Articles and whitepapers

Video Switching and Distribution (5/7/2004)

By Nick Mawer

"I want to be able to watch television in more than one room." "I have got a DVD player, VHS recorder and a Freeview box, and I want to choose which one to watch." Is this you or your customer? Perhaps the requirements are more sophisticated, but fundamentally these two scenarios are the jump off point into the world of switching and distribution, or 'signal management.'

Distribution

Distribution is the process of taking the signal from one source and sending it to more than one destination simultaneously, as in the first scenario above. A distribution amplifier (DA) would be used for this task. In order that the outgoing signals from a DA should be at the same level as the incoming signal, they are amplified after they are split.

DAs are described in terms of the number of inputs and outputs, and the signals that they handle, for example you might have a 1:5 Y/C and stereo audio DA. A DA will most probably have only one input, but some can be configured in more than one way: for example the Kramer VM-80V 1:8 composite video DA can also be configured as two 1:4 composite video DAs.

The Kramer VM-80V composite video DA (front and back)

DAs may also have controls such as gain and equalization which boost the outgoing signal to overcome losses incurred on long cable runs. DAs like this are often called 'line drivers.'

Switching

The second scenario above requires 'switching' or 'routing' as it is sometimes referred to. A definition from the ICIA Academy describes switchers in these terms: "Simple switchers can select one signal from multiple sources and send it to a specific destination e.g. plasma or projector. More sophisticated routers can send several signals to several different places." Switchers in this second group are also called 'matrix switchers.' A true matrix switcher has the ability to send any one of its inputs to any one or all of its outputs simultaneously. To achieve this means that they will be powered and have signal amplification circuitry inside.

Switchers are also described in terms of the number of inputs and outputs, and the signals that they handle. So, for example, you might have a 3x1 Y/C switcher or an 8x8 RGBS and stereo audio matrix.

The Kramer VP-88 8x8 RGBHV and Balanced Stereo Audio matrix switcher (front and back)

Switchers need control. This may be from a simple push button on the front panel, via IR, or via a proprietary control system from companies such as Vity, Cue, Crestron or AMX.

Choosing a DA or switcher

To choose which brand of switcher to buy, you will need to talk to a home cinema specialist or installer. The basic consideration is cost versus quality - not of the DA or switcher, but of the whole system. In other words, how much you are prepared to budget for your home cinema or multiroom installation? The more you can afford, the higher quality signals you will be able to enjoy, and the more expensive your switching and distribution system will inevitably be.

Analogue video signals

Let us take a quick look at video signals, from the best quality to the worst, with some brief comments.

At the top of the hierarchy of analogue signals with best quality we have the full bandwidth group. These include RGBHV, RGBS, RGsB and RsGsBs. RGB stands for Red Green and Blue, H,V and S stand for Horizontal Sync, Vertical Sync, and combined Sync. These types of signal require expensive components to handle them correctly, and will increase the cost of the installation because they will require three-, four- or five-core coax cables, or more if the RGBHV video is sent as a VGA-type signal.

The next step down is YUV which you will often find referred to as component, Y/Pr/Pb, or Y/R-Y/B-Y. This is a very good quality signal requiring three-core cables, but it is an artificially compressed signal. The sync information is combined in the Y channel.

Below this is Y/C, also know as S-video. It requires a two-core cable. Although not as good quality as the signals above, it is significantly better quality than composite video. With the latter, all of the picture information is combined on one cable. This makes it cheap to work with, but the colour reproduction is not as good, and is prone to artefacts.

Bottom of the heap in terms of quality is RF (Radio Frequency). This is the signal that combines composite video and audio on one cable, and is what comes out of the TV aerial.

Format conversion

All this is very well, but the average home cinema installation contains source machines that output signals of all different types. Fortunately, we do not need a separate switcher and/or distribution amplifier for each signal type. Format conversion machines (as opposed to standards converters which convert between PAL, NTSC and SECAM) are available to convert up and down between each of the signals in the video hierarchy. The general rule is, if quality is the prime concern, then convert up to the best signal you can, but if cost is the concern, then convert down.

Some switchers have format conversion built in (although this is rare), and some others can handle different signals by ensuring that signals of one type are only switched to outputs that are connected to devices that can handle that signal. The latter type is inevitably more expensive, as it must be able to handle the highest quality signals, i.e. RGBHV.

Some scalers, such as the recently-launched Kramer VP-740, accept virtually any video signal and output them as an RGBHV signal. The outputs of these devices can be tailored to match the exact pixel matrix of the projector, plasma or LCD display. Such switcher/scalers certainly simplify installations and should provide better output images than they receive in. They do however, introduce a certain delay into the video signal - as does any digital processing - such that you hear the soundtrack before the lips move on the film. This can be solved easily by installing audio line delays.

Digital signals

So far, we have just discussed analogue signals, but the future lies in the digital world. SDI (Serial Digital Interface) would certainly be making greater inroads into the home cinema world if only there were more DVD players outputting it, and display devices to accept it. SDI is based on a 270Mb/s transfer rate and supports both component and composite digital video and four groups each of four channels of embedded digital audio.

DVI (Digital Visual Interface) is finding increasing acceptance in the professional AV world, and more and more DVI-compatible display devices and accessories are being manufactured. There are however, difficulties with distributing and switching this signal. Not only is it only capable of being sent short distances without repeaters or incredibly expensive cables, but there are technical reasons why this signal requires all receptor devices to be the same. It is well nigh impossible to matrix.

HDMI (High-Definition Multimedia Interface) supports data rates up to 5Gb/s for high definition video and multichannel audio. It looks set to overcome some of the disadvantages of DVI, but not all of them, and as yet nobody has come up with a way to matrix the signals. However watch this space - this is the cutting edge of technology, and the problems around DVI and HDMI may already have been solved.

Cabling

Cabling is not just a concern for digital signals, but analogue as well. In a multiroom installation where cable runs could be over quite a long distance, some noise can be introduced onto the cables, but more importantly, the attenuation caused by the cable itself will cause the signal to lose amplitude. Many houses are now being 'flooded' (pre-wired) with structured cable (also called 'twisted pair'). Structured cable such as Cat5 is inexpensive and easy to pull. Video signals can also be transmitted further on a structured cable than on coax, provided proper active devices are used to transmit and receive the signal.

Choosing a system

If you are considering a multiroom installation that is going to require switching or distribution, you should speak to a company experienced in home cinema design. You can find such companies through CEDIA. CEDIA is a trade body for qualified, reputable and insured design and installation contractors, and promotes professionalism and honourable business practices in the field of custom installation.

To help choose your switching and distribution system, here is a suggestion: Start with the desire for a home cinema - simple - as in the two scenarios that opened this discussion. Or be as ambitious as you like. Research, talk with an installer and decide what quality of signal you want to view and can afford. Count up the number of source machines that you have, and the number of display devices, and you will have chosen your switching and distribution system. Just because it may be the last part of the home cinema that is chosen, does not mean that it is unimportant. A good switching and distribution system works best when you do not notice it working.

Nick Mawer is the Marketing Manager for Kramer Electronics UK Ltd, manufacturer of distribution amplifiers, switchers, matrix switchers, processors, encoders, decoders, transcoders, and a full range of interfaces in a variety of formats, sizes and prices.

www.kramerelectronics.co.uk