Radio Receivers
There is more 'folk-lore' about radio microphones than almost any other sound subject. Many of these so called rules rely on techniques that appear to work, without any thought as to what is really going on. Radio frequency engineers have been working with radio systems for many years and often discovered the solutions (and usually the reasons behind them) before they were 're-discovered' by the sound system designers. Although I have simplified a large amount of what follows, the basic rules hold true and these comments are factual and have been tested. Legal radio microphone system use low power and should have a range (as per the specification sheets) of 150m or so. For most uses this is certainly quite sufficient. The crucial part of the system is at the receiver end. Types are known as either Diversity or non-Diversity systems. One of the common problems with radio systems is that as the transmitter moves around the receiver picks up a varying strength signal. As long as the level is above a threshold the signal-to-noise ratio is quite satisfactory and many listeners may not realise the system is using wireless technology. However, if the signal level drops below this threshold, then it gets very noisy, very quickly. Very often this happens with little warning. A rock solid signal suddenly flutters and fades and make roaring noises similar to the sound we get when tuning from one radio station to another. Designers came up with a system which had two separate receivers in one box. The receiver switched silently from one to the other so that we always heard the strongest. This diversity system always ensured the strongest signal was the one that fed into the sound system. The real problem with them is that they still switch even when both receivers have a poor signal. In this case the signal-to-noise ratio remains poor. There are many reasons why a signal should appear to be weak, even when it only has to 'limp' a few metres across a stage. One of the reasons often occurs when the building itself has a high metal content. Steel girders and aluminium panels reflect the radio waves from the transmitter and create areas where signal strength can be higher than expected and others where radio waves actually partially cancel each other out. As the path between the source transmitter and the receiving aerial changes the signal strength goes up and down over a large range. Diversity receivers should solve this problem. The fact that they don't, in many cases is mainly due to the two separate aerials on the receiver simply being too close together. They both end up in the same pocket of strong or weak signal. This problem gets worse when we start to add multiple systems, all working on slightly different operating frequencies. It must be remembered that radio waves travel in three dimensions, like light, and as such follow the inverse square law. This means that if we increase the distance between the transmitter and receiver to double the original measurement, then the signal strength at the receiver reduces to a quarter of its original level. This means that the received signal strength from an actor standing next to the receivers could be hundreds or even thousands of time stronger than an actor standing on the other side of the stage. To make things worse, receivers exhibit an effect known as de-sensitisation (DE-SENSE) which effectively makes them 'deaf' when presented with a very strong signal on a nearby channel. The upshot of all this is that for a system to function efficiently it is important to separate the aerials and then get them up and out of the way. The fly rail is a good place in a theatre. They are still reasonably close to the acting area but not too close to any transmitter to allow de-sense to occur. Experiments have shown that the reduction in de-sense very often solves apparent weak signals and in some cases removes the need for diversity systems at all. Many sound people worry about the loss from running RF down long feeder cables, but experience seems to prove that these worries are groundless. Separating the aerials on a diversity system (at least 10m) and remoting both of them from the receiver racks does seem to prove itself sound practice. You may come across front of house engineers who insist on having the receivers at the mix position. Very often the main reason for this is so that they are able to watch the displays on the front of the receivers showing the received signal strength. The main peril to this course of action is that even in a moderate size house the distance between mix position and stage is quite a long path. It doesn't take too much extra absorption or attenuation to bring the system close to the danger threshold