2 FCC Red Vol.5 Federal Communications Commission Record FCC 87-27 the AM band limit the opportunities for such changes Before the without creating objectionable interference. MQreovel. Federal Communications Commission these conventional methods do not always provide sta- Washington D.C. 20554 tions with sufficient economic flexibility to improve their service to all areas. The use of additional transmitters that simultaneously broadcast the programs of a primary origi- nating station is an effective and economical method that MM Docket No. 87-6 could be applied in some circumstances to improve and extend a station's service area. In the Matter of 4. The technology relating to synchronized transmitters was discussed early in the United States in a paper written Amendment of Part 7 by Charles B. Aiken of Bell Telephone Laboratories in i933, In 1937. the first experimental authorization to to Authorize the use of explore this technology in the U.S. was granted to radio Multiple. Synchronous station WLLH. Lawrence. Massachusetts. which has con- Transmitters by AM tinued its synchronous operations to this day. Similar Broadcast Stations experiments have been conducted in Boston (WBZ). Charlotte. North Carolina (WBT). Cincinnati. Ohio (WSAI). and in Washington. D.C. (WINX and WWDC). NOTICE OF INQUIRY Synchronized groups of transmitters have been used in the AM band on a large scale in Europe for many years. Adopted January 15, 1987; ReIeased March 3, 1987 and more recently in Japan. These foreign operations have been based upon the same theoretical bases as the By the Commission: earlier Uniied States operations.4 The results of the inter- national experience have been largely consiStent with those obtained in the United States. These operations have I. INTRODUCTION further demonstrated the feasibility of synchronous oper- ations and have confirmed earlier predictions of perfor- 1. This action initiates a J'otice of Inquiry (Inquiry into ) mance to be expected from different system designs. the use of multiple. synchronous transmitters to enhance and extend the signal coverage of AM broadcast stations. 5. There are numerous examples where such In April 1986. the Commission released the Mass Media applications could be made. For instance, additional Bureau's Report on the Status of i/ic AM Broadcast Rules. transmitters simultaneously broadcasting thc programs of RM-5532. (Report ).' The Report discussed. among other a primary station could he located in or near the areas things. the use of multiple transmitters. Of the various where service improvements are desired. Such service uses of multiple transmitters discussed, the application of enhancements could be instituted along major highways "synchronous transmitters" has prompted the most inter- in order to permit a station to serve the traveling public est as a means of providing immediate benefits. Thus, this over long distances forming 'ribbons of service." Addi- proceeding will initially focus only on issues primarily tional transmitters also could he located in nearby com- related to the use of synchronous transmitters. Synchro- munities lacking sufficient population to support their nous transmitter systems involve the use of two 01. more own independent stations. Other uses could include use broadcasting transmitters on the same frequency in neal of low power transmitters within a stations predicted geographical proximity. broadcasting the same program service area at locations suffering from inferior service material. They employ precision carrier frequency and because of anomalous propagation conditions or to pro- phase control in order to minimize mutual interference. vide service in nulls of directional antenna patterns. Other uses ot multiple transmitters will he considered in 6. Although three distinct uses of multiple transmitters other rule making actions where appropriate. were discussed in the Report, synchronous operation stim- 2. Although there has been much interest expressed ulated the greatest interest, as revealed in the comments regarding the use of synchronous networks, the Commis- that were filed in response to the Report. The comments sion has concluded that ii is necessary to develop a more gave general support to the suggested use of multiple complete record on the several related technical and transmitters for improvement or extension of AM service non-technical issues before proposing specific rules. In areas. The commenters were encouraged by the prospect addition to this Inquiry, the Commission has encouraged for coverage enhancement that synchronous techniques applications for experimental authorizations to develop may offer. Other commenters to the Report recognized technical data.2 Additionally, the Federal Communica- the need for careful study and the value of experimenta- tions Commission Radio Advisory Committee is encour- tion. CBS Inc. and Association for Broadcast Engineering aged to study these issues and to report its Standards Inc (ABES). however, expressed concern that recommendations to the Commission. such uses might increase the overall level of interference in the AM frequency band. Although ABES agreed with the Report's suggested methods of minimizing mutual II. BACKGROUND interference within synchronous transmitter networks, it added that synchronous operations would offer only mar- 3. Conventional methods for enhancing and extending ginal improvement in the coverage of some stations. the service area of an AM broadcast station typically called for increasing the transmitter power, designing a directional antenna system to improve service in a desired direction, relocating the station, or any combination of these. However, the crowded conditions that now exist in I 2O FCC 87-27 Federal Communications Commission Record 2 FCC Rcd VoL 5 III. DISCUSSION hours, it may be feasible during the daytime hours for 7. There are both technical and non-technical issues on each transmitter to broadcast separate programming. de- which data and information are needed in order to estab- pending on the transmitters' proximity to each other. lish bases for specific rule proposals for the operation of 12. When phase synchronization is employed, there is synchronous transmitters. Of particular importance are no relative carrier frequency variation between the dif- issues pertaining to criteria used to define mutual inter- ferent transmitters, and the pattern of mutual-interference ference between transmitters in a synchronized network. remains fixed in time and place. Such interference would Of equal importance are the interference protection cri- not be very noticable on automobile receivers since auto- teria to be applied between synchronous networks and mobiles in motion would normally move quickly through stations outside of the synchronous systems. any areas of interference. For AM radios with ferrite rod 8. With respect to non-technical issues, the Commission antennas (virtually all modern table models and portable is of the view that restrictions on the use of synchronous radios), the effect of the stable pattern of interference can transmitters should be minimized to the extent possible. be greatly reduced by orienting the radio to improve the in order to maximize opportunities for innovation and signal strength received from one of the transmitters. service improvements to the public. Non-technical mat- 13. A difference among the carrier frequencies of the ters of importance include Ownership and licensing issues. transmitters in the group results in a variation in the total These are discussed in detail below. received signal at any point in the common service area over time. If the frequency difference is small enough (On Technical issues the order of 0.1 Hz). the variation can be compensated for 9. Previous experiences, both in the United States and by the automatic gain control (AGC) circuitry of the abroad, have demonstrated that there are areas of concern receiver and the listener will not notice significant distoi- in synchronous transmitter operation. While advance- tion. except at locations where the signals from the dif- ments in fixed point-to-point communication techniques ferent transmitters are nearly the same field strength. At made since the earlier experiments will solve some of the these latter points, the variation may exceed the dynamic earlier program distribution problems. e. g.. radio or land range of the AGC and distortion effects similar to slow line signal propagation delay. a principal consideration of fading will be experienced. synchronous operation is that of the zones of mutual- 14. The relative merits of these methods need to he interference. This interference occurs most prominently studied, Is there a significant advantage of one technique in the service areas of the synchronized transmitters at over the other or should the Commission establish rules locations where the signal levels from two or more syn- for both? Where possible. those stations operating under chronized transmitters are nearly equal. AM radio receiv- experimental authorizations with synchronous transmit- ers located in these interference zones may experience ters are encouraged to develop data on this issue. program signal fading or distortion, depending on the 15. Projection rajios for synchronized !ransniiuers. The method of transmitter synchronization employed. The term protection ratio" generally refers to the minimum boundaries of these zones or areas of interference as well ratio of the field strength of a desired signal to the