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When Wires Aren't the Worst Way to Go - TvTechnology

When Wires Aren't the Worst Way to Go

By all accounts in the technical literature and sales propaganda that crosses my desk, the 21st century is set to be the Wireless Age.
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By all accounts in the technical literature and sales propaganda that crosses my desk, the 21st century is set to be the Wireless Age. This should not be confused with the Radio Age that immediately preceded the Television Age, the tail end of which we can still see in our preview monitors. This Wireless Age is about not being shackled by the tyranny of wires or fibers to connect things together.

The current incarnation of a wireless age has arisen out of the ready availability of off-the-shelf, commodity-priced, radio frequency and infrared communication modules. These remove much of the pain and cost of developing an electronic device that incorporates a wireless link.

WIRELESS WIDGETS

Simply tack a wireless link module onto the circuit board of your existing widget and you have the new wireless widget, regardless of whether there's a need or a market for it.

Hence, we have Bluetooth earpieces for our cell phones, wireless keyboards and mice for our computers, wireless entry to our homes, offices and automobiles, wireless doorbells, cordless household phones that now travel in clusters, and outside our kitchen window is the wireless temperature sensor that once a minute, sends an update to the indoor/outdoor thermometer display.

Since the introduction of WiFi wireless data transmission standards, there has been a huge uptake of wireless local area computer networks, driving the development of devices to bridge Ethernet networks.

Since DMX512 is just another digital data stream, and a relatively undemanding one by modern computer data standards, several smart engineers came up with devices to pack one or more DMX data streams into an Ethernet stream.

In addition to allowing multiple DMX universes to be moved over a single Ethernet data cable, these devices also allowed DMX data to be carried wirelessly using ready-made radio link equipment.

A couple of DMX to Ethernet boxes and a couple of WiFi devices could be plugged together to produce a no-brainer wireless DMX link.

The last few years have seen a deluge of purpose-built wireless DMX (WDMX) devices come on to the market. Some are based on general purpose wireless communications modules, although many appear to be based on the 802.11b modules now available from many semiconductor manufacturers.

Unlike the systems built around standard bidirectional WiFi access points, most of the purpose-built WDMX systems consist of dedicated transmitter and receiver devices. As a cost reduction measure, this may have seemed like a good idea, as DMX512A is a broadcast-only protocol. However, the introduction of the remote device management extension to DMX512 has brought with it a need for data to be able to flow in both directions, a requirement that may entail major design modifications.

The main idea promoted by many WDMX marketers is that we can avoid the effort of running out DMX data cables to all those difficult-to-reach lighting positions.

Given that we have to get luminaires dimmers, scrollers, a power supply and a crew into these positions anyway, the elimination of one data feed doesn't seem all that compelling. The other selling point being pushed is that a wireless link eliminates any problems with earth loops, and offers galvanic isolation on the DMX network. This problem is usually solved by our optically isolated DMX buffers and splitters, or in the case of outdoor data runs subject to lighting, by using a run of nonconducting optical fiber.

While the notion of a cordless world may sound very appealing, the reality invariably falls well short of being glorious. For decades, I have privately chortled and publicly commiserated as the poor folk in the audio department struggle to get a reliable 15 kHz bandwidth radio signal from a wireless microphone to a receiver 20 feet away.

BANDWIDTH BEASTIE

As with all audio problems, I was glad that someone else was the reason for the hold-up, allowing me to continue with fine-tuning the lighting without being blamed for lost production time. After all, you can't always rely on the talent to blow their lines on the first three takes while you're waiting for that cutter to be put in place. To put this in perspective, a WDMX link requires 15 times the bandwidth of a radio mic.

Despite all protestations to the contrary, the black art of radio communication remains as unpredictable and unreliable as ever. To make matters worse, the very belief that the Wireless Age is upon us is beginning to look like a self-defeating prophesy. Every gullible wireless enthusiast who buys a cordless phone, a wireless mouse or a WiFi-equipped notebook computer is just adding more radio frequency pollution to the spectrum, especially in the 2.4 GHz industrial scientific and medical band shared by WiFi, Bluetooth, wireless DMX512 and cordless phones, to name a few.

Although many of the new generation of wireless devices use clever spread-spectrum, frequency-hopping modulation systems, there is a very real and finite limit to the amount of data that can be carried reliably in a given chunk of the radio spectrum, and we're well on the way to exceeding it.

As our audio, communications and video brethren discovered some years ago, production management for almost every event now has to include a radio spectrum management plan, to avoid RF anarchy. Bitter experience has already shown that a single unexpected freelance documentary crew at a big event has the potential to cause a domino effect with radio mic channel allocations that can shut down audio for everybody.

Given the limited reliability of wireless data links, the limited availability of usable wireless spectrum and the catastrophic impact of even the smallest glitch in the DMX feed to moving lights, scrollers or media servers, there has to be a very compelling reason to control any element of a lighting system with WDMX.

Maybe if part of your rig is on a barge in a fast moving river with so much water traffic that a length of fiber or data cable is impractical, you might be forced to consider WDMX. Similarly, if you have to control gear mounted on a moving vehicle, then WDMX may be the only answer.

Even in locations where there are few other sources of radio signals present, a sunspot, a dual-path reflection from the script assistant's notepad, or someone opening the trunk of their automobile 100 feet away can be enough to cause a glitch in the data reaching your equipment.

My personal rule is that wireless anything is great for other people, but you wouldn't want to use it yourself if there was any alternative.