Our client asked us to solve a problem whereby police TETRA handsets (hand portables) would emit a loud howling sound when used close together in Direct Mode Operation (DMO). This was a major problem wherever several police officers were grouped together. Our client needed a solution which could be implemented in a firmware upgrade for units in the field.
We set up units in our audio laboratory to research the problem. We found that the root cause was an interaction between the radio frame timing and more classic acoustic feedback. Since standard audio feedback elimination algorithms were unable to prevent the handset howl, we modelled the problem in MATLAB and created a number of candidate algorithms. The best performing of these (based on learning techniques) was prototyped in a real handset and further refined. We delivered the client a C-language reference for the algorithm and a set of tools for training the algorithm for different handset configurations.
Over a period of just 2-3 months we took our client from limited understanding of the problem to a robust solution. The solution was rapidly deployed to units in the field and offered a marked improvement in police handset usability through the reduction in audio howling.