 Multiway
Radio Mics - 4 - Radio Mic Problems - 2 |
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Inter-modulation (or Inter-mod), is actually a specific form of interference caused by the mixing of radio signals. If two RF signals are mixed, they produce further RF signals at predictable frequencies - being the sum and differences and multiples thereof. If we take two signals A and B, where A is 860MHz and B is 854MHz, then further signals will be produced at :- A+B = 1714MHz, A-B = 6MHz, 2A + B = 2574MHz, 2A - B = 866MHz, 2B + A = 2568MHz, 2B - A = 848MHz, 3A + B = 3434MHz and so on. Each of these signals will be relatively weak, but remember that each of these signals becomes a new signal which is again mixed. The more signals there are, the more mixing goes on and the more 'cluttered' the RF environment becomes. Remember also that metal objects (scaffolding, lighting trusses) can act as antennae and produce signals. As you can imagine, the use of more than one radio mic system is a potential source of multiple RF signals. Fortunately, as these frequencies are predictable, it is possible to produce frequency plans to try to avoid problematic channels. These are available from our download center, or there is a frequency plan software program which you can download from the Comtek web site. |
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Desensitization (or de-sense) occurs when a receiver gets too strong a signal. This signal can either be your wanted signal, an interfering signal, or a combination of the two. The signal is processed (or 'rectified') by the receiver and because it is too great in level, turns the amplifier transistor 'off'. There is obviously then no signal at the audio 'out' of the receiver. the solution to this problem is to reduce the level of the signal, by either changing the antennae placement, or to insert in-line attenuators. This is the reason why some authorities on radio mics will not recommend the use of active antennae boosters or amplified antennae distribution, as it is more likely that de-sense will happen. It is certainly true that range is very rarely a problem in radio mic systems, as operating ranges are quoted as 300' and in most theatrical situations this is rarely exceeded. |
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