The Potential Benefits of Precise Frequency Control of Television Transmitters John A. Pranke Theta-Com When the present television channel assignments After the Second World War, the FCC made were made by the FCC in 1952, the visual car­ extensive changes in the 40-300 MHz band, rier frequency tolerance was specified as ± 1 KHz, which resulted in the television assignments we with provision for a± 10KHz offset where neces­ have today, shown in the lower spectrum of sary to minimize co-channel interference. These Figure 1. Again, with the exception of Channels assignments have, with exceptions, proved satis­ 5 and 6, the format of 6 MHz channels with band factory for twelve channel reception and cable edges on 6 MHz multiples was retained. In ret­ transmission, since all second order products rospect, it almost appears that the 1940 assign­ fall out of band and the magnitude of triple beats ments were preferable to those of today, since had not been a limiting factor. Recent industry we would not have the problem of Channels 5 and efforts to standardize on a channeling plan for 6. It is also regrettable that the carrier fre­ expanded channel systems and an off air, cable quencies, instead of the band edges, were not compatible receiver have pointed out a number of put on 6 MHz multiples, but then hindsight is problems with the present transmission assign­ 20-20. ments. This paper reviews the various plans proposed within CTAC and other organizations, From 1940 to 1948 (with the exception of the war) their advantages and limitations, and concludes the standard picture carrier assignment was that operation of broadcast transmitters on pre­ 1. 25 MHz above the lower channel edge, with a cise frequencies or phase locked to a national tolerance of± 1KHz. In the period from 1947 to standard could effect a major improvement in off 1948, following the war, an ever increasing air and cable television performance. Methods number of stations went on the air. With more to achieve such frequency control are also and more stations, reports began to come in reviewed. about unexpected interference due to long dis­ tance propagation. (Marconi had predicted this as early as 1932.) ( 1) Consequently, in Sep­ Before going into the main subject matter of this tember, 1948, a "freeze" was put on further paper, it is desirable to briefly review the his­ station construction until the problem could be tory of television channel frequency assignments studied. in the United States. The FCC first assigned frequencies for commercial television broadcast After four years, in 1952, it was concluded that in 1937. At that time the channel width was made the interference could be considerably reduced 6 MHz, and the form of transmission was double by the use of± 10 KHz "offset carriers" for sideband AM, with the picture carrier 2. 5 MHz some channels to minimize co-channel interfer­ above the lower band edge and an AM modulated ence, and assignment locations based on sound carrier located 3. 25 MHz above the pic­ increased knowledge of VHF propagation. How­ ture carrier. In 1940, the original assighments ever, as we know, the solution was far from per­ were slightly modified, and, with the exception fect. In 1952, the "freeze" was lifted, and there of Channel 1, retained the original format of was a rapid increase in VHF station construction, 6 MHz channels with the channel edges on mult­ principally in the late 50's and early 60's. iples of 6 MHz. The form of transmission was changed to vestigial sideband with FM sound as we know it today. The upper spectrum of Figure 1 shows the 1940 assignments, which ( 1) Alexander A. McKenzie, "The Three provided 18 channels. It is interesting to note Jewels of Marconi", IEEE Spectrum, that in 1940 this band of frequencies was called PP 46-49, December 1974 UHF. NCTA 75-111 0 60 100 160 200 260 300 0 60 100 160 200 FIGURE 1. These assignments have, with exceptions, proved same Report and Order, the FCC also created a satisfactory for twelve channel reception and Cable Television Technical Advisory Committee cable transmission, since all second order pro­ (CTAC) to deal with engineering and operational ducts fall out of band and triple beats have not facts and make recommendations to the Com­ been a limiting factor. However, in the late mission regarding technical standards for cable 1960's, CATV started moving into the larger services, limitations on signal degradation, metropolitan areas and a demand arose for measurement techniques, and in the formulation greater channel capacity both where the VHF plus of future policies in these areas. One of the UHF complement of stations exceeded twelve, more important charters of CTAC was to recom­ and to provide extra services to attract revenue. mend a channeling plan and receiver requirements With the advent of push-pull amplifiers, use of which would result in a compatible off-air and the midband for these extra channels became cable television receiver having minimum econo­ feasible, since the push-pull configuration mic impact on the consumi ng public. reduced second order distortions below the inter­ ference level. There was some brief interest in Nine working panels were set up within the CTAC single octave systems ( 120-240 MHz) but they organization to investigate different aspects of never gained popularity since a standard TV the problem, and to coordinate with other organi­ receiver would be restricted to only seven chan­ zations such as EIA, IEEE, and various govern­ nels on the dial. The most generally accepted mental departments. CTAC Panel 5 was speci­ channelling plan was simply an extension of the fically charged with the task of recommending a existing FCC allocations both up and down from channeling plan for cable television use. This the high VHF band. paper is based on the work of Panel 5, as viewed by the author, and should not be construed as Recognizing the need for additional channels, the representing all points covered or the final panel FCC, in its Cable Television Report and Order of report to the CT AC Steering Committee, February 2, 1972, required that cable systems bui lt after that date have a minimum of 20 chan­ As a starting point, the panel first reviewed all of nel capacity, and that by 1977 all s y stems over a the previous ideas and proposals for channeling certain size have 20 channel capacity. In the plans from other groups of which it had knowledge. 112- NCTA 75 The majority of these were discussed in the A) Direct off-air pick-up paper "Channel Allocation Options" by B) Second order distortions RobertS. Powers of the Office of Telecommuni­ C) Third order distortions cations, presented at the 1972 NCTA convention. In the case of direct off-air pickup, Plan 1, After the first few meetings of Panel 5, it became assuming the cable is locked to the local, presents obvious that what at first appeared to be a rela­ only the problem of leading ghosts, since the tively simple task was indeed going to be a for­ broadcast and cable frequencies are the same. midable one. This was because of the many areas Plans 2 and 3 could create co-channel type inter­ that would be impacted by the choice of any chan­ ference since there is a frequency off-set. neling plan, some of them subtle, and some of them not so subtle. One of the not so subtle facts Second order distortions (Fl ± F2 and 2Fl pro­ was the large population of existing TV receivers, ducts) are a problem with Plans 1 and 2, and coupled with the mobility of the American citizen. especially with regard to Channels 5 and 6. A more subtle item was the limitations of the HRC is the only plan with potential for reducing receiver manufacturers in producing a compatible these interferences, but it in turn creates prob­ set without undue economic impact. While a beau­ lems in receiver compatibility and off-air pickup. tiful engineering solution could be achieved with Second order distortion is generally not a limi­ relative ease, the panel had to spend a great deal tation with modern push-pull equipment in of time considering the other factors involved. expanded channel systems, but is quite serious in older single ended equipment. Following the review o.f the various channeling plans brought forward, it was the consensus of Third order distortions (Fl ± F2 ± F3 and the panel that only three merited further, in-depth 2Fl ± F2 products) are a problem with Plan study. Briefly, these plans inay be described as since the carriers are not on precise frequen­ follows: cies a.nd the resultant beats appear as sidebands or "busy background". The number of these Plan 1 - Augmented FCC assignments with beats and their visible effect increase very standard tolerances and offsets. rapidly as the number of channels is increased. (2) This is essentially the "de-facto" Plan 2 substantially reduces this interference situation currently in use by most since the majority of carrier beats are zero beats CATV systems using the mid and and only have a minor effect on luminance. super bands. Plan 3 (HRC) makes all visual carrier beats zero beats. (Note: Cross modulation is a special Plan 2 - Constant interval assignments case of third order distortion and will occur with based on a 6N + 1. 25 MHz comb of any channeling plan. ) frequencies. This would put all picture carriers on the system in During discussions of the full panel, it was evi­ a phase stable condition, but dent that certain members of the panel, especially requires rather complicated those concerned with receivers, preferred equipment at the headend.