[99-25 NPRM] Appendix C: In Band On Channel Digital Radio Service
1. FM broadcast signals are classified as "analog" emissions, i.e., an emission which is characterized by a continuum of output parameter values. All current AM stations use analog emissions, as well as the majority of TV stations, although a limited number of TV stations have commenced digital transmissions since November 1, 1998. Digital emissions, which are characterized by discrete levels of output parameter values, are gradually replacing analog emissions in a variety of communications applications because they possess several technological advantages over analog emissions which make them more useful and reliable, and the Commission is committed to facilitating this transition in an orderly and systematic manner.122 On October 9, 1998, a petition for rule making was filed with the Commission by USA Digital Radio Partners, L.P. ("USADR"), requesting the initiation of a proceeding to amend Part 73 of the Rules to permit the introduction of digital audio broadcasting in the AM and FM radio services. A full discussion of this petition is beyond the scope of this Notice. However, because the petition raises important issues concerning the interference protection criteria used in the FM band, we are addressing aspects of this issue now, at least preliminarily, in conjunction with our proposals to create two new FM station classes and to consider a microradio service. See paragraphs 0-0 in the Notice, above. This Appendix provides some details of USADR's proposal.
2. USADR proposes the introduction of digital signals on the FM band using a technique whereby a station would transmit both its analog signal and two digital signals of lesser amplitude, one on each side of the existing FM signal. (Other systems in development of which we have cognizance would use a similar signal configuration). This arrangement is commonly called "in-band, on-channel," or IBOC. Using IBOC, the two digital signals would be positioned on frequencies slightly offset above and below the frequency modulated signal and would be sufficiently suppressed in magnitude so that they would fit within the emission mask currently required for all FM stations. Using this configuration, USADR argues that digital signals could be introduced into the FM band without disrupting the reception of FM signals or amending the current station-to-station interference protection criteria. USADR envisions that this dual transmission mode, which they refer to as the "hybrid mode," would be initiated in the next few years and would continue for a number of years, eventually being replaced by an "all-digital mode," when the analog FM signal would be eliminated and the power of the 2 digital channel-edge signals would be significantly increased.
3. USADR states that it has conducted analyses of its proposed system which "verify that restricting the digital carriers to the 70 kHz region between 129 and 199 kHz from the center frequency on either side of the analog spectrum minimizes interference to the host analog and adjacent channels without exceeding the existing FCC spectrum mask."123 In USADR's study of the interference impact of their hybrid and all-digital configurations on the existing FM station environment, and the interference impact of existing stations on the USADR system, it found that its system could be implemented without disrupting regular FM analog service and without suffering significant interference from FM analog service.124 The relevant issues from that study which impact this Notice are the effects arising from second- and third-adjacent channel interference, as we have not considered and are not proposing to permit any of the new classes of stations to cause co-channel or first-adjacent channel interference beyond those limits already applying to existing classes of FM stations.
4. With respect to third-adjacent channel interference, the USADR petition states: "Because of the design of the USADR IBOC system, digital reception is essentially not susceptible to third-adjacent channel interference; nor is IBOC likely to increase the potential for causing such interference to analog stations."125 In its comments, NAB argues that, because an IBOC system will add new energy around host analog signal, effectively widening this signal to some degree, it will increase the potential for an IBOC station to interfere with the reception of the analog signal from a third-adjacent channel station. They conclude that "[a]llowing third-adjacent channel stations to move closer together would increase the signal strength of third-adjacent channel interfering stations with respect to the signal strength of a desired station and would thus increase the potential for this interference to occur. For this reason, third-adjacent channel spacing requirements cannot be modified."126 Because no comprehensive operational test data is available for any form of IBOC system configuration, we do not know whether USADR or NAB is correct. We note that we are not proposing to alter the third-adjacent channel protection requirements between any of the existing classes of FM stations. Thus, under the proposals within this Notice concerning third-adjacent channels, the potential for interference would be from IBOC stations to the reception of analog LP1000, LP100, and microradio stations. This problem would present a minimal hinderance (or no hinderance at all, if USADR is correct) because the slight amount of additional noise caused by the digital signal within the third-adjacent channel would produce only a very marginal, if any, degradation of the received FM signal. Third-adjacent channel interference from LP1000, LP100, and microradio stations would be obviated by the significantly restricted occupied bandwidth and correspondingly tightened spectral mask we discuss for these stations.
5. USADR and NAB also address the issue of second-adjacent channel interference. NAB states that "second-adjacent channel interference is the primary challenge facing IBOC designers."127 The NAB's diagrammatic representations of second-adjacent signal magnitudes and spacings clearly indicate that the most important second-adjacent channel interference consideration would involve IBOC-to-IBOC interference, because the upper digital sideband of the victim signal is almost directly adjacent to the lower digital sideband of the interfering signal.128 NAB does not provide any analysis evaluating IBOC-to-FM or FM-to-IBOC second-adjacent interference. In the context of the current FM radio interference standards, USADR addresses the issue of second-adjacent FM interference to an IBOC signal, stating: "An analog second-adjacent interferer will have a negligible effect on the performance of the digital signal, since it does not overlap in frequency with the desired digital signal."129 USADR also addressed the issue of second-adjacent IBOC interference to FM signals, noting that the digital sidebands of the hybrid and all-digital IBOC second-adjacent signals fall well outside the victim FM channel, and saying that "as a result, the [interference] effects of second-adjacent hybrid and all-digital IBOC signals [to FM signals] should be negligible."130 We invite comment in this regard and the submission of relevant measurement data.
122 For example, the fidelity of digital audio recordings typically surpasses that of analog recordings, but this extra fidelity can be degraded or lost if the digital recording is transmitted by a station using an analog, rather than digital, emission. The comparative fidelity of the two emissions, given identical audio inputs, is a function of their relative bandwidths and other factors, as well as the quality of the listener's receiver and the strength of the signal.
123 USADR Petition, Appendix C at 8.
124 Id., Appendix E
125 Id., Appendix D at 3
126 NAB Comments at 23-24. Based on Figure 7 (page 24), it appears that the amount of third-adjacent channel digital energy which could cause interference within the victim receiver's FM analog channel is extremely small, and, in all likelihood, below the noise floor of the receiver.
127 Id. at 22.
128 Id. at 21.
129 USADR Petition, Appendix E at 22.
130 Id. at 67.