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Coaxial Cable Systems and Networks
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Figure 2-3 The block up-converter
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nels unable to be selected by standard TV sets For some early cable systems that placed added programming channels in the so-called mid-band, a simple UHF up-converter was used This device simply converted the mid-band channels that were spaced from the FM band to VHF channel 7 to the UHF band (470 to 890 MHz) This block converter, as it was known, connected between the cable and the TV set s UHF terminal, as shown in Figure 2-3 Since there were 9 equivalent 6MHz television channels spaced in the mid-band, these 9 channels, in addition to the 12 VHF channels, resulted in what was known as the 21-channel cable system At this point in time, available trunk and distribution cables had signi cant loss at UHF frequencies Available ampli ers had upper frequency limits of 220 to 300 MHz The reception of UHF broadcast stations was carried on the cable systems by converting each one of them to one of these 9 mid-band cable channels at the headend The block up-converter at the subscriber s television set would then convert them back from mid-band to UHF Back in the 1970s the 21-channel cable system offered a decent selection of programming with better reception than most indoor or outdoor antennas The success of 21-channel systems encouraged the expansion of cable operators to increase the upper frequency limit to 300 MHz Thus, another 14 television channels were added, making the 35-channel system This increase in channels beyond the frequency selection capabilities of the TV sets of the day made the tunable, or frequency-agile set top, converter necessary The method of connecting this device between the cable and the television set is shown in Figure 2-4
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Figure 2-4 Tunable subscriber converter
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Converters of this type had to have good noise speci cations, or low noise, simply because the cable signal has to go through the front end of the converter and then through the front end of the TV set, resulting in noise contributions from both sources These rst-generation tunable converters required the subscriber to select the desired program on the converter, which translated to a xed channel (usually channel 3 or 4) This situation essentially defeated the use of the set s remote control Thus, the rst big problem with subscriber devices resulted The natural progression to increase the upper frequency limit beyond 300 MHz led to the evolution from 52 channels at 400 MHz to 62 channels at 450 MHz to current state-of-the-art systems offering more than 150 channels 2123 With the tremendous increase in cable system channel capacity, subscriber service selections became enormous Many subscribers wanted only a few basic channels covering local news, weather, and sports Therefore, cable operators tiered the program selections according to their subscribers viewing habits Since all cable system programs were on the system, it was natural to use a lter installed at the service drop to prevent the undesired program bands from being fed to the subscriber s home In the industry this was referred to as negative trapping; that is, the unordered services were trapped out of the subscriber service drop With the arrival of the satellite system that made available premium (pay) channels to cable operators, the negative traps could prevent these channels getting into homes not subscribing to these channels System audits are conducted by driving the aerial plant, looking for the presence or absence of the traps The trap method can be defeated by a subscriber climbing a ladder to remove the trap Determined cheaters replace the
Coaxial Cable Systems and Networks
trap lter with a look-alike device to fool the cable operator Hence, the pilfering of cable services started and program security became an issue Today cable operators have developed various programs to control the theft of cable services From negative signal-trapping methods, the positive trap method was developed This method engages an interfering carrier system where an interfering carrier signal was injected in the band of the signal to be protected At the subscriber tap, a trap would lter out the interfering carrier, allowing the signal to enter the subscriber drop without the interfering signal so the subscriber could receive the premium program This method meant that only premium (pay) program subscribers would need this trap The ratio of premium subscribers to nonpremium subscribers was an important factor for making the economic decision for the best method In short, a system that does not have many pay-program subscribers needs a greater number of negative traps than positive-type traps Remember that the programming choices are the stock and trade of the cable television industry In today s market where the computer Internet connection is a desired service, many telephone system service providers have dif culty in delivering large data les because of the limiting transfer speeds or low bit rates A cable system s bandwidth can allow for faster downstream bit rates, but upstream ordering capabilities are in many cases not available For cable systems operating in the present computer environment, the expansion of two-way services is a must Bidirectional plants with suf cient bandwidth are necessary for cable operators to become players in the telecommunications eld
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