Kitplus - The TV-BAY Magazine

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MONITORING Wireless Production Tools Hints, tips and a word of warning by Alan March Some Do’s and Don’ts Over the years, there have been certain questions or queries that have had a habit of re-occurring. At the risk of teaching you, dear reader, to suck eggs, here are a few of them; Can I use two transmitters with just one receiver? No. With wireless microphones, a dedicated transmitter and receiver are required for each wireless audio link. Furthermore, as the channel count (number of wireless audio links) increases, with analogue wireless systems (probably 99.9% of all of the wireless equipment currently in the market) careful calculations need to be performed in order to ensure that the wireless links do not interfere with each other. This type of interference, also known as intermodulation, can be caused by the additional ‘spikes’ that are generated either side of the main carrier frequency for a given link. If one of these spikes coincides with another frequency that is being used to deliver another audio link, then interference to the second carrier will occur. Fortunately, most, if not all manufacturers, group frequencies into ‘banks’ of compatible (non self-interfering) frequencies. Consequently, when deploying multi-channel systems, users are strongly advised to stick to the frequencies in given banks in order to avoid intermodulation problems. The situation is somewhat different with regards to in-ear monitor systems. In this case, one transmitter can serve as many receivers as can be fi tted into the transmitter’s coverage range. A second, third or fourth (the count can go on) transmitter or transmitters are only required if different performers require a different mix, or content. However, as the channel count increases, the same rules with regards to intermodulation apply. Common practice is to keep any in-ear monitor systems well away, in terms of frequencies used, from wireless microphones. This makes things much easier from a frequency planning point of view. It’s all about antenna position – not receiver position. A common phone call sounds something like this: “I don’t understand it. We ran all of the wireless systems up in sound check and they were fi ne. But once all the conference delegates (you could substitute audience, politicians, press people, here) came into the room some of the systems started dropping out”. One of the biggest absorbers of RF energy is H20 – water. And we, as human beings, are 90% made up of the stuff. All too often, engineers will set their wireless systems up at the mix position, at the back of the room. Consequently, when the room fi lls with people, the relatively small RF output from the transmitter, in particular with body-pack systems given their close proximity to the human body of the presenter (also 90% water), can struggle to reach the receiver antennas – especially if those antennas are hidden away in the back of a rack. Far better to place the antennas and receivers at the stage end of the room and run the resulting audio back to the mix position via a balanced (or increasingly, digital) multicore. Problem solved. Another common industry mistake is to deploy multiple wireless microphone receivers each with their own set of antennas. If they are all in close proximity to each other, either stacked or in a rack, then the antennas will, inevitably, ‘talk’ to each other resulting in interference i.e. bad audio. It’s far better to employ an antenna splitter and have a single pair of remote antennas servicing all of your receivers. Increasingly, remote directional antennas are used to maximise the pick-up of wanted RF signals and to minimise the pick-up of unwanted ones. With in-ear monitors we have a similar situation, but in reverse. Here, for ultimate performance, antenna combiners are used resulting in all mixes (frequencies) being transmitted by a single antenna. Again, directional antennas can help to concentrate the delivery of the desired signal to a given performance area, therefore helping to ensure that the performers receive uninterrupted audio. 52 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 95 NOVEMBER 2014 TV-BAY095NOV14.indd 52 06/11/2014 13:05