Troubleshooting Video Systems

Troubleshooting VideoSystems

In this chapter, you will learn about

  • Common cable-related distributed video problems
  • Troubleshooting video cabling and distribution systems

Regardless of the actual source of a problem, a homeowner who can’t watch the big game, the latest episode of “24,” his favorite talk show, or whatever show he wants is experiencing a video system problem. With all of the connection and distribution points involved in a distributed video system, tracking down the source of the problem can be daunting.

In a structured wiring environment, tracking down the cause of a problem (assuming that the problem is cable- and distribution-related) is eased a bit because access points to all of the cable home runs are available for testing, as are all of the distribution and interconnection points.

This chapter extends the information from Chapters 9 and 19 to focus on the cabling and connection problems that can affect the performance of a distributed video system and the troubleshooting steps that can be used to identify a problem source.


Diagnosing Distributed Video Problems

As is the case with other distributed media systems, most of distributed video problems can be traced to cabling. How the cable was installed, how it was terminated, and how it is connected into the equipment distributing the signals to different rooms or zones of a home all can have an effect on the quality of the video seen on that big, expense high-definition TV (HDTV) plasma display.

As with all problems reported by the customer, you should listen, look, and analyze the situation (see Chapter 9 for more details on this process) before beginning your diagnostics. Once you have a clear understanding of what the customer “sees” as the problem, you can begin your diagnostics to isolate the cause and apply a remedy.

Rule Out the Obvious

As I’ve said a few times already in this book, before you begin performing cable tests, or worse, check out all possible systems that are related to the problem, and even those that aren’t, check for things such as unplugged alternating current (AC) cords, poorly mounted connections, and the like. In a majority of cases, especially in those where the system has been in place for more than 30 to 90 days, it is likely that the cause is external to the cabling and its termination and something fairly simple.

Check the Cable

Video systems are generally installed over coaxial cabling, either RG59 or RG6 (or using their newest nomenclatures, Series 59 or Series 6). However, some distributed video systems are being installed on twisted-pair (TP) cabling as well. In any of these cases, how the cable was installed and terminated should be the first check in your diagnostics.

Coaxial Cable

As illustrated in Figure 20-1, coaxial cable actually has two conductors: the center, solid-core copper conductor and the shield placed around the dielectric insulating material and the inner conductor. The center conductor is the primary signal carrier of the cable, and the shield provides a return path for the signal ground. When a coaxial cable is terminated, if attention isn’t paid to both conductors, the quality of the transmitted signal can be affected.

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Figure 20-1: The layers of a coaxial cable

Cable ShieldingThe shield can be one of several materials, but typically it is wire mesh, foil, or both (including two of one or both). Each of these different shield types is designed to resist certain kinds of radio frequency interference (RFI) and Electromagnetic Interference (EMI). A foil shield is primarily used to deflect RFI from the cable. Foil shields have a thin drain wire that allows the foil to be connected to the outer channel of an F-type or BNC connector. A wire-mesh or braid shield, which is usually made of tinned copper, is used to ward off EMI from the center conductor. In high-quality cables, both shields may be installed. In either case, the shield must be in contact with the metal body of the connectors to perform its task.

Center ConductorThe center conductor can also be a source of cable performance issues. If the center conductor extends too far from the connector, it can easily be bent or pushed to the side when the connector jack is attached to the connector plug. If the center conductor isn’t extended far enough out from the connector, the conductor may not meet the conductor on the other half of the connector. Notice the center conductor extending beyond the connector body in Figure 20-2.

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Figure 20-2: A coaxial cable terminated with an F-type connector

Cable PropertiesThe performance of a coaxial cable in a video system is determined by the properties of the cabling installed. The primary properties that can affect the performance of a coaxial video cable are

  • FrequencyAs the frequency of the signal transmitted over a coaxial cable increases, the greater the affect the cable’s capacitance and conductive resistance have on the signal.
  • InterferenceRFI and EMI can distort the signal if the cable is not properly shielded against them. Crosstalk is a type of interference that occurs when cables that are run adjacent aren’t properly shielded and pick up signals from one another, which can distort the signal on either or both cables.
  • LengthThe longer a signal travels through a cable, the more its quality decreases. All copper cabling is susceptible to attenuation, which is caused by an electrical signal being affected by the materials of the cable as the signal travels through the cable. Each series of coaxial cable has a specific maximum cable segment length. For example, RG6 coaxial cabling has a rating to about 200 meters.
  • SpecificationsThe four primary properties included in a manufacturer’s cable specifications are

    • AttenuationThis is perhaps the most important specification for a coaxial cable and is stated in the number of decibels (dB) of signal loss that occurs beyond a fixed length (typically around 30 meters or 100 feet) for different signal frequencies. An example of an attenuation specification is -2.2dB/100 feet @ 100 MHz, which means that a transmitted signal will lose 2.2 decibels per 100 feet of run length. In addition, the attenuation rating of a cable also changes as the run length of the cable increases. In effect, attenuation gradually decreases the quality of the signal as the signal travels over the line, which can impact the display produced by a signal transmitted over an extended length run.
    • CapacitanceCapacitance measures a cable’s capability to take an electrical charge, hold the charge, and then discharge. In a coaxial cable, capacitance indicates how well a cable can hold a change in its dielectric insulator to offset the potential difference between the conductors, which on a coaxial cable is the center conductor wire and its shielding. One video condition that can be affected by a cable with an improper capacitance is the black-to-white transition of the video signal, which might be displayed with a gray transition between the black and white. The capacitance of a coaxial cable is measured in picofarads per foot (pF/ft), or one-trillionth of a farad.
    • ImpendenceThis rating determines the amount of signal flow the cable supports. A possible source for system performance issues is unmatched impedance, which can cause the signal to be reflected back up the cable instead of on to its destination. Reflection signals can also be caused by improper termination. Coaxial cable typically has 75-ohm impedance.
    • ResistanceA cable’s resistance rating defines how resistive the cable is to signal flow. A cable with a very high resistance impairs the amplitude of a signal and converts the lost voltage to heat. The materials used in the cable and their dimensions, as well as the temperature of the area in which the cable is installed, determine a cable’s resistance. A cable’s resistance is stated in ohms per 1,000 feet (305 meters).
  • TemperatureHeat can affect the performance of a cable, which can also make troubleshooting difficult. In areas such as walls, ceilings, and equipment racks that may not be properly ventilated to prevent heat buildup, cables can be subject to higher heat. Special cabling types are available for installation in locations where heat may be a problem.

TerminationsThe different types of connectors used with coaxial cabling can impact the quality of the signal transmitted because they may create an impedance mismatch on the line. For example, a BNC connector has 75-ohm impedance, which matches that of the typical coaxial cable. However, an RCA connector typically has between 35- and 55-ohm impedance. This doesn’t mean that an RCA connector should not be used with coaxial cable, it just depends on the system being supported. Some systems include a 25-ohm resistor on the incoming cable jack.

Not all video connectors use crimp-style attachments. Many styles of connectors must be soldered into place, which creates another failure point for the cable. Improperly applied soldering can create problems with continuity, impedance, and resistance of a cable.

Another potential problem area that actually extends beyond the scope of structured cabling is the patch cords used with coaxial cabling used in a video system. The patch cords used with coaxial distribution cables should also be coaxial cable that matches the specification of the main distribution cable. The equipment cords provided with video devices by the manufacturer are good for use only directly between devices and aren’t designed or provided for use as patch cords between the distribution cable and the device.

The difference between a patch cord and an equipment cord boils down to connectors. BNC and F-type connectors are used with coaxial cabling on both distribution and patch cords. Equipment cords commonly have RCA connectors, like the cord shown in Figure 20-3, and are used for line-level audio and composite or component video interfaces.

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Figure 20-3: A video equipment cord terminated with RCA connectors

Photo courtesy of Canare Corporation of America.

Twisted-Pair Audio Cables

Unshielded twisted-pair (UTP) cable can be a less expensive alternative to coaxial cable for a distributed video system. Cat 5 or better (meaning Cat 5e, Cat 6, or Cat 7) can be used to distribute video signals if it’s properly installed and terminated.

Using UTP cable for a video system requires that one pair of wires is used for each signal. Or in other words, up to four video system signals can be transmitted on a UTP cable, one signal per wire pair.

AttenuationOne specification of UTP cabling that can be an issue for a video system is attenuation. If a system is performing poorly, and UTP cabling is in use for distributing the video signal, attenuation could be the problem. Table 20-1 compares the dB/100 feet loss of coaxial and UTP cabling.

Table 20-1: A DeciBels per 100 Feet for Common Cabling

MHz

RG6

RG59

Cat 5

1

0.2

0.4

1.8

10

0.6

1.4`

5.8

50

1.4

3.3

11.0

100

2.0

4.9

19.3

400

4.3

11.2

42.0

To offset the dramatic attenuation differences shown in Table 20-1, video baluns (transformers) should be placed between BNC connectors (coaxial cable) and RJ-45 connectors (UTP cable).

A video balun, besides providing an interface between the two termination types, converts the UTP line from unbalanced to balanced (see Chapter 19 for more on video baluns and their use with UTP cable).

Common UTP Video ProblemsPerhaps the most common problem for video systems installed on UTP cable is color separation. For example, light colored lines or the edges of light-colored objects appear to have a red or purple tinge to them. This is caused by the number of twists per inch of the wire pairs inside the Cat 5e cable. If the twists-per-inch of one wire pair, carrying one part of the video signal, is tighter than on another wire pair, the signals arrive at slightly different times, especially on long cable runs. This problem is more noticeable on high-resolution systems.

If the customer is complaining of this problem from a video system installed on UTP cable, you may need to install a video graphics array (VGA) to UTP adapter to correct the RGB conversion and signal timing. This device uses a potentiometer to adjust the skew of the signal timing.


Cable Testing

The cable tests recommended in Chapter 9 are the same procedures used when performing diagnostics on coaxial or UTP cabling in a video distribution system. However, the following few sections provide a brief overview of these tests and their use in diagnosing and troubleshooting a distributed video system.

Coaxial Cable Tests

The primary tests that should be performed on coaxial cabling as part of your diagnostics are

  • Test the cable for 75-ohm impedance
  • Test for continuity on both the center conductor and the shield
  • Test for attenuation
  • Test for cable length

If there are other tests that are required by your company, city, county, state, or federal regulations, you may want to repeat them at this time just to verify that none of the cable’s characteristics have been changed.

Testing Guidelines

When testing coaxial cabling, follow these guidelines:

  1. Perform a visual inspection and verify the correct cabling is installed, the connectors are the appropriate type, the terminations are correct, the patch or equipment cords are appropriate, and you don’t see any visible cable damage.
  2. Start your tests at the central distribution panel or the demarcation point of the video system, such as the cable television network interface device (NID).
  3. If you suspect that the problem may be attenuation on a very long run of cable, add a repeater to the link as near to the middle of the cable as you can. If the problem continues, the repeater may not be necessary.
  4. If you suspect a particular device or system is causing the problem, after eliminating the cable as the source of the problem, remove that device from the system, if possible, and retest.
  5. Connect each modulator or video hub directly to a television set to verify the device is working properly.

Coaxial Cable Test Tools

When testing coaxial cabling as a part of your diagnostics, certain tests are used to answer certain questions. Table 20-2 lists the troubleshooting questions and the test you can use to provide answers.

Table 20-2: Coaxial Cable Diagnostic Testing Tools

Question

Test Tools

Which cable is this?

Documentation, tone generator/probe

Is the cable run wired correctly?

Multimeter

Is the cable too long?

Multimeter, time domain reflectometer (TDR)

Is the impedance correct?

Multimeter, certification test set

Is there too much interference on the line?

Certification test set

  CROSS-REFERENCE

See Chapter 9 for more information on the use of a multimeter and the TDR test on structured wiring cable systems.


Troubleshooting Video Problems

Many video system performance problems are less directly related to the cable than they are to the overall system configuration. Table 20-3 lists several common video system problems, their causes, and what you should consider to resolve each problem.

Table 20-3: Common Video Power and Cable Issues

Problem

Cause

Possible Solution

Dark bars on video display

AC power interference

Move cable at least 18 inches from AC power lines.

Modulated signals not displaying on monitor

Cable TV box may not pass modulated signals

Add splitter in front of cable box and install high-pass filter to modulated line.

Modulated signals ghosting

Inadequate shielding on coaxial cable

Replace existing cable with cable that has appropriate shielding.

No picture on TV

Coaxial cable problem

Perform a signal strength test on each cable segment.

Test before and after each cable run, including testing before and after the cable demarc, splitter, amplifier, distribution panel, the patch cord to the TV.

No picture on some channels

Insufficient cable bandwidth

Replace RG59 cabling with RG6 and verify capacity of video splitters, if any. Also the problem may be that the TV set's tuner is not capable of displaying certain channels.

Rolling lines or patterns on many television channels

Signal strength is too high

Add a line attenuator to reduce signal amplitude.

Static or snow on some channels

Problem coaxial cable run

Damaged cable or broken connector.

Snowy (static) picture on all channels

Signal interruption between TV and modulator

Check connection at each end of cable. Verify modulator settings.

The SCART Connector

In European countries, the connector used to connect televisions, VCRs, and set-top boxes is the SCART (Syndicat des Constructeurs d'Appareils Radiorcepteurs et Tlviseurs) connector, which is illustrated in Figure 20-4. This connector, which is also called a Peritel connector, is used to terminate input lines, output lines, and lines that do both. The SCART connector is a 21-pin block that is a crimp on connector type.

click to expand
Figure 20-4: A SCART connector is used with TV systems in Europe.

Many brands of television sets, those manufactured for sale in both the United States and Europe, have a SCART connector on them.


Review

How the cabling is installed, terminated, and connected can effect the quality of the video display.

Before you start testing, check out all possible systems that may be related to the problem, such as unplugged AC cords, poorly mounted connections, and the like. If the system has been in place for more than 30 to 90 days, the problem is likely caused by external devices.

Video systems are generally installed over coaxial cabling, either RG59 or RG6, but distributed video systems are installed on TP cabling. How the cable was installed and terminated should be the first check in your diagnostics.

Coaxial cable actually has two conductors: the center, solid-core copper conductor and the shield placed around the dielectric insulating material and the inner conductor. The center conductor is the primary signal carrier of the cable, and the shield provides a return path for the signal ground. When a coaxial cable is terminated, if attention isn’t paid to both conductors, the quality of the transmitted signal can be affected.

The performance of a coaxial cable in a video system is determined by the properties of the cabling installed. The primary properties that can affect the performance of a coaxial video cable are: frequency, interference, length, specifications (which includes attenuation, capacitance, and impendence), resistance, and temperature.

The different types of connectors used with coaxial cabling can impact the quality of the signal transmitted because they may create an impedance mismatch on the line. Many styles of connectors must be soldered into place. Improperly applied soldering can create problems with continuity, impedance, and resistance of a cable.

UTP cable can be a less expensive alternative to coaxial cable for a distributed video system. Cat 5 or better (meaning Cat 5e, Cat 6, or Cat 7) can be used to distribute video signals if it’s properly installed and terminated. One specification of UTP cabling that can be an issue for a video system is attenuation. If a system is performing poorly, and UTP cabling is in use for distributing the video signal, attenuation could be the problem. A video balun, besides providing an interface between the two termination types, converts the UTP line from unbalanced to balanced.

The cable tests used during trim-out should be used when performing diagnostics on video cabling. The primary tests that should be performed on coaxial cabling as a part of your diagnostics are: test the cable for 75-ohm impedance, test for continuity on both the center conductor and the shield, test for attenuation, and test for cable length.

Questions

  1. Which of the following is not recommended for distributing video signals in a structured wiring environment?

    1. RG59
    2. RG6
    3. Speaker wire
    4. UTP
  2. In a coaxial cable, which layer carries the signal to ground current?

    1. Center conductor
    2. Dielectric insulation
    3. Shielding
    4. Outer jacket
  3. Which type of coaxial cable shielding is used to resist RFI?

    1. Foil
    2. Dielectric
    3. Wire mesh
    4. Wire braid
  4. Which of the following coaxial cable problems could create a continuity problem on a video distribution run?

    1. Cable pierced by a staple
    2. Center conductor not extended far enough
    3. Center conductor extended too far
    4. Wire mesh not in contract with the connector body
    5. All of the above
  5. Which of a cable’s properties limits the distance a cable is able to successfully carry a transmitted signal?

    1. Attenuation
    2. Capacitance
    3. Frequency
    4. Resistance
  6. What is the normal impedance level of a coaxial cable terminated with a BNC connector?

    1. 35-ohms
    2. 55-ohms
    3. 75-ohms
    4. 120-ohms
  7. When UTP cabling is used for distributing video signals, which of the following is true about the cable configuration?

    1. One signal per single wire
    2. One signal per wire pair
    3. One signal per cable (four pairs)
    4. Two signals per wire pair
  8. True or False. The dB loss per 100 feet of UTP cable is typically less than that with either RG6 or RG59 coaxial cable?

    1. True
    2. False
  9. Which of the following is a test that may not be performed on coaxial cabling during troubleshooting?

    1. Attenuation
    2. Crosstalk
    3. Impedance
    4. Continuity
  10. When troubleshooting a coaxial cable for length, what testing tool should be utilized?

    1. Cable certification set
    2. Time domain reflectometer
    3. Tone generator/probe
    4. Wiremap tester

Answers

  1. C. Speaker wire is not rated for in-wall installation, nor is it robust enough to carry the frequencies required for video transmissions. The other choices are used for video systems.
  2. C. Whether the shielding is foil or wire mesh or braid, it is used to provide a return for the signal to ground current.
  3. A. The foil shield resists RFI and the wire mesh or braid shield resists EMI. The dielectric insulator is used to provide capacitance.
  4. E. Any one of these conditions could cause continuity problems on a coaxial cable.
  5. A. On every copper cable, including UTP, there is a distance point at which the signal strength begins losing power.
  6. C. UTP cable is typically in the range of 35- to 55-ohms.
  7. B. Up to four signal paths can be provided by a single run of Cat 5 or better UTP cable.
  8. B. The attenuation on a UTP cable can be on the order of two or more times greater.
  9. B. Unless you believe that there are two coaxial cables carrying high frequencies that are placed too close together, this would not typically be a problem with coaxial cable.
  10. B. TDR is the most commonly used test for testing copper (and fiber optic) cabling length.






HTI+ Home Technology Integrator & CEDIA Installer I All-In-One Exam Guide
HTI+ Home Technology Integrator & CEDIA Installer I All-In-One Exam Guide
ISBN: 72231327
EAN: N/A
Year: 2003
Pages: 300
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