively, whether transmitters should be certified with built-in modulation limits.
52. Emission Limits. Outside of their assigned channels, the emissions of FM radio stations must be attenuated below the level of the unmodulated carrier frequency: (1) by at least 25 dB at any frequency removed from the center frequency by 120 kHz up to 240 kHz; (2) by at least 35 dB at any frequency removed from the center frequency by 240 kHz up to and including 600 kHz; and (3) by at least 43 dB + 10log(Power, in watts) dB on any frequency removed by more than 600 kHz from the center frequency. 47 C.F.R. Section 73.317. This emission mask ensures that FM broadcast emissions are reasonably confined within the 200 kHz channel width.
53. The center frequency of a second-adjacent channel is 400 kHz removed, and the minimum separation between the channel edges of 2nd-adjacent channels is 200 kHz. The current emission mask requires a minimum attenuation of 35 dB below the level of the unmodulated carrier for emissions extending over the second-adjacent channel. We invite comment on the extent to which an increased emission attenuation requirement would reduce the potential for 2nd-adjacent channel interference, assuming no 2nd-adjacent channel spacing requirements. By how much would this attenuation have to be increased in this regard? 10 dB? 20 dB? What would be the consequences of a more restrictive emissions mask for LPFM stations? For example, at what point would tighter emission limits become cost prohibitive?
54. We recognize the difficulty in quantifying the potential benefits of emission limitations to future digital systems in the absence of standards for these systems. Nonetheless, based on what is known about IBOC technology, commenters are invited to consider generally the relationship between an LPFM emission mask and protection to digital signals. Could a strict emission mask for LPFM stations significantly reduce the potential for interference to IBOC signals, presuming we did not impose 2nd-adjacent channel spacing requirements on LPFM stations?
55. Bandwidth Limits. FM broadcast channels have a bandwidth of 200 kHz. The center frequency of each channel is 100 kHz from the upper and lower edges of the channel, and the frequency modulated ("FM") signal in each channel swings in frequency from the center frequency toward the channel edges, and has its radiated power envelope shaped such that virtually all of the energy of the signal is contained within the channel. The potential for interference could be further reduced if LPFM stations operated with a reduced bandwidth, creating additional frequency separation to adjacent channels. A reduced bandwidth in combination with a strict emission mask would offer even more protection. For example, suppose the channel bandwidth were reduced by approximately half.74 This reduced bandwidth would ameliorate the possible adjacent channel interference impact of new LPFM stations. By reducing the bandwidth available to LPFM stations, and with the corresponding contraction in the spectral mask, we believe that all types of adjacent channel interference from these stations could be significantly reduced. We seek comment on the effectiveness of reduced bandwidth as an alternative means of interference protection, particularly with regard to 2nd-adjacent channels. What bandwidth reduction would best serve this purpose? What emission mask for a reduced channel bandwidth would be appropriate to further restrict emissions on adjacent channels?
56. We inquire about the operational effects of reduced bandwidth on LPFM stations. First, would LPFM signals be received by existing radios; for example, car radios, home stereo systems, and boom boxes? At what level of bandwidth reduction would LPFM stations not be able to transmit stereo signals? We would be particularly concerned with the impact of a narrowed bandwidth on the transmission of stereophonic sound. A narrowed channel bandwidth could also restrict or preclude the use of baseband subcarriers by LPFM operators; subcarriers are utilized by FM stations for a variety of purposes. Would prospective LPFM operators be willing to sacrifice the use of subcarriers in return for the ability to broadcast a narrow band radio signal? Could the loss of LPFM subcarrier services such as those typically provided by full power FM stations be detrimental to the public? In seeking comment on possibly narrowing the channel bandwidth for LPFM stations, we are interested in the optimal bandwidth that would strike the right balance between facilitating a larger number of potential stations and optimizing the services that could be offered by those stations. Commenters should address the specific stereophonic sound transmission standards which would be appropriate for a reduced channel bandwidth, including pilot tone, L/R subcarrier, highest modulating frequency, and maximum signal deviation. Establishing a reduced channel bandwidth for LPFM could necessitate the development and manufacture of new lines of transmitting equipment, at an unknown cost, and reduce the availability of transmitters for LPFM stations, especially used transmitters designed for a 200 kHz bandwidth. We seek comment on these matters and, generally, on whether any adverse effects of LPFM operations on a reduced channel bandwidth could outweigh the increased channel availability that could result.
72 See, e.g., para. 0, below.
73 See Equipment Authorization Streamlining R&O, FCC 98-338 (released Dec. 23, 1998).
74 We could specify, for example, that the signals from all LPFM stations be attenuated by at least 25 dB at spacings of 60 kHz to 180 kHz from the channel center frequency, by at least 35 dB at spacings of 180 kHz through 540 kHz from the channel center frequency, and by at least 43 dB + 10log(Power, in watts)dB on any frequency removed by more than 540 kHz from the channel center frequency.