Video setup

Many AV video setups are simple connections of a video source like a videotape player to some kind of display, and are “no brainers” that should be very simple. It is surprising how many technical problems can arise with a very basic setup and leave you looking like a fool.

Connections

The first part of understanding a video setup is to go over the world of video connectors. There are several ways that video moves from one box to another but in our humble AV world the most common is by a shielded coaxial wire with a connection device on the end.

The most common connection device by far is the RCA style that simply inserts like a peg in a hole with an outside metal sleeve that slides over the outside of the hole. Everyone and their uncle used these types of connection for their stereo or TV hookup at home. The bummer with these connections is that they tend to pop out with a little pull and then you are left with a high impedance air gap and no picture and the client or talent staring at you in anger. This style of connection is also commonly used for carrying consumer audio signals.

To make a more secure connection, most professional video people use the BNC connector that has a locking outer shield that is much harder to remove. For some large installations you may need a BNC tool to reach into tight places to secure the connections. There is a long running debate over the source of BNC’s name. The source I believe, Extron Electronics, said that it stands for its style and its inventor Bayonet, Niell Councelman.

There are adaptors that convert from BNC to RCA and visa versa to help you out when you need to convert a cable. The RCA Male to BNC Female is also known as a bullet. The BNC Female to BNC Female is also known as a Barrel

The BNC Female to Female is useful for joining two BNC video cables and extending a line. Beware, these “barrels” often cause reflection and distortion in the picture.

		BNC Female to BNC Female a.k.a Barrel
RCA Female BNC male	RCA Male BNC Female a.k.a Bullet

BNC Female – RCA Male

RCA Female –

BNC Male

If you are using one Coaxial cable to pass RF Video with audio on one cable, traditionally the connections are made via RF-connectors. These are the kind of connectors that the cable company uses and are probably all over your apartment. If your client wants to watch the superbowl or the sopranos from the hotel cable system you will probably start with a signal on one of these style of connectors.

Remember this is a modulated video signal, i.e. video and audio are encoded together as a Radio Frequency signal. To project this signal the RF video needs to be decoded with a TV tuner and the decoded (separated) video and audio will then be able to be sent to the projector and sound board.

A less common type of connection is the S-video or SVHS connection that like the RCA end is not locked to the device. THW S-Video is a consumer component video cable that keeps the video’s color and brightness, or luminance, information separated to maintain a higher signal bandwidth than coaxial cables. These separated signals are carried on 4 pins with an outer metal shield. The picture quality of S-video can be great but the cables are often less shielded from RF interference and the connections like the RCA type can come loose. Or as I have seen happen a waiter from the hotel kitchen cuts through backstage, hooks his foot on the cable you forgot to tape down and there goes your connection and you are back to the angry looks.

Termination: Off at the loop, on at the end of the line

Another very important part of understanding connections is the proper termination of the video signal at the connection points. The main rule is that all video paths need to end at a 75ohm resister. This is referred to as Terminated video. Many professional video devices have an ability to pass the video signal on to another device in the video signal chain. This is very common with professional video monitors like the Sony 2030 or 2530 video monitors that many companies use. If you see a video in and a video out next to each other, then normally the equipment can pass the signal to another device. This passive pass through, or “loop through”, requires that the device passing the video signal does not put a resistance on the signal, indicating that the video path does not end at that point. We refer to this as un-terminated, or an un-terminated connection. There sometimes is a physical switch on the side of the loop through that turns the resistor in the circuit on and off. On, or 75 ohm turns the resister on in the circuit and off, or Hi-Z, un-terminates the video line for loop through.

		75 ohm

VIDEO IN

Hi - Z

VIDEO LOOP

OUT

75 ohm

VIDEO IN

Hi - Z

VIDEO LOOP
OUT

A Typical video setup using loop through circuits might start with a video camera outputting to a video recorder, looping out to a monitor, then into a projector.

Remember un-terminate at the loop through and terminate at the end of the line. Many new display and processing equipment will self terminate and un-terminate by sensing whether there is a cable attached to the end of the connection of the loop through.

If there is no switch on the unit it is possible that the signal is self-terminating.

Beware that some old equipment is not self-terminating nor do they have the switches. These need to be terminated with BNC terminators that lock onto the loop through and terminate the line. These adaptors have a 75 ohm resistor built in to make the proper 75ohm termination that the video circuit needs.

The result of improper termination can be seen in the picture quality of all the display devices in the video signal chain. An overly bright or blown out look to the picture usually indicates that the signal is un-terminated. An overly dark picture can indicate that the signal is double terminated, terminated before the end on the video signal chain and then again at the display or device at the end. It is very important to check for these switches and make sure that when a monitor or device is removed from the end of the line that the cable is also removed at the loop through and the chain re-terminated at that point.

Cable Types:

There are dozens of coaxial video cables that are used for temporary and permanent installations with variations in core diameter and shielding. There are several guides available from manufacturers like Canare and Belden for the specifications and attributes of cables. You can get the latest information at http://www.belden.com/ or at www.canare.com . Belden even has a “cable college” for those who aspire to a higher level of video geek education that teaches everything you could possibly want to know about video and audio cables. The most common type of video cables used by video companies are the RG-59 (Belden 8241) or RG 58 (Belden 8240) coaxial cables. These are flexible cables with copper braided shields to reduce electrical interference in the signal. For longer runs of video, less flexible, double shielded Belden 8281 or Canare lv-77s are preferred. For a fixed facility, where the cables are not going to be wrapped, twisted, run over by lifts and carts, an additional foil shield is added to the cable to add shielding from RF and electrical interference. This foil makes the cable rather stiff and hard to wrap. Another common coaxial cable is the RG-6 type of cable which is a lower cost option common in Cable TV or Broadband cable transmission.

There are also new types of cables designed for Digital video transmission that incorporate slightly different construction to minimize errors in digital video signal transmission. The Canare L-4CFB RG-59 size cables and several Belden cables are recommended for digital transmission. Regular coaxial cables may work fine for shorter distances. Concerning SDI Digital video in cables there is a recommended limit to transmission distance that needs to be consulted for installations. Unlike analog video, digital video signal errors are imperceivable on a standard display until they reach a critical level, a shelf so to speak, and then the signal disappears.

Video Hum

Video ground loops, otherwise known as video hum, is a common problem for video systems in the temporary setups that are installed in hotels or convention centers. The problem for video hum is usually seen as a horizontal bar of distortion that moves through a video picture from the bottom to the top. The level of hum can vary from almost imperceivable to so severe that the image is completely distorted. There are many potential causes for video hum in a signal but the usual cause is setting up a video system that has more than one path to ground. If you set up a video player in one ballroom and send the signal through a video distribution amplifier, then into an overflow room down the hall, it is likely that you will have a projector or monitor in that location that will add a new path to ground to the video system. The result can be seen when you connect the video cable to the amplifier and look at the systems output. One common solution is to use special video inline transformers, also know as hum-buckers, to break the ground circuit on the shield of the video cable going to the unit powered on a different power service stopping the voltage to ground that is causing the hum to appear

in the video picture.

Here are some videohumbuckers, the first for bucking single coaxial lines and the second is for 5 wire RGBHV lines.

A simpler method for fixing this condition could be to un-grounding the display television monitor at the overflow to break the ground loop. This may not be an option if there is a large amount of equipment being ground-lifted and you might create an unsafe stage environment. NEVER DISCONNECT THE GROUND AT THE MAIN POWER DISTRIBUTION PANEL ! I repeat NEVER DISCONNECT THE GROUND AT THE MAIN POWER DISTRIBUTION PANEL ! If you were to do this you could kill someone. This will remove all the grounding in the video, audio computer and projection systems, making you or the folks on stage the ground path for voltage. Yikes, not a good thing. This is how people can get killed. Talk to the stage electrician before you make ANY changes to the grounding of the electrical system in the setup.

The best way to prevent ground hum is to plan the distribution of power for all the video equipment so that everything is on a common ground. This usually means that all the video equipment is on its own power service and not set up with some things on a power distro panel and some things powered from the wall outlets. Also, take some care in placing video cables on paths that avoid high wattage power cables. Placing a video projection feed on top of the lighting cables that are coiled on the floor is a bad idea to say the least. I once found a hum was caused by a camera cable placed on the power cables for a sub woofer. Every time the kick drum doomed and the base played, a hum distortion appeared in the video screen.

Balancing the load of power on the distro service is also key to avoiding problems with hum in video. Try to have all the equipment spread out on the distro, with sound, lighting and video and their wattage demands even over the three-phase power service. Loading up one phase and neglecting another can cause system wide hum that you cant remove with a humbucker device.

The best description I have found to explain the source and solutions for video hum is from a publication from Video Accessory Corporation and their publication Sync Times.

I have included a copy of this publication with this review.