What is Diversity and why is it used for Wireless Mics?

Radio Waves are electromagnetic radiation, the same as Light and just as with light, areas can fall into shadow, particularly where there are metal objects which can reflect and absorb RF energy.  But more importantly and in common with the other wave propagated medium we are all familiar with, Sound, radio waves suffer from constructive and destructive interference, or phase cancellation.  This can mean that even in a location of relatively high signal strength loss of signal can still occur.  Where the transmitter or receiver or other RF reflecting objects are moving then phase cancellations can come and go very quickly causing the all too familiar burst of noise or dropout.

There are actually two basic types of diversity applied to radio reception: Frequency diversity and Spacial diversity.  In both cases the aim is to provide different numbers of whole wavelengths from the transmitter to the receiver antenna, or even physically different paths.

Frequency diversity which was used on HF transmissions achieved this by transmitting the same signal on two or more different frequencies.  Any reflections from the ionosphere causing phase cancellation on one frequency would have different phase on the other frequency and therefore not cancel.  Widely different frequencies will also take a different path completely and suffer from a quite different set of losses and reflections.

Spacial diversity traditionally used two antennae and two receivers at the receiving end.  Providing the antennae were far enough apart to ensure that a different combination of phases of signals were present on each, then phase cancellation occurring at the receiving end could be eliminated.  This however was of less use against cancellations which had occurred far away in the ionospheric disturbances of the signals.  It is clear that this system can only work if the antennae are a good proportion of a wavelength apart.

The situation is quite different for VHF and UHF radio mics.  Here we are not concerned with strange distortions of the signals by the upper atmosphere.  The main problem is simple reflection.  RF energy bounces around the rooms and streets where a recordist needs to work.  This can mean that phase cancellation is a major problem in any built up environment.  However, when electromagnetic signals reflect they change their polarisation.  This is why polarising sunglasses reduce reflected glare.  It also means that the mix of signals appearing at the receiver antenna are not simply polarised in the same plane as the transmitting antenna.  Audio Ltd. recognised that this means that the traditional received wisdom that antennae must be at least a wavelength apart for diversity to work is no longer valid for radio mics.  In fact polarisation diversity works exceptionally well with Audio's Dx2020 and Dx2000 receivers.

Diversity is not however an excuse for making insensitive receivers.  There are also shortcut ways of achieving some of the benefits of diversity reception.  For example simply having two antennae and switching to the other one as soon as the signal gets weak or drops sharply on one, hoping that the signal will be better on the other, is a system which has been practiced by some exponents.  There are a number of problems, one in particular, with this scheme which we won't detail here.  Audio took the approach of taking the same full receiver design with full sensitivity and selectivity, and then duplicating it.  Careful monitoring of RF level over a huge dynamic range which is individually tracked on each unit means that the receiver can never switch to a worse signal.  A noiseless switching circuit was developed to take full advantage of the accurate level detection and allow switching as often as needed without noise or clicks.

The combination of these techniques has resulted in a full true dual diversity system which has changed the way many recordists work.

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