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INGRESS - SOURCES and SOLUTIONS by John w. Ward Jr. Comcast Cablevision of Montgomery County, Inc. Signal ingress is a problem which has external radio signals are especially high, always plagued cable systems. While there even defects that are otherwise insignifi­ has always been potential for problems cant must be found and corrected. from VHF television stations, with the modern cable system encountering the UHF Interference due to ingress can be television band, the susceptibility of classified into two basic forms, either co­ cable systems to ingress interference is channel or discrete carrier. When there are increasing. Immunity from ingress problems one or more local VHF TV stations located can only be achieved by maintaining system near a cable system which uses a channel integrity at levels better than those occupied by one of these VHF stations, required by specification. there will without doubt, sooner or later, be need to correct co-channel interference Interfering signals leak into a cable between the two. Discrete carriers from system not only thru flaws in the cable communications transmitters will cause system but also by way of consumer problems on mid-band and super-band cable equipment, cable ready televisions and channels. As communications transmitters VCRs. The increase in the number of these include everything from car phones and devices as well as other factors lead to a personal pagers to amateurs and the need to be able to efficiently diagnose and National Weather Service, discrete carrier cure signal ingress problems. ingress can occur anywhere and often at random times. Cable systems near the VHF TV WHAT IS INGRESS transmitters of a large city are the systems that will likely suffer from co­ Ingress, as far as the CATV community channel ingress related problems. At two to is concerned, is the entrance into a cable five miles from a full power TV transmitter system of any undesired external radio it is not uncommon to have a field source. Ingress will occasionally be in strength of 35 to 40 dBmv or more, very the form of static or electrical noise, but often 25 dB more than than what is inside it is normally considered to be interfer­ the cable. Levels from TV transmitters as ence from a radio frequency signal. Ingress far away as 30 miles may exceed the average of such signals will result in interference levels of a CATV plant. Beyond that range to cable pictures. Modern cable systems the effects of ingress related co-channel have sufficient isolation from ingress, or interference become less noticeable. shielding, to prevent ingress of carriers in even the noisiest of radio environments. Co-channel type ingress interference But, an awareness of ingress related in its most basic form will appear on a TV problems is required by the cable system picture as a strong beating pattern , when operator to enable repair of inevitable, the cable channel is not phaselocked with natural, flaws in the system. the interfering station. If the cable pro­ graming is phaselocked to but not sync Ingress is the opposite of egress, or locked to the unwanted carrier, a wiping of system radiation. The principle by which the interfering stations' sync bar through both phenomena operate is the same, related the background of the desired picture is by the principle of antenna reciprocity, the first effect noticed. If cable pro­ which is that antennas transmit and receive graming is both phase and sync locked to equally well. That cable systems do radiate the local station, as it is when operation energy is an established fact, that cable is "on channel", the first effect will be systems are susceptible to ingress there­ faint ghosts in the picture, either of text fore follows. As system egress levels are characters with their high energy edges, or reduced to within regulation, effects of of the horizontal sync bar, stablized, but ingress are reduced toward acceptable in the middle of the screen. The difference levels. Still, in order to completely over­ in the time it takes the signal to arrive come the effects of ingress in areas where at the set both through the cable and 1985 NCTA Technical Papers-11 through the air allows the channel to HOW DOES INGRESS GET IN interfere with itself. If the two signals arrived at the same time they would mesh Ingress gets into the cable system by perfectly, all the interference would be way of poor shielding and faulty connec­ hidden. tions. The cable acts as an antenna and will have currents from external radio When operating "on channel", or with fields induced onto it's shield. Electron alternate programing phase-locked and sync flow, or currents, of radio frequency locked to a local VHF transmitter, the energy, happens only on the surface of a cable signal must be at least 50 dB or more conductor. Under normal conditions, the above any ingress from the airwave signal cable signal energy flows on the inside in order to suppress sync bar ghosting of surface of the shield and broadcast radio the picture. Without sync lock, a separa­ signals flow on the outer surface of the tion of 55 dB is a minimum to prevent an shield. A "hole" in the shield will join annoying wiping pattern in the background. the two surfaces, allowing undesired cur­ If the cable channel operates with altern­ rents to flow both out and in. Unbalanced ate programing not phaselocked to the local current flow between the shield and center TV station sharing the same channel, the conductor of the cable will cause the un­ cable video carrier should be 60 dB higher desired signal to be added to the cable than any interference. In extreme cases of signal. co-channel ingress interference, where an off air signal is only 40 or 45 dB down Experience has shown the most common from the cable signal, strong ghosts or point of ingress to be a slightly loose other distortions may be expected, regard­ connector. The connection is normally tight less of the operating mode. Customers will enough not to noticeably effect the cable surely call for service when the interfer­ signals, if not for the ingress problem. ence is this bad. The connection may be just loose enough to permit air molecules to permeate between the threads and, given time, form a layer The effect of ingress due to discrete of corrosion. The improperly made connector carrier interference is similar to second will also permit gasses to corrode the order beat problems in that both will aluminum of the shield itself, forming appear on the customers set as a herring­ aluminum oxide, a poor electrical conduc­ bone pattern or wavy series of vertical tor. Corrosion will create a point of lines. If several unwanted carriers are resistive and/or capacitive nature in the present, as is often the case with channels shield of the cable. This breakdown of the shared by the communications bands, the outside conductor is the unwanted hole in effect may be a soft distortion similar in the shield. symptoms to third order product accumula­ tion. A discrete carrier located near the Theory and experiment show that a color sub-carrier of a cable channel may mismatch on the inner conductor will not cause a beating pattern in picture tint or, permit signals to enter the cable, only the if sufficiently strong, may even drive the signals inside the cable already will be picture into black and white. affected. This can be demonstrated by cut­ ting the center conductor short at a splice in the middle of a section of drop. The Picture distortions due to discrete isolation is as good as the shield in this beats vary depending upon the level of the experiment. It can also be demonstrated beat as well as the position of the beat in that a single crack or hole in the shield, the band of the cable channel. For example, an interfering carrier 30 dB down and 10 khz. from the cable video carrier will 60 probably not cause any noticeable picture low band distortions. Interference located 1/2 mhz so high band --- above the video carrier with a level as low as 40 dB down from it, will cause strong d 40 ..._..,, -- beats in the picture. When the interference B is from a carrier located in the middle of m 30 '~,,, the cable channel, 1-1/2 mhz away from the v ,, video, a rejection of up to 55 to 60 dB is 20 principal area limit necessary to prevent a "busy background" grade "A" signal effect. The amount of immunity required by 10 --'~-~' -- the cable system from ingress beats is the ,, same as and can be compared to FCC specifi­ 0 cations regarding carrier to second order beat ratios. miles 1 2 4 8 16 32'' 64 FIELD STRENGTH OF TV TRANSMITTERS [3] 12-1985 NCTA Technical Papers not completely around the cable, is in and it is next to impossible to find a itself not a significant source of ingress. proper fitting. The fitting must not only However, when the small cracks are spaced pass signals but must properly seal the at regular distances, an effective amount shield from ingress as well as survive of energy is transferred into the cable (as through time. well as out). Other parts of the cable system Improper handling or installation of responsible for ingress are loose amplifier drop cable can cause periodic cracks in covers and tap plates. Although experience several ways. One common way a flaw may is that an amplifier housing must be open arise is when a staple gun, faulty itself and the amp's module cover almost off in or improperly used, causes a severe sharp order to get a significant amount of in­ dent in a drop wire as the staple is fired.

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