Spectrum efficiency of analogue and digital wireless microphones

The study sought to quantify the relative and absolute spectrum efficiency of digital and analogue wireless microphones.  It was required in the context of the general background of reduced availability of UHF spectrum for PMSE use, and the specific requirement to ensure adequate spectrum resources for the London Olympics in 2012.


The study considered examples of incidences of high density usage of radio microphones, the types of devices available and their technical parameters and undertook laboratory measurements and field trials with the object of understanding the maximum spectrum density at which radio microphones can operate without mutual interference.

Laboratory measurements found that the actual performance of typical professional microphone equipment obtained from hire companies was in close agreement with manufacturers? specifications for sensitivity and adjacent channel selectivity. A more important parameter for determining „spectrum packing density? is the degree to which transmitters or receivers generate „intermodulation products? (IPs). This information is seldom quoted by manufacturers, but laboratory measurements found that radio microphone transmitters generate such IPs at significant levels when two or more devices are separated by less than around 1 metre. If such re-radiated IPs fall on the channel being used by another microphone, interference may be caused, particularly to analogue systems which are more sensitive to interference than the digital systems that are starting to appear on the market. The risk of such interference occurring can often be reduced by simple measures such as ensuring that artists do not leave transmitters active while standing together in the wings.

A series of trials were also conducted to determine the risk of interference between microphones operating at higher-than-usual spectral densities in realistic environments. It was found that up to 15 analogue or digital devices could be used within an 8 MHz TV channel without interference. For the analogue case in particular, the conditions to allow this packing density needed careful control, particularly in respect of the mutual separation between transmitters.

An examination of existing criteria for the frequency assignment of radiomicrophone systems found that the complicated statistics of the situation are not adequately accounted for, and a new approach to modelling was proposed.