Is digital definitely better?

Along with time, there never seems to be enough room to install all the equipment associated with an AV system; at least not where you need it. Steve Montgomery reports on solutions to the problem.

The AV extender, that useful little pair of transmitter-receiver devices, has for years allowed us to take a composite or component signal and send it over a Cat5 cable to a remote location without any fuss. But the world moves on and we are now all keen to distribute digital signals. Soon everyone and everything will be digital. In the words of Hagai Gefen, President of Gefen Inc.: “We are at the transition stage, with around 40% of our product sales for digital devices.

Take up is increasing as installers become more confident and the price disparity with analogue falls.” Other companies agree on a rise in product sales, however the penetration rates are generally much lower; at around 20% or less. Kamran Ahmed, Senior Technology Advisor, Magenta Research: Digital extenders are nowhere near being mainstream devices yet in commercial digital signage networks, with estimates being only around 20% of sales by revenue, less by volume. We estimate that digital signage, the bulk of our applications, employs around 90-95% VGA extenders.”

Price is an issue, digital devices are, on average, 50-80% more expensive. Another factor is acceptance in an industry where proven technology prevails over new, as Kamran Ahmed points out: “When it comes to ease of installation, analogue VGA extenders, unlike digital ones, are forgiving, plug-and-play devices but like their digital counterparts, still operate at HD rates, right up to 1080p. While the use of resident media players or digital extension systems can be appealing due to the technology hype, VGA based extension systems provide equal capability and quality at a fraction of the price with noticeably lower installation issues - or in the case of HDMI, nightmares – hence, integrators should consider analogue VGA distribution, whenever possible. Another point of note is that the cost of signal distribution is often higher than the cost of the displays or the source. Selection of the right architecture, particularly the cable type and extender capability, can also help future proof the network. Limiting the distribution infrastructure to 720p on cost grounds, when the incremental cost of 1080p is not high, will prove extremely expensive as upgrading the distribution infrastructure is significantly more expensive than upgrading the display.”

Rainer Stiehl, Vice President of Marketing EMEA, Extron Electronics: “Analogue signal distribution is straightforward and with the right equipment and cables, the majority of the systems work flawlessly. Even if the specified maximum length for a transmission system is exceeded it does not mean that the image will be lost: there will be signal quality degradation but still you will get a viewable image in most cases. On the face of it, digital signal transmission offers various benefits over analogue signals, such as no loss in image quality and pixel perfect matching for the displays. However there are serious complications on transmitting digital signals over longer distances. Apart from SDI and HD-SDI all other digital signals were designed for point-to-point connections with relatively short distances. With digital signals the so called ‘cliff’ effect occurs: if you exceed the maximum specified cable length in a digital distribution system you will quickly come to an area where the image is completely lost. There is a very small area where you encounter some visual artifacts caused by transmission errors before you lose the image entirely. DVI and HDMI use up to 6 parallel data streams plus the clock frequency. From the outside it’s very tempting to apply the same system design methodology from the analogue world to digital signals, however digital is an entirely different vehicle and requires a more detailed approach.”

In addition, communication like the DDC communication, based on the I2C bus, which was never intended for longer signal runs in the first place, has to be accommodated. Dave Jones, of distributor Digi-Box explains: “DVI and HDMI transmission includes error correction and requires a bidirectional link as the display communicates with the source to indicate its primary resolution. HDCP requires a continual handshaking between the two so whilst digital extenders do not suffer in the same way from voltage loss and skew, they do require high quality to sustain constant image quality and to avoid signal loss in both directions.” This has brought about an opportunity for additional devices, “Devices such as DVI Detective remove the need for the full return back channel by emulating the display at the transmitter end. It learns the parameters of the display device then sits in the path and tricks the source into believing there is a display present, even if has been removed completely. As a result it can prevent loss of pictures in critical situations.” The Virtual EDID is a similar device from Kramer Electronics.

By far the largest problem at the moment is the use of Cat5 and Cat 6 cables, a fact not taken lightly by Robert Drake, R&D Manager at TV One: “Cat 5 and 6 cables were designed for Ethernet data, not video. Cat 6 cable has different densities of twist which means that individual pairs are of different lengths and cause skew in the electrical signals. It is critical that the digital clock is on the right pair in the cable and that a high quality, well shielded cable is used, after a point the clock cannot be recovered and the cable won’t work. Cat 5 cable has equal twist densities but doesn’t have the bandwidth capability, however we often achieve similar results between the cables. In practice 50 metres is the maximum achievable without repeaters for full HD signals.”

Despite these reservations, all manufacturers are furiously working to produce digital extenders with a variety of interconnection choices. Rainer Stiehl: “The most popular distribution method is the use of inexpensive Category network cables. With our range of DVI and HDMI Twisted Pair extenders we can bridge distances up to 60m depending on the signal resolution and the cable used. These extenders use one UTP or STP Cable for timing and critical high bandwidth signals and a second one for the DDC communication, remote control and remote power if needed. They include an EDID Minder to provide control data from a memory in case no display is connected.” Gefen also offers Category cable devices, Hagai Gefen: “Technology has advanced to the point now where a single cable, rather than two, can be used to transmit a full HDCP-encoded 1920 x 1080 progressive signal over a single Cat 6 cable. Built in signal equalisation and pre-emphasis permits this to be achieved, with the overall effect that colour resolution, balance and stability are superior to analogue.”

The next upward step is to utilise fibre connections, which are capable of transmitting full HD video signals of 1600 x 1200 and 1080p/60 with audio up to 150 metres using multimode cable and 30Km using single mode fibre, Hagai Gefen continues: “In high end installations fibre is a good choice as it does not suffer bandwidth limitations or EMI problems and provides a cleaner, more reliable and robust signal than copper, as well as removing problems associated with different electrical ground potentials that can cause unwanted current flow between conventional transmitters and receivers with the associated risk of device ‘burn-out’ and failure.” Options are available using single or dual fibre cables, again EDID management in the transmitter obviates the need for the second cable if preferred. Typically single mode is used between buildings and multimode or copper would be used within them. Fibre has the added advantage of being nearly impossible to tap unnoticed.

We are also observing a lot of activity in delivering digital signals over non-standard cables. Nick Mawer Marketing Manager of Extron: ”Fibre cables are fragile and need very careful handling and large bend radii in installations. Cat 5/6 is adequate but not robust. In rental and staging applications you cannot drive a fork lift truck over either of these.“ As a consequence, manufacturers offer options that work on BNC cables, including traditional 5-wired RGBHV. This makes them more suited to adverse installations and also the legacy market, Hagai Gefen: “We make a range of extenders that will work on virtually any cable already in the wall. It is seen as a good idea, although as yet there is little take up which we believe is down to industry awareness, so expect greater penetration soon.”

Other manufacturers have looked at applications and customer requirements for signal extenders and solved them using alternative designs. One such is Avocent; another company with a long background in IT telecoms and KWM products. Avocent produce a digital extender that operates over Cat5/6 using IP protocols rather than straight digital transmission. The effect is the same at the receiver end when the signal is decoded back to a DVI or HDMI format. In addition, and because of their heritage, USB, keyboard, mouse, IR blaster and audio are included as well. Chuck Pheterson, VP of Product Marketing at Avocent explains the concept: “The ECMS digital extender operates from point-to-point or through a switched router to offer point-to-multipoint over local networks. Bidirectional operation with the USB facility makes this an ideal device for interactive kiosks and signage applications. Because it is IP based it is better suited to slow-changing content requiring a lower transmission bandwidth, such as computer graphics. For full motion video up to 1080i at 30 fps the wired/wireless MPX range is HDCP compliant. It includes functionality for centrally managing the link and attached devices over the internet, such as turning displays on or off. It can even perform a VGA to DVI conversion if the source is a computer and display needs to be digital. The MPX system works like a video splitter and a video extender by wirelessly delivering audio and video to multiple displays up to 1000 feet away.” Adder is also extending its KVM technology know-how to include IP distribution, Nigel Dickens, Adder’s Technical Director outlines a new product line due for launch later this year: “The Adderlink Infinity range of extenders will distribute digital video together with audio and USB control across any distance, with capacity for thousands of receivers & transmitters, and all in real time so content can be truly live”. This new multicast technology takes advantage of gigabit IP networks and leverages silicon developed by Adventiq to deliver a flexible alternative to more traditional point to point extenders.

Matrox has come up with an entirely different approach to solve the same problem, as explained by Rob Moodey, at Matrox Graphics: “Rather than extend the DVI signal from the source device we take the approach in our Extio Remote Graphics Unit, of extending the computer bus by up to 250 metres over fibre to a dual-head graphic card situated close to the display. This has many advantages: it maintains isolation of the source and display over distance whilst keeping the maximum graphic bandwidth, maintaining internal error correction and allowing multi-display operation over a single fibre link, as well as including a USB2 link.”

In the future there is some evidence that DisplayPort, a lockable digital connector supported by NEC, Dell, Apple and other major manufacturers and already appearing on source and display devices, will be more suited to professional installations in place of HDMI. Digital extenders are becoming available with this type of extender as manufacturers gear up for the expected. Kramer’s VM4DH is one such device, Gefen also has extenders with these connectors and anticipates more widespread use.
We are becoming a little cynical about the fact that ‘digital is always better’ (CD, DAB, Freeview etc) and it is right to question the case here too. The fact is that analogue is more than capable in many cases and should still be the first consideration in installations where special requirements that can only be provided digitally are not needed. The next generation of video extenders could well be network orientated. Chuck Pheterson, amongst others, firmly believes that: “We are at the cusp of a new wave of digital transmission technology which will be IP based.”

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