Cable Television (CATV) PDF

Cable televison (CATV) started as a means of providing signals to communities that could not receive broadcast stations, either because of distance or shadow areas in which the signal was too weak. Then a community antenna was used at a remote location to feed TV signals to receivers in the area. To...

Cable televison (CATV) started as a means of providing signals to communities that could not receive broadcast stations, either because of distance or shadow areas in which the signal was too weak. Then a community antenna was used at a remote location to feed TV signals to receivers in the area. Today, cable TV has developed far beyond that into huge systems that cover large areas, even for locations having good reception. The reason is that cable TV does not have the restriction of channel allocations for broadcasting. The cable systems offer up to 36 channels. A cable converter box permits selection of the desired channel. Premium pay services such as Home Box Office, Spotlight, Prism, Cinemax, and others also offer current movies and sports events not available on broadcast television. These programs reach the cable operator via satellite transmission. CABLE FREQUENCIES Many older cable systems distribute TV signals on the same VHF channel frequencies that are used for broadcasting. The UHF channels are converted to VHF channels for distribution because cable losses are too high in the UHF band. This method is a 12-channel system, including the lowband and highband VHF channels 2 to 13. Subscribers in the system do not need a con-verter. Direct cable connections are made to the TY receiver, where the RF tuner can be used to select the desired channel. ADJACENT CABLE CHANNELS With a 12-chan-nel system, some receivers may have adjacent-channel interference, since all the VHF channels are used. The ability to reject adjacent-channel frequencies depends on the IF selectivity of the receiver. The interference produces a windshield-wiper or venetian-blind effect in the picture. See Figs. 11-7 and 11-8 in Chap. 11. In the cable system, interference is minimized by balancing the signals for all channels at a common level. Also, compared with TV broadcast signals, the sound carrier level is usually much lower than the picture carrier signal. MIDBAND AND SUPERBAND CABLE CHANNELS Since the cable signal is not radiated, at east not intentionally, the cable system can use frequencies that are assigned to other radio services without interference. Therefore, the mid-band cable channels are used in the gap between VHF channels 6 and 7. These frequencies from 88 to 174 MHz include 88 to 108 MHz for the FM radio broadcast band plus various marine and aircraft communications services. However, the FM radio band generally is not used for TY cable channels. As listed in Table 15-1, the midband cable TV channels start with number 14 or letter designation A for 120 to 126 MHz, with the video or picture carrici frequency set at 121.25 MHz. Although not listed, the sound carrier frequency is automatically 4.5 MHz higher, or 125.75 MHz. Included are channel numbers 14 to 22 or letters A to I. Additional midband channels are numbers 00, 01, and 54 to 59. Channels 00 and 01 are above the FM radio band. Double digits are used for all cable channel numbers to allow for a digital control board for tuning. Channels 54 to 59 occupy spot frequencies in gaps of the regular mid-band channel allotments. Superband just means cable TV channels above the VHF broadcast channel 13. This band starts with cable channel letter J or number 23. The letters continue to Z and the numbers to 53. The use of VHF broadcast channels 2 to 13 and cable channels 14 to 37 provides 12 + 24 - 36 channels in a typical large cable TV system. These frequencies are up to approximately 300 MHz. Systems using the higher cable channels up to 400 MHz are more sophisticated. They require special cable and better amplifiers with closer spacing to offset greater losses at the higher frequencies. TUNING TO THE CABLE CHANNELS In conventional TV receivers, the RF tuner usually is not made to select the midband and superband cable channels. Therefore, the cable operator provides a separate converter unit. It converts all cable frequencies to a designated VHF channel, such as channel 2, 3, or 4. The subscriber keeps the receiver tuned to the specified channel, and all channel selection is done at the converter. CABLE-READY TV RECEIVERS Many late-model receivers offer a tuner that can select the midband and superband cable channels directly without the need for a converter. However, there is another practical problem. The premium pay services usually have a signal that is scrambled elec-tronically. The circuits required for descrambling are built into the converter or attached to it. As a result, the system-oriented converter would be needed anyway to watch the scrambled-signal premium channels CABLE RADIATION The cable operator must be especially careful that the system does not radiate TV signals. Radiation can occur if cables are open, short-circuited, or even partially mismatched at their termination. Damage to the cables may result from strong winds, storms, or other accidents. To detect radiation, a selected midband chan-el may be used just for an FM tone-modulated dicator signal. Then a simple portable FM ra-ho can be used as a "sniffer" to locate any radia-on, just by riding along the cable route in the ervice truck. Some midband channels are particularly sensi-ve to the radiation problem. For instance, the and for channel A or 14 includes the aircraft stress frequency at 121.5 MHz. Some cable op-rations may not use that channel when the possi-mity of interference exists. ARMONICALLY RELATED CHANNELS Cable sterns have the option of operating slightly off e frequencies assigned for TV broadcasting, but ose enough to allow TV receivers and converts to tune to the frequencies. One such choice harmonically related channels (HIRCs), where the picture carrier frequencies are an integral altiple of 6 MHz. These are listed in Table 3-2. Note that channel 04 with the 66- MHz armonic picture carrier is only 1.25 MHz away from the 67.25-MHz broadcast carrier frequeney. he advantage of using the HRC system in cable stems is simplification of the frequency synthe-circuits used for tuning at the head end and the converter. COAXIAL CABLE FOR CATV The conduit used for distributing the CATV signal is at RF channel frequencies is coaxial cable. is an efficient wideband transmission line that has the advantage of shielding. There are several types of coaxial line, generally called coax, but all are constructed as in Fig. 15-1. A central wire is surrounded by a cylindrical or tubular conductor, and the two conductors are separated by an insulator. The type of cable generally used in a main signal route, called a trunk line, is shown in Fig. 15-2. It consists of a heavy central aluminum conductor that is copper-clad, meaning that it is coated with copper. The outer conductor or shield is also aluminum and is shaped in a solid tube. A polyethylene foam fills the internal space and supports the inner conductor exactly at the center. The cable diameter is about 3/4 in. [19.1 mm]. Some other types of trunk cable are hollow, with the inner conductor supported by plastic beads at regular spacings. The larger the cable diameter, the less the attenuation. How- ever, large cables are not flexible and are difficult to install. For installations with a long, unsupported span of cable, a steel messenger cable is encased within the outer jacket. The steel wire actually suppor the weight of the cable. Trunk cables also are made with a waterprod polyethylene jacket for use underground or un derwater. In addition, armored cables with a sp ral layer of steel are used In some systems, two cables are combined i a single outer jacket. These are called Siame cables. They can be used in two-cable system in which each cable carries different progran in the range of channel 2 to channel 13. The the system has a capacity of 24 channels witho the need for a cable converter. A simple tw position A-B switch can be used by the subscribe to choose one cable or the other, for 12 channe on each. The cable used in the branch lines from di main trunk are similar to that in Fig. 15-2 b smaller in diameter. Thinner cable can be us because the run is not too long. The line from a branch to the subscriber called a drop line. The drop line is genera RG-59U coaxial cable, as shown in Fig. IS This cable is flexible because a copper braid used for the outer shield. Its diameter is 1/4 in [6.35 mm], including the outer polyethylen jacket to make the cable weatherproof. CABLE LOSSES In our imaginary lossless line, all energy sent down the line from the source is consumed in the terminating load. With practical lines, though, some energy is dissipated in the line it-self. The result is attenuation of the signal. There are three causes of attenuation: 1. PR losses produced by current in the conduc-tors. 2. Dielectric losses in the insulator between con- ductors. Remember that the signals are high radio frequencies in the VHF band. 3. Skin effect. The RF current flows more on the circumference of the conductor than its center. Because of the smaller area for current, the ac resistance of the conductor increases. The aluminum cable in Fig. 15-2 has a copper coating around the inside conductor to reduce losses from the skin effect. LOSSES INCREASE WITH FREQUENCY Specifi-cally, the losses increase in proportion to the square root of frequency f. For a practical case, compare channel 13 at 210 to 216 MHz which is about four times higher in frequency than channel 2 at 54 to 60 MHz. At 4/. the line losses for channel 13 equal V4, or double the losses for channel 2. The values of cable attenuation for several different types of CATV cable are listed in Table 15-3 for frequencies from 5 to 500 MHz. CABLE DISTANCE Desigers of cable distribution systems think in terms of signal attenuation per unit distance of line. For example, suppose that a particular cable has an attenuation of 1 dB per 100 ft [30.5 m]. The loss is 6 dB for 600 ft [183 m) and 20 dB for 2000 ft [610 mj-Incidentally a 6-B loss in voltage means one-half the signal. Rather than continuously make the conversion from decibel loss to distance, it is more convenient to work in terms of cable loss directly. In our example, the cable 2000 ft [610 m] long is referred to as a 20-dB run. It is the function of the trunk amplifiers to make up for the cable losses and restore signal levels to standard values throughout the cable system. CABLE DISTRIBUTION SYSTEM Refer to Fig. 15-6. The starting point for cable signals is called the head end. Here the broadcast signals picked up by the antenna are amplified adjusted for level, and fed into the trunk lines. The UHF channels are converted to VHF chan- nels. Also included are local-origination signals from a studio. The video and audio signals modulate separate carriers in a VHF channel not being used. The main routes of signal from the head end are the trunk lines. TRUNK AMPLIFIERS The trunk amplifiers are in serted at regular intervals along the trunk route to make up for cable losses. For the example in Fig. 15-7, a 20- dB amplifier is placod at the end of a cable run with a loss of 20 dB. The decibel unit used for cable signals is dBmV. which means "decibels above 1 mV. Amplifien are placed at regular intervals to keep the signal up to the standard level of 1 to 3 mV. Figure 15-8 shows a weatherproof housing for a trunk amplifier in an aerial system, which i mounted on a pole. Power is obtained from a tap on electric service lines on the same pole. BRIDGING AMPLIFIERS This type of amplifier a for a branch from the main trunk to feed a articular neighborhood in the cable system. The pical gain is 20 to 40 dB. The output is for e branch lines to individual subscribers. In many cases, the trunk and bridging amplifi-as are located in the same weatherproof housing. An additional attenuator may be used at the input to the bridging amplifier to balance the signal levels. UNE AMPLIFIERS Long line runs from the bridging amplifier may require that line-extender am plifiers be inserted in the branch line to make up for cable losses in that branch. This amplitier extends the number of drop lines that can be usod on a branch line. The typical gain for a line amplifer is 20 to 40 dB. DIRECTIONAL COUPLERS Signal power taken from the trunk must be kept very small so that the line is not loaded by all the branches. The device used to tap off the signal is a directional coupler. Its construction is illustrated in Fig. 15-9a, and its symbol is shown in Fig. 15-9b. It is a three-terminal device. One terminal is for signal input. Another carries the signal through the trunk line. The third terminal has tapped output signal for a branch. The directional coupler is so named because it feeds a sample of the direct, downstream signal out at the tap but ignores reflected energy in the trunk line. This is accomplished by a small loep placed in the wall of the coaxial assembly, as shown in Fig. 15-9a. The loop is terminated with a 75-0 resistor. The loop acts as both a capacitor and an inductor. Its capacitance charges to the potential difference between the inner and outer conductors at that point on the line. As a one-turn coil, the loop is magnetically coupled to the center conductor to tap off the signal. Directional couplers have a very small insertion loss between the input and output signals on the trunk line. A typical value is -1 dB for insertion loss at 300 MHz. The tap loss from input to output at the tap is typically -13 dB, but this loss is made up in the bridging amplifier. POWER SUPPLIES The cable amplifiers are solid-state and require little power. The power supplie may be placed at long intervals and the cable itself used to carry the de power. Input for the power supply is 120 V ac tapped from the powe line on the same pole in an aerial system. The typical de supply voltage for the cable amplihen is 24 V. The power supply module often is located a the same weatherproof housing that encloses the trunk and bridging amplifiers. In some cases storage batteries are kept on constant charge with two 12-V batteries in series to make 24 V They are switched into service when there is a failure in ac power. LINE TAPS The final tap on the system feeds the drop line for the subscriber, usually with RG 59U cable. Figure 15-10 shows a multitap with four taps for four houses close to one another As with directional couplers, the line tap has a low insertion loss but high tap loss. The tap-to-tap loss is made high to provide isolation between the individual subscriber lines Isolation is necessary so that a misterminates cable at the subscriber's TV receiver will not se up reflections in the cable system. Two possible terminations are a cable that is not connected or leads that are short-circuited. The tap units are available with many values of tap loss, so the signal levels can be balanced for different subscriber drop lines along the branch. BALUN UNITS The word balun is actually an abbreviation of balanced-to-unbalanced connec-mons. A typical unit is shown in Fig. 15-11a. Most TV receivers are designed for 300-ohm anced input at the antenna connections, for twin-lead transmission line with neither side grounded. The coaxial cable in a CATV system, however, is single-ended or unbalanced with one side grounded. The balun is used to match the 75-1 coaxial cable to the 300-0 receiver input. As shown in Fig. 15- 116, the balun is constructed as two sections of 150-0 line, usually coiled to make the unit smaller. The lines are connected in parallel at the 75-1 side and in series at the 300-52 side. Actually, the balun can match the impedances in either direction. F CONNECTOR The coaxial connector for the 75-0 line in Fig. 15-11a is the standard F connee-tor used in cable work. Its advantage is that no soldering is required. The solid center wire of the cable is the center pin of the connector. Also, the grounded sleeve slides into the braided shield of the cable. Either the sleeve of the plug is screwed onto the jack, or a press-fit connector can be used. Many TV receivers have an input jack for an F connector so that a direct cable connection can be made without the need for a balun. WAVE TRAPS AND SCRAMBLING METHODS Cable systems offer for a minimum fee the so- called basic service, which includes the local TV broadcast channels, some out-of-town stations, and local-origination programs. In addition, pre mium services are offered, such as Home Ba Office, Spotlight, Cinemax, Rainbow, and othes They feature special sports events and move uncut and without commercial interruption However, these premium channels require a to be paid that is added to the basic charge. serve only those subscribers who pay for the a service, two techniques are used. One m is to insert a wave trap that attenuates the l channel. The trap is in the feed line to each scriber who does not have premium service. I ure 15-20 shows such a trap for aerial mous on the outside pole. The trap method is not much anymore, though, because it requires on the pole to change the service. Also, the wave traps can be bypassed by illegal tampering at the subscriber tap on the feed line. Today, the preferred method of security is to scramble the signal. The picture is not intelligible unless it is descrambled with a unit supplied by the cable operator. SCRAMBLING The most common method of scrambling the signal is known as sync suppres-sion. Syne is compressed only in the RF modulation envelope of the video carrier in the cable channel. Then the receiver cannot lock in with the sync- suppressed signal. The picture is usually out of sync, both vertically and horizontally, as evidenced by rolling and diagonal bars. In addi-tion, the loss of sync upsets the receiver AGC circuit and produces the effects of AGC overload distortion. The picture is dark, possibly reversed in white and black values, like a negative, and out of sync. Figure 15-21 shows such a scrambled picture. DESCRAMBLING The descrambler unit reverses the effect of the scrambler at the head end by restoring syne to the RF signal. Syne is restored by means of a keyed RF attenuator bypassed with a diode switch, indicated as R, and S in Fig. 15-22. In this method, the pulses needed for the switched attenuator are sent to the de scrambler unit by a separate route. A pilot carrier signal having a frequency below that of the channel is used. An example of the pilot carrier frequency is 114 MHz for midband channel A at 120 to 126 MHz. Other choices are possible, though, and cable operators choose their own pilot frequencies for security reasons. In Fig. 15-22, the descrambler contains a narrowband receiver tuned to the assigned pilot fre- quency. The receiver has an amplitude detector and pulse-shaping circuits to drive the diode switch. The decoding pulse in the pilot signal is the sync needed for descrambling. As a result, syne is restored in the RF signal for the TV re-ceiver. LONG-DISTANCE LINKS Large cable systems often cover long distances which could result in prohibitive cable attenua tion. Examples include the distance from a remote antenna tower to the head end and that from the head end to major hubs, which are the distribution points for local service. The methods used by the cable operator to reduce losses for long-distance links include super-trunks, microwave links, and fiber-optic links using light waves. SUPERTRUNKS In this method, larger cables are used and the cable channels are heterodyned down to lower frequencies. Both techniques reduce the cable losses. As shown in Fig. 15-24, the cable channels are changed to 6 to 78 MHz in the down con-verter. This band has space for 12 channels. Cable losses in the supertrunk are reduced for the lower frequencies, in proportion to the square root of the frequency change. For instance, 54 MHz when reduced by a factor of 9 to 6 MHz, would have one-third the cable losses. Note that a second heterodyne cireuit is needed as an up converter in Fig. 15-24, to provide the cable channels at their standard frequencies. From the hub, channels 2 to 12 are fed to the cable distribution system. In the supertrunk line, special low-loss cable is used. For example, a 1-in. [25.4-mm] coaxial cable with fused insulator disks has an attenuation of only 0.32 dB per 100 ft [30.5 m) at 78 MHz. The low attenuation allows wider spacing between amplifiers on the trunk. MICROWAVE LINKS Frequency allocations by the FCC permit operation in the band of 12.7 to 13.2 GHz. Relay stations of this type are called community antenna relay services (CARS). Note that 1 GHz - 1000 MHz = 1 x 10° Hz. Frequencies in the range of 0.3 to 300 GHz are called the microwave band because the short wavelengths are a fraction of 1 m. The advantage of microwave transmission is that parabolic reflector dish antennas can be used to provide very high gain with a very narrow beam. In effect, the energy travels in a pencil-like beam from the transmitter dish antenna to the receiver dish. The line-of-sight transmission requires that no physical obstructions be in the path between transmitter and receiver. Microwave propagation is similar to light radiation, since the microwave frequencies are not much lower. However, microwaves are not visible. Some typical signal levels can be calculated for a microwave link operating at 13 GHz. The transmitter output is 1 W, or 1000 mW. With a 1-mW reference for dBmW units, this power is°30 dBmW. Antenna gain in decibel units for a dish with 6-ft [1.8-m] diameter is 43, in either decibels or dBmW. The antenna gain applies at the transmitter and receiver, for a total of 2 * 43 = 86 dBmW. Attenuation of the microwave signal for a 20-mi [32.2-m] distance can be taken as —146 dBmW. This value is the approximate free-space loss at 13 GHz, as listed in reference tables. The resulting signal at the receiver is 30+ 86-146=-30dBmW The signal power at -30 dBmW below the refer. ence of 1 mW is equal to 0.00l mW, or. 1 p.W. In terms of voltage across 75 l, V = 0.115 mV of signal. Special low-noise amplifiers are required for the receiver in the microwave link. Note that individual transmitter-receiver combi--nations are used for the different cable channels. Either a frequency-modulation link (FML) or an amplitude-modulation link (AML) can be used. The FML generally has a better signal-to-noise ratio, especially if preemphasis and deem-phasis are used for the baseband video signal. However, the AML can actually have superior performance compared with an FM system using a narrow frequency deviation. FIBER OPTICS. The latest type of communications link uses a cable made with thin glass fibers that serve as a conduit for light over long distances with little losses. In this system with a fiber-optic link, the full cable- channel bandwidth can be used for amplitude modulation of the light source. For trans-mission, a modulated light beam is the source that introduces light into the glass fibers. At the receiving end, a photoelectric detector converts the variations in light amplitude back to the cable signals. The light serves as a super carrier wave for the entire cable passband. There are important advantages to using fiber-optic cable as a long-distance link. The cable is not as heavy as copper conductors, making it convenient for installation. Attenuation of the light is much less than the losses with conduction or radiation of an RF carrier wave. A big factor is that the exceptionally high frequency for light makes it possible to have modulation that in: cludes a tremendous range of frequencies. REFRACTION AND INTERNAL REFLECTION OF LIGHT The reason for the low attenuation is the internal reflection of light inside the fiber cable. Thus no light can escape, and the losses are extremely small. To analyze this effect, the basic laws of refraction of light are illustrated in Fig. 15-25a, and internal reflection is shown in Fig. 15-25b. Light rays are shown going from air into a slab of glass. The velocity of light is reduced in the glass because it is a denser medium. In Fig. 15-25a, light rays enter at an angle from the normal line. This direction is perpendicular to the interface where light enters or leaves the glass. Going into the denser medium. the light rays do not continue their original angular direction. Instead, they are bent to an angle closer to the normal line because of the reduced velocity of the rays. As each wavefront of light reaches the air-glass interface, the effect is like a squad of people abreast marching in a line. The first one to hit the interface begins going slower, but the last continues with the original speed up to the interface. As a result, the line of the wavefront turns to its right, and the light beam bends toward the normal. At the bottom of the glass slab, the light going out is bent away from the normal. This direction is opposite to the bending of the incident light. The reason is that the light goes to a less dense medium that allows it to travel at higher velocity The bending of the light is called refraction How much the light bends when it meets a differ. ent medium is determined by the index of refrac-tion, whose symbol is n. Its value is speed of light in vacuum speed of light in the medium Typical values of n are 1 for air or vacuum. 1.8 for glass, 2.4 for diamond, and 1.3 for water Now consider a light source actually inserted in the slab of glass, as in Fig. 15-25b. The light radiates in all directions. The light ray marked A approaches the interface at a right angle. Such rays along the normal are not refracted. Ray B is refracted but still leaves the glass. Note that refraction bends the light away from the normal line. For ray C, however, the refraction is just enough to make the light follow along the glass surface. At D and smaller angles for the rays, the light is reflected internally. None of these light rays can leave the glass. The angle at which the internal reflection begins is the critical internal angle. The corresponding action is shown in Fig. 15-26 for fiber-optic cable. Light entering the conduit at angles less than twice the critical angle will be reflected internally. Then the light is propagated along the cable in zigzag directions, bouncing off the walls but without leaving the glass. This angle that allows complete internal reflectión is the acceptance angle. All incident light with a smaller angle, or along the central axis, is transmitted in the optic cable. MODAL DISPERSION Light going into the optical cable or the central axis takes the shortest route: At other angles within the açceptance an-gic, the light must travel a longer path because of the internal reflections. The time difference between the direct and reflection paths is called the modal dispersion. This factor limits the bandwidth of the cable. To minimize the modal dispersion, practical optic cable is made with fine fibers of small diam-eter. In addition, the fiber is encased in a cladding material that has a high index of refraction in order to increase internal reflections. In effect, all the fibers are in parallel to provide a cable with very low losses. OPTIC TRANSMITTER The light source is often a special light-emitting diode (LED), operating in the infrared part of the spectrum, where the wavelength is greater than that for visible light. In construction, the LED is at the bottom of a conical pit to produce light in the cable. The LED current and its light output can be modulated with the full passband of all the cable chan-nels. Actually, the LED is the limiting factor. The bandwidth of the cable itself is far greater than the bandwidth of the LED modulation cir-cuits. Another method of transmitting the optic signal uses an injection laser diode as the light source. This system can accept modulating frequencies well into the UHF range. CABLE CONNECTORS One of the problems with fiber optics is splicing, or joining, cables. Care must be taken to make precise optical alignment at the connection, or excessive light loss will result. However, special connector