Avoiding Frequency Cancellation Using Multiple Microphones in a Surveillance Camera

Sammanfattning: In surveillance it is important that the recordings are accurate and in as high quality as possible. The surveillance surroundings and equipment can alter and change the audio and needs to be compensated for. This master thesis project investigates whether using multiple microphones could improve the audio quality. The project was limited to only use analog electronics and constricted itself to the audible frequency range of 20 Hz to 20 kHz. The projects started with a theoretical study of different concepts of multiple microphones, then one concept was implemented and evaluated. The chosen concept was how to avoid destructive interference using multiple microphones achieving a frequency response without cancelled frequencies. After the prestudy, the concept was realised by filtering the signals from two microphones that were exposed to destructive interference in different frequency ranges. The filter was based on the Linkwitz-Riley crossover filter. Simulations were used to evaluate the filter, witch was then realised in a prototype that was tested in an audio lab. The test result showed that the concept functioned successfully. The filter produced a signal without the frequency losses that each of the microphones experienced.