THE IMPACT OF MULTICHANNEL SOUND ON CATV SYSTEMS
.Joaeph Van Loan
Vlacom Cable
Pleaaanton, California
A proposed Multichannel Sound (MCS) System for August, 1982 the Committee published a 1,000 page televi sian is described, and differences between report which contained results of tests conducted three proponent systems are discussed. Each of on three proponent systems. 1 EIA-J, Telesonics the proponent systems includes provision for and Zenith have each proposed systems which are transmission of: similar in many respects and different in others. 1. L+R information to maintain compatibility with existing receivers Each system will maintain compatibility with the 2. L-R information for stereo existing monophonic system by transmitting an L+R 3. A Second Audio Program (SAP) channel for a channel with the same characteristics as the second language, quadraphonic sound or as a present monophonic channel. Each system will tutorial channel; and transmit stereophonic information using an L-R 4. A non-public channel for voice and/or data channel and a corresponding pilot in the spectrum telemetry. above the L+R channel. A channel called a Second Audio Program (SAP) channel wi 11 permit the Although immediate wide-scale implementation is simultaneous transmission of additional aural not expected, the planned system may have a information such as a second language or a substantial financial impact on the CATV narrative. The performance of this channel is industry. Headend equipment (including limited both in fidelity and noise performance processors, modulators, demodulators and when compared with the stereo channe 1. Finally, microwave transmitters and receivers) may have to a channe 1 ca 11 ed the non-pub 1i c channe 1 is be modified or replaced before passing an MCS proposed to transmit data and voice information. signa 1. Present day descramb lers will 1ike ly suffer deteriorated performance when used on MCS Figure 1 depicts the baseband spectra of each of signals, and the MCS signals themselves may the proposed systems. Each of the systems has suffer degradation when used with descrambling undergone changes to improve performance as test equipment. The introduction of MCS carriers with results are analyzed. Most noteworthy among the their increased bandwidth and possible higher differences are: the EIA-J systems uses an FM levels on a CATV system raise questions about the subcarrier for L-R transmission while the ability of present day receivers to trap out Telesonics and Zenith systems use double sideband sound carriers on lower adjacent channels. suppressed carrier amplitude modulation (DSB-SC JIM) for L-R transmission. Note, current The MCS Subcommittee of the NCTA Engineering practice for FM broadcast stereo in the U.S.A. Committee is conducting tests in cooperation with uses DSB-SC AM for stereo transmissions. In the the Electronics Industries Association and the EIA-J and Zenith systems the L-R subcarrier is National Association of Broadcasters to determine centered at twice the horizontal scanning the impact MCS signals will have on the CATV frequency (2fH), but in the Telesonics system, industry and to recommend which of the three the L-R subcarrier is positioned at 2.5 the proponent systems, if any, should be selected. hori zonta 1 frequency. The SAP channe 1 has been subjected to numerous changes. Figure 1 INTRODUCTION indicates the proposed configurations at the time of this writing. Multi-Channel Sound for Television and its Implications for CATV In August, 1982 the NCTA Engineering Committee formed the MCS Subcommittee to study the impact In 1979 the Consumer Electronics Group of the that MCS will have on CATV system operation and Electronics Industries Association, the National Association of Broadcasters and the Joint Council l"Multichannel Television Sound: The Basis for of Inter-Society Coordination formed the Selection of a Single Standard", by Electronics Multichannel Sound Subcommittee to study Industries Association's BTS Committee, Published propos a 1s to introduce mu lt ichanne 1 sound by the National Association of Broadcasters, (stereo) into the U.S. television system. In Vol. I, July 16, 1982; Vol. II, August 6, 1982.
254 to recommend one of the three systems if deemed stereo SNR or approximately 55dB and a SAP appropriate. The Subcommittee set a precedent in SNR of approximately 45dB. These are early 1983 when professional help was retained to certainly more tolerable conditions and could supplement the volunteer activity in studying the be improved even more if companding is used. impact. The EIA/NAB Committee has made its (A separate working group is developing a laboratory facilities, located near Chicago, companding system). available to aid in the study. Tests are now being conducted in that facility with joint From the calculations it appears that cooperation between the two groups. threshold margins are adequate, both for new sets with sound IF bandwidths sufficient to TECHNICAL IMPLICATIONS FOR THE CATV INDUSTRY support the broadband signal and for old sets. A. Audio Signal/Noise Ratio2 B. Visual/Sound Ratio In Volume I of the Multichannel Television It is recognized that one way to improve the Sound report, it was indicated3 that for multichannel sound SNR is to increase the high quality sound reception, the signal to sound carrier level. Above threshold the noise ratio should be at least 60dB, audio SNR is increased one dB for each dB we preferably 70dB or better, for the principal raise the sound carrier level. In community of viewers. For comparison, EIA Volume I5 of the EIA report, use of the RS-250B specifies for monaural sound that the highest feasible sound power is encouraged. minimum unweighted audio SNR for end-to-end television relay facilities be 56dB, It must be recognized that increasing the including buzz. We should deliver at least a sound carrier level in a cable system is 60dB SNR to the TV receivers connected at the undesirable and would create a multitude of extremities of our systems. To determine the problems, as explained in the following: expected SNR (thermal noise only) in cable systems, calculations were performed by l. TV Receiver Adjacent Channel Rejection several members of the NCTA Subcommittee. All cable systems operate with TV channels For the case of 36 dB NCTA Video RF -SNR4, adjacent to one another. In the early with the sound carrier l5dB below the video days of cable it was determined that most peak envelope power, we get the following TV receivers lacked sufficient selectivity unweighted audio SNR's. to provide for beat-free pictures, unless the lower adjacent sound carrier was Stereo reduced in level below the visual/sound Monophonic L or R SAP ratio being transmitted by broadcast ::.epa rate stations. Although the NCTA col1lllittee Mixing 63.8dB 49.2 39.6 recognized that receiver selectivity has Intercarri er, been considerably improved over the years, Video at one must recognize that the proposed Blanking Level 63.6 49.0 39.4 multichannel sound standards with their Intercarn er, increased deviations (approximately 70kHz) Video at creates sound carriers whose energy White Level 59.0 44.4 34.8 occupies a much wider bandwidth than before (approximately 320 kHz for 70 kHz Obviously we are in trouble if there are deviation as compared to 80kHz for 25kHz subscribers whose ears demand stereo but deviation). In a paper written in 1972 by whose eyes will tolerate 36dB. In addition, Will Hand of Sylvania entitled "Television the Separate Audio Program SNR will be Receiver Requirements for CATV Systems", approximately lOdB less than stereo signal. tests were performed on a number of color If we assume that a more typical situation is receivers which represented collectively for the cable system to deliver a 43dB Video over 50% of receiver designs in the SNR, then we find ourselves dealing with a industry at that time. The data shown below is the weighted results of these 2rn order to simplify analysis and due to measurements. The data was weighted to similarities between systems the Zenith was used reflect the proportion of the market held for discussion and analysis except where clear by the receiver manufacturers. differences are noted. 5Multichannel Television Sound Report, p. 164. 3Multichannel Television Sound: "The Basis for Selection of a Single Standard", Volume I, The National Association of Broadcasters, July 16, 1982, p. 51. 4Minimum allowed CNR for Cable Systems. Part 76 of FCC Regulations.
255 highly selective bandpass filters on the Frequency (kHz) output of these devices but not without an
1 AdJacent Sound null to 4ldB increase in the envelope delay distortion point on low frequency side of inherent in these filters. Equipment IF trap 94 already in place may have to be replaced Mj acent Sound nu I I to 41 dB or undergo major modification to meet new point on high frequency side of performance standards. IF trap ll5 The 4ldB reJeCtlOn bandwldth ot 3. AML Equipment this trap is 94 + 115 = 209kHz AML equipment is used to transmit channels One can only conjecture what the rejection of television from one area of a community of this trap would be to a sound carrier to another. Increases in aural carrier adhering to the proposed multichannel levels would cause corresponding increases sound standards; however, it goes without in the lower 4.5MHz products, as described question that raising the sound level in 2. above. For systems operating at would only aggravate what already may be full rated power, it would likely be an unacceptable operating condition. necessary to reduce output power thereby reducing fade margin. The performance of Set-top converters with adjacent channel this equipment is impaired by scrambling traps would help, although only a few systems which cause a 6dB or more increase presently in the field have this feature. in the aural level during transmission of Separate traps connected to the output of descrambling timing pulses. This practice converters and tuned to the lower adjacent creates a marginal condition to exist with sound would help; however, they suffer today's practices. Increasing levels for from the following problems: Multichannel Televi sian Sound would be unacceptable. a. AFC and fine tuning errors would diminish effectiveness. 4. Distribution Equipment
b. They would introduce additional group Measurements were conducted to determine delay errors. the perceptibility of sound carrier beats when the aural carrier levels were c. The overall cost would be substantial. increased above l5dB below the visual carriers. From these measurements, d. Subscribers might confuse them with pay perceptibility of sound-carrier beats TV traps and remove them. occurs at a system level 3 to 3-l/2dB above that level which produces barely 2. Headend Equipment (Si1nal Processors, perceptible video carrier beats Strip Amps and Modulators (CTB-composite triple beat distortion) when not phase locked. In the same test The majority of these devices operate at the system level could be elevated an output level between +50dBmV to +60dBmV approximately 5dB when phase locked before (+60dBmV = 1 volt rms @ 75 ohms) for the background images (modulation cross-over) video carrier with the sound carrier could be seen. These tests were performed typically l5dB below this level. Most with 54 channel Harmonically Related manufacturers also quote a specification Carriers (HRC) loading. The conclusion to which states that when the unit is be drawn from these tests indicate 1f operated at maximum output (this is not aural levels must be increased to uncommon), all spurious outputs will be at accommodate Multichannel Sound, then the least 60dB below the desired video advantage gained from phase locking is carrier. One component of this distortion lost. falls l.5MHz above the video carrier of the lower adjacent channel. Any attempt Another problem may surface if aural to raise the sound carrier level causes a levels are increased. Many cable one dB rise in this undesired signal for operators find it possible to use certain each dB the sound level is increased. channels in the aeronautical radio service This is a particularly sensitive area of bands by lowering aural carrier levels to the video spectrum and any interfering +28.75dBmV maximum. In fact, several carrier must be 55dB to 60dB below the channels can sometimes only be used this desired carrier not to be perceptible. way due to conflicts with both the visual This problem is only aggravated when and aural carriers. If aural levels are scrambling systems are employed which raised, this practice would be eliminated amplitude-modulate the sound carrier with resulting in the loss of these channels descrambling timing information. The NCTA for stereo service. Subcommittee recognizes that this potential problem can be solved with
256 C. Increased Deviations Committee described the use of unequalized filters in home stereo In order to achieve the highest possible rece1vers. This practice results in stereo SNR, plus provide for auxiliary stereo separation of 40dB, a number services, all proponents of the multichannel considered adequate. Genera 1 practice sound systems intend to increase the peak has been to control amplitude deviation to approximately 70kHz. By doing characteristics of filters, but no so, technical problems may be created in both special care is used in controlling cable headends and set-top converters, not to delay characteristics. This practice mention the selectivity problems of TV has not been reported to have caused receivers. The following section discusses any problems of which the Committee is the nature of these problems in detail. aware. 1. TV Receiver Selectivity (See Section B.l.) In general, the NCTA Subcommittee beheves IF circu1try in all existing 2. Headend Equipment processors would have to be redesigned for successful MCS operation. a. Signal Processors b. Modulators Two types are currently in use. One type uses a sp 1 it sound system where If extern a 1 MCS signa 1s are generated the aura 1 carrier is trapped and and pre-emphasis circuits are removed processed separately. The second type from modern modu 1a tors, it may be processes the video and sound combined possible to use them for MCS and uses adjustable traps to reduce the transmission without difficulty. Older sound carrier 1eve l. Both systems will designs may have problems caused by suffer when deviations are increased transformers used for coupling and from 25kHz to 70kHz. uncontrolled amplitude and phase characteristics in filters. For split sound units, the sound notch must not introduce amplitude or group c. Demodulators de 1 ay errors in chroma i nf ormation while attenuating the sound carrier and Volume I of the referenced MCS report its sidebands at least 40dB. This has discusses the impact MCS wi 11 have on been achieved for 25kHz deviation with TV receivers; since most demodulators a Carson bandwidth of 80kHz. When the use similar circuitry, they will new Carson bandwidth for stereo experience many of the same problems. (approximately 350kHz) is measured, This includes insufficient sound IF attenuation of the upper sideband is on bandwidth, problems with buzz caused by the order of lOdB. With incomplete i ntercarri er sound processing and trapping, a portion of the aural inadequate baseband response to pass carrier passes through the visual MCS subcarrier components. processing circuitry. When this signal recombines with the processed aural 3. Set-Top Converters(Scrambling/Descrambling) signal, impairment of the received aural s1gnal w1ll result. This Of all the possible problems, this one has impairment could result in a the potential for the greatest impact on substant1al reduction in the amount of the CATV i ndustrx;. Devices with the separation between the 1eft and right greatest proliferat1on use either pulse or channels. More testing needs to be s i newave sync suppression. Prob 1ems with conducted to confirm or dispel this both systems can arise when FM to .AJ-1 concern. conversion occurs in headend processors, modu 1 ators and set-top converter bandpass One processor measured had 2% .AJ-1 on the filters. These products manifest FM sound carrier with 25kHz deviation themselves in different ways. In the case and 16% .AJ-1 with 70kHz deviation. This of pulse systems, the AM pulse on the would clearly cause a problem When sound carrier, and any spurious AM using analog descrambling information products, fire a trigger circuit, usually on the aural carrier. a monostable multivibrator. Any stray .AJ-1 products or noise can cause descramb 1 i ng In addition to the notch problem, the pulse jitter due to slicer uncertainty. 3dB bandwidth of the sound path was measured on one type of processor to be An attempt was also made to simulate approximately 350kHz. No attempt is stereo susceptibility to stray tagging and made to control delay characteristics descrambling pulses. Measurement data at the band edge. One member of the seems to indicate compatibility between MCS and pulse scrambling systems; however,
257 reports from the field indicate buzzing is subchanne 1 was observed. The interference being experienced . on some sets imported was especially noticeable in split-carrier from Japan which include stereo reception. This was due to amplitude and demodulator circuitry using the Japanese phase nonlinearities of traveling-wave standard. These reports must be tube (TWT) power amplifiers used in old investigated prior to reaching any final trans 1 a tors and a 1so due to cascaded use conclusions. of such trans 1 a tors." There is reason to believe a similar problem may exist with The sinewave sync suppression system may AML equipment. Tests are underway to be most susceptible to FM to AM conversion determine what impact this equipment will products. These systems wi 11 undoubtedly have on future TV receiver design. suffer from the almost certain increase in AM products on the aural carrier. Although most operators use phase locked Additional testing is to be performed in receivers for AML transmission, some this area by the Subcommittee. operators occasionally use receivers with free running 1oca 1 osc i 11 ators. In such Several manufacturers have baseband cases, the levels of IPM increase systems in the field. Since these units substantially. One cable operator have demodulators, the performance and measured substantial increases in noise problems will be similar to those 1eve 1 s using these receivers with experienced with TV sets. Special MCS synchronous demodu 1ators. MCS TV units will likely be similar to the tuner, receivers using sp 1it sound systems would IF amp and detector circuitry developed experience impaired noise performance as a for TV receivers intended for MCS result of this phenemonon. operation. This suggests units already in the field can continue to be used for 2. Headend Equipment subscribers with monophonic TV receivers, while units of a new, compatible design Most signa 1 processors and modu 1a tors use would be required for subscribers wishing oscillator designs which are crystal to avail themselves of the opportunity to controlled; however, there are presently have multichannel sound. The one unknown, on the market several multichannel units to be investigated, was discussed earlier (typically used for standby purposes) in section B.l. of this report. The which use oscillators that are either adjacent channel sound traps in these frequency synthesized or under AFC products could be expected to behave like contra 1. These devices wi 11 almost those in TV sets. One manufacturer uses certainly increase the amount of crystal stabilized local oscillators with incidental phase modulation present on the AFC and SAW IF filters. When compared sound carrier. with older tube-type television sets with unstable IF circuitry, these units may 3. Set-Top Converters offer satisfactory performance. This must be verified before concerns can be relaxed. Converters which use synthesized tuning systems introduce substantia 1 1eve 1s of D. Phase Noise Considerations incidental phase modulation (ICPM) on the TV signal. Older varactor tuned products In Volume I of the Multichannel Sound Report, also introduce ICPM, but to a lesser mention was made that a return to split-sound degree. TV receivers using intercarrier TV receiver design could eliminate sound buzz sound detection are not affected by high due to incidental phase modulation of the levels of ICPM when both the visual and sound carrier generated by mixing with the aural carriers are subjected to the video carrier. This would certainly be the introduction of ICPM: however, in TV case; however, a 11 concerned must a 1 so receivers using split sound IF and recognize there are additional problems detection systems ICPM products introduced created by this technique which will likely by CATV converters can contribute to be made worse by equipment presently used in deteriorated noise performance. It is not cable systems. Indicated below are several uncommon for varactor converters to areas which need additional investigation introduce frequency modu 1at ion on the TV before split-sound receiver design should be signal in excess of 10kHz. This suggests considered. a serious problem for the cable industry if split sound TV receivers are introduced 1. AML Equipment in the marketplace.
On page 161 of Volume I of the MCS report, the EIA Committee discusses a problem with SOME ADDITIONAL THOUGHTS ON MULTICHANNEL SOUND translators in Japan. "During the rebroadcast tests of MCS signals, a. large One must ask if the course of action the increase in buzz interference w1th1n t~e television industry is about to embark on is
258 appropriate now. We are introducing 40-year old for digital stereo audio which are compatible technology at a time when a quantum leap in with the existing monophonic system, then a digital technology is about to take place. The quantum leap will have been realized which would TV sound system in the United States is already be a direct benefit to the American viewing being strained to capacity with performance which public. It will be a challenge to develop can best be described as marginal. The course of cost-effective digital technology compatible with action we are about to embark on appears to take the TV sound system a 1 ready in p 1 ace, but a marginal situation and make it worse. This engineers using the latest technology have the action is taking place at a time when the capacity to overcome difficult challenges. It is telephone industry is moving in the direction ·of true there are difficult compatibility issues to digitized voice communications, at a time when be overcome, but this is a real opportunity to the consumer stereo industry is on the verge of enhance the quality of television in the United moving away from the traditional analog methods States. into digital technology using laser disks and CONCLUSION tape, and when satellite delivery mechanisms such as Public Broadcasting Service (PBS), Home Box This paper has described several major elements Office (HBO) and Direct Broadcast Satellite (DBS) in CATV systems where the carriage of services are moving to digital audio. multi-channel sound might create problems. At the time of this writing the NCTA Engineering The CATV industry is facing some serious problems Committee has outlined a test plan to investigate with the present methods of premium program these topics. The tests, which began in February security. Our premi urn TV security systems face and will last about six months, will be performed serious threats not only by tampering but also by by representatives of the cable industry working units manufacturered and sold on the black in conjunction with EIA Committee engineers. The market. The industry is also facing pressure in results will be described in a comprehensive the regu 1a tory and 1egis 1 at i ve area to make the report including both subjective and objective sound on certain channels unavailable to the evaluations of each area of concern. If testing owners of cable-ready sets and users of results are negative, it may be appropriate for conventional converters. The task of keeping our the industry to lead the drive to develop premium signals secure eventually may come to standards for a digital audio system. depend on scrambled digital audio. If the TV industries could successfully develop standards
259 MAIN CARRIE"R. PfAK DEV. KH1 50l TELESONlCS I.--S.3UI ~ 1: MAIN CIII-!RIER 25 25 L-R AM 1 [W:B] DSB.-SC SO-ISkHs PE"AK DEV. Kill L+R 75}'5ec See Note So- 15KHz 15 75}-IS Tl: LiME:'1RY ,., ...,. SAP FM VOICE~ J.£1 KMt, 1S.OJoiS ,l)ATA : :Z k ... , ...... 50-81<. i 10 I
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Figure 1
Baseband Spectra (Proposed Systems)
Source: Multichannel Television Sound, Volume I, p. 57
260