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Video signal distribution

The How & Why of Video Switching, Video Cables and Distribution
Many times it is desirable to send video signals long distances or to multiple pieces of video equipment. Whenever this involves cables lengths of over 6 ft or high bandwidth signals such as HDTV, many factors require special consideration to achieve distortion free video. This guide will help show you how to design your home distribution network properly or to troubleshoot problems you have.

The goal in video signal distribution is to propagate a video waveform from a 75 ohm source impedance amplifier down a shielded 75 ohm intrinsic impedance conductor to a matched 75 ohm real termination load. Given ideal components, the original signal can be perfectly reproduced at the termination load regardless of cable length.

Basic networks:

All video connections can be modeled as a point to point connection between an active driver (amplifier/source) and a termination load (end device, such as a TV). In this model, each part of the connection effects the end result as follows.

Video source: The amplifier in the source must provide an exact 2.0 gain to all components of the original signal. The source impedance of the amplifier must be 75 ohms, and the connector should have gold contacts and a good ground connection. If the gains are not correct, this will cause contrast and color shifts. If the source impedance is not correct, reflections from the load will not be cancelled, which looks like smearing or shadowing in the image. Poor connectors can also cause reflections.

It is important that the output is AC coupled. DC coupling is not good for long distance video distribution because ground potential differences will be dissipated in the termination load, which can damage the end device.

Most video devices do not meet these requirements. The most common problem are amplifiers which cannot provide true 75 ohm source impedance due to insufficient current drive. During large transitions, the amplifiers source impedance increases, which limits the ability to eliminate signal reflections.

Cable: The cable used in the connection should be 75 ohm intrinsic impedance, properly shielded, and of matched length to the other cables in the set (Y,Pb,Pr). For most video applications, this means use RG-6 cable for long distances. The shielding specified for RG-6 is sufficient for almost all networks, and enhanced shielded varieties can also be purchased if your environment is very noisy. Cable length matching is very important. For HDTV, a difference in length of >5 feet can be visible to the eye. Only cables in "sets" must be matched, such as Y,Pb,Pr or Y/C. Cables to different monitors etc do not need to be matched.

Cable loss is also an important factor for very long runs (>100 ft). Cable loss is specified in dB/100ft for a specific frequency. For HDTV, the frequency of interest is 30Mhz. For cvbs, only 7 Mhz is needed. Try to purchase cable such that your cable loss is 3dB or less at the maximum frequency of interest. Too much loss will reduce the amplitude of fine lines in the image.

Video termination: Video termination devices are generally assumed to be a source of error in your network. These are the monitors, DVD-Rs, and other display devices you use. The video termination is internal, and typically not specified. The quality will vary due to manufacturing differences of the connectors and electronics used. Most consumer electronics do not have perfect 75 ohm termination across frequency, and this will cause primary reflections. The good news is that if your source amplifier and cables are designed properly, these reflections will be canceled before they cause signal degradation.

Splitting: Passive splitters are devices that connect more than one video termination to your video source. Never use these devices, they will degrade your signal even for short cables. For long cables they also produce very large reflections.

Active splitters are a combination of a video termination with more than one video amplifier. A properly designed active splitter enables you to drive multiple terminations from a single source. All the requirements for the video source above must be met by the active splitter for best performance. Video Storm CB003 is an example of a professional quality active splitter.

Switching: Switches are also split into two categories, passive and active. Passive switches use mechanical or electronic relays to selectively connect one of many inputs to one output. They do not terminate or amplify the video signal. For some applications, passive switching is ok. If none of the inputs cables are < 6ft long, and the output cable is also < 6 ft long, then a passive switch is an acceptable solution. The problem with passive switches is they do not match the 75 ohm intrinsic impedance of your cables. When the signal passes through the switch, there will be a voltage reflection from the mismatch. By itself, this is not a problem since the source can cancel it. However, this reflection is in addition to the reflection from the video termination connected to the output. The interaction is complicated to explain, but the net result is degradation. A popular myth is that if your output cable is short, you will be ok. The truth is all your cables must be short, otherwise use an active switch.

Active switches combine multiple video terminations with a single amplified video output. As long as the active switch meets the requirements for a good video source, these provide an optimal solution for any cable lengths. Even for short cables active switches give better performance than passive switches. Video Storm CSW02 is a professional quality active switch & splitter in one.

Please see our review of the Pelican System Selector Pro for an example of passive switch problems.

Not all active switches are well designed either! Please see our review of the Shinybow HD54-70 for an example of active switch problems.

Daisy chains: Daisy chaining is connecting "through" a video device. Some VCRs and TVs have video outputs that can be driven from the video inputs. An example is the antenna (RF) input on most VCRs. You can connect your antenna to the VCR, then connect the CVBS output of the VCR to your TV. In general, don't plan on using daisy chained outputs to drive long cables. Although some equipment will properly amplify the signals in all modes, many will only amplify in some modes and passive switch others.

Bandwidth considerations: This one is easy, just make sure all the components of your network are specified by enough bandwidth to support the type of video you are sending. HDTV should have at least 30Mhz (65Mhz for 1080p), cvbs needs only 7 Mhz.

DC restoration: All video signals are defined as AC only, with the signal power residing entirely above 60Hz. However, before the signal can be displayed a DC level (< 60 Hz) must be added back in so the monitor can bring the signal within the voltage range of it's input circuits. It is the job of the receiving equipment to perform this task of "DC restoration". Most video displays have properly designed DC restoration circuits, but there are some which do not work fast enough. When this circuit fails, the display will lose sync during some scene transitions. The table below shows a comparison of Video Storm products and competing products with respect to how DC frequencies are handled.

Common problems:

Grounding & Power supplies: Improper grounding can cause a lot of problems for video distribution. If your equipment is not fuse protected and AC coupled, improper grounding can destroy attached devices. With protection, too much ground potential difference can temporarily shut down your amplifiers or cause video artifacts. Usually these problems are resolved if you are powering all your devices from grounded outlets. If you have an electrical wiring problem with the power grounds, please consult a certified electrician.

Poor cable: Using cable that is not shielded can result in noise or interference. Noise is graininess in the image, interface is a moving pattern that overlays the video. Note that for your cable shielding to be effective it must have good connection to the rca plug jacket.

Cable with insufficient bandwidth will remove high frequencies (fine lines/details) from your signal. Use RG-6 for all long cable runs.

Cable with the wrong intrinsic impedance will cause reflections and other loss (shadowing/smearing). Make sure your cable is 75 Ohm.

Passive switches & splitters: Using passive switches and splitters is a very common cause of signal degradation. This errors are due to un-cancelled reflections and cause shadowing and color smearing. See the section on splitters above for more details.

Weak drivers: Weak video amplifiers or poor source termination will fail to cancel termination reflections. Long cable runs with these sources will have shadowing and color smearing. Only use good quality amplifiers such as the Video Storm CB003.

Connectors: Poor quality manufactured connectors can have too much real resistance or poor grounding. If you wiggle the cable and the signal changes visibly, it is probably the connector or the RCA plug. To prevent these issues always buy video components with gold plated connectors.

Checklist for quality distribution:
1. Start with the right amplifier as your source
2. Make sure source and all sinks are connected to a good quality common ground
3. Use RG-6 cable, match lengths for component video
4. Avoid multiple connectors or cable changes
5. If you need switching at the sink end, use a good quality amplified switch to avoid reflections.
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