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The Impact of Multichannel Tv Sound on the Cable System Headend

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The Impact of Multichannel Tv Sound on the Cable System Headend THE IMPACT OF MULTICHANNEL TV SOUND ON THE CABLE SYSTEM HEADEND Thomas R. Stutz, Product Manager JERROLD DISTRIBUTION SYSTEMS DIVISION GENERAL INSTRUMENT CORPORATION on a particular transmission system. To ABSTRACT avoid a repeat of the present AM stereo radio controversy, however, the FCC Although the Federal Communications protected the EIA's recommended MTS system Commission (FCC) did not rule on a and, in effect, created a de facto Multichannel Television Sound (MTS) standard. standard, they did protect the Electronic Industry Association's {EIA) recommended All problems solved, right? Not system. As such, the FCC has created a de quite. During the decades which followed facto standard for the transmission and FM stereo radio, an entire cable reception of MTS. television industry was developing. By 1984, more than 30 million subscribers This paper summarizes the effect of were being served by over 7000 cable the MTS de facto standard on the cable systems throughout the nation. These system's headend equipment. Although systems employ various generations of other cable carriage schemes are in use equipment and technology. Many are small today to supply satellite stereo services, 12 channel systems with limited bandwidth this paper will only consider in-band and outdated technology, while others have carriage of the MTS signal. No judgements excess capacity and utilize will be made concerning the audio state-of-the-art technology. Regardless performance of the MTS signal. Also of the available bandwidth or technology discussed will be an overview of the employed, these systems will have to deal modifications and equipment necessary for with the issue of MTS. The driving forces upgrading the headend to accommodate the behind cable system implementation of MTS MTS signal. could be any or all of the following: Since the FCC recently decided to 1) A future must-carry mandate from indefinitely delay the decision to impose the FCC. cable must-carry for MTS, no attempt will be made in this paper to anticipate any 2) subscriber demand. future FCC decision concerning this issue. 3) Revenue generation. 4) Franchise requirement. THE HISTORY OF MULTICHANNEL TELEVISION SOUND - Must-Carry Mandate A BRIEF CHRONOLOGY The decision regarding whether cable In 1961, the Federal Communications systems should be forced to carry the MTS Commission (FCC) decided on and approved a signal, and in what format, was single stereo transmission system for FM indefinitely delayed recently by the FCC. radio. More than two decades later, in a The cable industry had strongly argued time when Federal intervention was being against must-carry on the grounds of downplayed, the FCC approved the use of system incompatibility and resultant high sub-carriers for AM stereo, but did not cost to upgrade. The broadcast industry, choose a single AM stereo radio on the other hand, had argued that transmission system. Rather, they allowed copyright laws prohibit the modification the marketplace to decide. On March 29, of protected programs and, therefore, 1984, the FCC approved the expanded use of cable systems should be required to supply the television aural baseband for stereo, the MTS signal to the subscriber. The FCC second language service, and any other compromised by directing its Mass Media broadcast or non-broadcast purpose and Bureau (MMB) to annually monitor market also, like AM stereo radio, did not decide penetration of MTS braodcasts and MTS 110-1985 NCTA Technical Papers compatible receivers. The MMB was also THE MULTICHANNEL TELEVISION SOUND SYSTEM directed to monitor the voluntary implementation of MTS by cable systems. The objective of the EIA's Broadcast In the meantime, the must-carry issue will Television Systems Committee (BTSC) was to remain open until such time the FCC arrive at a single multichannel television solicits a new round of comments o·n the sound transmission standard which would subject. Cable operators should, incorporate a compatible main channel, a therefore, not overlook the possibility of full quality stereo sub-channel, a lesser a future must-carry mandate from the FCC. quality seperate audio program (SAP) sub-channel and the potential for a Subscriber Demand professional sub-channel for telemetry purposes. Following initial testing, it Most TV receiver manufacturers have was determined that some form of noise recently announced the availability of reduction would be necessary to compensate multichannel sets. Some consumer for the lower signal-to-noise ratio electronics manufacturers have also associated with the wider audio bandwidth announced availability of MTS adapters to of the MTS signal. convert monophonic sets to stereo. These announcements have undoubtedly created a Of the three proposed transmission consumer awareness of MTS. With this systems considered by the EIA, a single awareness comes unrealistic expectations system was recommended which met the above from the cable subscriber that they will objectives. As illustrated in Figure 1, soon be receiving MTS via cable. the recommended BTSC MTS system consists Subscriber demand for MTS, whether founded of a main channel containing both left and or unfounded, will be a factor which right audio signals. The main channel cannot be overlooked by the industry. pre-emphasis, bandwidth and audio deviation are identical to the current Revenue Generation monophonic standard, i.e. 75 usee, 15 KHz and +25 KHz respectively, and is therefore MTS consumer awareness and resultant fully compatible with non-stereo TV demand has created an opportunity for receivers. operators to generate additional revenue. Broadcasters are well aware that To separate the left and right audio advertising revenues can be increased when signals, the stereo sub-channel contains native language programming is accompanied the difference of left and right and has by native language commercials. Cable an effective bandwidth of 15 KHz with a operators serving non-English speaking sound carrier deviation of +50 KHz. The communities, in the same way could combined audio deviation of-the main and increase their advertising revenues by stereo sub-channel is +50 KHz, since by utilizing the second language program of definition, both signals cannot be maximum the MTS standard. Major networks have at the same time. The stereo sub-carrier already test-marketed bilingual broadcasts uses double sideband suppressed carrier in non-English speaking markets and have amplitude modulation with the sub-carrier had excellent results. locked to twice the horizontal scanning frequency. Dbx companding or noise Those ignored stereo satellite reduction is added to this channel to services could also be offered in or improve the signal-to-noise ratio. The out-of-band for additional revenue. For companding process also provides an the consumer not sold on cable, these improvement to the usual pre-emphasis expanded audio services could help add technique for the stereo sub-channel. A subscribers and increase penetration. pilot signal at the horizontal scanning frequency, causing a deviation of +5 KHz, Franchise Requirement provides the necessary reference sTgnal to control the stereo decoder in the TV Although program material with receiver. bilinqual audio is limited in its availability at present, major city The SAP sub-carrier is locked at 5 systems, in the future, may be required by times the horizontal scanning frequency franchising authorities to carry second and is frequency modulated with a language service for non-English speaking bandwidth of 10 KHz and audio deviation of residents. The transmission of MTS will +15 KHz. To boost the signal to noise proliferate the availability of bilinqual ratio of this carrier, dbx companding is programming as broadcasters seek to expand also incorporated and provides an their market penetration and advertising improvement to the usual pre-emphasis revenues. technique. Finally, the professional sub-channel has its sub-carrier located at 6.5 times the horizontal scanning 1985 NCTA Technical Papers-111 THE BROADCAST TELEVISION SYSTEMS COMMITTEE (BTSC) MULTICHANNEL TELEVISION SOUND SYSTEM AUDIO CARRIER OE.VIAl\ON I~Ht 1.5KHt :1: MAIN CAAAIL A 73.0KHt FfAK OLVIATION FROFE SSIONAL SUBCHANNLl 25 1....--------.l r AUDIO BANDWIDTH PAElMF f L • A r I AM VOICE: 0-3.4K 150us L +A 50-15KHz I DSB-SC OAT A: 0-1.5K NON£: AUDIO db• '::10-15KHL BANDWIDlH NOISE ~ AUDIO r---1-,-~,. A[DUCT ION BANDWIDTH SAP 75""set: PRE-EMPHASIS ~0-10 KH.t IOI(Hl PROf. SUR AUOIO SUtJ- i'IAttOWIOlH f"N :JI( llEVIAltOH SUR· CARAafl 1 PilOT BASE HAND t AEOUENC 'f I H = HOA!LONTAL SCANNING fAE:.QUENCY {15:134 KHt) FIGURE 1 system may not pass the sound trap of the frequency and has a bandwidth of 1.5 KHz strip amplifier processor without to 3.4 KHz (depending on the type of distortion, and thus, only the main modulating signal) and audio deviation of channel (L + R) signal would be +3 KHz. unaffected. Depending on the particular design of the strip amplifier processor, IMPACT OF THE BTSC MULTICHANNEL TELEVISION modification to the sound trap circuit may SOUND SYSTEM ON THE CABLE SYSTEM HEADEND be possible so that both the main channel and the stereo sub-channel would have The cable system headend has acceptable distortion. Most likely it experienced many technological changes would not be possible to pass the SAP over the last 25 years. From tne earliest sub-channel. However, caution should be strip amplifier processing, utilizing given to the effects that widening the vacuum tubes, to today's surface sound trap·will have on the video acoustical wave (SAW) filter processing passband. Widening the sound trap will utilizing integrated circuitry, the decrease the video passband. It is headend has evolved to meet the increased recommended that the strip amplifier performance and bandwidth requirements of processor manufacturer be consulted to today's sophisticated systems. Because of determine if the design will support a the various technologies of headend modification for MTS and to what extent systems in operation today, the BTSC MTS the modification will support MTS, i.e: system will impact each in different ways.
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